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making an initial appearance in a population or that is increasing in incidence or geographic range endemic disease disease that is constantly present, usually at low incidence, in a population epidemic disease that occurs in an unusually high number of individuals in a population at the same time extremophile organism that grows under extreme or harsh conditions foodborne disease any illness resulting from the consumption of contaminated food, or of the pathogenic bacteria, viruses, or other parasites that contaminate food Gram negative bacterium whose cell wall contains little peptidoglycan but has an outer membrane Gram positive bacterium that contains mainly peptidoglycan in its cell walls halophile organism that require a salt concentration of at least 0.2 M hydrothermal vent fissure in Earth’s surface that releases geothermally heated water 924 Chapter 22 | Prokaryotes: Bacteria and Archaea hyperthermophile organism that grows at temperatures between 80–122 °C microbial mat multi-layered sheet of prokaryotes that may include bacteria and archaea MRSA (methicillin-resistant Staphylococcus aureus) very dangerous Staphylococcus aureus strain resistant to multiple antibiotics nitrification conversion of ammonium into nitrite and nitrate in soils nitrogen fixation ammonia process by which gaseous nitrogen is transformed, or “fixed” into more readily available forms such as nodule novel structure on the roots of certain plants (legumes) that results from the symbiotic interaction between the plant and soil bacteria, is the site of nitrogen fixation nutrient essential substances for growth, such as carbon and nitrogen osmophile organism that grows in a high sugar concentration pandemic widespread, usually worldwide, epidemic disease peptidoglycan material composed of polysaccharide chains cross-linked to unusual peptides phototroph organism that is able to make its own food by converting solar energy to chemical energy pilus surface appendage of some prokaryotes used for attachment to surfaces including other prokaryotes pseudopeptidoglycan different sugars component of archaea cell walls that is similar to peptidoglycan in morphology but contains psychrophile organism that grows at temperatures of –15 °C or lower radioresistant organism that grows in high levels of radiation resuscitation process by which prokaryotes that are in the VBNC state return to viability S-layer surface-layer protein present on the outside of cell walls of archaea and bacteria serotype strain of bacteria that carries a set of similar antigens |
on its cell surface, often many in a bacterial species stromatolite layered sedimentary structure formed by precipitation of minerals by prokaryotes in microbial mats teichoic acid polymer associated with the cell wall of Gram-positive bacteria thermophile organism that lives at temperatures between 60–80 °C transduction process by which a bacteriophage moves DNA from one prokaryote to another transformation process by which a prokaryote takes in DNA found in its environment that is shed by other prokaryotes viable-but-non-culturable (VBNC) state survival mechanism of bacteria facing environmental stress conditions zoonosis disease that primarily infects animals that is transmitted to humans CHAPTER SUMMARY 22.1 Prokaryotic Diversity Prokaryotes existed for billions of years before plants and animals appeared. Hot springs and hydrothermal vents may have been the environments in which life began. Microbial mats are thought to represent the earliest forms of life on Earth, and there is fossil evidence of their presence about 3.5 billion years ago. A microbial mat is a multi-layered sheet of prokaryotes that grows at interfaces between different types of material, mostly on moist surfaces. During the first 2 billion years, the atmosphere was anoxic and only anaerobic organisms were able to live. Cyanobacteria evolved from early This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 925 phototrophs and began the oxygenation of the atmosphere. The increase in oxygen concentration allowed the evolution of other life forms. Fossilized microbial mats are called stromatolites and consist of laminated organo-sedimentary structures formed by precipitation of minerals by prokaryotes. They represent the earliest fossil record of life on Earth. Bacteria and archaea grow in virtually every environment. Those that survive under extreme conditions are called extremophiles (extreme lovers). Some prokaryotes cannot grow in a laboratory setting, but they are not dead. They are in the viable-but-non-culturable (VBNC) state. The VBNC state occurs when prokaryotes enter a dormant state in response to environmental stressors. Most prokaryotes are social and prefer to live in communities where interactions take place. A biofilm is a microbial community held together in a gummy-textured matrix. 22. |
2 Structure of Prokaryotes Prokaryotes (domains Archaea and Bacteria) are single-celled organisms lacking a nucleus. They have a single piece of circular DNA in the nucleoid area of the cell. Most prokaryotes have a cell wall that lies outside the boundary of the plasma membrane. Some prokaryotes may have additional structures such as a capsule, flagella, and pili. Bacteria and Archaea differ in the lipid composition of their cell membranes and the characteristics of the cell wall. In archaeal membranes, phytanyl units, rather than fatty acids, are linked to glycerol. Some archaeal membranes are lipid monolayers instead of bilayers. The cell wall is located outside the cell membrane and prevents osmotic lysis. The chemical composition of cell walls varies between species. Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have peptidoglycan, but they may have pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based cell walls. Bacteria can be divided into two major groups: Gram positive and Gram negative, based on the Gram stain reaction. Gram-positive organisms have a thick cell wall, together with teichoic acids. Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins. 22.3 Prokaryotic Metabolism Prokaryotes are the most metabolically diverse organisms; they flourish in many different environments with various carbon energy and carbon sources, variable temperature, pH, pressure, and water availability. Nutrients required in large amounts are called macronutrients, whereas those required in trace amounts are called micronutrients or trace elements. Macronutrients include C, H, O, N, P, S, K, Mg, Ca, and Na. In addition to these macronutrients, prokaryotes require various metallic elements for growth and enzyme function. Prokaryotes use different sources of energy to assemble macromolecules from smaller molecules. Phototrophs obtain their energy from sunlight, whereas chemotrophs obtain energy from chemical compounds. Prokaryotes play roles in the carbon and nitrogen cycles. Carbon is returned to the atmosphere by the respiration of animals and other chemoorganotrophic organisms. Consumers use organic compounds generated by producers and release carbon dioxide into the atmosphere. The most important contributor of carbon dioxide |
to the atmosphere is microbial decomposition of dead material. Nitrogen is recycled in nature from organic compounds to ammonia, ammonium ions, nitrite, nitrate, and nitrogen gas. Gaseous nitrogen is transformed into ammonia through nitrogen fixation. Ammonia is anaerobically catabolized by some prokaryotes, yielding N2 as the final product. Nitrification is the conversion of ammonium into nitrite. Nitrification in soils is carried out by bacteria. Denitrification is also performed by bacteria and transforms nitrate from soils into gaseous nitrogen compounds, such as N2O, NO, and N2. 22.4 Bacterial Diseases in Humans Devastating diseases and plagues have been among us since early times. There are records about microbial diseases as far back as 3000 B.C. Infectious diseases remain among the leading causes of death worldwide. Emerging diseases are those rapidly increasing in incidence or geographic range. They can be new or re-emerging diseases (previously under control). Many emerging diseases affecting humans, such as brucellosis, are zoonoses. The WHO has identified a group of diseases whose re-emergence should be monitored: Those caused by bacteria include bubonic plague, diphtheria, and cholera. Biofilms are considered responsible for diseases such as bacterial infections in patients with cystic fibrosis, Legionnaires’ disease, and otitis media. They produce dental plaque; colonize catheters, prostheses, transcutaneous, and orthopedic devices; and infect contact lenses, open wounds, and burned tissue. Biofilms also produce foodborne diseases because they colonize the surfaces of food and food-processing equipment. Biofilms are resistant to most of the methods used to control microbial growth. The excessive use of antibiotics has resulted in a major global problem, since resistant forms of bacteria have been selected over time. A very dangerous strain, methicillin-resistant Staphylococcus aureus (MRSA), has wreaked havoc recently. Foodborne diseases result from the consumption of contaminated food, pathogenic bacteria, viruses, or parasites that contaminate food. 926 Chapter 22 | Prokaryotes: Bacteria and Archaea 22.5 Beneficial Prokaryotes Pathogens are only a small percentage of all prokaryotes. In fact, our life would not be possible without prokaryotes. Nitrogen is usually the most limiting element in terrestrial ecosystems; atmospheric nitrogen, |
the largest pool of available nitrogen, is unavailable to eukaryotes. Nitrogen can be “fixed,” or converted into ammonia (NH3) either biologically or abiotically. Biological nitrogen fixation (BNF) is exclusively carried out by prokaryotes. After photosynthesis, BNF is the second most important biological process on Earth. The most important source of BNF is the symbiotic interaction between soil bacteria and legume plants. Microbial bioremediation is the use of microbial metabolism to remove pollutants. Bioremediation has been used to remove agricultural chemicals that leach from soil into groundwater and the subsurface. Toxic metals and oxides, such as selenium and arsenic compounds, can also be removed by bioremediation. Probably one of the most useful and interesting examples of the use of prokaryotes for bioremediation purposes is the cleanup of oil spills. Human life is only possible due to the action of microbes, both those in the environment and those species that call us home. Internally, they help us digest our food, produce crucial nutrients for us, protect us from pathogenic microbes, and help train our immune systems to function correctly. REVIEW QUESTIONS 1. Which is the best evidence that prokaryotes evolved about 3 billion years ago? a. Scientists believe photosynthesis evolved about 3.0 billion years ago. b. There is fossil evidence of mammalian forms going back about 4.0 billion years. c. Earth and its moon are thought to be about 4.5 billion years old. d. There is fossil evidence of microbial mats—large multi-layered sheets of prokaryotes—starting about 3.5 billion years ago. 2. Which statement describing the environment of early Earth is false? a. The atmosphere contained much less molecular oxygen. b. Strong volcanic activity was common. c. It was subject to mutagenic radiation from the Sun. d. There was little to no geologic activity. 3. Which type of extremophile grows optimally at temperatures of –15 to 10 ºC or lower? a. alkaliphiles b. thermophiles c. hyperthermophiles d. psychrophiles 4. Which is an example of a relatively moderate environmental condition to which some prokaryotes are adapted and can survive as spores? a. extremely low temperature b. hypersalinity c. high doses of radiation d. normal drought 5. Over _____ percent of bacteria and archaea |
cannot be successfully cultured in a laboratory setting. a. 9 b. 19 c. 91 d. 99 6. The most substantial difficulty in culturing prokaryotes in laboratory settings is related to _____. a. the lack of knowledge about their needs for growth b. growth requirements that are too difficult to meet c. d. inefficient methods for resuscitation of viable but nonculturable (VBNC) organisms the expense of techniques such as polymerase chain reaction (PCR) 7. Which of the following represents the earliest forms of life on Earth? a. hydrothermal vent b. microbial mat c. meteorite d. stromatolite 8. Which best summarizes the conditions of early Earth at the time that life first evolved? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 927 a. The atmosphere of early Earth was very different a. eukaryotic cells from today’s atmosphere, but most other conditions (such as geologic upheaval and volcanic activity) were very much the same. b. The atmosphere of early Earth was very much like today’s atmosphere, but many other conditions (such as geologic upheaval and volcanic activity) were very different. c. Early Earth had a very different atmosphere, was subject to extreme radiation, and had a lot of geologic upheaval and volcanic activity. d. Early Earth had a very different atmosphere and was subject to extreme radiation, but there was very little geologic upheaval or volcanic activity. 9. Halophiles prefer conditions in which there is a _____. a. high sugar concentration b. salt concentration of at least 0.2 M c. pH of 3 or below d. high level of radiation 10. The presence of a membrane-enclosed nucleus is a characteristic of ____. a. prokaryotic cells b. eukaryotic cells c. all cells d. viruses 11. All prokaryotic and eukaryotic cells have four structures in common: the plasma membrane, the cytoplasm, nucleic acid, and ____. a. b. c. the cell wall ribosomes the nucleus d. organelles 12. Which statement comparing the prokaryotes Bacteria and Archaea is false? a. The cytoplasm of both bacterial and archaean prokaryotic cells has a high concentration of dissolved sol |
utes. b. Osmotic pressure in both types of prokaryotic cells is relatively high. c. The domains Bacteria and Archaea differ in the use of fatty acids versus phytanal groups in their cell membranes. d. The domains Bacteria and Archaea have very similar cell wall structure. 13. Pseudopeptidoglycan is a characteristic of the walls of some _____. b. bacterial prokaryotic cells c. archaean prokaryotic cells d. bacterial and archaean prokaryotic cells 14. The cell wall, a feature of most prokaryotes, is _______. a. interior to the cell membrane b. exterior to the cell membrane c. a part of the cell membrane d. interior or exterior, depending on the particular cell 15. Which statement summarizes what is known about macronutrient needs of prokaryotes? a. Boron is required in small amounts by some prokaryotic organisms. b. Manganese is required in small amounts by some prokaryotic organisms. c. Iron is required in small amounts by some prokaryotic organisms. d. Sulfur is needed in large amounts by prokaryotic organisms. It is part of the structure of some amino acids and is also present in some vitamins and coenzymes. 16. Which statement about the importance of particular nutrients is false? a. Carbon is a macronutrient and major element in all macromolecules. b. Nitrogen is a macronutrient and necessary component of proteins and nucleic acids. c. Hydrogen is a macronutrient and key component of many organic compounds, including water. d. Iron is a macronutrient necessary for the function of cytochromes. 17. What are prokaryotes that obtain their energy from chemical compounds called? a. phototrophs b. autotrophs c. chemotrophs d. heterotrophs 18. What uses organic compounds as both an energy source and as a carbon source? a. chemolithotrophs b. photoautotrophs c. photoheterotrophs d. chemoorganotrophs 19. A primary role of many prokaryotes in the carbon cycle is that of ____. 928 Chapter 22 | Prokaryotes: Bacteria and Archaea a. producers b. decomposers c. d. fixers synthesizers 20. Ammonification is |
the process by which ____. a. ammonia is released during the decomposition of nitrogen-containing organic compounds b. ammonium is converted to nitrite and nitrate in soils c. nitrate from soil is transformed to gaseous nitrogen compounds d. gaseous nitrogen is fixed to yield ammonia 21. Which is a macronutrient needed by prokaryotes? a. phosphorus b. iron c. chromium d. boron a. Biofilms are related to foodborne illnesses because they colonize food surfaces and foodprocessing equipment. b. In healthcare environments, biofilms grow on ventilators, shunts, and other medical equipment. c. Biofilms tend to colonize medical devices such as prostheses, contact lenses, and catheters. d. Biofilms form in open wounds, burned tissue, or internal medical devices such as pacemakers. 26. Which best describes the crisis related to antibiotics? a. b. It is becoming too expensive to manufacture effective antibiotics. It takes too much time to develop effective antibiotics; infections spread before treatment is available. c. Bacteria are increasingly resistant to antibiotics used to treat and eradicate infections. d. People are increasingly allergic to antibiotics commonly used in treatment. 22. A disease that is constantly present in a population is called _______. 27. Which statement about the cause of resistant bacteria is false? a. pandemic b. endemic c. emerging d. re-emerging 23. Which set of terms names diseases caused by bacteria? a. diptheria, bubonic plague, yellow fever b. yellow fever, dengue fever, bubonic plague c. bubonic plague, diptheria, cholera d. cholera, diptheria, dengue fever 24. Which of the following health issues is caused by biofilm colonization? a. dental plaque b. dry scalp c. skin rash d. prosthetic discomfort 25. Which of the statements about the loci of biofilmrelated disease is false? a. The excessive use of antibiotics has resulted in the natural selection of resistant forms of bacteria. b. Antibiotics are used by patients with colds or the flu, the treatment for which antibiotics are useless. c. There is excessive use of antibiotics in livestock and in animal feed. d. Antibiotics are used by patients of different ages and the fact that their ages differ increases resistance. 28. Which statement about diseases is false? a. An epidemic is |
a disease that occurs in a high number of individuals in a population at a time. b. A pandemic is a widespread, usually worldwide, epidemic. c. An endemic disease is a disease that is constantly present, usually at high incidence, in a population. d. An emerging disease is a disease that has appeared in a population for the first time. 29. Which statement best explains which organisms need nitrogen fixation and why? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 929 a. The foods taste better. b. Nutrients are preserved. c. The food is less stable. d. Nutrients were safer. 33. Which best defines bioremediation? a. b. c. the use of microbial metabolism to clean up oil spills the use of microbial metabolism to ferment food the use of microbial metabolism to remove pollutants d. the use of microbial metabolism to fix nitrogen 34. Which statement about bioremediation is false? a. b. c. d. It includes removing agricultural chemicals. It includes removing industrial by-products. It includes cleaning up oil spills. It includes cleaning up ammonia in soil. 35. Nitrogen is an essential element that is widely available in the atmosphere. Because eukaryotes cannot use nitrogen in its gaseous form, they benefit from prokaryotes’ conversion of gaseous nitrogen to ____. a. nitrates, a form of nitrogen they can use b. phosphate, a different essential element they can use c. ammonia, a form of nitrogen they can use d. hydrogen, a different essential element they can use a. Prokaryotes cannot use gaseous nitrogen to synthesize macromolecules, so it must be converted into ammonia. b. Prokaryotes cannot use ammonia to synthesize macromolecules, so it must be converted into gaseous nitrogen. c. Eukaryotes cannot use ammonia to synthesize macromolecules, so it must be converted into gaseous nitrogen. d. Eukaryotes cannot use gaseous nitrogen to synthesize macromolecules, so it must be converted into ammonia. 30. Which statement about nitrogen fixation is false? a. b. c. d. It can be accomplished abiotically, as a result of lightning. It can be accomplished abiotically, as a result of industrial processes |
. It can be accomplished biologically, by algae. It can be accomplished biologically, by cyanobacteria. 31. Which are three foods for which prokaryotes are used in their processing? a. cheese, yogurt, and milk b. cheese, yogurt, and bread c. wine, bread, and butter d. milk, wine, and beer 32. What was the initial benefit for humans in processing foods with prokaryotes? CRITICAL THINKING QUESTIONS 36. Explain the relationship between Earth’s ancient atmosphere and the evolution of some of the first life forms on Earth. Use the terms anaerobicandphototrophic, and explain the effect of cyanobacteria on the atmosphere. 930 Chapter 22 | Prokaryotes: Bacteria and Archaea a. Phototrophic organisms appeared during the first two billion years of Earth’s existence. Anaerobic organisms appeared within one billion years of Earth’s formation. From these organisms evolved the cyanobacteria which produce oxygen as a by-product of photosynthesis, leading to the oxygenation of the atmosphere. b. For the first two billion years of Earth’s existence, the atmosphere had no molecular oxygen. Thus, the first organisms were anaerobic. Cyanobacteria appeared within one billion years of Earth’s formation. From these evolved the phototrophic organisms which produce oxygen as a by-product of photosynthesis, leading to the oxygenation of the atmosphere. c. For the first two billion years of Earth’s existence, the atmosphere had no molecular oxygen. Thus, the first organisms were anaerobic. Phototrophic organisms appeared within one billion years of Earth’s formation. From these organisms evolved the cyanobacteria, which produce oxygen as a byproduct of photosynthesis, leading to the oxygenation of the atmosphere. d. For the first two billion years of Earth’s existence, the atmosphere had no molecular oxygen. Thus, the first organisms were anaerobic. Cyanobacteria which produce oxygen as a by-product of photosynthesis, leading to the oxygenation of the atmosphere, appeared within one billion years of Earth’s formation. From these organisms evolved phototrophic organisms. 37. Extremophiles are considered an important area for research in the development of therapeutic drugs or industrial applications. Why do you think this is so? a. Extremophiles can be altered genetically in vitro to allow them to live in extreme conditions and this capability of alteration can be |
used to help humans. For example, some water-resistant prokaryotes have developed DNA repair mechanisms. Also, they could be developed and used in the treatment of human disease. b. Extremophiles have specialized adaptations that allow them to live in extreme conditions. These adaptations can be mobilized to help humans. For example, some water-resistant prokaryotes have developed DNA repair mechanisms. Also, they could be developed and used in the treatment of human disease. c. Extremophiles can be altered genetically in vitro to allow them to live in extreme conditions and this capability of alteration can be used to help humans. For example, some radiation-resistant prokaryotes have developed DNA repair mechanisms. Also, they could be developed and used in the treatment of human disease. d. Extremophiles have specialized adaptations that allow them to live in extreme conditions. These adaptations can be mobilized to help humans. For example, some radiation-resistant prokaryotes have developed DNA repair mechanisms. Also, they could be developed and used in the treatment of human disease. 38. Describe briefly how you would detect the presence of a non culturable prokaryote in an environmental sample. a. Recombinant DNA techniques are used to detect the presence of a non-culturable prokaryote in an environmental sample. Polymerase chain reaction is used to amplify selected portions of prokaryotic DNA. b. Molecular biology techniques are used to detect the presence of a non-culturable prokaryote in an environmental sample. Electrophoresis is used to amplify selected portions of prokaryotic DNA. c. Molecular biology techniques are used to detect the presence of a non-culturable prokaryote in an environmental sample. Polymerase chain reaction is used to amplify selected portions of prokaryotic DNA. d. Recombinant DNA techniques are used to detect the presence of a non-culturable prokaryote in an environmental sample. Electrophoresis is used to amplify selected portions of prokaryotic DNA. 39. Why do scientists believe that the first organisms on Earth were extremophiles? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 931 a. Typical cells in Archaea and Bacteria contain a cell wall, cell membrane, nucleoid region, ribosomes, and often a capsule, flage |
llum, and pili. However, these are sometimes made from different chemical compounds. Cell walls of Bacteria contain peptidoglycan while Archaea do not. Plasma membrane lipids of Bacteria are fatty acids while those of Archaea are phytanyl groups. b. Typical cells in Archaea and Bacteria contain a cell wall, cell membrane, nucleoid region and often a capsule, flagellum, and pili but in some instances different chemical compounds make them. Cell walls of Bacteria contain peptidoglycan while Archaea do not. Bacteria contain 70S ribosomes while Archaea contain 80S ribosomes. c. Typical cells in Archaea and Bacteria contain a cell wall, nuclear membranes, nucleoid region and often a capsule, flagellum, and pili but in some instances different chemical compounds make them. Cell walls of Bacteria contain peptidoglycan while Archaea do not. Plasma membrane lipids of bacteria are fatty acids, while the plasma membrane lipids of Archaea are phytanyl groups. d. Typical cells in Archaea and Bacteria contain a cell wall, cell membrane, nucleoid region and often a capsule, flagellum, and pili but in some instances different chemical compounds make them. Cell walls of Bacteria contain peptidoglycan while Archaea do not. Plasma membrane lipids of Bacteria are phytanyl groups, while the plasma membrane lipids of Archaea are fatty acids. 42. Three basic prokaryotic categories are cocci, spirilli, and bacilli. Describe the basic structural features of each category. a. Earth’s early environment was full of extreme places with much oxygen in the atmosphere, no ozone to shield Earth’s surface from mutagenic radiation, much geologic upheaval and volcanic activity. Extremophiles are bacteria and archaea that are adapted to grow in extreme environments. b. Earth’s early environment was full of extreme places with little oxygen in the atmosphere, no ozone to shield Earth’s surface from mutagenic radiation, much geologic upheaval and volcanic activity. Extremophiles are bacteria and archaea that are adapted to grow in extreme environments. c. Earth’s early environment was full of extreme places with little oxygen in the atmosphere, no ozone to shield Earth’s surface from mutagenic radiation, less geologic upheaval and volcanic activity. Extremophiles are bacteria and archaea that are adapted to grow in |
extreme environments. d. For the first two billion years of Earth’s existence, the atmosphere had no molecular oxygen. 40. Describe a typical prokaryotic cell. a. b. c. d. It has a cell wall enclosing cell membrane, cytoplasm, ribosomes and nucleoid region with genetic material. It may have a protective capsule, flagellum, pili and plasmids. It has a cell wall enclosing cell membrane, cytoplasm, ribosomes and nucleus containing genetic material. It may have a protective capsule, flagellum, pili and plasmids. It has a cell wall enclosing nuclear membrane, cytoplasm, ribosomes and nucleoid region with genetic material. It may have a protective capsule, flagellum, pili and plasmids. It has a cell wall enclosing nuclear membrane, cytoplasm, mitochondria, vacuoles and nucleoid region with genetic material. It may have a protective capsule, flagellum, pili and plasmids. 41. Explain the statement that both Archaea and Bacteria have the same basic structures, but these structures are built from different chemical components. 932 Chapter 22 | Prokaryotes: Bacteria and Archaea a. These three prokaryote groups have similar basic structural features. They typically have cell walls enclosing nuclear membranes, cytoplasm, ribosomes, mitochondria and nucleoid region with genetic material. They may have a protective capsule, flagellum, pili and plasmids. b. Cocci and spirilli have similar basic structural features. They typically have cell walls enclosing cell membranes, a flagellum for locomotion and pili for attachment. Bacilli are rod shaped which contain ribosomes and a nucleoid region with genetic material. c. These three prokaryote groups have similar basic structural features. They typically have cell walls enclosing cell membranes, cytoplasm, ribosomes and a nucleoid region with chromosomes. They may have a protective capsule, flagellum, pili and plasmids. d. Bacilli and spirilli have similar basic structural features. They typically have cell walls enclosing nuclear membranes, a flagellum for locomotion and pili for attachment. Cocci are spherical containing ribosomes and a nucleoid region with genetic material. 43. Which macronutrient do you |
think is most important? What evidence can you offer to support your choice? a. Carbon because it represents 12 percent of the total dry weight of a typical cell and is a component of all macromolecules. b. Oxygen because it is necessary and is a major component for all macromolecules. It also accounts for 50% of the total composition of a cell. c. Carbon because it is necessary and is a major component for all macromolecules. It also accounts for 50% of the total composition of a cell. d. Nitrogen because it is necessary and is a major component for all macromolecules. It also accounts for 50% of the total composition of a cell. 44. A bacterium requires only a particular amino acid as an organic nutrient and lives in a completely lightless environment. What mode of nutrition (free energy and carbon) does it use? Justify your response. a. Chemoheterotroph, as it must rely on chemical sources of energy living in a lightless environment and a heterotroph if it uses organic compounds for its carbon source. b. Chemoorganotroph, as it must rely on chemical sources of energy living in a lightless environment and an organotroph if it uses organic compounds other than carbon dioxide for its carbon source. c. Chemolitoautotroph, as it must rely on chemical sources of energy living in a lightless environment and an autotroph if it uses organic compounds other than carbon dioxide for its carbon source. d. Chemoheterotroph, as it must rely on chemical sources of energy living in a lightless environment and a heterotroph if it uses organic compounds other than carbon dioxide for its carbon source. 45. Assuming that you could synthesize all of the nitrogencontaining compounds needed if you had nitrogen, what might you eat for a typical meal if you could fix nitrogen like some prokaryotes? a. My meal might be fruits or vegetables and water as nitrogen is present in the highest amount in water. b. My meal might be fruits or vegetables, water and air as atmospheric nitrogen could be simply absorbed. c. My meal might be fruits or vegetables, cheese, meat, water, and air as atmospheric nitrogen could be simply absorbed. d. My meal might be cheese or meat, water, and air as atmospheric nitrogen could be simply absorbed. 46. Which are more important: macronutrients or micronutrients? Explain your reasoning. a. Neither are important, as |
cells can survive as well as carry out essential functions without both types of nutrients. b. Micronutrients, even though they are required in lesser amounts, without them cells cannot survive and carry out functional processes. c. Macronutrients, as they are required in larger amounts by cells and thus are more essential than micronutrients. d. Neither is more important as both types of nutrients are absolutely necessary for prokaryotic cell structure and function. 47. Identify and discuss a bacterial disease that caused a historically important plague or epidemic. What is the modern distribution of this disease? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 933 50. What was the Plague of Athens? What is the modern distribution of this disease? a. The Plague of Athens was a disease caused by Yersinia pestis that killed one-quarter of Athenian troops in 430 BC. Between 10 and 15 million cases of typhoid fever occur today, resulting in over 10, 000 deaths annually. b. The Plague of Athens was a disease caused by Salmonella entericaserovar typhi that killed onequarter of Athenian troops in 430 BC. Between 5 and 10 million cases of typhoid fever occur today, resulting in over 20, 000 deaths annually. c. The Plague of Athens was a disease caused by Yersinia pestis that killed one-quarter of Athenian troops in 430 BC. Between 16 and 33 million cases of typhoid fever occur today, resulting in over 200,000 deaths annually. d. The Plague of Athens was a disease caused by Salmonella entericaserovar typhi that killed onequarter of Athenian troops in 430 BC. Between 16 and 33 million cases of typhoid fever occur today, resulting in over 200,000 deaths annually. 51. Identify three beneficial results of symbiotic nitrogen fixation. a. Plants benefit from an endless supply of nitrogen; soils benefit from being naturally fertilized; and bacteria benefit from using potassium from plants. b. Plants benefit from a limited supply of nitrogen; soils benefit from being naturally fertilized, and bacteria benefit from using photosynthates from plants. c. Plants benefit from an endless supply of carbon; soils benefit from being naturally fertilized; and bacteria benefit from using photosynthates from plants. d. Plants benefit from an endless supply of nitrogen; soils benefit from being naturally |
fertilized; and bacteria benefit from using photosynthates from plants. 52. Why is the processing of foods with prokaryotes considered an example of early biotechnology? a. Bubonic plague caused by Yersinia pestis was a pandemic that occurred in the 14th century. In modern times, there are only about 100 cases of bubonic plague each year. The bacterium responds well to modern antibiotics. b. Bubonic plague caused by Yersinia enterocolitica was a pandemic that occurred in the 14th century. In modern times, there are about 1,000 to 3,000 cases of bubonic plague each year. The bacterium responds well to modern antibiotics. c. Pneumonic plague caused by Yersinia pestis was a pandemic that occurred in the 14th century. In modern times, there are about 1,000 to 3,000 cases of pneumonic plague each year. The bacterium responds well to modern antibiotics. d. Bubonic plague caused by Yersinia pestis was a pandemic that occurred in the 14th century. In modern times, there are about 1,000 to 3,000 cases of bubonic plague each year. The bacterium responds well to modern antibiotics. 48. Have foodborne illnesses related to biofilms changed over time? Explain. a. Yes, better sterilization and canning procedures have reduced the incidence of botulism. Most cases of foodborne illness now are related to small-scale food production. b. No, better sterilization and canning procedures have reduced the incidence of botulism. Most cases of foodborne illness now are related to small-scale food production. c. No, better sterilization and canning procedures have increased the incidence of botulism. Most cases of foodborne illnesses now are related to large-scale food production. d. Yes, better sterilization and canning procedures have reduced the incidence of botulism. Most cases of foodborne illnesses now are related to large-scale food production. 49. What is the relationship between MRSA and the problem of antibiotic resistance? a. b. c. d. Indiscriminate use of antibiotics results in the population growth of resistant bacteria like MRSA. Infrequent use of antibiotics results in the population growth of resistant bacteria like MRSA. Indiscriminate use of antibiotics results in the population decline of resistant bacteria like MRSA. Infrequent use of antibiotics results in the population stability of resistant bacteria like MR |
SA. 934 Chapter 22 | Prokaryotes: Bacteria and Archaea 54. Why is the relationship between sustainable agriculture and nitrogen fixers called a mutualism? a. Due to agrobacterium which are nitrogen fixers, plants benefit from an endless supply of nitrogen; soils benefit from being naturally fertilized and bacteria benefit from using photosynthates from plants. b. Due to rhizobia, which are nitrogen fixers, plants benefit from an endless supply of nitrogen; soils benefit from being naturally fertilized and bacteria benefit from using photosynthates from plants. c. Due to rhizobia, which are nitrogen fixers, plants benefit from an endless supply of nitrogen; soils benefit from being naturally fertilized and bacteria benefit from using potassium from plants. d. Due to rhizobia, which are nitrogen fixers, plants benefit from a limited supply of nitrogen; soils benefit from being naturally fertilized and bacteria benefit from using potassium from plants. a. Prokaryotes have been used to only make specific food products like cheese, wine, bread, beer and yogurt since before the term biotechnology was coined. b. Prokaryotes have been used to make and alter specific food products like cheese, wine, single cell proteins, beer and yogurt since before the term biotechnology was coined. c. As prokaryotes have been used to make and alter specific food products like cheese, wine, bread, beer and yogurt since before the term biotechnology was coined. d. As prokaryotes have been used to alter specific food products like cheese, wine, bread, beer and yogurt since before the term biotechnology was coined. 53. On what does the success of bioremediation of oil spills depend? a. Success depends on the presence of only aromatic and highly branched hydrocarbon chain compounds and the temperature. b. Success depends on the presence of less nonvolatile and more aromatic and highly branched hydrocarbon chain compounds and the temperature. c. Success depends on the type of oil compounds, the presence of naturally-occurring oilsolubilizing prokaryotes in the ocean, and the type of water body. d. Success depends on the type of oil compounds, the presence of naturally-occurring oilsolubilizing prokaryotes in the ocean and the temperature. TEST PREP FOR AP® COURSES 55. Which of the following pieces of evidence is the BEST support for the alternative scenario of early life formation, in |
which organic compounds on early Earth formed near submerged volcanoes? and Urey repeated their experiment without the electrical discharge, no organic compounds were found. Hypothesize what might explain this result. Consider your answer in the context of the conditions of early Earth. a. Some prokaryotes that live near deep-sea vents today use hydrogen as an energy source. b. Fossilized stromatolites that are 3.5 billion years a. The lack of organic compounds without the sparks indicates that organic components are formed from biotic components. old are found near deep-sea vents. b. The first trial of the experiment must have been c. Extremophiles that exist today live in a variety of extreme environments, including those that are high in salinity. d. The chemical composition of water around deepsea vents is the same as it was on early Earth. 56. Stanley Miller and Harold Urey conducted experiments which demonstrated that several organic compounds could be formed spontaneously by simulating the conditions of Earth's early atmosphere. When Miller done incorrectly. c. Abiotic molecules can only develop into organic molecules in the presence of oxygen, so oxygen should be added. d. Lightning, or some form of energy, is needed for the inorganic molecules in the atmosphere to interact with each other. This indicates that a similar energy source was present on early Earth which stimulates the interaction and development. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 935 57. Laboratory experiments have demonstrated that the abiotic synthesis of organic molecules in condition similar to those of early Earth is possible. Which of the following provides additional support for the idea of abiotic synthesis of organic compounds? a. Analysis of the chemical composition of meteorites sometimes yields amino acids. b. A hydrothermal vent in the Sea of Cortés releases hydrogen sulfide and iron sulfide. c. Researchers have dripped solutions of amino acids onto hot surfaces to produce amino acid polymers. d. Some present-day prokaryotes live and reproduce in very extreme and unforgiving environments, such as the Arctic. 58. Which of the following cell types does Figure 22.10 illustrate? how this strain could pose a health risk to people is false? a. Genes for antibiotic resistance are transferred from the nonpathogenic bacterium to a pathogenic bacterium via transduction. b. Genes for antibiotic resistance are |
transferred from the nonpathogenic bacterium to a pathogenic bacterium via transformation. c. Genes for antibiotic resistance are transferred from the nonpathogenic bacterium to a pathogenic bacterium via conjugation. d. Genes for antibiotic resistance are transferred from the nonpathogenic bacterium to a pathogenic bacterium via binary fission. 61. In a rapidly changing environment, which prokaryotic population would you hypothesize likely to be more successful: one that included individuals capable of conjugation or one that did not? a. A population including individuals capable of conjugation would be more successful because all of its members would form recombinant cells having new gene combinations advantageous in a new environment. b. A population including individuals capable of conjugation would be more successful as some members could form recombinant cells having new gene combinations advantageous in a new environment. c. A population including individuals not capable of conjugation would be more successful as the members undergoing conjugation would form new recombinant cells having gene combinations lethal in the new environment. d. A population including individuals not capable of conjugation would be more successful because conjugation will result in an increase in genetic diversity of the prokaryotic population which will be disadvantageous in a new population. 62. Plates that have only ampicillin-resistant bacteria growing include which of the following? a. Plant cell b. Animal cell c. Bacterial cell d. Fungal cell 59. Which option best describes the function and presence of cell organelles among prokaryotes and eukaryotes? a. Ribosomes are the sites of protein synthesis found in prokaryotic and eukaryotic cells. Cell wall is a protective layer, typical in prokaryotic cells and in some eukaryotes. Chromosomal DNA, the genetic material of the cell is present in a nucleoid region in prokaryotes while enclosed in a nucleus in eukaryotes. b. Ribosomes are the sites of protein synthesis found in prokaryotic and eukaryotic cells. The cell wall is a protective layer found in some prokaryotic and eukaryotic cells. Chromosomal DNA is the genetic material of the cell present in a nucleoid region in prokaryotes while in eukaryotes, it is enclosed in a nucleus. c. Ribosomes are sites of ATP production found in both prokaryotic and eukaryotic cells. |
The cell wall is a protective layer, typically found in prokaryotic cells and in some eukaryotes. Chromosomal DNA is present in a nucleoid region while enclosed in a nucleus in eukaryotes. It is the genetic material of the cell. d. Ribosomes are the sites of protein synthesis found in prokaryotic and eukaryotic cells. The cell wall is a protective layer, typically found in prokaryotic cells and in some eukaryotes. Chromosomal DNA, the genetic material of the cell is present in a nucleus in prokaryotes while it is enclosed in a nucleoid region in eukaryotes. 60. A nonpathogenic bacterium acquires resistance to antibiotics. Which of the following scenarios describing 936 Chapter 22 | Prokaryotes: Bacteria and Archaea a. All E. coli cells were not successfully transformed on plate IV. b. The nutrient agar medium inhibited the growth of some bacteria on plate IV. c. All E. coli cells were successfully transformed on plate IV. d. The bacteria in plate III were naturally resistant to ampicillin. 64. Which of the labeled structures in the diagram allows you to positively identify the cell as a prokaryote? a. b. c. d. I only III only IV only I and II 63. Given your understanding of the experiment and of bacterial genetic recombination, explain why there are fewer colonies on plate IV than on plate III. a. A, circular DNA b. B, ribosome c. C, cell wall d. D, cytoplasm 65. A bacterial species that is a methanogen is discovered. If you wanted to build on this discovery to better understand the evolution of mechanisms related to the ability to capture, store, and use free energy in prokaryotes, which question would you pose to answer? a. Have metabolic pathways evolved separately in Bacteria and Archaea? b. Should all methanogens be classed as Archaea in evolutionary phylogeny? c. Have methanogens evolved to live in both moderate and extreme environments? d. Did the methanogenic bacteria species also evolve as a strict anaerobe? 66. What is another question you might pose to learn more about the structural features that allow for the capture, storage, and use of free energy by archaean methanogens? This OpenStax book is available for free at http://cnx.org/content/col |
12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 937 a. Do archaean methanogens differ from other Archaea structurally, and if so, in what way? Is one or more of these structural differences related to these methanogens’ ability to use H2 to oxidize CO2? b. Do archaean methanogens differ from other Bacteria structurally, and if so, in what way? Is one or more of these structural differences related to these methagens’ ability to use CO2 to oxidize H2? c. Do archaean methanogens differ from other Archaea structurally, and if so, in what way? Is one or more of these structural differences related to these methagens’ ability to use CO2 to oxidize H2? d. Do archaean methanogens differ from other Archaea structurally, and if so, in what way? Is one or more of these structural differences related to these methagens’ ability to use H2O to oxidize H2? 67. Which set of phrases related to nutritional and metabolic adaptations best fits the organisms described? a. chemoautotrophs, obligate anaerobes b. chemoautotrophs, faculative anaerobes c. chemoheterotrophs, faculative anaerobes d. chemoheterotrophs, obligate anaerobes 68. In an experiment, researchers grew plant seedlings in soils to which one of two strains of bacteria were added. A control group had no bacteria added to the soil. The seedlings’ uptake of the nutrient potassium increased dramatically in the soil with Strain 1 and decreased dramatically in the soil with Strain 2. What specific and broad inferences about the relationship between the bacteria, the seedlings, and available nutrients can you make? a. The Strain 2 bacteria increased the availability of potassium in the soil and this nutrient was needed and used by the seedlings in the soil. The Strain 1 bacteria decreased the availability of potassium in the soil. b. The soil with Strain 1 bacteria must have had more potassium in comparison to soil with Strain 2 bacteria. The seedlings took up more potassium in Soil 1 than in 2 due to this. c. The Strain 1 bacteria increased the availability of potassium in the soil and this nutrient was needed and used by the seedlings in the soil. The |
Strain 2 bacteria decreased the availability of potassium in the soil. d. The Strain 1 bacteria decreased the availability of potassium in the soil and this nutrient was needed and used by the seedlings in the soil. The Strain 2 bacteria increased the availability of potassium in the soil. 69. In a hypothetical research situation, scientists discover bacterial endospores in silt at the bottom of a marsh that has been contaminated with a pollutant for 25 years. The silt of the marsh was deposited in annual layers. The age of the endospores can be estimated, then, by identifying the layer of silt in which the endospores are found. In flask A, researchers place 20-year-old endospores along with growth medium and the pollutant. In flask B, researchers place 100-year-old endospores along with growth medium and the pollutant. Which statement describes the results you would expect to see in the growth of the flasks? a. The growth in flask A will exceed that of flask B. b. The growth in flask B will exceed that of flask A. c. The growth each flask will be about equal. d. There will be little to no growth in each flask. 70. In a hypothetical research situation, scientists discover bacterial endospores in silt at the bottom of a marsh that has been contaminated with a pollutant for 25 years. The silt of the marsh was deposited in annual layers. The age of the endospores can be estimated, then, by identifying the layer of silt in which the endospores are found. In flask A, researchers place 20-year-old endospores along with growth medium and the pollutant. In flask B, researchers place 100-year-old endospores along with growth medium and the pollutant. Explain why you would expect to see more growth in one particular flask than in the other. 938 Chapter 22 | Prokaryotes: Bacteria and Archaea a. Because endospores formed 20 years ago would be more dormant compared to endospores formed 100 years ago, before the marsh was polluted. b. Because endospores formed 20 years ago would be less adapted to polluted conditions compared to endospores formed 100 years ago, before the marsh was polluted. c. Because endospores formed 20 years ago would be more adapted to polluted conditions compared to endospores formed 100 years ago, before the marsh was polluted. d. Because endospores formed 20 years ago would |
be less dormant compared to endospores formed 100 years ago, before the marsh was polluted. 71. In a hypothetical research situation, scientists discover bacterial endospores in silt at the bottom of a marsh that has been contaminated with a pollutant for 25 years. The silt of the marsh was deposited in annual layers. The age of the endospores can be estimated, then, by identifying the layer of silt in which the endospores are found. In flask A, researchers place 20-year-old endospores along with growth medium and the pollutant. In flask B, researchers place 100-year-old endospores along with growth medium and the pollutant. Suppose the researchers observe the flasks for a while, continuing to replenish growth medium and pollutant as necessary. Which statement describes the results you would expect to see in the growth of the flasks after some time? a. The growth in flask A will continue to exceed that of flask B. b. The growth in flask B will exceed continue to exceed that of flask A. c. Eventually, the difference in the growth in each flask will lessen. d. Eventually, will be little to no growth in each flask. 72. In a hypothetical research situation, scientists discover bacterial endospores in silt at the bottom of a marsh that has been contaminated with a pollutant for 25 years. The silt of the marsh was deposited in annual layers. The age of the endospores can be estimated, then, by identifying the layer of silt in which the endospores are found. In flask A, researchers place 20-year-old endospores along with growth medium and the pollutant. In flask B, researchers place 100-year-old endospores along with growth medium and the pollutant. Suppose the researchers observe the flasks for a while, continuing to replenish growth medium and pollutant as necessary. If the difference in the growth in each flask lessened after some time, which statement explains why? a. Because the endospores formed 100 years ago, before the marsh was polluted, they would evolve resistance to the pollutant fairly quickly. The bacteria in flask B would then grow more prolifically, and the difference in population size of each flask would lessen. b. Because the endospores formed 20 years ago would lose their resistance to the pollutant. The bacteria in flask A would die, and the difference in population size of each flask would lessen. c. Because the endosp |
ores formed 100 years ago, before the marsh was polluted, they would lose their resistance to the pollutant. The bacteria in flask B would then grow more prolifically, and the difference in population size of each flask would lessen. d. Because the endospores formed 20 years ago would evolve resistance to the pollutant fairly quickly. The bacteria in flask A would die, and the difference in population size of each flask would lessen. 73. How does resistance spread in bacteria? a. By undergoing genetic recombination through conjugation, transduction, and transformation. b. By undergoing reproduction through binary fission. c. By undergoing genetic recombination through conjugation and transformation only. d. Reproduction among bacteria through any mechanism results in the spread of antibiotic resistance genes. 74. Health officials worldwide are concerned about antibiotic resistance in bacteria that cause disease. In patients infected with nonresistant strains of the bacterium that causes tuberculosis, antibiotics can relieve symptoms fast—in as short of a time as a few weeks. However, it takes much longer to stop infection entirely, and patients may discontinue treatment once symptoms are abated. In a hypothetical study, researchers found a much higher incidence of recurrent tuberculosis infection in patients who discontinued treatment once symptoms were relieved, but before the planned course of treatment was complete. Which statement best explains this result? a. The wrong course of antibiotics was used on the patient, so the infection was never treated. b. Not all of the bacteria were killed, and the remaining ones reproduced and bring back the symptoms of infection. c. The antibiotics were not prescribed for a long enough time to treat the infection. d. The infection was actually viral in nature, and so the antibiotics were a useless treatment. 75. Human intestines are home to hundreds of species of This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea 939 bacteria. One of these, Bacteriodes thetaiotaomicron, has the capability of digesting complex plant materials that human enzymes cannot digest. Its presence in the human guts makes a significant contribution to human metabolic processes.Which term best describes the relationship between humans and B. thetaiotaomicron? a. commensalistic b. mutualistic c. parasitic d. pathogenic 76. If you suddenly and dramatically changed your diet, how might this affect the diversity of prokaryotic species that live |
in your intestine? a. The diversity would not get altered and would remain the same. b. Species abundance and relative distribution may increase. c. Species abundance and relative distribution may get affected. d. Species abundance and relative distribution may decrease. 77. More than 100 bacterial species live on the surface of the human body. Bacteria cover portions of human skin in concentrations of up to 8 million cells per square centimeter. In particular, human sebaceous glands support the growth of the bacterium Propionibacterium acnes, which uses oil from the glands for food. Two strains of P. acnes are associated with the development of acne on human skin, but other strains are associated with healthy skin. Which statement best describes the relationship between humans and P. acnes? a. b. c. d. In some cases it is commensal and in others it is parasitic. In some cases it is mutualistic and in others it is commensalistic. It is almost always parasitic. It is almost always mutualistic. SCIENCE PRACTICE CHALLENGE QUESTIONS 78. That the uniformity of cell size in prokaryotes is independent of the conditions of cell growth has long been a puzzle. Suppose that cells grew for a random period of time and then divided. The largest and smallest, by sometimes dividing to make even larger or smaller cells, would be expected to broaden the distribution of cell sizes, as shown in the diagram on the left for a time, t2, after a time t1. Competing claims are made to explain the fact, however, that the distribution does not broaden: 1) There is a “timer” that initiates cell division, and 2) there is a volume threshold that, when reached, initiates cell division. Recently (Amir, Phys. Rev. Lett, 2014), a third model was suggested: From the end of the last cell division to the next, the cell volume increases by a constant value. Figure 22.30 A. Justify the claim of the third model by i) rejecting the two alternative claims, using the fact that growth rate depends on the availability of resources and considering that regulation of expression at a critical volume would require measurement of total volume by the cell, and ii) arguing that adding a constant volume before each cell division would narrow the cell size distribution. B. Design a plan to test both the most recent model and the timer model. 79. Gram-negative bacteria have an inner cytoplasm |
ic membrane separated by a peptidoglycan layer from a second outer membrane. In addition, transport proteins called efflux pumps span this double membrane and actively eliminate chemicals such as antibiotics that pass through porins on the outer membrane. These efflux pumps can confer multi-drug resistance, a situation that is threatening human health. A. Explain how combining a drug that disrupts ATP synthesis in bacteria with antibiotics is a possible strategy for the treatment of bacterial infections caused by antibiotic-resistant gram-negative bacteria. ATP synthesis in prokaryotes is accomplished by a protein that connects the extracellular space to the cytoplasm. In gram-negative bacteria, the proton gradient that supplies the free energy to convert ADP into ATP is established across the inner membrane. B. Predict differences in the interactions of eukaryotic and prokaryotic cells with a drug molecule that successfully targets ATP synthesis and provide reasoning for your 940 predictions. In gram-positive bacteria, ATP synthesis is accomplished by a protein that spans the single membrane and the outer cell wall. During the production of yogurt and wine, which rely on gram-positive bacteria, the pH is controlled. Sodium bicarbonate secretions from the pancreas maintain the pH of the human intestine, where many beneficial methanogens are gram-positive bacteria. C. Explain why homeostasis for gram-positive bacteria requires control of extracellular pH. 80. Cyanobacteria are single-celled organisms with the capacity to fix nitrogen, N2. Some cyanobacteria cooperatively aggregate as filaments, and heterocysts may form at intervals along the filament between a pair of vegetative (actively growing) cells. Heterocysts are specialized cells that express certain genes when nitrogen becomes limiting. The nitrogenase complex converts the nitrogen in N2 into NH3 (ammonia). This enzyme functions only under anaerobic conditions that are, in part, enforced by an O2 barrier surrounding the cytoplasm of the heterocyst, as shown below. Figure 22.31 A. Other modifications displayed in the diagram maintain an anaerobic state and synthesize ammonia from N2. Identify four modifications of vegetative cells, either by their addition to or omission from the heterocyst. Refine This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 22 | Prokaryotes: Bacteria and Archaea the representation by drawing a line between |
each of the three numbered circles and the feature. B. Further refine the representation by providing a brief description of the role of each modification in either regulating oxygen or synthesizing ammonia. The Krebs cycle in prokaryotes and eukaryotes differs. In prokaryotes, the Krebs cycle occurs in the cytoplasm and the intermediate 2-oxoglutarate (?-ketoglutarate) is absent. C. Construct a representation of the regulation of genes encoding the nitrogen fixation system using the elements below. The irregular shapes are either metabolites or transcription factors, NtcA, HetR, and PatS. In your representation, label each shape using the names on the left in the figure below. Your representation must account for these observations: • when nitrogen is limiting, 2-oxoglutarate concentration in the cytoplasm increases • HetR is transcribed when 2-oxoglutarate concentrations are low • PatS is transcribed when 2-oxoglutarate concentrations are low • nitrogenase is transcribed when HetR concentrations are high and PatS concentrations are low • when PatS concentrations are high, nifX genes are not transcribed Figure 22.32 D. Heterocysts form along the filament separated by a fixed number of vegetative cells. Based on your model of the regulation of heterocyst development, make and support a claim that accounts for this pattern. 81. Escherichia coli Strain A is able to grow in a minimal medium only when supplemented with methionine and biotin. Strain B is able to grow in a minimal medium only when supplemented with threonine, leucine, and thiamine. The two strains are incubated together in a medium containing each supplement. They are then transferred to a minimal medium with no supplements, and each strain is able to grow under these conditions. A. Describe the evidence that supports information exchange between Strain A and Strain B, and the mechanisms that can account for this behavior demonstrated by Lederberg and Tatum (Nature, 1946). Colistin is regarded as a last-resort antibiotic in the treatment of multi-drug-resistant, gram-negative bacteria. The MCR-1 gene that confers colistin resistance was recently detected in a plasmid found in E. coli from the Chapter 22 | Prokaryotes: Bacteria and Archaea 941 intestines of human patients (Liu et al, Lancet Infect |
. Dis., 2016). Colistin is cheap to produce, is often used as a feed supplement for domesticated animals (12,000 metric tons per year in 2015), and its use is increasing. Colistin is also unstable in water (Healan et al., Antimicrob. Agents Chemother, 2012). B. Describe the possible biological consequences of an immediate ban on the use of colistin in agriculture. 82. Life on Earth is sustained by four processes that are unique to prokaryotes: 1) methanogens reduce hydrogen or carbon atoms to produce methane; 2) methanotrophs combine methane with oxygen to form formaldehyde; 3) nitrogen fixation converts N2 into ammonia; and 4) nitrification converts ammonia into nitrates. These processes recycle matter, maintaining the carbon (1 and 2) and nitrogen (3 and 4) cycles. Methanogens are strictly anaerobic. Estimates of global fluxes of methane from major sources (Kirschke, Nature Geoscience, 2013, in units of 1012 g C/year) are shown in the figure below. Agricultural sources are predominately the microbiomes of ruminants (cows, goats, etc.) and rice cultivated in shallow ponds where anoxic compost and crop residues promote methanogen growth on roots. Other major human activities that contribute to atmospheric methane levels are landfills and natural gas drilling. Figure 22.33 The fate of this methane is also shown. Most reacts with OH in the lower atmosphere to make formic acid, which then decomposes into carbon dioxide and water. Methanotrophs consume the remaining methane. Methane is a component of the carbon cycle, but it is much less significant than carbon dioxide, whose major fluxes are shown in units of 1015 g C/year (NASA, 2015). Oceanic uptake and loss of CO2 are primarily abiotic. Prokaryotic marine organisms account for approximately 50% of the biotic exchanges. Figure 22.34 A. Compare quantitatively the rates of carbon cycling as methane between the biosphere and atmosphere. Calculate the percentage of methane production that is anthropocentric (due to human actions). B. Assuming that the rates of carbon dioxide exchange shown in the diagram are accurate, analyze these data to identify a missing contribution to the carbon budget. Recently, it was discovered that ruminants fed nitrooxypropanoic acid reduced their methane release from digestion by approximately 50% and increased the rate of meat production by |
as much as 80% (E. Duin et al., Proc. Natl. Acad. Sci, 2016). C. Since methane is a greenhouse gas, its release into the atmosphere further increases global temperatures. It has been claimed that a feed supplement program will reduce the effects of climate change. Predict the consequences of such a program and provide reasoning for your prediction. A vertica<|endoftext|>l profile of methane and oxygen below the surface of a rice paddy are shown in the graph below (Lee et al., Front. Microbiol, 25, 2015). Also shown are estimates of the relative abundance of all genera of methanotrophs (red line) and methanogens (blue line) as a function of depth. Rice paddies are the largest contributor to agricultural methane production. The estimates were based on extraction and analysis of ribosomal RNA from the soil. 942 Chapter 22 | Prokaryotes: Bacteria and Archaea undigested fraction. The large intestine of the adult human has a length of approximately 1.5 meters with a volume between 6 and 7 liters. The total volume of gut microbes is just a few hundred milliliters. B. Predict the length of a large intestine with equivalent recovery of resources and the same transit times through the bowel if, rather than 100 trillion organisms with a total volume of 1 L, there were 100 billion (1011) organisms, each with a volume of 10-8 mL (the approximate volume of the epithelial cells lining the intestine). The relationship between gut microbes and their host is more complex than simple resource recovery, as shown in the figure of the microbiome below. PYY is a hormone that works with the enteric nervous system lining the intestinal wall to cause changes in the period of contractions of muscles (motility) that push material through the intestine. C. Based on the diagram, summarize the regulation of appetite by the microbiome and the elimination of waste by the host in terms of feedback loops and chemical signaling. Figure 22.35 D. Justify the selection of these measurements of the concentrations of two types of microbes and the gases that are consumed or produced to the development of a quantitative understanding of the habitat range of both groups and the control of methane release from rice fields. 83. The human gut provides a habitat for approximately 100 trillion bacteria. Some sources claim that the surface area of the cells lining the small and large intestines is between 150 and 300 square meters and compare this area to that of a |
tennis court. Recent measurements, however, show that the surface area of the gut is closer to that of a studio apartment (Helander and Fandriks, Jour. Gastro, 2014) and is roughly 50 square meters. A. Calculate the cellular surface area of the 100 trillion (1014) microbes in the typical human gut, assuming that the cells are spherical with an average radius of 0.001 mm. Use this calculated surface area to predict the relative rates of procurement of nutrients by both microbes and the host cells lining the large and small intestines. Humans compete with microbes for nutrients, but the relationship is mutually beneficial. Between 10 and 30% of ingested food remains undigested before reaching the large intestine. Some microbial waste products, particularly H2 and CH4, are not resources for the host. But short-chain fatty acids like acetic, propionic, and butyric acids are resources that microbes extract from the This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Figure 22.36 The microbial population of the intestine is referred to as the microbiome. Undernutrition and obesity are both symptoms of malnutrition, and populations of the microbiome vary with the type of malnutrition (Brown et al., Nutr Clin Pract, 2012). The microbiome of humans Chapter 22 | Prokaryotes: Bacteria and Archaea 943 can be transplanted into germ-free (GF) mice to observe the effects of diet in a controlled experiment of relatively short duration. The microbiomes of healthy and undernourished 6-month-old children were transplanted into GF mice whose growth is graphed below. Growth in both length and weight were reduced when the source of the microbiome was the undernourished child (after Blanton et al., Science, 2016). Both groups of mice were provided with the same nutritional resources. The data (after Schwarzer et al., Science, 2016) show concentrations of this growth factor in mice with no microbiome (GF), wild-type mice whose microbiome and growth provide a control (WT), and mice whose microbiome population is composed entirely of Lactobacillus plantarum (two strains labeled L1 and L2). Lactobacillus is one of many hundred genera of microbial inhabitants of a healthy human intestine. Figure 22.37 D. Pose two scientific questions that, when investigated, could lead to a solution for the stunting of growth caused by undernourishment in early infancy that |
affects millions of children. Human growth hormone stimulates the release of insulinlike growth factor 1 (IGF-1). IGF-1 is a messenger that activates the production of bone cells called osteocytes. Figure 22.38 E. Analyze these data in terms of the potential for disruption of human bone growth due to loss or reduction in diversity of the microbiome. 944 Chapter 22 | Prokaryotes: Bacteria and Archaea This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 945 23 | PLANT FORM AND PHYSIOLOGY Figure 23.1 A locust leaf consists of leaflets arrayed along a central midrib. Each leaflet is a complex photosynthetic machine, exquisitely adapted to capture sunlight and carbon dioxide. An intricate vascular system supplies the leaf with water and minerals, and exports the products of photosynthesis. (credit: modification of work by Todd Petit) Chapter Outline 23.1: The Plant Body 23.2: Stems 23.3: Roots 23.4: Leaves 23.5: Transport of Water and Solutes in Plants 23.6: Plant Sensory Systems and Responses Introduction Plants are as essential to human existence as land, water, and air. Without plants, our day-to-day lives would be impossible because without oxygen from photosynthesis, aerobic life cannot be sustained. From providing food and shelter to serving as a source of medicines, oils, perfumes, and industrial products, plants provide humans with numerous valuable resources. When you think of plants, those that first come to mind are likely to be vascular plants. These plants have tissues that conduct food and water (the word “vascular” means “having vessels”). While each vascular plant species is unique, all are made up of a plant body consisting of stems, roots, and leaves. They also all transport water, minerals, and sugars produced through photosynthesis through the plant body using the same mechanism, and they all respond to environmental factors, such as light, gravity, competition, temperature, and predation. Scientists recently discovered that two plants, the giant goldenrod and spicebush, each make five different compounds that disrupt the life cycles of insect pests. Further investigation into the chemicals could lead to a new class of pesticides. You can read more about it in ScienceNews magazine (http://openstaxcollege.org/l/32pest) |
. [1] 1. Seok-Hee Lee, Hyun-Woo Oh, Ying Fang, Saes-Byeol An, Doo-Sang Park, Hyuk-Hwan Song, Sei-Ryang Oh, Soo-Young Kim, Seonghyun Kim, Namjung Kim, Alexander S. Raikhel, Yeon Ho Je, and Sang Woon Shin. “Identification of Plant Compounds that Disrupt the Insect Juvenile Hormone Receptor Complex.” PNAS 112.6 (2015) : 1733–1738; published ahead of print January 26, 2015, doi: 10.1073/pnas.1424386112 946 Chapter 23 | Plant Form and Physiology 23.1 | The Plant Body In this section, you will explore the following questions: • What are differences between the shoot organ system and the root organ system? • What are differences between meristematic tissues and permanent tissues? • What are the three regions where plant growth occurs? • What are the role of dermal tissues, vascular tissues, and ground tissues? • What is the difference between simple plant tissues and complex plant tissues? Connection for AP® Courses Much of the content in this chapter is not within the scope of AP®, including information about the different kinds of tissues that comprise the plant body. However, the evolution of vascular tissue made possible the transition of plants from aquatic to terrestrial environments. Xylem and phloem transport water, minerals, and sugars produced through photosynthesis through the plant body (see the Transport of Water and Solutes in Plants module). In addition all plant species respond to environmental factors, such as light, gravity, competition, temperature, and predation (see the Plant Sensory Systems and Reponses module). Like animals, plants contain cells with organelles, in which specific metabolic activities occur, and specialized tissues and organs. Unlike animals, plant use energy from sunlight to synthesize sugars during photosynthesis—creating the food that supports life on this planet. Without plants, life on Earth could not exist. With the exception of vascular tissue—which we will explore in detail in the Transport of Water and Solutes in Plants module—information presented in this section, and the examples highlighted, does not align to the content and AP® Learning Objectives outlined in the AP® Curriculum Framework. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for |
the AP exam. These questions address the following standards: [APLO 2.3][APLO 2.4][APLO 2.28][APLO 4.15][APLO 4.14][APLO 4.21] Like eukaryotes, plants contain cells with organelles in which specific metabolic activities take place. Unlike animals, however, plants use energy from sunlight to form sugars during photosynthesis. In addition, plant cells have cell walls, plastids, and a large central vacuole: structures that are not found in animal cells. Each of these cellular structures plays a specific role in plant structure and function. Watch Botany Without Borders (http://openstaxcollege.org/l/botany_wo_bord), a video produced by the Botanical Society of America about the importance of plants. When the link opens to the page “Botany Without Borders” click on the menu item, “Plants Are Cool Too!” View the videoAngiosperms: The Secrets of Flowers, by Botanical Society of America (BSA) member Kate March, and answer the question below.Which group of plants dominates the landscape on Earth? a. conifers b. mosses c. ferns d. flowering plants This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 947 Plant Organ Systems In plants, just as in animals, similar cells working together form a tissue. When different types of tissues work together to perform a unique function, they form an organ; organs working together form organ systems. Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists of two portions: the vegetative (nonreproductive) parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which supports the plants and absorbs water and minerals, is usually underground. Figure 23.2 shows the organ systems of a typical plant. Figure 23.2 The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the plant while absorbing water and minerals from the soil. Plant Tissues Plants are multicellular eukaryotes with tissue systems made of |
various cell types that carry out specific functions. Plant tissue systems fall into one of two general types: meristematic tissue, and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing. Meristematic tissues consist of three types, based on their location in the plant. Apical meristems contain meristematic tissue located at the tips of stems and roots, which enable a plant to extend in length. Lateral meristems facilitate growth in thickness or girth in a maturing plant. Intercalary meristems occur only in monocots, at the bases of leaf blades and at nodes (the areas where leaves attach to a stem). This tissue enables the monocot leaf blade to increase in length from the leaf base; for example, it allows lawn grass leaves to elongate even after repeated mowing. Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main types: dermal, vascular, and ground tissue. Dermal tissue covers and protects the plant, and vascular tissue transports water, minerals, and sugars to different parts of the plant. Ground tissue serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to store water and sugars. Secondary tissues are either simple (composed of similar cell types) or complex (composed of different cell types). Dermal tissue, for example, is a simple tissue that covers the outer surface of the plant and controls gas exchange. Vascular tissue is an example of a complex tissue, and is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes three different cell types: vessel 948 Chapter 23 | Plant Form and Physiology elements and tracheids (both of which conduct water), and xylem parenchyma. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of four different cell types: sieve cells (which conduct photos |
ynthates), companion cells, phloem parenchyma, and phloem fibers. Unlike xylem conducting cells, phloem conducting cells are alive at maturity. The xylem and phloem always lie adjacent to each other (Figure 23.3). In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder. Figure 23.3 This light micrograph shows a cross section of a squash (Curcurbita maxima) stem. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue. (credit: modification of work by "(biophotos)"/Flickr; scale-bar data from Matt Russell) 23.2 | Stems In this section, you will explore the following questions: • What is the main function and basic structure of a plant stem? • What are the roles of dermal tissues, vascular tissues, and ground tissues? • What is the difference between primary growth and secondary growth in stems? • What is the origin of annual rings in stems? How are annual rings used to approximate the age of a tree? • What are examples of modified stems? Connection for AP® Courses Much content described in this section is not within the scope of AP®. You are not required to memorize the different types of tissues that comprise the plant stem. However, in the Transport of Water and Solutes in Plants module we will explore in detail the roles vascular tissues (xylem and phloem), epidermal guard cells, stomata, and trichomes play in transpiration, the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes—hair-like structures on the epidermal surface—also defend leaves against predation (see the Plant Sensory Systems and Reponses module). Except for the concepts described in the AP® Connection, information presented in this module, and the examples highlighted, does not align to the content and AP® Learning Objectives outlined in the AP® Curriculum Framework |
. Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 949 provide support to the plant, holding leaves, flowers and buds; in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant. Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (Figure 23.4). Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud. Figure 23.4 Leaves are attached to the plant stem at areas called nodes. An internode is the stem region between two nodes. The petiole is the stalk connecting the leaf to the stem. The leaves just above the nodes arose from axillary buds. Stem Anatomy The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are the most common plant cells (Figure 23.5). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such |
as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch. 950 Chapter 23 | Plant Form and Physiology Figure 23.5 The stem of common St John's Wort (Hypericum perforatum) is shown in cross section in this light micrograph. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis) are composed of parenchyma cells. Vascular tissue composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith. (credit: Rolf-Dieter Mueller) Collenchyma cells are elongated cells with unevenly thickened walls (Figure 23.6). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells. Figure 23.6 Collenchyma cell walls are uneven in thickness, as seen in this light micrograph. They provide support to plant structures. (credit: modification of work by Carl Szczerski; scale-bar data from Matt Russell) Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope (Figure 23.7). This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 951 Figure 23.7 The central pith and outer cortex of the (a) flax stem are made up of parenchyma cells. Inside the cortex is a layer of sclerenchyma cells, which make up the fibers in flax rope and clothing. Humans have grown and harvested flax for thousands of years. In (b) this drawing, fourteenth-century women prepare linen. |
The (c) flax plant is grown and harvested for its fibers, which are used to weave linen, and for its seeds, which are the source of linseed oil. (credit a: modification of work by Emmanuel Boutet based on original work by Ryan R. MacKenzie; credit c: modification of work by Brian Dearth; scale-bar data from Matt Russell) Students are examining stem cross-sections under the microscope and sketching their observations. As they are labeling the different tissues, they realize that they labeled different parts of the stem as parenchyma. Which part of the stem is made of parenchyma cells? a. The cortex and pith are made of parenchyma cells. b. The companion cells of the phloem are parenchyma cells. c. Fiber cells of the sclerenchyma d. Sieve elements and tracheids of the xylem Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival. Dermal Tissue The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark, which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place (Figure 952 Chapter 23 | Plant Form and Physiology 23.8). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores. Figure 23.8 Openings called stomata (singular: stoma) allow a plant to take up carbon dioxide and release oxygen and water vapor. The (a) colorized scanning-electron micrograph shows a closed stoma of a dicot. Each stoma is flanked by two guard cells that regulate its (b) opening and |
closing. The (c) guard cells sit within the layer of epidermal cells (credit a: modification of work by Louisa Howard, Rippel Electron Microscope Facility, Dartmouth College; credit b: modification of work by June Kwak, University of Maryland; scale-bar data from Matt Russell) Vascular Tissue The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue (Figure 23.9). Figure 23.9 In (a) dicot stems, vascular bundles are arranged around the periphery of the ground tissue. The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Sclerenchyma fibers cap the vascular bundles. In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue. Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 953 xylem cells with thinner walls; they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant. Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sievetube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The |
sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles. Ground Tissue Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith, while the layer of tissue between the vascular tissue and the epidermis is known as the cortex. Growth in Stems Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth, and is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. Figure 23.10 shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots. Figure 23.10 In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis. Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size. Primary Growth Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also |
contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots). 954 Chapter 23 | Plant Form and Physiology The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape. Watch this BBC Nature video (http://openstaxcollege.org/l/motion_plants) showing how time-lapse photography captures plant growth at high speed. The video you watched showed time lapse photography of the growth of a stem. Which of these is a fast response in a plant that was not recorded in the video? a. opening of a flower b. tendrils looping around a support c. growth of an apical bud d. closing of leaflets on a lightly touched mimosa leaf Secondary Growth The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see Figure 23.10). The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength. In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects |
the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere (Figure 23.11). This supplies oxygen to the living and metabolically active cells of the cortex, xylem and phloem. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 955 Figure 23.11 Lenticels on the bark of this cherry tree enable the woody stem to exchange gases with the surrounding atmosphere. (credit: Roger Griffith) Annual Rings The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem (Figure 23.12). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season. Figure 23.12 The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate—note how the rings vary in thickness. (credit: Adrian Pingstone) Stem Modifications Some plant species have modified stems that are especially suited to a particular habitat and environment (Figure 23.13). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of |
some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are 956 Chapter 23 | Plant Form and Physiology more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris. Figure 23.13 Stem modifications enable plants to thrive in a variety of environments. Shown are (a) ginger (Zingiber officinale) rhizomes, (b) a carrion flower (Amorphophallus titanum) corm (c) Rhodes grass (Chloris gayana) stolons, (d) strawberry (Fragaria ananassa) runners, (e) potato (Solanum tuberosum) tubers, and (f) red onion (Allium) bulbs. (credit a: modification of work by Maja Dumat; credit c: modification of work by Harry Rose; credit d: modification of work by Rebecca Siegel; credit e: modification of work by Scott Bauer, USDA ARS; credit f: modification of work by Stephen Ausmus, USDA ARS) Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants were cultivated for food hundreds of years ago in the Arizona desert in this video: (http://openstaxcollege.org/l/ ancient_crop) Finding the Roots of an Ancient Crop. Agave plants were cultivated for hundreds of years by Pre-Columbian American populations. The sap was considered a good source of _____. a. sweetener for drinks and cooking b. proteins to supplement the daily diet c. lipids for cooking and baking d |
. starch for thickening desserts and stews Some aerial modifications of stems are tendrils and thorns (Figure 23.14). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant; common examples include roses, Osage orange and devil’s walking stick. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 957 Figure 23.14 Found in southeastern United States, (a) buckwheat vine (Brunnichia ovata) is a weedy plant that climbs with the aid of tendrils. This one is shown climbing up a wooden stake. (b) Thorns are modified branches. (credit a: modification of work by Christopher Meloche, USDA ARS; credit b: modification of work by “macrophile”/Flickr) 23.3 | Roots In this section, you will explore the following questions: • What are the two types of root system? • What are the three zones of the root tip and what is the role of each in root growth? • What is the structure of the root? • What are examples of modified roots? Connection for AP® Courses Much content described in this section, specifically root anatomy, is not within the scope of AP®. However, in the Transport of Water and Solutes in Plants module, we will explore the role of roots in absorbing water necessary for photosynthesis, transporting minerals and other nutrients, and storing carbohydrates. When we explored mitosis in the Cell Reproduction chapter, you may have examined a root meristem under the microscope. The meristem is comprised of actively dividing cells. However, just like animal organs and organ systems, plant structures, including roots, interact to provide specific functions. For example, water is absorbed through the root system and travels upward through xylem to the leaves, where it is used in photosynthesis. (See the Transport of Water and Solutes in Plants module.) With the exceptions described in the AP® Connection, information presented in this section, and the examples highlighted, does not align to the content and AP® Learning Objectives outlined in the AP® Curriculum Framework. The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting |
them upwards, and storing the products of photosynthesis. Some roots are modified to absorb moisture and exchange gases. Most roots are underground. Some plants, however, also have adventitious roots, which emerge above the ground from the shoot. Types of Root Systems Root systems are mainly of two types (Figure 23.15). Dicots have a tap root system, while monocots have a fibrous root system. A tap root system has a main root that grows down vertically, and from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds, and they can regrow another shoot from the remaining root). A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, and forms a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). Some plants have a combination of tap roots and fibrous roots. Plants that grow 958 Chapter 23 | Plant Form and Physiology in dry areas often have deep root systems, whereas plants growing in areas with abundant water are likely to have shallower root systems. Figure 23.15 (a) Tap root systems have a main root that grows down, while (b) fibrous root systems consist of many small roots. (credit b: modification of work by “Austen Squarepants”/Flickr) Root Growth and Anatomy Root growth begins with seed germination. When the plant embryo emerges from the seed, the radicle of the embryo forms the root system. The tip of the root is protected by the root cap, a structure exclusive to roots and unlike any other plant structure. The root cap is continuously replaced because it gets damaged easily as the root pushes through soil. The root tip can be divided into three zones: a zone of cell division, a zone of elongation, and a zone of maturation and differentiation (Figure 23.16). The zone of cell division is closest to the root tip; it is made up of the actively dividing cells of the root meristem. The zone of elongation is where the newly formed cells increase in length, thereby lengthening the root. Beginning at the first root hair is the zone of cell maturation where the root cells begin to differentiate into special cell types. All three zones are in the first centimeter or so of the root tip. Figure 23.16 A longitudinal view of the |
root reveals the zones of cell division, elongation, and maturation. Cell division occurs in the apical meristem. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 959 The root has an outer layer of cells called the epidermis, which surrounds areas of ground tissue and vascular tissue. The epidermis provides protection and helps in absorption. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals. Inside the root, the ground tissue forms two regions: the cortex and the pith (Figure 23.17). Compared to stems, roots have lots of cortex and little pith. Both regions include cells that store photosynthetic products. The cortex is between the epidermis and the vascular tissue, whereas the pith lies between the vascular tissue and the center of the root. Figure 23.17 Staining reveals different cell types in this light micrograph of a wheat (Triticum) root cross section. Sclerenchyma cells of the exodermis and xylem cells stain red, and phloem cells stain blue. Other cell types stain black. The stele, or vascular tissue, is the area inside endodermis (indicated by a green ring). Root hairs are visible outside the epidermis. (credit: scale-bar data from Matt Russell) The vascular tissue in the root is arranged in the inner portion of the root, which is called the stele (Figure 23.18). A layer of cells known as the endodermis separates the stele from the ground tissue in the outer portion of the root. The endodermis is exclusive to roots, and serves as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded. The outermost cell layer of the root’s vascular tissue is the pericycle, an area that can give rise to lateral roots. In dicot roots, the xyle |
m and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith. Figure 23.18 In (left) typical dicots, the vascular tissue forms an X shape in the center of the root. In (right) typical monocots, the phloem cells and the larger xylem cells form a characteristic ring around the central pith. Root Modifications Root structures may be modified for specific purposes. For example, some roots are bulbous and store starch. Aerial roots 960 Chapter 23 | Plant Form and Physiology and prop roots are two forms of aboveground roots that provide additional support to anchor the plant. Tap roots, such as carrots, turnips, and beets, are examples of roots that are modified for food storage (Figure 23.19). Figure 23.19 Many vegetables are modified roots. Epiphytic roots enable a plant to grow on another plant. For example, the epiphytic roots of orchids develop a spongy tissue to absorb moisture. The banyan tree (Ficus sp.) begins as an epiphyte, germinating in the branches of a host tree; aerial roots develop from the branches and eventually reach the ground, providing additional support (Figure 23.20). In screwpine (Pandanus sp.), a palm-like tree that grows in sandy tropical soils, aboveground prop roots develop from the nodes to provide additional support. Figure 23.20 The (a) banyan tree, also known as the strangler fig, begins life as an epiphyte in a host tree. Aerial roots extend to the ground and support the growing plant, which eventually strangles the host tree. The (b) screwpine develops aboveground roots that help support the plant in sandy soils. (credit a: modification of work by "psyberartist"/Flickr; credit b: modification of work by David Eikhoff) 23.4 | Leaves In this section, you will explore the following questions: • What are the parts of a typical leaf? • What is the internal structure and function of a leaf? • What are differences between simple leaves and compound leaves? Similarities? • What are examples of modified leaves? Connection for AP® Courses Plants have specialized leaves adapted to their particular environments. For example, the leaves of plants growing in tropical rainforests have a larger surface area than cacti growing in |
the desert or in very cold conditions, whose smaller surface area This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 961 minimizes water loss through transpiration. A waxy cuticle covers the surface of all plant species to reduce the rate of water loss from the leaf surface. Other leaves may have small hairs called trichomes on the leaf surface; not only do trichomes also help reduce water loss; they also protect the leaf from herbivory by restricting insect movements or by storing toxic or bad-tasting chemicals. The information presented in this module does not align to the content and AP® Learning Objectives outlined in the AP® Curriculum Framework. The role of stomata, guard cells, and trichomes in transpiration are explored in the Transport of Water and Solutes in Plants module. Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll. The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss. Structure of a Typical Leaf Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole. Leaves that do not have a petiole and are directly attached to the plant stem are called sessile leaves. Small green appendages usually found at the base of the petiole are known as stipules. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin. Figure 23.21 shows the structure of a typical eudicot leaf. Figure 23.21 Deceptively simple in appearance, a leaf is a highly efficient structure. Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation pattern. Monocots and dicots differ |
in their patterns of venation (Figure 23.22). Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant, the Ginkgo biloba, has dichotomous venation where the veins fork. 962 Chapter 23 | Plant Form and Physiology Figure 23.22 (a) Tulip (Tulipa), a monocot, has leaves with parallel venation. The netlike venation in this (b) linden (Tilia cordata) leaf distinguishes it as a dicot. The (c) Ginkgo biloba tree has dichotomous venation. (credit a photo: modification of work by “Drewboy64”/Wikimedia Commons; credit b photo: modification of work by Roger Griffith; credit c photo: modification of work by "geishaboy500"/Flickr; credit abc illustrations: modification of work by Agnieszka Kwiecień) Leaf Arrangement The arrangement of leaves on a stem is known as phyllotaxy. The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said to be either alternate—meaning the leaves alternate on each side of the stem in a flat plane—or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled. Leaf Form Leaves may be simple or compound (Figure 23.23). In a simple leaf, the blade is either completely undivided—as in the banana leaf—or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a compound leaf, the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk, but is attached to the rachis. A palmately compound leaf resembles the palm of a hand, with leaflets radiating |
outwards from one point Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant Schefflera sp. (common name “umbrella plant”). Pinnately compound leaves take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves (Rosa sp.), or the leaves of hickory, pecan, ash, or walnut trees. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 963 Figure 23.23 Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (credit a: modification of work by "BazzaDaRambler"/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes) Leaf Structure and Function The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata (Figure 23.24): openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing. Figure 23.24 Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac (Rhus glabra) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre- |
leaved sand cress (Arabidopsis lyrata) have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an A. lyrata leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf. (credit: modification of work by Robert R. Wise; part c scale-bar data from Matt Russell) The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the cuticle covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface (Figure 23.25). 964 Chapter 23 | Plant Form and Physiology Figure 23.25 Trichomes give leaves a fuzzy appearance as in this (a) sundew (Drosera sp.). Leaf trichomes include (b) branched trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes on a mature Quercus marilandica leaf. (credit a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell) Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or “middle leaf.” The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma (Figure 23.26). The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere |
through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 965 Figure 23.26 In the (a) leaf drawing, the central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy layer, comprised of loosely packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all aerial surfaces of land plants to minimize water loss. These leaf layers are clearly visible in the (b) scanning electron micrograph. The numerous small bumps in the palisade parenchyma cells are chloroplasts. Chloroplasts are also present in the spongy parenchyma, but are not as obvious. The bumps protruding from the lower surface of the leave are glandular trichomes, which differ in structure from the stalked trichomes in Figure 23.25. (credit b: modification of work by Robert R. Wise) Like the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure 23.27). The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues. 966 Chapter 23 | Plant Form and Physiology Figure 23.27 This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle from the lyreleaved sand cress (Arabidopsis lyrata). (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell) Leaf Adaptations Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area: two |
attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water. Watch “The Pale Pitcher Plant” episode of the video (http://openstaxcollege.org/l/plants_cool_too) series Plants Are Cool, Too, a Botanical Society of America video about a carnivorous plant species found in Louisiana. How do pale pitcher plants (Sarracinia alata) make sure that insects do not escape after consuming the nectar that attracted them? a. The insects are skewered on spikes and thorns that rim the funnel-shaped leaf. b. The insects ingest narcotics secreted by the leaf and fall into the funnel-shaped leaf. c. The insects are poisoned by lethal compounds in the nectar. d. The insects are immobilized by sticky substances on the rim of the funnel-shaped leaf. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 967 Plant Adaptations in Resource-Deficient Environments Roots, stems, and leaves are structured to ensure that a plant can obtain the required sunlight, water, soil nutrients, and oxygen resources. Some remarkable adaptations have evolved to enable plant species to thrive in less than ideal habitats, where one or more of these resources is in short supply. In tropical rainforests, light is often scarce, since many trees and plants grow close together and block much of the sunlight from reaching the forest floor. Many tropical plant species have exceptionally broad leaves to maximize the capture of sunlight. Other species are epiphytes: plants that grow on other plants that serve as a physical support. Such plants are able to grow high up in the canopy atop the branches of other trees, where sunlight is more plentiful. Epiphytes live on rain and minerals collected in the branches and leaves of the supporting plant. Bromeliads (members of the pineapple family), ferns, and orchids are examples of tropical epiphytes (Figure 23.28). Many epiphytes have specialized tissues that enable them to efficiently capture and store water. Figure 23.28 One of the most well known |
bromeliads is Spanish moss (Tillandsia usneoides), seen here in an oak tree. (credit: Kristine Paulus) Some plants have special adaptations that help them to survive in nutrient-poor environments. Carnivorous plants, such as the Venus flytrap and the pitcher plant (Figure 23.29), grow in bogs where the soil is low in nitrogen. In these plants, leaves are modified to capture insects. The insect-capturing leaves may have evolved to provide these plants with a supplementary source of much-needed nitrogen. 968 Chapter 23 | Plant Form and Physiology Figure 23.29 The (a) Venus flytrap has modified leaves that can capture insects. When an unlucky insect touches the trigger hairs inside the leaf, the trap suddenly closes. The opening of the (b) pitcher plant is lined with a slippery wax. Insects crawling on the lip slip and fall into a pool of water in the bottom of the pitcher, where they are digested by bacteria. The plant then absorbs the smaller molecules. (credit a: modification of work by Peter Shanks; credit b: modification of work by Tim Mansfield) Many swamp plants have adaptations that enable them to thrive in wet areas, where their roots grow submerged underwater. In these aquatic areas, the soil is unstable and little oxygen is available to reach the roots. Trees such as mangroves (Rhizophora sp.) growing in coastal waters produce aboveground roots that help support the tree (Figure 23.30). Some species of mangroves, as well as cypress trees, have pneumatophores: upward-growing roots containing pores and pockets of tissue specialized for gas exchange. Wild rice is an aquatic plant with large air spaces in the root cortex. The air-filled tissue—called aerenchyma—provides a path for oxygen to diffuse down to the root tips, which are embedded in oxygenpoor bottom sediments. How do pale pitcher plants (Sarracinia alata) make sure that insects do not escape after consuming the nectar that attracted them? a. The insects are skewered on spikes and thorns that rim the funnel-shaped leaf. b. The insects ingest narcotics secreted by the leaf and fall into the funnel-shaped leaf. c. The insects are poisoned by lethal compounds in the nectar. d. The insects are immobilized by sticky substances on the rim of the funnel-shaped leaf. Figure 23.30 The branches of (a) mangrove trees |
develop aerial roots, which descend to the ground and help to anchor the trees. (b) Cypress trees and some mangrove species have upward-growing roots called pneumatophores that are involved in gas exchange. Aquatic plants such as (c) wild rice have large spaces in the root cortex called aerenchyma, visualized here using scanning electron microscopy. (credit a: modification of work by Roberto Verzo; credit b: modification of work by Duane Burdick; credit c: modification of work by Robert R. Wise) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 969 Watch Venus Flytraps: Jaws of Death (http://openstaxcollege.org/l/venus_flytrap), an extraordinary BBC close-up of the Venus flytrap in action. Why do many ornamental plants that thrive indoors originate on the floor of tropical rainforest, where they grow under the canopy of trees? a. Growing under the rainforest’s canopy made these plants adapt to less water and nutrients. b. With their narrow leaves, these plants are adapted to grow in low light. c. With their broad leaves, these plants are adapted to grow in low light, like that usually found indoors. d. Growing under the rainforest’s canopy provides the plants with more water and nutrients, which they also need as indoor plants. 23.5 | Transport of Water and Solutes in Plants In this section, you will explore the following questions: • What is water potential, and how is it influenced by solutes, pressures, gravity, and the matric potential? • How do water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants? • How are photosynthates transported in plants? Connection for AP® Courses Information in this section applies to concepts we explored in previous chapters by connecting them to the transport of water and solutes through a plant, showing ways that plants take up and transport materials. These concepts include the processes of photosynthesis and cellular respiration, the chemical and physical properties of water, and the coevolution of plants with mutualistic bacteria and fungi. The vascular system of terrestrial plants allows the efficient absorption and delivery of water through the cells that comprise xylem, whereas phloem delivers sugars produced in photosynthesis to all parts of the plant |
, including the roots for storage. The physical separation of xylem and phloem permits plants to move different nutrients simultaneously from roots to shoots and vice versa. Nearly all plants use related mechanisms of osmoregulation, and we will focus on the transport of water and other nutrients. You likely remember the concept of water potential (Ψ) from our exploration of diffusion and osmosis in the chapter where we discuss the structure and function of plasma membranes. Water potential is a measure of the differences in potential energy between a water sample with solutes and pure water. Water moves via osmosis from an area of higher water potential (more water molecules, less solute) to an area of lower water potential (less water, more solutes). The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and other factors (matrix effects). Water potential and transpiration influence how water is transported through the xylem. Carbohydrates synthesized in photosynthesis, primarily sucrose, move from sources to sinks through the plant’s phloem. Sucrose produced in the Calvin cycle is loaded into the sieve-tube elements of the phloem, and the increased solute concentration causes water to move by osmosis from the xylem into the phloem. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 4 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and 970 Chapter 23 | Plant Form and Physiology AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 2 Enduring Understanding 2.A Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. Essential Knowledge 2.A.3 Molecules and atoms from the environment are necessary to build new molecules; the movement of water in a plant depends on the properties of water. Science Practice Learning Objective Big Idea 2 Enduring Understanding 2.A 4.1: The student can justify the selection of the kind of data needed to answer a particular scientific question. 2.8 The student is able to justify the selection of data regarding the types of molecules that an animal, plant or bacterium will take up as |
necessary building blocks and excrete as waste products. Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth, reproduction and maintenance of living systems require free energy and matter. Essential Knowledge 2.A.3 Molecules and atoms from the environment are necessary to build new molecules; the movement of water in a plant depends on the properties of water. Science Practice Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A 1.1: The student can create representations and models of natural or man-made phenomena and systems in the domain. 1.4: The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively. 2.9 The student is able to represent graphically or model quantitatively (or qualitatively) the exchange of molecules between an organism and its environment, and the subsequent use of these molecules to building new molecules that facilitate dynamic homeostasis, growth and reproduction. Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.4 Interactions and coordination between organs and organ systems provide essential biological activities. Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A 3.3: The student can evaluate scientific questions. 4.8 The student is able to evaluate scientific questions concerning organisms that exhibit complex properties due to the interaction of their constituent parts. Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.4 Interactions and coordination between organs and organ systems provide essential biological activities. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 971 Science Practice Learning Objective Big Idea 4 Enduring Understanding 4.A 3.3: The student can evaluate scientific questions. 4.9 The student is able to predict the effects of a change in the component(s) of a biological system on the functionality of an organism(s). Biological systems interact, and these systems and their interactions possess complex properties. Interactions within biological systems lead to complex properties. Essential Knowledge 4.A.4 Interactions and coordination between organs and organ systems provide essential biological activities. Science Practice Science Practice Learning Objective 1.3: The student can refine representations and models of natural or man-made phenomena and systems in the domain. 6.4 |
: The student can make claims and predictions about natural phenomena based on scientific theories and models. 4.10 The student is able to refine representations and models to illustrate biocomplexity due to interactions of the constituent parts. The Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.40][APLO 4.12][APLO 2.1][APLO 2.8][APLO 2.9][APLO 2.41][APLO 1.2][APLO 1.22][APLO 1.25][APLO 2.19][APLO 2.32] The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential. Water Potential Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure 23.31a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 23.31b). Plants achieve this because of water potential. Figure 23.31 With heights nearing 116 meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.) 972 Chapter 23 | Plant Form and Physiology Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ (psi) and is expressed in units of pressure pure H2O) is, by convenience of (pressure is |
a form of energy) called megapascals (MPa). The potential of pure water (Ψw definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψw The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation: pure H2O. Ψsystem = Ψtotal = Ψs + Ψp + Ψg + Ψm where Ψs, Ψp, Ψg, and Ψm refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψsoil), root water (Ψroot), stem water (Ψstem), leaf water (Ψleaf) or the water in the atmosphere (Ψatmosphere): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψsoil must be > Ψroot > Ψstem > Ψleaf > Ψatmosphere. Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψsystem, by manipulating the individual components (especially Ψs), a plant can control water movement. Solute Potential Solute potential (Ψs), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψw) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydroph |
obic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψs decreases with increasing solute concentration. Because Ψs is one of the four components of Ψsystem or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content (Figure 23.32). Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential. Plant cells can metabolically manipulate Ψs (and by extension, Ψtotal) by adding or removing solute molecules. Therefore, plants have control over Ψtotal via their ability to exert metabolic control over Ψs. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 973 Figure 23.32 In this example with a semipermeable membrane between two aqueous systems, water will move from a region of higher to lower water potential until equilibrium is reached. Solutes (Ψs), pressure (Ψp), and right or left), and therefore, the difference gravity (Ψg) influence total water potential for each side of the tube (Ψtotal between Ψtotal on each side (ΔΨ). (Ψm, the potential due to interaction of water with solid substrates, is ignored in this example because glass is not especially hydrophilic). Water moves in response to the difference in water potential between two systems (the left and right sides of the tube). Positive water potential is applied on the left side of a tube by increasing Ψp so that the water level rises on the right side. The equation for water potential is: Ψsystem = Ψtotal = Ψs + Ψp + Ψg + Ψm where Ψs, Ψp, Ψg, and Ψm refer to the solute, pressure, gravity, |
and matric potentials, respectively. Could you equalize the water level on each side of the tube by adding solute? a. Yes, water level can be equalized by adding solute to the right side of the tube so that water moves toward the left until the water levels are equal. b. No, water level cannot be equalized on both sides of the tubes by adding solutes with no other action. c. Yes, water level can be equalized by adding solute to the left side of the tube so that water moves toward the left until the water levels are equal. d. No, water level cannot be equalized by adding solutes because solutes are always pulled down by gravity, thereby not letting water equalize. Pressure Potential Pressure potential (Ψp), also called turgor potential, may be positive or negative (Figure 23.32). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψtotal, and a negative Ψp (tension) decreases Ψtotal. Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψp of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in-2 MPa-1 = 210 lb/in-2). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure 23.33). Water is lost from the 974 Chapter 23 | Plant Form and Physiology leaves via transpiration (approaching Ψp = 0 MPa at the wilting point) and restored by uptake via the roots. A plant can manipulate Ψp via its ability to manipulate Ψs and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψs will decline, Ψtotal will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψp will increase. Ψp is also under indirect |
plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψp and Ψtotal of the leaf and increasing ΔΨ between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf. Figure 23.33 When (a) total water potential (Ψtotal) is lower outside the cells than inside, water moves out of the cells and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the cells, resulting in turgor pressure (Ψp) and keeping the plant erect. (credit: modification of work by Victor M. Vicente Selvas) Gravity Potential Gravity potential (Ψg) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψtotal). The taller the plant, the taller the water column, and the more influential Ψg becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m-1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψg. Matric Potential Matric potential (Ψm) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψm is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψm, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψm is very large (negative) in |
dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψm cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves. Movement of Water and Minerals in the Xylem Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ. Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent. Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 975 required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells (Figure 23.34), thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure |
. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional. Figure 23.34 The cohesion–tension theory of sap ascent is shown. Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem. Which statement is false? a. Negative water potential draws water into the root hairs, cohesion and adhesion draw water up the xylem, and transpiration draws water from the leaf. b. Negative water potential draws water into the root hairs, cohesion and adhesion draw water up the phloem, and transpiration draws water from the leaf. c. Water potential decreases from the roots to the top of the plant. d. Water enters the plant through the root hairs and exits through stomata. Transpiration—the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled. 976 Chapter 23 | Plant Form and Physiology Control of Transpiration The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration. Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into |
the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss. Plants have evolved over time to adapt to their local environment and reduce transpiration(Figure 23.35). Desert plants (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations. Figure 23.35 Plants are suited to their local environment. (a) Xerophytes, like this prickly pear cactus (Opuntia sp.) and (b) epiphytes such as this tropical Aeschynanthus perrottetii have adapted to very limited water resources. The leaves of a prickly pear are modified into spines, which lowers the surface-to-volume ratio and reduces water loss. Photosynthesis takes place in the stem, which also stores water. (b) A. perottetii leaves have a waxy cuticle that prevents water loss. (c) Goldenrod (Solidago sp.) is a mesophyte, well suited for moderate environments. (d) Hydrophytes, like this fragrant water lily (Nymphaea odorata), are adapted to thrive in aquatic environments. (credit a: modification of work by Jon Sullivan; credit b: modification of work by L. Shyamal/Wikimedia Commons; credit c: modification of work by Huw Williams; credit d: modification of work by Jason Hollinger) This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 977 Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants. Transportation of Photosynthates in the Phloem Plants need an energy source to |
grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose. Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season. The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage. Translocation: Transport from Source to Sink Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H+ symporter. Phloem STEs have reduced cytopl |
asmic contents, and are connected by a sieve plate with pores that allow for pressuredriven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs (Figure 23.36). Figure 23.36 Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells. 978 Chapter 23 | Plant Form and Physiology Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψs, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink (Figure 23.37). Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap. Figure 23.37 Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels. Activity Based on water’s molecular properties, create a visual diagram/model to illustrate how water travels up a 300-foot California redwood tree through xylem. Lab Investigation AP® Biology Investigative Labs: Inquiry-Based, Investigation 11: Transpiration. Design and conduct a series of |
experiments to investigate the effects of environmental variables on transpiration rates. Think About It Desert travelers claim that cactus juice tastes sweeter during the day than at night. Based on your understanding of photosynthesis, transpiration, and the regulation of stomata by guard cells in response to environmental conditions, is there any validity to this claim? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 979 23.6 | Plant Sensory Systems and Responses In this section, you will explore the following questions: • How do red and blue light affect plant growth and metabolic activities? • What is gravitropism? • What are examples of plant hormones, and how do they affect plant growth and development? • What are the differences between thigmotropism, thigmonastism, and thigmogenesis? • How do plants defend themselves against predators and respond to wounds? Connection for AP® Courses Why do some flowering plants bloom in the spring, whereas others bloom in the summer or fall? Why do the stems of plants grow upward and bend toward light, while roots grow downward? Why do the leaves of deciduous plants in northern climates turn colors in the fall and eventually fall off, while evergreens keep their needles all year around? Is it true that putting an unripe avocado in a paper bag will hasten the ripening process? Can the same tactic work on bananas? Like most other organisms, plants have the ability to detect and respond to environmental change; this ability is an adaptation favored by natural selection. Flowering plants react to light, gravity, infection by pathogens, drought, and, as with the Venus flytrap, touch. Animals have two systems on which they can rely to detect and respond to stimuli: the nervous and endocrine systems. Plants, however, only have chemical control mechanisms at their disposal. In addition to the phytochromes, plants evolved hormones that allow them to respond to environmental changes. Like animal hormones, plant hormones are chemical signaling molecules that trigger a cellular response through a signal transduction pathway. Although you do not have to memorize a laundry list of plant hormones and their activities, you should understand the basic mechanism of how they trigger a response. Examples that we’ll learn about in this chapter include auxins, which cause plant stems to grow and bend toward light, and gibberellins, which stimulate cell |
growth and breaks seed dormancy. Plants also produce other chemicals to protect against biotic stimuli, including herbivory and parasitism. The fact that plants produce chemicals should not be surprising; our pharmaceutical industry depends on these substances. For example, species of the foxglove plant produce digitalis, a powerful heart medicine, whereas the opium from the poppy is the source of many narcotics. Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 3 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices. Big Idea 2 Enduring Understanding 2.C Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. Essential Knowledge 2.C.1 Plants use negative and positive feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Science Practice Science Practice 4.2 The student can design a plan for collecting data to answer a particular scientific question. 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 6.1 The student can justify claims with evidence. 980 Chapter 23 | Plant Form and Physiology Learning Objective 2.17 The student is able to evaluate data that show the effect(s) of changes in concentrations of key molecules on negative feedback mechanisms. Big Idea 2 Enduring Understanding 2.C Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis. Essential Knowledge 2.C.1 Plants use negative and positive feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Science Practice Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. Learning Objective 2.18 The student can make predictions about |
how organisms use negative feedback mechanisms to maintain their internal environments. Big Idea 2 Enduring Understanding 2.D Essential Knowledge Science Practice Science Practice Science Practice Science Practice Learning Objective Big Idea 2 Enduring Understanding 2.D Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth and dynamic homeostasis of a biological system are influenced by changes in the system’s environment. 2.D.1 Cellular activity in plants is affected by interactions with biotic and abiotic factors. 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain. 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models. 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. 2.22 The student is able to refine scientific models and questions about the effect of complex biotic and abiotic interactions on all biological systems, from cells and organisms to populations, communities, and ecosystems. Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Growth and dynamic homeostasis of a biological system are influenced by changes in the system’s environment. Essential Knowledge 2.D.4 Plants have nonspecific immune responses to defend against infections and other threats. Science Practice Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain. 1.2 The student can describe representations and models of natural or man-made phenomena and systems in the domain. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 981 Science Practice Learning Objective Big Idea 2 Big Idea 2 Enduring Understanding 2.E 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. 2.30 The student can create representations or models to describe nonspecific immune defenses in plants and animals. Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Biological systems utilize free energy and molecular building blocks to grow |
, to reproduce, and to maintain dynamic homeostasis. Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination. Essential Knowledge 2.E.3 Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes. Science Practice Science Practice Science Practice Science Practice 4.2 The student can design a plan for collecting data to answer a particular scientific question. 5.3 The student can evaluate the evidence provided by data sets in relation to a particular scientific question. 6.1 The student can justify claims with evidence. 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. Learning Objective 2.39 The student is able to justify scientific claims, using evidence, to describe how timing and coordination of behavioral events in organisms are regulated by several mechanisms. Big Idea 2 Enduring Understanding 2.E Essential Knowledge Science Practice Learning Objective Big Idea 3 Enduring Understanding 3.B Essential Knowledge Science Practice Learning Objective Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis. Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination. 2.E.3 Plant responses to stimuli are adaptations favored by natural selection. 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas. 2.40 The student is able to connect concepts in and across domain(s) to predict how environmental factors affect responses to information and change behavior. Living systems store, retrieve, transmit and respond to information essential to life processes. Expression of genetic information involves cellular and molecular mechanisms. 3.B.2 Signal transmission within and between plant cells mediates gene expression. 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices. 3.22 The student is able to explain how signal pathways mediate gene expression, including how this process can affect protein production. The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for 982 Chapter 23 | Plant Form and Physiology the AP exam. These questions address the following standards: [APLO 2.29][APLO 2.30] Animals can respond to environmental factors by moving to a new location. Plants, however, are rooted in place and must respond to the surrounding environmental factors. Plants have sophisticated systems to detect |
and respond to light, gravity, temperature, and physical touch. Receptors sense environmental factors and relay the information to effector systems—often through intermediate chemical messengers—to bring about plant responses. Plant Responses to Light Plants have a number of sophisticated uses for light that go far beyond their ability to photosynthesize low-molecularweight sugars using only carbon dioxide, light, and water. Photomorphogenesis is the growth and development of plants in response to light. It allows plants to optimize their use of light and space. Photoperiodism is the ability to use light to track time. Plants can tell the time of day and time of year by sensing and using various wavelengths of sunlight. Phototropism is a directional response that allows plants to grow towards, or even away from, light. The sensing of light in the environment is important to plants; it can be crucial for competition and survival. The response of plants to light is mediated by different photoreceptors, which are comprised of a protein covalently bonded to a lightabsorbing pigment called a chromophore. Together, the two are called a chromoprotein. The red/far-red and violet-blue regions of the visible light spectrum trigger structural development in plants. Sensory photoreceptors absorb light in these particular regions of the visible light spectrum because of the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophylls peaks in the blue and red regions of the spectrum. As light filters through the canopy and the blue and red wavelengths are absorbed, the spectrum shifts to the farred end, shifting the plant community to those plants better adapted to respond to far-red light. Blue-light receptors allow plants to gauge the direction and abundance of sunlight, which is rich in blue–green emissions. Water absorbs red light, which makes the detection of blue light essential for algae and aquatic plants. The Phytochrome System and the Red/Far-Red Response The phytochromes are a family of chromoproteins with a linear tetrapyrrole chromophore, similar to the ringed tetrapyrrole light-absorbing head group of chlorophyll. Phytochromes have two photo-interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) and is immediately converted to Pfr. Pfr absorbs far-red light (~730 nm) and is quickly converted back to Pr. Absorption of |
red or far-red light causes a massive change to the shape of the chromophore, altering the conformation and activity of the phytochrome protein to which it is bound. Pfr is the physiologically active form of the protein; therefore, exposure to red light yields physiological activity. Exposure to far-red light inhibits phytochrome activity. Together, the two forms represent the phytochrome system (Figure 23.38). The phytochrome system acts as a biological light switch. It monitors the level, intensity, duration, and color of environmental light. The effect of red light is reversible by immediately shining far-red light on the sample, which converts the chromoprotein to the inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark, or break down over time. In all instances, the physiological response induced by red light is reversed. The active form of phytochrome (Pfr) can directly activate other molecules in the cytoplasm, or it can be trafficked to the nucleus, where it directly activates or represses specific gene expression. Once the phytochrome system evolved, plants adapted it to serve a variety of needs. Unfiltered, full sunlight contains much more red light than far-red light. Because chlorophyll absorbs strongly in the red region of the visible spectrum, but not in the far-red region, any plant in the shade of another plant on the forest floor will be exposed to red-depleted, far-red-enriched light. The preponderance of far-red light converts phytochrome in the shaded leaves to the Pr (inactive) form, slowing growth. The nearest non-shaded (or even less-shaded) areas on the forest floor have more red light; leaves exposed to these areas sense the red light, which activates the Pfr form and induces growth. In short, plant shoots use the phytochrome system to grow away from shade and towards light. Because competition for light is so fierce in a dense plant community, the evolutionary advantages of the phytochrome system are obvious. In seeds, the phytochrome system is not used to determine direction and quality of light (shaded versus unshaded). Instead, is it used merely to determine if there is any light at all. This is especially important in species with very small seeds, such as lettuce. Because of their size, lettuce seeds have few food reserves. Their seedlings |
cannot grow for long before they run out of fuel. If they germinated even a centimeter under the soil surface, the seedling would never make it into the sunlight and would die. In the dark, phytochrome is in the Pr (inactive form) and the seed will not germinate; it will only germinate if exposed to light at the surface of the soil. Upon exposure to light, Pr is converted to Pfr and germination proceeds. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 983 Figure 23.38 The biologically inactive form of phytochrome (Pr) is converted to the biologically active form Pfr under illumination with red light. Far-red light and darkness convert the molecule back to the inactive form. Plants also use the phytochrome system to sense the change of season. Photoperiodism is a biological response to the timing and duration of day and night. It controls flowering, setting of winter buds, and vegetative growth. Detection of seasonal changes is crucial to plant survival. Although temperature and light intensity influence plant growth, they are not reliable indicators of season because they may vary from one year to the next. Day length is a better indicator of the time of year. As stated above, unfiltered sunlight is rich in red light but deficient in far-red light. Therefore, at dawn, all the phytochrome molecules in a leaf quickly convert to the active Pfr form, and remain in that form until sunset. In the dark, the Pfr form takes hours to slowly revert back to the Pr form. If the night is long (as in winter), all of the Pfr form reverts. If the night is short (as in summer), a considerable amount of Pfr may remain at sunrise. By sensing the Pr/Pfr ratio at dawn, a plant can determine the length of the day/night cycle. In addition, leaves retain that information for several days, allowing a comparison between the length of the previous night and the preceding several nights. Shorter nights indicate springtime to the plant; when the nights become longer, autumn is approaching. This information, along with sensing temperature and water availability, allows plants to determine the time of the year and adjust their physiology accordingly. Short-day (longnight) plants use this information to flower in the late summer and early fall, when nights exceed |
a critical length (often eight or fewer hours). Long-day (short-night) plants flower during the spring, when darkness is less than a critical length (often eight to 15 hours). Not all plants use the phytochrome system in this way. Flowering in day-neutral plants is not regulated by daylength. 984 Chapter 23 | Plant Form and Physiology Horticulturist The word “horticulturist” comes from the Latin words for garden (hortus) and culture (cultura). This career has been revolutionized by progress made in the understanding of plant responses to environmental stimuli. Growers of crops, fruit, vegetables, and flowers were previously constrained by having to time their sowing and harvesting according to the season. Now, horticulturists can manipulate plants to increase leaf, flower, or fruit production by understanding how environmental factors affect plant growth and development. Greenhouse management is an essential component of a horticulturist’s education. To lengthen the night, plants are covered with a blackout shade cloth. Long-day plants are irradiated with red light in winter to promote early flowering. For example, fluorescent (cool white) light high in blue wavelengths encourages leafy growth and is excellent for starting seedlings. Incandescent lamps (standard light bulbs) are rich in red light, and promote flowering in some plants. The timing of fruit ripening can be increased or delayed by applying plant hormones. Recently, considerable progress has been made in the development of plant breeds that are suited to different climates and resistant to pests and transportation damage. Both crop yield and quality have increased as a result of practical applications of the knowledge of plant responses to external stimuli and hormones. Horticulturists find employment in private and governmental laboratories, greenhouses, botanical gardens, and in the production or research fields. They improve crops by applying their knowledge of genetics and plant physiology. To prepare for a horticulture career, students take classes in botany, plant physiology, plant pathology, landscape design, and plant breeding. To complement these traditional courses, horticulture majors add studies in economics, business, computer science, and communications. The Blue Light Responses Phototropism—the directional bending of a plant toward or away from a light source—is a response to blue wavelengths of light. Positive phototropism is growth towards a light source (Figure 23.39), while negative phototropism (also called skototropism) is growth away from light |
. The aptly-named phototropins are protein-based receptors responsible for mediating the phototropic response. Like all plant photoreceptors, phototropins consist of a protein portion and a light-absorbing portion, called the chromophore. In phototropins, the chromophore is a covalently-bound molecule of flavin; hence, phototropins belong to a class of proteins called flavoproteins. Other responses under the control of phototropins are leaf opening and closing, chloroplast movement, and the opening of stomata. However, of all responses controlled by phototropins, phototropism has been studied the longest and is the best understood. In their 1880 treatise The Power of Movements in Plants, Charles Darwin and his son Francis first described phototropism as the bending of seedlings toward light. Darwin observed that light was perceived by the tip of the plant (the apical meristem), but that the response (bending) took place in a different part of the plant. They concluded that the signal had to travel from the apical meristem to the base of the plant. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 985 Figure 23.39 Azure bluets (Houstonia caerulea) display a phototropic response by bending toward the light. (credit: Cory Zanker) In 1913, Peter Boysen-Jensen demonstrated that a chemical signal produced in the plant tip was responsible for the bending at the base. He cut off the tip of a seedling, covered the cut section with a layer of gelatin, and then replaced the tip. The seedling bent toward the light when illuminated. However, when impermeable mica flakes were inserted between the tip and the cut base, the seedling did not bend. A refinement of the experiment showed that the signal traveled on the shaded side of the seedling. When the mica plate was inserted on the illuminated side, the plant did bend towards the light. Therefore, the chemical signal was a growth stimulant because the phototropic response involved faster cell elongation on the shaded side than on the illuminated side. We now know that as light passes through a plant stem, it is diffracted and generates phototropin activation across the stem. Most activation occurs on the lit side, causing the plant hormone indole acetic acid |
(IAA) to accumulate on the shaded side. Stem cells elongate under influence of IAA. Cryptochromes are another class of blue-light absorbing photoreceptors that also contain a flavin-based chromophore. Cryptochromes set the plants 24-hour activity cycle, also know as its circadian rhythem, using blue light cues. There is some evidence that cryptochromes work together with phototropins to mediate the phototropic response. Use the navigation menu in the left panel of this website (http://openstaxcollege.org/l/plnts_n_motion) to view images of plants in motion. If green light was used rather than white light to irradiate a sunflower seedling, what would happen? a. Green light is absorbed by the plant. The seedling would not bend, but it would grow tall and spindly as if grown in the dark. b. Green light is not absorbed by the plant. The seedling would not bend, but it would grow tall and spindly as if grown in the dark. c. Green light is not absorbed by the plant. The seedling would not grow tall and spindly, but it would bend. d. Green light is absorbed by the plant. The seedling would not grow tall and spindly, but it would bend. Plant Responses to Gravity Whether or not they germinate in the light or in total darkness, shoots usually sprout up from the ground, and roots 986 Chapter 23 | Plant Form and Physiology grow downward into the ground. A plant laid on its side in the dark will send shoots upward when given enough time. Gravitropism ensures that roots grow into the soil and that shoots grow toward sunlight. Growth of the shoot apical tip upward is called negative gravitropism, whereas growth of the roots downward is called positive gravitropism. Amyloplasts (also known as statoliths) are specialized plastids that contain starch granules and settle downward in response to gravity. Amyloplasts are found in shoots and in specialized cells of the root cap. When a plant is tilted, the statoliths drop to the new bottom cell wall. A few hours later, the shoot or root will show growth in the new vertical direction. The mechanism that mediates gravitropism is reasonably well understood. When amyloplasts settle to the bottom of the gravity-sensing cells in the |
root or shoot, they physically contact the endoplasmic reticulum (ER), causing the release of calcium ions from inside the ER. This calcium signaling in the cells causes polar transport of the plant hormone IAA to the bottom of the cell. In roots, a high concentration of IAA inhibits cell elongation. The effect slows growth on the lower side of the root, while cells develop normally on the upper side. IAA has the opposite effect in shoots, where a higher concentration at the lower side of the shoot stimulates cell expansion, causing the shoot to grow up. After the shoot or root begin to grow vertically, the amyloplasts return to their normal position. Other hypotheses—involving the entire cell in the gravitropism effect—have been proposed to explain why some mutants that lack amyloplasts may still exhibit a weak gravitropic response. Growth Responses A plant’s sensory response to external stimuli relies on chemical messengers (hormones). Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially every cell in a plant can produce plant hormones. They can act in their cell of origin or be transported to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a distant site for action, and they act alone. Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins (particularly IAA), cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors. Auxins The term auxin is derived from the Greek word auxein, which means "to grow." Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue, and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, IAA is the only naturally occurring auxin that shows physiological activity. Apical dominance—the inhibition of lateral bud formation—is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of |
auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses. Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping to coordinate the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins. Cytokinins The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally occurring or synthetic cytokinins are known to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds, and cytokinins that promote bushier growth. Gibberellins Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning. GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 987 light to germinate. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods and contain inconspicuous seeds that fail |
to develop. Because GAs are produced by the seeds, and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the instance of mildew infection (Figure 23.40). Figure 23.40 In grapes, application of gibberellic acid increases the size of fruit and loosens clustering. (credit: Bob Nichols, USDA) Abscisic Acid The plant hormone abscisic acid (ABA) was first discovered as the agent that causes the abscission or dropping of cotton bolls. However, more recent studies indicate that ABA plays only a minor role in the abscission process. ABA accumulates as a response to stressful environmental conditions, such as dehydration, cold temperatures, or shortened day lengths. Its activity counters many of the growth-promoting effects of GAs and auxins. ABA inhibits stem elongation and induces dormancy in lateral buds. ABA induces dormancy in seeds by blocking germination and promoting the synthesis of storage proteins. Plants adapted to temperate climates require a long period of cold temperature before seeds germinate. This mechanism protects young plants from sprouting too early during unseasonably warm weather in winter. As the hormone gradually breaks down over winter, the seed is released from dormancy and germinates when conditions are favorable in spring. Another effect of ABA is to promote the development of winter buds; it mediates the conversion of the apical meristem into a dormant bud. Low soil moisture causes an increase in ABA, which causes stomata to close, reducing water loss in winter buds. Ethylene Ethylene is associated with fruit ripening, flower wilting, and leaf fall. Ethylene is unusual because it is a volatile gas (C2H4). Hundreds of years ago, when gas street lamps were installed in city streets, trees that grew close to lamp posts developed twisted, thickened trunks and shed their leaves earlier than expected. These effects were caused by ethylene volatilizing from the lamps. Aging tissues (especially senescing leaves) and nodes of stems produce ethylene. The best-known effect of the hormone, however, is the promotion of fruit ripening. Ethylene stimulates the conversion of starch and acids to sugars. Some people store unripe fruit, such as |
avocadoes, in a sealed paper bag to accelerate ripening; the gas released by the first fruit to mature will speed up the maturation of the remaining fruit. Ethylene also triggers leaf and fruit abscission, flower fading and dropping, and promotes germination in some cereals and sprouting of bulbs and potatoes. 988 Chapter 23 | Plant Form and Physiology Ethylene is widely used in agriculture. Commercial fruit growers control the timing of fruit ripening with application of the gas. Horticulturalists inhibit leaf dropping in ornamental plants by removing ethylene from greenhouses using fans and ventilation. Nontraditional Hormones Recent research has discovered a number of compounds that also influence plant development. Their roles are less understood than the effects of the major hormones described so far. Jasmonates play a major role in defense responses to herbivory. Their levels increase when a plant is wounded by a predator, resulting in an increase in toxic secondary metabolites. They contribute to the production of volatile compounds that attract natural enemies of predators. For example, chewing of tomato plants by caterpillars leads to an increase in jasmonic acid levels, which in turn triggers the release of volatile compounds that attract predators of the pest. Oligosaccharins also play a role in plant defense against bacterial and fungal infections. They act locally at the site of injury, and can also be transported to other tissues. Strigolactones promote seed germination in some species and inhibit lateral apical development in the absence of auxins. Strigolactones also play a role in the establishment of mycorrhizae, a mutualistic association of plant roots and fungi. Brassinosteroids are important to many developmental and physiological processes. Signals between these compounds and other hormones, notably auxin and GAs, amplifies their physiological effect. Apical dominance, seed germination, gravitropism, and resistance to freezing are all positively influenced by hormones. Root growth and fruit dropping are inhibited by steroids. Plant Responses to Wind and Touch The shoot of a pea plant winds around a trellis, while a tree grows on an angle in response to strong prevailing winds. These are examples of how plants respond to touch or wind. The movement of a plant subjected to constant directional pressure is called thigmotropism, from the Greek words thigma meaning “touch,” and tropism implying “direction.” Tendrils are one example of this. The meristematic region |
of tendrils is very touch sensitive; light touch will evoke a quick coiling response. Cells in contact with a support surface contract, whereas cells on the opposite side of the support expand (Figure 23.14). Application of jasmonic acid is sufficient to trigger tendril coiling without a mechanical stimulus. A thigmonastic response is a touch response independent of the direction of stimulus Figure 23.24. In the Venus flytrap, two modified leaves are joined at a hinge and lined with thin fork-like tines along the outer edges. Tiny hairs are located inside the trap. When an insect brushes against these trigger hairs, touching two or more of them in succession, the leaves close quickly, trapping the prey. Glands on the leaf surface secrete enzymes that slowly digest the insect. The released nutrients are absorbed by the leaves, which reopen for the next meal. The mimosa plant is also known as the sensitive plant, because its leaves are sensitive to touch and will fold inward and droop. Leaves in their normal state are shown on the left. Figure 23.41 Thigmomorphogenesis is a slow developmental change in the shape of a plant subjected to continuous mechanical stress. When trees bend in the wind, for example, growth is usually stunted and the trunk thickens. Strengthening tissue, especially xylem, is produced to add stiffness to resist the wind’s force. Researchers hypothesize that mechanical strain induces growth and differentiation to strengthen the tissues. Ethylene and jasmonate are likely involved in thigmomorphogenesis. This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 989 Use the menu at the left to navigate to three short movies: (http://openstaxcollege.org/l/nastic_mvmt) a Venus fly trap capturing prey, the progressive closing of sensitive plant leaflets, and the twining of tendrils. A Venus fly trap response is triggered by touching the leaves __. a. anywhere, because the whole surface of the leaf responds to touch b. only on the margins of the leaves where insects usually land c. in the center of the leaf, where the touch-sensitive hairs are located d. on the petiole followed by the center of leaf which signal the presence of a wandering insect Defense Responses against Herbivores and Pathogens Plants face two types of enemies: herbivores and pathogens. Herb |
ivores both large and small use plants as food, and actively chew them. Pathogens are agents of disease. These infectious microorganisms, such as fungi, bacteria, and nematodes, live off of the plant and damage its tissues. Plants have developed a variety of strategies to discourage or kill attackers. The first line of defense in plants is an intact and impenetrable barrier. Bark and the waxy cuticle can protect against predators. Other adaptations against herbivory include thorns, which are modified branches, and spines, which are modified leaves. They discourage animals by causing physical damage and inducing rashes and allergic reactions. A plant’s exterior protection can be compromised by mechanical damage, which may provide an entry point for pathogens. If the first line of defense is breached, the plant must resort to a different set of defense mechanisms, such as toxins and enzymes. Secondary metabolites are compounds that are not directly derived from photosynthesis and are not necessary for respiration or plant growth and development. Many metabolites are toxic, and can even be lethal to animals that ingest them. Some metabolites are alkaloids, which discourage predators with noxious odors (such as the volatile oils of mint and sage) or repellent tastes (like the bitterness of quinine). Other alkaloids affect herbivores by causing either excessive stimulation (caffeine is one example) or the lethargy associated with opioids. Some compounds become toxic after ingestion; for instance, glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore. Mechanical wounding and predator attacks activate defense and protection mechanisms both in the damaged tissue and at sites farther from the injury location. Some defense reactions occur within minutes: others over several hours. The infected and surrounding cells may die, thereby stopping the spread of infection. Long-distance signaling elicits a systemic response aimed at deterring the predator. As tissue is damaged, jasmonates may promote the synthesis of compounds that are toxic to predators. Jasmonates also elicit the synthesis of volatile compounds that attract parasitoids, which are insects that spend their developing stages in or on another insect, and eventually kill their host. The plant may activate abscission of injured tissue if it is damaged beyond repair. 990 Chapter 23 | Plant Form and Physiology Think About It 1. Owners and managers of plant nurseries have to plan a lighting schedule for a long-day plant that will flower in February. What lighting periods will be most effective? |
examples include dermal tissue and ground tissue. Dermal tissue provides the outer covering of the plant. Ground tissue is responsible for photosynthesis; it also supports vascular tissue and may store water and sugars. Complex tissues are made up of different cell types. Vascular tissue, for example, is made up of xylem and phloem cells. 23.2 Stems The stem of a plant bears the leaves, flowers, and fruits. Stems are characterized by the presence of nodes (the points of attachment for leaves or branches) and internodes (regions between nodes). Plant organs are made up of simple and complex tissues. The stem has three tissue systems: dermal, vascular, and ground tissue. Dermal tissue is the outer covering of the plant. It contains epidermal cells, stomata, guard cells, and trichomes. Vascular tissue is made up of xylem and phloem tissues and conducts water, minerals, and photosynthetic products. Ground tissue is responsible for photosynthesis and support and is composed of parenchyma, collenchyma, and sclerenchyma cells. Primary growth occurs at the tips of roots and shoots, causing an increase in length. Woody plants may also exhibit secondary growth, or increase in thickness. In woody plants, especially trees, annual rings may form as growth slows at the end of each season. Some plant species have modified stems that help to store food, propagate new plants, or discourage predators. Rhizomes, corms, stolons, runners, tubers, bulbs, tendrils, and thorns are examples of modified stems. 23.3 Roots Roots help to anchor a plant, absorb water and minerals, and serve as storage sites for food. Taproots and fibrous roots are the two main types of root systems. In a taproot system, a main root grows vertically downward with a few lateral roots. Fibrous root systems arise at the base of the stem, where a cluster of roots forms a dense network that is shallower than a taproot. The growing root tip is protected by a root cap. The root tip has three main zones: a zone of cell division (cells are actively dividing), a zone of elongation (cells increase in length), and a zone of maturation (cells differentiate to form different kinds of cells). Root vascular tissue conducts water, minerals, and sugars. In some habitats, the roots of certain plants may be modified to form aerial roots or epiphy |
tic roots. 23.4 Leaves Leaves are the main site of photosynthesis. A typical leaf consists of a lamina (the broad part of the leaf, also called the blade) and a petiole (the stalk that attaches the leaf to a stem). The arrangement of leaves on a stem, known as phyllotaxy, enables maximum exposure to sunlight. Each plant species has a characteristic leaf arrangement and form. The pattern of leaf arrangement may be alternate, opposite, or spiral, while leaf form may be simple or compound. Leaf tissue consists of the epidermis, which forms the outermost cell layer, and mesophyll and vascular tissue, which make up the inner portion of the leaf. In some plant species, leaf form is modified to form structures such as tendrils, spines, bud scales, and needles. 23.5 Transport of Water and Solutes in Plants Water potential (Ψ) is a measure of the difference in potential energy between a water sample and pure water. The water This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 995 potential in plant solutions is influenced by solute concentration, pressure, gravity, and matric potential. Water potential and transpiration influence how water is transported through the xylem in plants. These processes are regulated by stomatal opening and closing. Photosynthates (mainly sucrose) move from sources to sinks through the plant’s phloem. Sucrose is actively loaded into the sieve-tube elements of the phloem. The increased solute concentration causes water to move by osmosis from the xylem into the phloem. The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is unloaded into the sink, and the water returns to the xylem vessels. 23.6 Plant Sensory Systems and Responses Plants respond to light by changes in morphology and activity. Irradiation by red light converts the photoreceptor phytochrome to its far-red light-absorbing form—Pfr. This form controls germination and flowering in response to length of day, as well as triggers photosynthesis in dormant plants or those that just emerged from the soil. Blue-light receptors, cryptochromes, and phototropins are responsible for phototropism. Amyloplasts, which contain heavy starch granules, sense |
gravity. Shoots exhibit negative gravitropism, whereas roots exhibit positive gravitropism. Plant hormones—naturally occurring compounds synthesized in small amounts—can act both in the cells that produce them and in distant tissues and organs. Auxins are responsible for apical dominance, root growth, directional growth toward light, and many other growth responses. Cytokinins stimulate cell division and counter apical dominance in shoots. Gibberellins inhibit dormancy of seeds and promote stem growth. Abscisic acid induces dormancy in seeds and buds, and protects plants from excessive water loss by promoting stomatal closure. Ethylene gas speeds up fruit ripening and dropping of leaves. Plants respond to touch by rapid movements (thigmotropy and thigmonasty) and slow differential growth (thigmomorphogenesis). Plants have evolved defense mechanisms against predators and pathogens. Physical barriers like bark and spines protect tender tissues. Plants also have chemical defenses, including toxic secondary metabolites and hormones, which elicit additional defense mechanisms. REVIEW QUESTIONS 1. Students are sketching diagrams of the shoot system of angiosperms for a plant anatomy class. These lists describe diagrams made by four students. Which diagram represents the shoot system incorrectly? a. b. c. d. leaves, stem, fruit, flowers stem, fruit, leaves, branches flowers, leaves, branches, stem stem, hair roots, leaves, flowers, branches 2. An herbicide causes roots to shrivel and die. What is the most direct consequence for a plant treated with the herbicide? a. The plant will grow normally but will not bloom. b. The plant will dry out because water is not reaching all its organs. c. New leaves will form to compensate for the dying of roots d. The plant will grow normally but will not produce fruit 3. Scientists label cells in the lateral meristem of a sapling with a dye to follow the developmental fate of the cells. After several weeks, sections are prepared from the sapling and observed under the microscope. Which tissues are most likely to be stained by the dye that was injected into the lateral meristem? a. Vascular tissue to transport nutrients and water b. The tip of plant to promote growth of plant c. Secondary xylem to increase girth of stem d. Epidermis to cover the plant 4. A lab technician is looking for a slide that shows an example of permanent tissue. Which slide is the best choice? a. a slide of the |
apical bud of a stem b. a slide obtained from the intercalary meristems c. d. lateral meristem in the vascular cambium secondary xylem 5. Which region of a plant is most likely to contribute to an increase in its length? a. tip of leaves b. dermal layer c. vascular bundles d. tip of the root 6. You are measuring the effect of a new fertilizer on the growth of lawns. Which of the following tissues should be the target of the fertilizer? a. apical meristem b. c. lateral meristem intercalary meristem d. vascular bundle 7. The dermal tissue of a plant provides __ for the plant. 996 Chapter 23 | Plant Form and Physiology a. b. c. transport of water transport of minerals support d. protection 8. A branch of celery is soaked in a glass of water containing food dye. Soon, the tough fibers in celery branch are colored. What tissue do the tough fibers contain? 13. a. the thin epidermis that covers an onion bulb b. a sample of fruit tissue c. a sample of actively dividing cells located at the tip of an onion root d. a region of the mesenchyme a. dermal tissue b. xylem c. phloem d. ground tissue 9. A plant biologist is examining sections of plant tissue under the microscope. The slides are not labeled and the biologist is interested in simple tissues. Which of the following slides is a sample of a simple tissue? a. cells dividing rapidly in a stem b. root cambium showing different types of cells c. parenchyma showing only one type of cell d. leaf displaying the vascular bundle where diverse types of cells are involved in transport 10. Students are asked to sort tissue slides into simple and complex tissues. How should they recognize a complex tissue through the microscope? a. Complex tissue has a variety of cell types that fulfill different functions. b. Only complex tissue is observed in adult plants. c. Complex tissue appears only in lateral roots and branches. d. Complex tissues contain cells that are strikingly different in appearance but perform the same function. 11. Students are sketching diagrams of the reproductive system of angiosperms for a plant anatomy class. These lists describe diagrams made by four students. Which diagram represents the reproductive system correctly? a. hair roots, lateral roots, and taproot b. c. d. stem, branches, and leaves flowers |
and fruit leaves, petioles, and branches 12. Plant scientists are interested in isolating meristematic tissue for an experiment. They sample several regions of a plant. Which sample is most likely to contain meristematic tissue? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 This sketch of a stem shows the region to which leaves are attached. Which version of the sketch is correctly labeled? a. version A b. version B c. version C d. version D 14. A student examines a plant part and concludes that it is part of a stem. The presence of _____fully justifies the student’s conclusion. a. vascular tissue b. nodes and internodes c. epidermal layer d. stored carbohydrates 15. A student reported vascular tissue while inspecting a cross-section of a plant stem under the microscope. Which cells would allow the student to identify vascular tissue? a. tracheids, vessel elements, sieve-tube cells, and companion cells b. cells actively dividing at the apex of the stem c. parenchyma cells at the center of the section d. cells covered by a cuticle at the outside edge of the section 16. While using a microscope to observe a stem section stained with a dye that binds lignin, a student notices that some cells with thick cell walls and large hollow centers are preferentially stained. He concludes that those cells belong to the ____. Chapter 23 | Plant Form and Physiology 997 a. meristematic tissue b. vascular tissue c. ground tissue d. dermal tissue 17. Scientists are cataloguing slides of plant crosssections. They are interested in finding examples of secondary growth. Which example contributes to secondary growth? a. apical meristem, which contributes to increase in length b. vascular cambium, which contributes to increase in thickness or girth c. d. root region, which shows an increase in root hairs stems, which show an increase in number of leaves 18. Where is the vascular cambium located in an established woody plant? a. between the primary xylem and the primary phloem b. between the secondary xylem and the primary phloem a. an underground stem with fleshy leaves modified for food storage as in onions b. a solid, underground stem covered with scales formed by some plants such as crocuses c. an aboveground stem with buds as seen |
in strawberry plants d. a modified horizontal stem that grows underground as seen in irises 22. Modified organs are part of survival strategies of plants. Which of these plants has a flattened, photosynthetic stem that could be mistaken for a leaf? a. fern b. cactus c. potato d. iris 23. Analyzing cross-sections of different parts of a plant in a plant anatomy class, students categorized the most frequently encountered types of cells in plant tissues. Which student gave the most accurate report? a. Student A reported that meristematic cells were the most abundant. b. Student B tallied mostly collenchyma cells. c. between the secondary xylem and the secondary c. Student C noticed mostly sclerenchyma cells. phloem d. Student D observed that parenchyma cells were d. between the primary xylem and the secondary the most abundant. phloem 19. Dendrochronology is the science of dating the age of a tree by counting the annual rings in a tree trunk. If scientists are determining the age of a tree by dendrochronology, what tissue are they looking at? a. primary xylem b. secondary xylem c. primary phloem d. vascular cambium 20. While examining the stump of a recently cut tree, you count four thick rings alternating with four rings that are much narrower and appear denser. From this observation, you should conclude that the tree is __. a. b. c. two years old, because each ring corresponds to a season three years old, because the first ring you observe is the primary xylem four years old, because secondary xylem grows only in the spring and fall of each year d. eight years old, because there are eight rings in all 21. Many forms of modified organs exist in plants. What is a rhizome? 24. A carrot is an example of a tap root. Which of these can also be classified as a tap root? a. the large network of superficial roots of a cactus b. a dandelion anchored by a long main root that penetrates deep into the soil c. a banyan tree’s system of roots that dangle from the branches d. a round organ that stores carbohydrates 25. Some weeds are anchored by taproots. They cause problems to gardeners because they are ___. a. easy to pull up because the root system is shallow b. difficult to pull |
up because their taproots penetrate deep into the soil c. difficult to pull up because they are anchored by an extensive network of roots d. easy to pull up because there is not a large network to anchor the plant 26. One of the major concepts of biology is that form follows function. If that is so, what can be deduced from the shape and location of the root cap? 998 a. b. c. d. Chapter 23 | Plant Form and Physiology It provides protection to the root tip. the blue granules indicative of starch? It absorbs water and minerals. It acts as a storage tissue. a. parenchymal cells of the cortex b. cells of phloem It replicates actively to elongate the root. c. cells of the epidermis 27. A technician is preparing microscope slides that will display the different stages of mitosis from root samples. He compares sections from several areas of the root. Which is the best prediction of his observation? a. The technician will see mostly mitotic cells in the root cap. b. The technician will observe mitotic figures in the meristematic tissue below the cap. c. The technician will observe cell division in the elongation zone. d. The technician will see that most mitotic cells are in the maturation zone. 28. Selective uptake of minerals in the root is measured and the results are analyzed. If you analyze the data, what should you see? a. Pericycle is the tissue where selectivity takes place. b. The endodermis acts as a selective barrier for minerals taken up by the root. c. The epidermis acts as a selective barrier for minerals. d. The root cap functions as a selective barrier for minerals taken up by the root. 29. Sudan Red dye stains primarily waxy, hydrophobic material. A root is soaked in Sudan Red and analyzed for stain retention. What is a scientist observing sections of the root under a microscope likely to see? a. The cells in the cortex show the deepest stain. b. The tracheids in the xylem contain mostly lipid droplets stained with Sudan Red. c. The Casparian strip will show the deepest coloring. d. The sieve elements in the phloem show staining with Sudan Red because of transported oil droplets. 30. In environments where light is scarce, some plants grow on other plants to reach light. Which root system would best support this mode of life? a. Epiphy |
tic root system in the air b. Prop roots that support the trees to stand in muddy soil c. Adventitious roots that grow above ground d. Taproots that penetrate the soil d. cells of the endodermis and pericycle 32. Which of the following best describes a fibrous root system? a. covers a limited surface and contains few roots b. consists of a single main root with adjacent smaller roots c. covers a large area and contains an extensive network of roots d. contains several major, interconnected roots 33. Ethylene promotes the fall of leaves by triggering the death of cells and abscission. What region of the leaf responds to ethylene? a. b. c. d. the lamina, where photosynthesis takes place the vein, which carries nutrients and water in and out of the leaf the petiole, which attaches the leaf to the stem the margin, which is serrated and may be sharp 34. A horticulture student is classifying plants as dicots or monocots according to their leaf structure. How is a dicot leaf recognizable? a. It does not have stipules b. The veins form a network pattern. c. The veins are parallel. d. The veins form forks and fan out. 35. Multiple leaves attached to the same node are fairly unusual. One example is found on the macadamia nut tree. The leaf arrangement in the macadamia tree is best characterized as ___. a. whorled b. opposite c. tripled d. alternate 36. You picked leaves while on a hike. One specimen appears to show an opposite arrangement. On closer inspection, you notice that those are not leaves, but leaflets attached to a midrib vein. What type of leaf arrangement are you observing? a. palmately compound b. pinnately compound c. d. simple whorled simple spiral 31. A section of buttercup root is stained with iodine, which stains starch blue. Where would you expect to find 37. Chlorophyll, the primary photosynthetic pigment, This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 999 emits light in the red region of the visible spectrum. The presence of chlorophyll correlates with photosynthetic capacity. Under a fluorescent microscope, what part of a leaf would fluoresce in the red region of the spectrum? 43. Which of the following physical components of the |
total water potential cannot be manipulated by the plant because it represents the interaction between water and hydrophilic molecules lining the vessels and tracheids? a. vascular bundle b. epidermis c. mesophyll d. cuticle a. pressure b. solute concentration c. gravity d. matric potential 38. A pulse of radioactive carbon dioxide (CO2) is provided to isolated leaves. In which tissue would you expect to see radioactive glucose appear first? 44. If the concentration of solute increases in a cell, the water potential will ________ inside the cell and water will move ________ the cell. a. b. in the cells of the mesophyll in the sieve elements of the phloem c. epidermis d. vessels of the xylem a. b. increase; out of increase; into c. decrease; into d. decrease; out of 39. Which adaptation is most likely to be found in a desert environment? a. broad leaves to capture sunlight b. spines instead of leaves c. needle-like leaves d. wide, flat leaves that can float 40. In the collection of a botanical garden, plants are classified according to the environments in which they thrive. What plant would have large leaves covered with a thick upper cuticle and wide flat blades and possess large air spaces (chambers) within its mesophyll tissue? a. a water lily floating on water b. a pine tree growing in the cold and dry taiga c. a cactus growing in a hot, sunny, and dry environment d. an orchid hanging from a tree in a tropical forest 41. If a gardener trims leaves off of the stem of a rose, which part of the leaf is cut? a. petiole b. c. lamina stipule d. midrib 42. On a field trip, students collect a few samples to analyze back in their classroom. One student picks a blade of grass in the field and identifies it as a dicot leaf, but his partner thinks it is a monocot. Which explanation supports his partner’s opinion? a. The leaf displays a thin lamina. b. There is no petiole. c. The margins are serrated. d. The venation is parallel. 45. Plants can modify their water potential by opening and closing their stomata to modulate the rate of respiration according to environmental conditions. Which of the following environmental conditions would cause the stomata to |
close? a. increased temperature b. high oxygen concentration c. high relative humidity d. high light levels 46. Plants regulate their internal water potential by opening and closing stomata. Which events take place when stomata open? a. Water vapor is lost to the external environment, increasing the rate of transpiration. b. Water vapor is lost to the external environment, decreasing the rate of transpiration. c. Water vapor enters the spaces in the mesophyll, increasing the rate of transpiration. d. The rate of photosynthesis drops when stomata open. 47. A pulse of sugars labelled with a fluorescent dye is supplied to leaves of young plants. After a brief interval, tissue sections are obtained from the plant and examined under the fluorescence microscope. Tissues are scored for the presence of fluorescence and ranked from very high to low fluorescence. Which cells would contain the most fluorescence? a. xylem b. companion cells c. sieve elements d. epidermis 48. Sugars produced in the leaf are distributed throughout the plant body. An experimenter supplies plants with a 1000 Chapter 23 | Plant Form and Physiology pulse of radiolabeled CO2 in a control chamber. The movement of radioactively labeled sugar is monitored in the plant by analyzing different cells content over time. Where will the radiolabeled sugar appear immediately after detection in the leaf cells? a. b. tracheids and vessel elements tracheids and companion cells c. vessel elements and companion cells d. sieve-tube elements and companion cells 49. Solute potential decreases when solutes are added to a cell. The consequence is to draw water into the cell. Which of these terms corresponds to solute potential? a. water potential b. pressure potential c. osmotic potential d. negative potential 50. increasingly longer nights will most likely flower in _____. a. b. spring summer c. autumn d. winter 52. Gravitropism is plant growth in response to gravity. A dahlia stem was toppled by the wind and is lying lies on the ground. After a few days, you would likely notice that ________. a. b. c. d. the stem is growing by curving toward the roots the stem is growing by trailing on the ground the stem is growing by curving upward the plant is wilting 53. Plants most likely detect gravity by sensing the direction in which some components respond to gravity. A mutant plant has roots that grow in |
all directions. Which organelle would you expect to be missing in the cell? a. mitochondria b. amyloplast c. chloroplast d. nucleus 54. Plants have many light responses, including photosynthesis, photoperiodism, and phototropism (growing toward a light source). Specific wavelengths of light absorbed by different photoreceptors trigger responses. This table shows some of the most common photoreceptors and pigments and the major regions of the spectrum in which they are active. Research shows that plants bend toward blue light. Even mutant plants that lack carotenoids will bend toward blue light. The photoreceptor is likely _____. a. phytochrome b. chlorophyll c. phototropin d. carotenoids 51. Plant flowering is an example of photoperiodism, the response to the length of nights or periods of darkness. A plant that responds to short nights followed by This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 In an experiment to release seeds from dormancy, several hormones were applied to seeds and germination rates were computed. Which plate likely showed the highest rate of germination? a. abscisic acid b. cytokinin c. ethylene d. gibberellic acid 55. Green bananas or unripe avocadoes can be kept in a Chapter 23 | Plant Form and Physiology 1001 a. thorns and spines b. cutin and suberin c. neurotoxic compounds d. bitter-tasting alkaloids 59. Many secondary alkaloids are poisonous to the nervous system. What organisms are targeted by the alkaloids? a. bacteria b. herbivores c. fungi d. viruses 60. Red light converts phytochrome red (Pr) to __. a. an inactive form of Pr b. a breakdown product c. the far red light absorbing form called Pfr d. cryptochrome 61. Circadian rhythm refers to a pattern of behavior that recurs on a daily schedule in the absence of an external stimulus. Flowers open and close according to a circadian rhythm. If a plant is transferred to a dark environment, what will happen? a. Flowers will stay closed. b. Flowers will stay open. c. Flowers will open and close every day at the same time. d. Flowers will open and close at random times. a. dermal tissue b. |
meristematic tissue c. vascular tissue d. ground tissue 64. How do the locations and the functions of the three types of meristematic tissues compare? brown bag to ripen faster. What hormone is involved? a. cytokinin b. abscisic acid c. ethylene d. gibberellic acid 56. A lab teacher wants to demonstrate thigmonastic behavior of a plant. Which of these experiments is the best choice? a. Observe flowering of a plant after a brief red light irradiation in the middle of a dark period. b. Observe whether seedlings bend towards blue light. c. Observe whether a tree grows bent in the direction of the prevailing wind. d. Touch the plant Mimosa pudicaand observe the closing of the leaflets. 57. A lab teacher wants to demonstrate thigmotropic behavior of a plant. Which of these experiments is the best choice? a. roots growing downwards b. venus fly trap snapping on an insect c. seedling germinating under a stone and growing upward and away from the stone d. plant growing towards a shaded area 58. Which is a protection against microbial pathogens? CRITICAL THINKING QUESTIONS 62. Why are plants with shallow roots more easily damaged by some herbivores? a. Shallow roots do not anchor the plant to the ground and can be easily uprooted. Once the plant is no longer in the ground, the roots are unable to grow back. b. Plants with shallow roots do not anchor the plant to the ground; meristems can be easily damaged and cannot grow back when not in the ground. c. Shallow roots do not anchor the plant to the ground and can be easily uprooted. Once the plant is no longer in the ground, roots take a long time to grow back. d. Shallow roots anchor the plant to the ground strongly but can be easily uprooted, and they grow back very slowly. 63. A researcher intends to test the effects of several growth factors on the differentiation of plant tissue. What would be the best choice of experimental tissue? 1002 Chapter 23 | Plant Form and Physiology a. Apical meristems found in the tip of stems and roots promote growth by elongation; lateral meristems found at nodes and bases of leaf blades promote increase in length and intercalary meristems found in the vascular and cork cambia promote increase in girth. b. Apical meristems found |
at nodes and bases of leaf blades promote growth by elongation; lateral meristems found in the vascular and cork cambia promote increase in girth and intercalary meristems found in the tip of stems and roots promote increase in length. c. Apical meristems found in the tip of stems and roots promote growth by elongation; lateral meristems found in the vascular and cork cambia promote increase in girth and intercalary meristems found at nodes and bases of leaf blades promote increase in length. d. Apical meristems found in the tip of stems and roots promote growth by elongation; lateral meristems found in the vascular and cork cambia promote increase in length and intercalary meristems found at nodes and bases of leaf blades promote increase in length. 65. In an experiment on transport in plants, seedlings are exposed to radiolabeled minerals. In a second experiment, plants are provided with CO2 that is labeled with 14C. At the end of each experiment, tissue slices are analyzed for the presence of radiolabeled minerals and radioactive sucrose. Which plant tissue would show the presence of labeled minerals and which would show the presence of radioactive sucrose? a. Phloem tissue would show the presence of labeled minerals and xylem tissue would show the presence of radioactive sucrose. b. Xylem tissue would show the presence of labeled minerals and phloem tissue would show the presence of radioactive sucrose. c. Parenchyma would show the presence of labeled minerals and sclerenchyma would show the presence of radioactive sucrose. d. Sclerenchyma would show the presence of labeled minerals and parenchyma would show the presence of radioactive sucrose. 66. How could the morphology of cells observed microscopically indicate that the specimen is probably simple tissue? a. Simple tissue is made of cells that have different shapes, so the specimen will show oval, polygonal, and other shapes. b. Simple tissue is made of cells that have intercellular spaces, so the specimen will contain spaces. c. Simple tissue is made of cells that are elongated and tapered, so the specimen will show elongated cells. d. Simple tissue is made of cells that are morphologically similar, so the specimen will appear uniform. 67. Which statements list two advantages of a taproot? a. b. c. d. It anchors the plant, so that it |
is not easily uprooted by predators or wind. It is a sink for proteins that is protected from herbivores by being underground. It anchors the plant, so that it is not easily uprooted by predators or wind. It is a source of starches that is protected from herbivores by being underground. It anchors the plant, so that it cannot be uprooted by predators or wind. It is a sink for starches that is protected from herbivores by being underground. It anchors the plant, so that it is not easily uprooted by predators or wind. It is a sink for starches that is protected from herbivores by being underground. 68. Students observe several slides of tissue cross-sections under the microscope. They are asked to develop a key system to classify the slides as coming from either monocot or dicots. What key system should the students develop? a. b. c. d. In monocots, the vascular bundles form a distinct ring. In dicots, the vascular bundles are scattered in the ground tissue. In monocots, the vascular tissue forms a characteristic X shape in the center. In dicots, the phloem and xylem cells are scattered in the pith. In monocots, the vascular bundles are scattered in the ground tissue. In dicots the vascular bundles form a distinct ring. In monocot roots, the pith is absent or very small. In dicots, the pith is large and well developed. 69. What are the functions of stomata and guard cells, and what would happen to a plant if these cells did not function correctly? This OpenStax book is available for free at http://cnx.org/content/col12078/1.6 Chapter 23 | Plant Form and Physiology 1003 a. Rhizomes, stolons and runners give rise to new plants that are the clones of the parents and they store food. Corms, tubers, and bulbs can also produce new plants. b. Rhizomes, stolons, and runners give rise to new plants that are the different from the parents. Corms, tubers, and bulbs can also produce new plants as well as store food. c. Rhizomes, stolons and runners give rise to new plants that are the clones of the parents. Corms, tubers, and bulbs can also produce new plants as well as store food. d. Rhizomes |
, stolons and runners give rise to new plants that are similar to the parents but show genetic variability. Corms, tubers, and bulbs can also produce new plants as well as store food. 73. A time course is developed to follow the fate of the vascular bundles in the stem of dicots. Sections along the stem are fixed, stained, and observed under a microscope. What happens to the vascular bundles in the stem of a dicot as the plant matures? a. The vascular bundles join to form growth rings. b. The vascular bundles divide into primary xylem and primary phloem. c. The vascular bundles divide into secondary xylem and primary phloem. d. The vascular bundles die out. 74. Which description correctly compares a tap root system with a fibrous root system? a. Guard cells allow carbon dioxide to enter and exit the plant. Stomata regulate the opening and closing of guard cells. If the cells didn’t function, photosynthesis and transpiration would cease, which would interfere with the necessary continuous flow of water upward from roots to leaves. b. Stomata allow oxygen to enter and exit the plant. Guard cells regulate the opening and closing of stomata. If the cells didn’t function, photosynthesis would continue but transpiration would cease, which would interfere with the necessary continuous flow of water upward from roots to leaves. c. Guard cells allow carbon dioxide to enter and exit the plant. Stomata regulate the opening and closing of guard cells. Transpiration and in turn, photosynthesis would not occur which is necessary to maintain a continuous flow of water upwards from the roots to the leaves. d. Stomata allow gases to enter and exit the plant. Guard cells regulate the opening and closing of stomata. Photosynthesis and, in turn, transpiration, would not occur which is necessary to maintain a continuous flow of water upwards from the roots to the leaves. 70. An herbicide is developed that impairs the function of the cork cambium in woody plants. Which changes in the plant should be monitored to gauge the effectiveness of the herbicide? a. Cork will not be produced and the plant will not increase in girth. b. Excess cork will be produced and annual rings will not be formed. c. Cork will not be produced and the plant will not be able to exchange gases. d. Excess cork will be produced and the plant will not increase in |
girth. 71. Besides the age of a tree, what additional information can annual rings reveal? a. Annual rings can also indicate the height of the tree. b. Annual rings can also indicate the climatic conditions that prevailed during each growing season. c. Annual rings can also indicate in which season the tree was sown. d. Annual rings can also give an estimation of how long a particular tree is going to live. 72. Modified stems give an advantage to plants. What advantage do rhizomes, stolons, and runners provide? What advantages do corms, tubers, and bulbs provide? 1004 Chapter 23 | Plant Form and Physiology a. A tap root system, such as that of carrots, has a single main root that grows down. A fibrous root system, such as that of wheat, forms a dense network of roots that is closer to the soil surface. Fibrous root systems are found in monocots and tap root systems are found in dicots. b. A fibrous root system, such as that of a carrot, has a single main root that grows down. A taproot system, such as that of wheat, forms a dense network of roots that is closer to the soil surface. Fibrous root systems are found in monocots and tap root systems are found in dicots. c. A taproot system, such as that of rice, has a single main root that grows down. A fibrous root system, such as that of a carrot, forms a dense network of roots that is closer to the soil surface. Fibrous root systems are found in monocots and tap root systems are found in dicots. d. A taproot system, such as that of a carrot, has a single main root that grows down. A fibrous root system, such as that of wheat, forms a dense network of roots that is closer to the soil surface. Taproot systems are found in monocots and fibrous root systems are found in dicots. 75. What is the advantage of a root cap covering the apical meristem of a root? a. b. c. d. It provides protection and helps in absorption. It increases the surface area of root for absorption of water and minerals. It protects meristem against injury and provides lubrication for the growing root to dig through soil. It protects the meristem against injury and helps in absorption. 76. How does selective uptake of water and mineral take place |
in a root? a. Water and minerals must follow entirely a path between cells, where selectivity occurs. b. Water and minerals must follow entirely a path between cells, where no selectivity occurs. c. Water and minerals must cross the endodermis. d. Water and minerals must cross the tracheids of the xylem. 77. What are the advantages to a plant of storing a food reserve underground? a. Food reserves are more nutritious underground. The soil conditions make these food reserves abundant. b. Food reserves underground are hidden from potential predators. The soil conditions make these food reserves abundant. c. Food reserves are more nutritious underground. The soil conditions such as moisture and temperature are less variable. d. Food reserves underground are hidden from potential predators. Soil conditions such as moisture and temperature are less variable. 78. Some desert plants have taproots that extend up to 20-30 feet underground. Others have fibrous root systems that cover wide areas. What are the advantages of a deep taproot and the advantages of a fibrous root system in a desert? a. A deep taproot can reach the deeper soil regions that stay moist after several rainfalls. A shallow fibrous system provides additional support to anchor the plant in the desert. b. A deep taproot provides additional support to anchor the plant in the desert. A shallow fibrous system increases the amount of water that can be absorbed after a light rainfall when the soil dries quickly in the desert. c. A deep taproot increases the amount of water that can be absorbed after a light rainfall when the soil dries quickly in the desert. A shallow fibrous system can reach the deeper soil regions that stay moist after several rainfalls. d. A deep taproot can reach the deeper soil regions that stay moist after several rainfalls. A shallow fibrous system increases the amount of water that can be absorbed after a light rainfall when the soil dries quickly in the desert. 79. Samples of leaves from monocots and dicots are piled on the table in a laboratory and students are sorting the leaves. What information will help them know which leaves to identify as monocots? a. Bulliform cells are usually absent from monocots whereas they are present on the upper epidermis of dicot leaves. b. Monocots have leaves with parallel venation and dicot leaves have reticulate, net-like venation. c. Dorsiventral symmetry is observed in monocot leaves |
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