page_content stringlengths 12 2.63M | metadata unknown |
|---|---|
Before a eukaryotic cell divides, it must duplicate its membrane-enclosed organelles. As cells grow, membrane-enclosed organelles enlarge by incorporation of new molecules; the organelles then divide and, during cell division, are distributed between the two daughter cells. Organelle growth requires a supply of new lip... | {
"Header 1": "Intracellular Compartments and Protein Transport",
"Header 3": "Protein Sorting",
"token_count": 420,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The synthesis of virtually all proteins in the cell begins on ribosomes in the cytosol. The exceptions are the few mitochondrial and chloroplast proteins that are synthesized on ribosomes inside these organelles; most mitochondrial and chloroplast proteins, however, are made in the cytosol and subsequently imported. Th... | {
"Header 1": "Intracellular Compartments and Protein Transport",
"Header 3": "Proteins Are Transported into Organelles by Three Mechanisms",
"token_count": 1027,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The typical sorting signal on a protein is a continuous stretch of amino acid sequence, typically 15–60 amino acids long. This **signal sequence** is often (but not always) removed from the finished protein once it has been sorted. Some of the signal sequences used to specify different destinations in the cell are show... | {
"Header 1": "Signal Sequences Direct Proteins to the Correct Compartment",
"token_count": 350,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The nuclear envelope, which encloses the nuclear DNA and defines the nuclear compartment, is formed from two concentric membranes. The *inner nuclear membrane* contains some proteins that act as binding sites for the chromosomes (discussed in Chapter 5) and others that provide anchorage for the *nuclear lamina*, a fine... | {
"Header 1": "Signal Sequences Direct Proteins to the Correct Compartment",
"Header 3": "Proteins Enter the Nucleus Through Nuclear Pores",
"token_count": 1426,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Both mitochondria and chloroplasts are surrounded by inner and outer membranes, and both organelles specialize in the synthesis of ATP. Chloroplasts also contain a third membrane system, the thylakoid membrane (discussed in Chapter 14). Although both organelles contain their own genomes and make some of their own prote... | {
"Header 1": "Signal Sequences Direct Proteins to the Correct Compartment",
"Header 3": "Proteins Unfold to Enter Mitochondria and Chloroplasts",
"token_count": 713,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Peroxisomes generally contain one or more enzymes that produce hydrogen peroxide, hence their name. These organelles are present in all eukaryotic cells, where they break down a variety of molecules, including toxins, alcohol, and fatty acids. They also synthesize certain phospholipids, including those that are abundan... | {
"Header 1": "Signal Sequences Direct Proteins to the Correct Compartment",
"Header 3": "Proteins Enter Peroxisomes from Both the Cytosol and the Endoplasmic Reticulum",
"token_count": 386,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The endoplasmic reticulum is the most extensive membrane system in a eukaryotic cell (Figure 15–12A). Unlike the organelles discussed so far, it serves as an entry point for proteins destined for other organelles, as well as for the ER itself. Proteins destined for the Golgi apparatus, endosomes, and lysosomes, as well... | {
"Header 1": "Signal Sequences Direct Proteins to the Correct Compartment",
"Header 3": "Proteins Enter the Endoplasmic Reticulum While Being Synthesized",
"token_count": 971,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Two protein components help guide ER signal sequences to the ER membrane: (1) a *signal-recognition particle* (*SRP*), present in the cytosol, binds
Figure 15-13 A common pool of ribosomes is used to synthesize all the proteins encoded by the nuclear genome. Ribosomes that are translating proteins with no ER signal s... | {
"Header 1": "Signal Sequences Direct Proteins to the Correct Compartment",
"Header 3": "Soluble Proteins Made on the ER Are Released into the ER Lumen",
"token_count": 594,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Not all proteins made by ER-bound ribosomes are released into the ER lumen. Some remain embedded in the ER membrane as transmembrane proteins. The translocation process for such proteins is more complicated than it is for soluble proteins, as some parts of the polypeptide chain must be translocated completely across th... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"token_count": 1209,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Entry into the ER lumen or membrane is usually only the first step on a pathway to another destination. That destination, initially at least, is generally the Golgi apparatus; there, proteins and lipids are modified and sorted for shipment to other sites. Transport from the ER to the Golgi apparatus—and from the Golgi ... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Vesicular Transport",
"token_count": 224,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Vesicular transport between membrane-enclosed compartments of the endomembrane system is highly organized. A major outward *secretory pathway* starts with the synthesis of proteins on the ER membrane and their entry into the ER, and it leads through the Golgi apparatus to the cell surface; at the Golgi apparatus, a sid... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Transport Vesicles Carry Soluble Proteins and Membrane Between Compartments",
"token_count": 522,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.p... |
Vesicles that bud from membranes usually have a distinctive protein coat on their cytosolic surface and are therefore called coated vesicles. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse. Cells produce several kin... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Vesicle Budding Is Driven by the Assembly of a Protein Coat",
"token_count": 883,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
After a transport vesicle buds from a membrane, it must find its way to its correct destination to deliver its contents. Often, the vesicle is actively transported by motor proteins that move along cytoskeletal fibers, as discussed in Chapter 17.
Figure 15-20 Clathrin-coated vesicles transport selected cargo molecule... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Vesicle Docking Depends on Tethers and SNAREs",
"token_count": 1495,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Most proteins that enter the ER are chemically modified there. *Disulfide bonds* are formed by the oxidation of pairs of cysteine side chains (see Figure 4–30), a reaction catalyzed by an enzyme that resides in the ER lumen. The disulfide bonds help to stabilize the structure of proteins that will encounter degradative... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Most Proteins Are Covalently Modified in the ER",
"token_count": 283,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
In the ER, individual sugars are not added one-by-one to the protein to create the oligosaccharide side chain. Instead, a preformed, branched oligosaccharide containing a total of 14 sugars is attached *en bloc* to all proteins that carry the appropriate site for glycosylation. The oligosaccharide is originally attache... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "branched oligosaccharide side chains composed of multiple sugars. This process of *glycosylation* is carried out by glycosylating enzymes present in the ER but not in the cytosol. Very few ... |
#### Question 15–6
Why might it be advantageous to add a preassembled block of 14 sugar residues to a protein in the ER, rather than building the sugar chains step-by-step on the surface of the protein by the sequential addition of sugars by individual enzymes?
Figure 15–23 Many proteins are glycosylated on asparag... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "CYTOSOL ER LUMEN NH2 NH2 growing polypeptide chain lipid-linked oligosaccharide dolichol dolichol = glucose = mannose = *N*-acetylglucosamine KEY: oligosaccharyl transferase P P P P Asn Asn... |
Some proteins made in the ER are destined to function there. They are retained in the ER (and are returned to the ER whenever they escape to the Golgi apparatus) by a C-terminal sequence of four amino acids called an *ER retention signal* (see Table 15–3, p. 494). This retention signal is recognized by a membrane-bound... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Exit from the ER Is Controlled to Ensure Protein Quality",
"token_count": 463,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Although chaperones help proteins in the ER fold properly and retain those that do not, this quality control system can become overwhelmed. When that happens, misfolded proteins accumulate in the ER. If the buildup is large enough, it triggers a complex program called the unfolded protein response (UPR). This program p... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "The Size of the ER Is Controlled by the Demand for Protein",
"token_count": 484,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The Golgi apparatus is usually located near the cell nucleus, and in animal cells it is often close to the centrosome, a small cytoskeletal structure near the cell center (see Figure 17–12). The Golgi apparatus consists of a collection of flattened, membrane-enclosed sacs called cisternae, which are piled like stacks o... | {
"Header 1": "Start and Stop Signals Determine the Arrangement of a Transmembrane Protein in the Lipid Bilayer",
"Header 3": "Proteins Are Further Modified and Sorted in the Golgi Apparatus",
"token_count": 786,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
In all eukaryotic cells, a steady stream of vesicles buds from the *trans* Golgi network and fuses with the plasma membrane in the process of **exocytosis**. This *constitutive exocytosis pathway* supplies the plasma membrane with newly made lipids and proteins (Movie 15.7), enabling the plasma membrane to expand prior... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"token_count": 277,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Over the years, biologists have taken advantage of a variety of techniques to untangle the pathways and mechanisms by which proteins are sorted and transported into and out of the cell and its resident organelles. Biochemical, genetic, molecular biological, and microscopic techniques all provide ways to monitor how pro... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "TRACKING PROTEIN AND VESICLE TRANSPORT",
"token_count": 309,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The most commonly used method for tracking a protein as it moves throughout the cell involves tagging the polypeptide with a fluorescent protein, such as green fluorescent protein (GFP). Using the genetic engineering techniques discussed in Chapter 10, this small protein can be fused to other cell proteins. Fortunately... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "At the movies",
"token_count": 1284,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Eukaryotic cells are continually taking up fluid, as well as large and small molecules, by the process of endocytosis. Specialized cells are also able to internalize large particles and even other cells. The material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first buds in... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Endocytic Pathways",
"token_count": 326,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The most dramatic form of endocytosis, phagocytosis, was first observed more than a hundred years ago. In protozoa, phagocytosis is a form of feeding: these unicellular eukaryotes ingest large particles such as bacteria by taking them up into phagosomes (Movie 15.9). The phagosomes then fuse with lysosomes, where the f... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Specialized Phagocytic Cells Ingest Large Particles",
"token_count": 303,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
What would you expect to happen in cells that secrete large amounts of protein through the regulated secretory pathway if the ionic conditions in the ER lumen could be changed to resemble those in the lumen of the *trans* Golgi network?
Figure 15–32 Specialized phagocytic cells can ingest other cells. (A) Electron mi... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Question 15–7",
"token_count": 756,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Pinocytosis, as just described, is indiscriminate. The endocytic vesicles simply trap any molecules that happen to be present in the extracellular fluid and carry them into the cell. In most animal cells, however, pinocytosis via clathrin-coated vesicles also provides an efficient pathway for taking up specific macromo... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Receptor-mediated Endocytosis Provides a Specific Route into Animal Cells",
"token_count": 681,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Iron (Fe) is an essential trace metal that is needed by all cells. It is required, for example, for synthesis of the heme groups and iron-sulfur centers that are part of the active site of many proteins involved in electron-transfer reactions; it is also required in hemoglobin, the main protein in red blood cells. Iron... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Question 15–8",
"token_count": 433,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Because most extracellular material taken up by pinocytosis is rapidly delivered to endosomes, it is possible to visualize the endosomal compartment by incubating living cells in fluid containing an electron-dense marker that will show up when viewed in an electron microscope. When examined in this way, the endosomal c... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Endocytosed Macromolecules Are Sorted in Endosomes",
"token_count": 698,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Many extracellular particles and molecules ingested by cells end up in lysosomes, which are membranous sacs of hydrolytic enzymes that carry out the controlled intracellular digestion of both extracellular materials and worn-out organelles. They contain about 40 types of hydrolytic enzymes, including those that degrade... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Lysosomes Are the Principal Sites of Intracellular Digestion",
"token_count": 885,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
- • Eukaryotic cells contain many membrane-enclosed organelles, including a nucleus, an endoplasmic reticulum (ER), a Golgi apparatus, lysosomes, endosomes, mitochondria, chloroplasts (in plant cells), and peroxisomes. The ER, Golgi apparatus, peroxisomes, endosomes, and lysosomes are all part of the *endomembrane syst... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Essential Concepts",
"token_count": 2006,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
ER signal sequences cannot therefore be reused to import ER proteins after mitosis, when cytosolic and ER proteins have become intermixed; these ER proteins must therefore be degraded and resynthesized."
#### Question 15–15
Consider a protein that contains an ER signal sequence at its N-terminus and a nuclear local... | {
"Header 1": "Secretory Proteins Are Released from the Cell by Exocytosis",
"Header 3": "Essential Concepts",
"token_count": 771,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Individual cells, like multicellular organisms, need to sense and respond to their environment. A free-living cell—even a humble bacterium—must be able to track down nutrients, tell the difference between light and dark, and avoid poisons and predators. And if such a cell is to have any kind of "social life," it must b... | {
"Header 1": "Cell Signaling",
"token_count": 747,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Cells in multicellular organisms use hundreds of kinds of *extracellular sig-nal molecules* to communicate with one another. The signal molecules can be proteins, peptides, amino acids, nucleotides, steroids, fatty acid derivatives, or even dissolved gases—but they all rely on only a handful of basic styles of communic... | {
"Header 1": "Cell Signaling",
"Header 3": "Signals Can Act over a Long or Short Range",
"token_count": 1374,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
A typical cell in a multicellular organism is exposed to hundreds of different signal molecules in its environment. These may be free in the extracellular fluid, embedded in the extracellular matrix in which most cells reside, or bound to the surface of neighboring cells. Each cell must respond very selectively to this... | {
"Header 1": "Cell Signaling",
"Header 3": "Each Cell Responds to a Limited Set of Extracellular Signals, Depending on Its History and Its Current State",
"token_count": 1803,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Extracellular signal molecules generally fall into two classes. The first and largest class consists of molecules that are too large or too hydrophilic to cross the plasma membrane of the target cell. They rely on receptors on the surface of the target cell to relay their message across the membrane (Figure 16–8A). The... | {
"Header 1": "Cell Signaling",
"Header 3": "Some Hormones Cross the Plasma Membrane and Bind to Intracellular Receptors",
"token_count": 1918,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
In contrast to NO and the steroid and thyroid hormones, the vast majority of signal molecules are too large or hydrophilic to cross the plasma membrane of the target cell. These proteins, peptides, and small hydrophilic molecules bind to cell-surface receptor proteins that span the plasma membrane (see Figure 16–8A). T... | {
"Header 1": "Cell Signaling",
"Header 3": "Cell-Surface Receptors Relay Extracellular Signals via Intracellular Signaling Pathways",
"token_count": 648,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Many of the key intracellular signaling proteins behave as molecular switches: receipt of a signal causes them to toggle from an inactive to an active state. Once activated, these proteins can stimulate—or in other cases suppress—other proteins in the signaling pathway. They then persist in an active state until some o... | {
"Header 1": "Cell Signaling",
"Header 3": "Some Intracellular Signaling Proteins Act as Molecular Switches",
"token_count": 1175,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
All cell-surface receptor proteins bind to an extracellular signal molecule and transduce its message into one or more intracellular signaling molecules that alter the cell's behavior. Most of these receptors belong to one of three large classes, which differ in the transduction mechanism they use.
- 1. *Ion-channel-... | {
"Header 1": "Cell Signaling",
"Header 3": "Cell-Surface Receptors Fall into Three Main Classes",
"token_count": 586,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Of all the types of cell-surface receptors, ion-channel-coupled receptors (also known as transmitter-gated ion channels) function in the simplest and most direct way. As we discuss in detail in Chapter 12, these receptors are responsible for the rapid transmission of signals across synapses in the nervous system. They ... | {
"Header 1": "Cell Signaling",
"Header 3": "Ion-channel–coupled Receptors Convert Chemical Signals into Electrical Ones",
"token_count": 785,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
G-protein-coupled receptors (GPCRs) form the largest family of cellsurface receptors. There are more than 700 GPCRs in humans, and mice have about 1000 involved in the sense of smell alone. These receptors mediate responses to an enormous diversity of extracellular signal molecules, including hormones, local mediators,... | {
"Header 1": "Cell Signaling",
"Header 3": "G-protein-coupled Receptors",
"token_count": 200,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The signaling mechanisms used by a steroid-hormone-type nuclear receptor and by an ionchannel-coupled receptor are relatively simple as they have few components. Can they lead to an amplification of the initial signal, and, if so, how?
Figure 16–18 All G... | {
"Header 1": "Cell Signaling",
"Header 3": "Question 16–4",
"token_count": 522,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
When an extracellular signal molecule binds to a GPCR, the receptor protein undergoes a conformational change that enables it to activate a G protein located on the other side of the plasma membrane. To explain how this activation leads to the transmission of a signal, we must first consider how G proteins are construc... | {
"Header 1": "Cell Signaling",
"Header 3": "Stimulation of GPCRs Activates G-Protein Subunits",
"token_count": 755,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
G proteins demonstrate a general principle of cell signaling mentioned earlier: the mechanisms that shut a signal off are as important as the mechanisms that turn it on (see Figure 16–15B). The shut-off mechanisms also offer as many opportunities for control—and as many dangers for mishap. Consider cholera, for example... | {
"Header 1": "Cell Signaling",
"Header 3": "Some Bacterial Toxins Cause Disease by Altering the Activity of G Proteins",
"token_count": 317,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
GPCRs activate G proteins by reducing the strength of GDP binding to the G protein. This results in rapid dissociation of bound GDP, which is then replaced by GTP, because GTP is present in the cytosol in much higher concentrations than GDP. What consequences would result from a mutation in the α subunit of a G protein... | {
"Header 1": "Cell Signaling",
"Header 3": "Question 16–5",
"token_count": 412,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The target proteins recognized by G-protein subunits are either enzymes or ion channels in the plasma membrane. There are about 20 different types of mammalian G proteins, each activated by a particular set of cellsurface receptors and dedicated to activating a particular set of target proteins. Consequently, the bindi... | {
"Header 1": "Cell Signaling",
"Header 3": "Some G Proteins Directly Regulate Ion Channels",
"token_count": 333,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
When G proteins interact with ion channels, they cause an immediate change in the state and behavior of the cell. Their interactions with enzymes, in contrast, have consequences that are less rapid and more complex, as they lead to the production of additional intracellular signaling molecules. The two most frequent ta... | {
"Header 1": "Cell Signaling",
"Header 3": "Many G Proteins Activate Membrane-bound Enzymes that Produce Small Messenger Molecules",
"token_count": 669,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Many extracellular signals acting via GPCRs affect the activity of the enzyme **adenylyl cyclase** and thus alter the intracellular concentration of the small messenger molecule **cyclic AMP**. Most commonly, the activated G-protein $\alpha$ subunit switches on the adenylyl cyclase, causing a dramatic and sudden incr... | {
"Header 1": "The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes",
"token_count": 1453,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Some GPCRs exert their effects via a G protein called Gq, which activates the membrane-bound enzyme phospholipase C instead of adenylyl cyclase. Examples of signal molecules that act through phospholipase C are given in Table 16–4.
#### Question 16–6
Explain why cyclic AMP must be broken down rapidly in a cell to a... | {
"Header 1": "The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes",
"Header 3": "The Inositol Phospholipid Pathway Triggers a Rise in Intracellular Ca2+",
"token_count": 1131,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Ca2+ has such an important and widespread role as an intracellular messenger that we will digress to consider its functions more generally. A surge in the cytosolic concentration of free Ca2+ is triggered by many kinds of cell stimuli, not only those that act through GPCRs. When a sperm fertilizes an egg cell, for exam... | {
"Header 1": "The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes",
"Header 3": "A Ca2+ Signal Triggers Many Biological Processes",
"token_count": 667,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The steps in the *signaling cascades* associated with GPCRs take a long time to describe, but they often take only seconds to execute. Consider how quickly a thrill can make your heart race (when adrenaline stimulates the GPCRs in your cardiac pacemaker cells), or how fast the smell of food can make your mouth water (t... | {
"Header 1": "The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes",
"Header 3": "GPCR-Triggered Intracellular Signaling Cascades Can Achieve Astonishing Speed, Sensitivity, and Adaptability",
"token_count": 242,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4t... |
Why do you suppose cells have evolved intracellular Ca2+ stores for signaling even though there is abundant extracellular Ca2+?
Figure 16–29 Calcium binding changes the shape of the calmodulin protein.
(A) Calmodulin has a dumbbell shape, with two globular ends connected by a long α helix. Each end has two Ca2+-bin... | {
"Header 1": "The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes",
"Header 3": "Question 16–7",
"token_count": 1519,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Like GPCRs, enzyme-coupled receptors are transmembrane proteins that display their ligand-binding domains on the outer surface of the plasma membrane (see Figure 16–17C). Instead of associating with a G protein, however, the cytoplasmic domain of the receptor either acts as an enzyme itself or forms a complex with anot... | {
"Header 1": "The Cyclic AMP Signaling Pathway Can Activate Enzymes and Turn On Genes",
"Header 3": "Enzyme-coupled Receptors",
"token_count": 387,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
To do its job as a signal transducer, an enzyme-coupled receptor has to switch on the enzyme activity of its intracellular domain (or of an associated enzyme) when an external signal molecule binds to its extracellular domain. Unlike the seven-pass transmembrane GPCRs, enzyme-coupled receptor proteins usually have only... | {
"Header 1": "Activated RTKs Recruit a Complex of Intracellular Signaling Proteins",
"token_count": 916,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
As we have seen, activated RTKs recruit and activate many kinds of intracellular signaling proteins, leading to the formation of large signaling complexes on the cytosolic tail of the RTK. One of the key members of these signaling complexes is Ras—a small GTP-binding protein that is bound by a lipid tail to the cytopla... | {
"Header 1": "Activated RTKs Recruit a Complex of Intracellular Signaling Proteins",
"Header 3": "Most RTKs Activate the Monomeric GTPase Ras",
"token_count": 973,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Many of the extracellular signal proteins that stimulate animal cells to survive and grow act through RTKs. These include signal proteins belonging to the insulin-like growth factor (IGF) family. One crucially important signaling pathway that these RTKs activate to promote cell growth and survival relies on the enzyme ... | {
"Header 1": "RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane",
"token_count": 454,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
An extracellular survival signal, such as IGF, activates an RTK, which recruits and activates PI 3-kinase. PI 3-kinase then phosphorylates an inositol phospholipid that is embedded in the cytosolic side of the plasma membrane. The resulting phosphorylated inositol phospholipid then attracts intracellular signaling prot... | {
"Header 1": "RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane",
"Header 2": "Figure 16–35 RTKs activate the PI-3-kinase–Akt signaling pathway.",
"token_count": 240,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Most signaling pathways depend on proteins that physically interact with one another. There are several ways to detect such direct contact. One involves using a protein as "bait." For example, to isolate the receptor that binds to insulin, one could attach insulin to a chromatography column. Cells that respond to the h... | {
"Header 1": "RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane",
"Header 2": "Figure 16–35 RTKs activate the PI-3-kinase–Akt signaling pathway.",
"Header 3": "Close encounters",
"token_count": 369,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th... |
Ultimately, one wants to assess what part a particular protein plays in a signaling pathway. A first test may involve using recombinant DNA technology to introduce into cells a gene encoding a constantly active form of the protein, to see if this mimics the effect of the extracellular signal molecule. Consider Ras, for... | {
"Header 1": "RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane",
"Header 2": "Figure 16–35 RTKs activate the PI-3-kinase–Akt signaling pathway.",
"Header 3": "Jamming the pathway",
"token_count": 431,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__... |
One powerful strategy that scientists use to identify proteins that participate in cell signaling involves screening tens of thousands of animals—fruit flies or nematode worms, for example (discussed in Chapter 19)—to search for mutants in which a signaling pathway is not functioning properly. By examining enough mutan... | {
"Header 1": "RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane",
"Header 2": "Figure 16–35 RTKs activate the PI-3-kinase–Akt signaling pathway.",
"Header 3": "Making mutants",
"token_count": 807,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_e... |
Not all receptors trigger complex signaling cascades to carry a message to the nucleus. Some use a more direct route to control gene expression. One such receptor is the protein Notch.
Notch is a crucially important receptor in all animals, both during development and in adults. Among other things, it controls the de... | {
"Header 1": "RTKs Activate PI 3-Kinase to Produce Lipid Docking Sites in the Plasma Membrane",
"Header 2": "Figure 16–35 RTKs activate the PI-3-kinase–Akt signaling pathway.",
"Header 3": "Some Receptors Activate a Fast Track to the Nucleus",
"token_count": 414,
"source_pdf": "datasets/websources/biochem/Al... |
Plants and animals have been evolving independently for more than a billion years, the last common ancestor being a single-celled eukaryote that most likely lived on its own. Because these kingdoms diverged so long ago—when it was still "every cell for itself"—each has evolved its own molecular solutions to the complex... | {
"Header 1": "Cell–Cell Communication Evolved Independently in Plants and Animals",
"token_count": 638,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Although the signaling pathways we have described thus far may seem dauntingly complex, the complexity of cell signaling is actually much greater than we have let on. First, we have not discussed all of the intracellular signaling pathways that operate in cells, even though many of these are critical for normal develop... | {
"Header 1": "Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors",
"token_count": 707,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
- • Cells in multicellular organisms communicate through a large variety of extracellular chemical signals.
- • In animals, hormones are carried in the blood to distant target cells, but most other extracellular signal molecules act over only a short distance. Neighboring cells often communicate through direct cell– ce... | {
"Header 1": "Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors",
"Header 3": "Essential Concepts",
"token_count": 1429,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
#### Question 16–10
If some cell-surface receptors, including Notch, can rapidly signal to the nucleus by activating latent transcription regulators at the plasma membrane, why do most cellsurface receptors use long, indirect signaling cascades to influence gene transcription in the nucleus?
#### Question 16–11
W... | {
"Header 1": "Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors",
"Header 3": "Questions",
"token_count": 999,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Discuss the following statement: "Membrane proteins that span the membrane many times can undergo a conformational change upon ligand binding that can be sensed on the other side of the membrane. Thus, individual protein molecules can transmit a signal across a membrane. In contrast, individual single-span membrane pro... | {
"Header 1": "Protein Kinase Networks Integrate Information to Control Complex Cell Behaviors",
"Header 3": "Question 16–17",
"token_count": 559,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The ability of eukaryotic cells to adopt a variety of shapes, organize the many components in their interior, interact mechanically with the environment, and carry out coordinated movements depends on the cytoskeleton—an intricate network of protein filaments that extends throughout the cytoplasm (Figure 17–1). This fi... | {
"Header 1": "Cytoskeleton",
"token_count": 1172,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Intermediate filaments have great tensile strength, and their main function is to enable cells to withstand the mechanical stress that occurs when cells are stretched. The filaments are called "intermediate" because, in the smooth muscle cells where they were first discovered, their diameter (about 10 nm) is between th... | {
"Header 1": "Cytoskeleton",
"Header 3": "Intermediate Filaments",
"token_count": 287,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
An intermediate filament is like a rope in which many long strands are twisted together to provide tensile strength (Movie 17.1). The strands of this cable are made of intermediate filament proteins, fibrous subunits each containing a central elongated rod domain with distinct unstructured domains at either end (Figure... | {
"Header 1": "Cytoskeleton",
"Header 3": "Intermediate Filaments Are Strong and Ropelike",
"token_count": 816,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Intermediate filaments are particularly prominent in the cytoplasm of cells that are subject to mechanical stress. They are present in large numbers, for example, along the length of nerve cell axons, providing essential internal reinforcement to these extremely long and fine cell extensions. They are also abundant in ... | {
"Header 1": "Cytoskeleton",
"Header 3": "Intermediate Filaments Strengthen Cells Against Mechanical Stress",
"token_count": 627,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
A mutant gene encoding a truncated keratin protein was introduced into a mouse. The defective protein assembles with the normal keratins and thereby disrupts the keratin filament network in the skin. (A) Light micrograph of a cross section of normal skin, which is resistant to mechanical pressure. (B) Cross section of ... | {
"Header 1": "Cytoskeleton",
"Header 3": "Figure 17–6 A mutant form of keratin makes skin more prone to blistering.",
"token_count": 388,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm? Explain your answers.
- A. Amoeba proteus (a free-living amoeba)
- B. Skin epithelial cell
- C. Smooth muscle cell in the digestive tract
- D. Escherichia coli
- E. Nerve cell in the spinal c... | {
"Header 1": "The Nuclear Envelope Is Supported by a Meshwork of Intermediate Filaments",
"Header 2": "QUESTION 17–1",
"token_count": 634,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Microtubules have a crucial organizing role in all eukaryotic cells. These long and relatively stiff hollow tubes of protein can rapidly disassemble in one location and reassemble in another. In a typical animal cell, microtubules grow out from a small structure near the center of the cell called the *centrosome* (Figu... | {
"Header 1": "The Nuclear Envelope Is Supported by a Meshwork of Intermediate Filaments",
"Header 2": "QUESTION 17–1",
"Header 3": "Microtubules",
"token_count": 732,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Microtubules are built from subunits—molecules of tubulin—each of which is itself a dimer composed of two very similar globular proteins called $\alpha$ -tubulin and $\beta$ -tubulin, bound tightly together by noncovalent interactions. The tubulin dimers stack together, again by noncovalent bonding, to form the wall ... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"token_count": 707,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Inside cells, microtubules grow from specialized organizing centers that control the location, number, and orientation of the microtubules. In animal cells, for example, the centrosome—which is typically close to the cell nucleus when the cell is not in mitosis—organizes an array of microtubules that radiates outward t... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 3": "The Centrosome Is the Major Microtubule-organizing Center in Animal Cells",
"token_count": 733,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
A newly formed microtubule will persist only if both its ends are protected from depolymerization. In cells, the minus ends of microtubules are generally protected by the organizing centers from which the microtubules grow. The plus ends are initially free but can be stabilized by binding to specific proteins. Here, fo... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"token_count": 375,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Once a microtubule has been nucleated, it typically grows outward from the organizing center for many minutes by the addition of $\alpha\beta$ -tubulin dimers to its plus end. Then, without warning, the microtubule can suddenly undergo a transition that causes it to shrink rapidly inward by losing tubulin dimers from ... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Growing Microtubules Display Dynamic Instability",
"token_count": 425,
"source_pdf": "datasets/websources/biochem/Alberts_-_E... |
The dynamic instability of microtubules stems from the intrinsic capacity of tubulin dimers to hydrolyze GTP. Each free tubulin dimer contains one GTP molecule tightly bound to $\beta$ -tubulin, which hydrolyzes the GTP to GDP shortly after the dimer is added to a growing microtubule. This GDP remains tightly bound to... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Dynamic Instability is Driven by GTP Hydrolysis",
"token_count": 684,
"source_pdf": "datasets/websources/biochem/Alberts_-_Es... |
Drugs that prevent the polymerization or depolymerization of tubulin dimers can have a rapid and profound effect on the organization of microtubules—and thereby on the behavior of the cell. Consider the mitotic spindle, the microtubule-based apparatus that guides the chromosomes during mitosis (see Figure 17–10C). If a... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Microtubule Dynamics Can be Modified by Drugs",
"token_count": 327,
"source_pdf": "datasets/websources/biochem/Alberts_-_Esse... |
Why do you suppose it is much easier to add tubulin to existing microtubules than to start a new microtubule from scratch? Explain how γ-tubulin in the centrosome helps to overcome this hurdle.
| Table 17–1 Drugs<br>That Affect<br>Microtubules | | |
|-----------... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Question 17–2",
"token_count": 279,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf... |
Cells are able to modify the dynamic instability of their microtubules for particular purposes. As cells enter mitosis, for example, microtubules become initially more dynamic, switching between growing and shrinking much more frequently than cytoplasmic microtubules normally do. This change enables microtubules to dis... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Microtubules Organize the Cell Interior",
"token_count": 781,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_... |
If a living cell is observed in a light microscope, its cytoplasm is seen to be in continual motion. Mitochondria and the smaller membraneenclosed organelles and vesicles travel in small, jerky steps—moving for a short period, stopping, and then moving again. This *saltatory* movement is much more sustained and directi... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Motor Proteins Drive Intracellular Transport",
"token_count": 309,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essen... |
Dynamic instability causes microtubules either to grow or to shrink rapidly. Consider an individual microtubule that is in its shrinking phase.
- A. What must happen at the end of the microtubule in order for it to stop shrinking and to start growing again?
- B. How would a change in the tubulin concentration affect ... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Question 17–3",
"token_count": 548,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf... |
Microtubules and motor proteins play an important part in positioning organelles within a eukaryotic cell. In most animal cells, for example, the tubules of the endoplasmic reticulum (ER) reach almost to the edge of the cell (Movie 17.4), whereas the Golgi apparatus is located in the cell interior, near the centrosome ... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Microtubules and Motor Proteins Position Organelles in the Cytoplasm",
"token_count": 760,
"source_pdf": "datasets/websources... |
The movement of organelles throughout the cell cytoplasm has been observed, measured, and speculated about since the middle of the nineteenth century. But it was not until the mid-1980s that biologists identified the molecules that drive this movement of organelles and vesicles from one part of the cell to another.
W... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "pursuing MICROTUBULE-ASSOCIATED motor proteins",
"token_count": 347,
"source_pdf": "datasets/websources/biochem/Alberts_-_Ess... |
Neuroscientists interested in the electrical properties of nerve cell membranes have long studied the giant axon from squid (see How We Know, pp. 406–407). Because of its large size, researchers found that they could squeeze the cytoplasm from the axon like toothpaste, and then study how ions move back and forth throug... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Teeming cytoplasm",
"token_count": 443,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._... |
More work was needed to identify the individual components that drive the transport system in squid axoplasm. What kind of filaments support this movement? What are the molecular motors that shuttle the vesicles and organelles along these filaments? Identifying the filaments was relatively easy: antibodies to tubulin r... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Snaking tubes",
"token_count": 677,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf... |
Combining such assays with ever more refined microscopic techniques, researchers can now monitor the movement of individual motor proteins along single microtubules, even in living cells.
Observation of kinesin molecules coupled with green fluorescent protein (GFP) revealed that this motor protein marches along micro... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Lights, camera, action",
"token_count": 1401,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4t... |
Actin filaments, polymers of the protein actin, are present in all eukaryotic cells and are essential for many of the cell's movements, especially those involving the cell surface. Without actin filaments, for example, an animal cell could not crawl along a surface, engulf a large particle by phagocytosis, or divide in... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Actin Filaments",
"token_count": 912,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.p... |
Actin filaments appear in electron micrographs as threads about 7 nm in diameter. Each filament is a twisted chain of identical globular actin monomers, all of which "point" in the same direction along the axis of the chain. Like a microtubule, therefore, an actin filament has a structural polarity, with a plus end and... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Actin Filaments Are Thin and Flexible",
"token_count": 518,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Ce... |
Although actin filaments can grow by the addition of actin monomers at either end, like microtubules, their rate of growth is faster at the plus end than at the minus end. A naked actin filament, like a microtubule without associated proteins, is inherently unstable, and it can disassemble from both ends. In living cel... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Actin and Tubulin Polymerize by Similar Mechanisms",
"token_count": 811,
"source_pdf": "datasets/websources/biochem/Alberts_-... |
About 5% of the total protein in a typical animal cell is actin; about half of this actin is assembled into filaments, and the other half remains as actin monomers in the cytosol. Thus unlike the situation for tubulin dimers, the concentration of actin monomers is high—much higher than the concentration required for pu... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Many Proteins Bind to Actin and Modify Its Properties",
"token_count": 943,
"source_pdf": "datasets/websources/biochem/Albert... |
Although actin is found throughout the cytoplasm of a eukaryotic cell, in most cells it is highly concentrated in a layer just beneath the plasma membrane. In this region, called the cell cortex, actin filaments are linked by actin-binding proteins into a meshwork that supports the plasma membrane and gives it mechanic... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "A Cortex Rich in Actin Filaments Underlies the Plasma Membrane of Most Eukaryotic Cells",
"token_count": 246,
"source_pdf": "... |
Many eukaryotic cells move by crawling over surfaces, rather than by swimming by means of beating cilia or flagella. Carnivorous amoebae crawl continually, in search of food. The advancing tip of a developing axon migrates in response to growth factors, following a path of chemical signals to its eventual synaptic targ... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Cell Crawling Depends on Cortical Actin",
"token_count": 1703,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential... |
All actin-dependent motor proteins belong to the myosin family. They bind to and hydrolyze ATP, which provides the energy for their movement along actin filaments toward the plus end. Myosin, along with actin, was first discovered in skeletal muscle, and much of what we know about the interaction of these two proteins ... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Actin Associates with Myosin to Form Contractile Structures",
"token_count": 309,
"source_pdf": "datasets/websources/biochem/... |
We have seen that myosin and other actin-binding proteins can regulate the location, organization, and behavior of actin filaments. But the activities of these proteins are, in turn, controlled by extracellular signals, allowing the cell to rearrange its actin cytoskeleton in response to the environment.
The extracel... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Extracellular Signals Can Alter the Arrangement of Actin Filaments",
"token_count": 988,
"source_pdf": "datasets/websources/b... |
Muscle contraction is the most familiar and best understood of animal cell movements. In vertebrates, running, walking, swimming, and flying all depend on the ability of *skeletal muscle* to contract strongly and move various bones. Involuntary movements such as heart pumping and gut peristalsis depend on *cardiac musc... | {
"Header 1": "Microtubules Are Hollow Tubes with Structurally Distinct Ends",
"Header 2": "Figure 17–14 The selective stabilization of microtubules can polarize a cell.",
"Header 3": "Muscle Contraction",
"token_count": 803,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed.... |
Skeletal muscle fibers are huge, multinucleated individual cells formed by the fusion of many separate smaller cells. The nuclei of the contributing cells are retained in the muscle fiber and lie just beneath the plasma membrane. The bulk of the cytoplasm is made up of **myofibrils**, the contractile elements of the mu... | {
"Header 1": "Actin Filaments Slide Against Myosin Filaments During Muscle Contraction",
"token_count": 1108,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The force-generating molecular interaction between myosin and actin filaments takes place only when the skeletal muscle receives a signal from a motor nerve. The neurotransmitter released from the nerve terminal triggers an action potential (discussed in Chapter 12) in the muscle cell plasma membrane. This electrical e... | {
"Header 1": "Actin Filaments Slide Against Myosin Filaments During Muscle Contraction",
"Header 3": "Muscle Contraction Is Triggered by a Sudden Rise in Cytosolic Ca2+",
"token_count": 1752,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The highly specialized contractile machinery in muscle cells is thought to have evolved from the simpler contractile bundles of myosin and actin filaments found in all eukaryotic cells. The myosin-II in nonmuscle cells is also activated by a rise in cytosolic Ca<sup>2+</sup>, but the mechanism of activation is differen... | {
"Header 1": "Different Types of Muscle Cells Perform Different Functions",
"token_count": 470,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
- • The cytoplasm of a eukaryotic cell is supported and organized by a cytoskeleton of intermediate filaments, microtubules, and actin filaments.
- • Intermediate filaments are stable, ropelike polymers—built from fibrous protein subunits—that give cells mechanical strength. Some intermediate filaments form the nuclear... | {
"Header 1": "Different Types of Muscle Cells Perform Different Functions",
"Header 3": "Essential Concepts",
"token_count": 813,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.