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Although the electrical charges inside and outside the cell are generally kept in balance, tiny excesses of positive or negative charge, concentrated in the neighborhood of the plasma membrane, do occur. Such electrical imbalances generate a voltage difference across the membrane called the **membrane potential**.
| ... | {
"Header 1": "The Ion Concentrations Inside a Cell Are Very Different from Those Outside",
"Header 3": "Differences in the Concentration of Inorganic Ions Across a Cell Membrane Create a Membrane Potential",
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Membrane transport proteins occur in many forms and are present in all cell membranes. Each provides a private portal across the membrane for a particular small, water-soluble molecule—an ion, sugar, or amino acid, for example. Most of these proteins allow passage of only select members of a particular molecular class:... | {
"Header 1": "The Ion Concentrations Inside a Cell Are Very Different from Those Outside",
"Header 3": "Cells Contain Two Classes of Membrane Transport Proteins: Transporters and Channels",
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Transporters and channels allow small hydrophilic molecules to cross the cell membrane, but what controls whether these solutes move into the
Figure 12–3 Inorganic ions and small, polar organic molecules can cross a cell membrane through either a transporter or a channel. (A) A transpo... | {
"Header 1": "The Ion Concentrations Inside a Cell Are Very Different from Those Outside",
"Header 3": "Solutes Cross Membranes by Either Passive or Active Transport",
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For an uncharged molecule, the direction of passive transport is determined solely by its concentration gradient, as we have implied above. But for electrically charged molecules, whether inorganic ions or small organic molecules, an additional force comes into play. As mentioned earlier, most cell membranes have a vol... | {
"Header 1": "Both the Concentration Gradient and Membrane Potential Influence the Passive Transport of Charged Solutes",
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Cells are mostly water (generally about 70% by weight), and so the movement of water across cell membranes is crucially important for living things. Because water molecules are small and uncharged, they can diffuse directly across the lipid bilayer—although slowly (see Figure 12–2). However, some cells also contain spe... | {
"Header 1": "Water Moves Passively Across Cell Membranes Down Its Concentration Gradient—a Process Called Osmosis",
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An important example of a transporter that mediates passive transport is the *glucose transporter* in the plasma membrane of many mammalian cell types. The protein, which consists of a polypeptide chain that crosses the membrane at least 12 times, can adopt several conformations—and it switches reversibly and randomly ... | {
"Header 1": "Water Moves Passively Across Cell Membranes Down Its Concentration Gradient—a Process Called Osmosis",
"Header 3": "Passive Transporters Move a Solute Along Its Electrochemical Gradient",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf... |
Cells cannot rely solely on passive transport. An active transport of solutes against their electrochemical gradient is essential to maintain the appropriate intracellular ionic composition of cells and to import solutes that are at a lower concentration outside the cell than inside. For these purposes, cells depend on... | {
"Header 1": "Water Moves Passively Across Cell Membranes Down Its Concentration Gradient—a Process Called Osmosis",
"Header 3": "Pumps Actively Transport a Solute Against Its Electrochemical Gradient",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pd... |
The ATP-driven Na+ pump plays such a central part in the energy economy of animal cells, that it typically accounts for 30% or more of their total ATP consumption. This pump uses the energy derived from ATP hydrolysis to transport Na+ out of the cell as it carries K+ in. The pump is therefore also known as the *Na+-K+ ... | {
"Header 1": "Water Moves Passively Across Cell Membranes Down Its Concentration Gradient—a Process Called Osmosis",
"Header 3": "The Na+ Pump in Animal Cells Uses Energy Supplied by ATP to Expel Na+ and Bring in K+",
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The Na $^+$ pump functions like a bilge pump in a leaky ship, ceaselessly expeling the Na $^+$ that is constantly entering the cell through other transporters and ion channels in the plasma membrane. In this way, the pump keeps the Na $^+$ concentration in the cytosol about 10–30 times lower than in the extracellula... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
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Ca<sup>2+</sup>, like Na<sup>+</sup>, is also kept at a low concentration in the cytosol compared with its concentration in the extracellular fluid, but it is much less plentiful than Na<sup>+</sup>, both inside and outside cells (see Table 12–1). The movement of Ca<sup>2+</sup> across cell membranes is nonetheless cru... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Ca<sup>2+</sup> Pumps Keep the Cytosolic Ca<sup>2+</sup> Concentration Low",
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A gradient of any solute across a membrane, like the electrochemical Na+ gradient generated by the Na+ pump, can be used to drive the active transport of a second molecule. The downhill movement of the first solute down its gradient provides the energy to power the uphill transport of the second. The active transporter... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Coupled Pumps Exploit Solute Gradients to Mediate Active Transport",
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Symports that make use of the inward flow of Na+ down its steep electrochemical gradient have an especially important role in driving the import of other solutes into animal cells. The epithelial cells that line the gut, for example, pump glucose from the gut lumen across the gut epithelium and, ultimately, into the bl... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "The Electrochemical Na+ Gradient Drives Coupled Pumps in the Plasma Membrane of Animal Cells",
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Plant cells, bacteria, and fungi (including yeasts) do not have Na+ pumps in their plasma membrane. Instead of an electrochemical Na+ gradient, they rely mainly on an electrochemical gradient of H+ to import solutes into the cell. The gradient is created by H+ pumps in the plasma membrane that pump H+ out of the cell, ... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Electrochemical H+ Gradients Drive Coupled Pumps in Plants, Fungi, and Bacteria",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Bi... |
In principle, the simplest way to allow a small water-soluble molecule to cross from one side of a membrane to the other is to create a hydrophilic channel through which the molecule can pass. Channel proteins, or channels, perform this function in cell membranes, forming transmembrane pores that allow the passive move... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Ion Channels and the Membrane Potential",
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Two important properties distinguish ion channels from simple holes in the membrane. First, they show *ion selectivity*, permitting some inorganic ions to pass but not others. Ion selectivity depends on the diameter and shape of the ion channel and on the distribution of the charged amino acids that line it. Each ion i... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Ion Channels Are Ion-selective and Gated",
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A transmembrane protein has the following properties: it has two binding sites, one for solute A and one for solute B. The protein can undergo a conformational change to switch between two states: either both binding sites are exposed exclusively on one side of the membrane or both binding sites are exposed exclusively... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Question 12–3",
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Changes in membrane potential are the basis of electrical signaling in many types of cells, whether they are the nerve or muscle cells in animals, or the touch-sensitive cells of a carnivorous plant (Figure 12–20). Such electrical changes are mediated by alterations in the permeability of membranes to ions. In an anima... | {
"Header 1": "The Na<sup>+</sup> Pump Generates a Steep Concentration Gradient of Na<sup>+</sup> Across the Plasma Membrane",
"Header 3": "Membrane Potential Is Governed by the Permeability of a Membrane to Specific Ions",
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Measuring changes in electrical current is the main method used to study ion movements and ion channels in living cells. Amazingly, electrical recording techniques can detect and measure the electric current flowing through a single channel molecule. The procedure developed for doing this is known as **patch-clamp reco... | {
"Header 1": "Ion Channels Randomly Snap Between Open and Closed States",
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There are more than a hundred types of ion channels, and even simple organisms can possess many different types. The nematode worm *C. elegans*, for example, has genes that encode 68 different but related K+ channels alone. Ion channels differ from one another primarily with
Figure 12–2... | {
"Header 1": "Ion Channels Randomly Snap Between Open and Closed States",
"Header 3": "Different Types of Stimuli Influence the Opening and Closing of Ion Channels",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Voltage-gated ion channels play a major role in propagating electrical signals along all nerve cell processes, such as those that relay signals from our brain to our toe muscles. But voltage-gated ion channels are present in many other cell types, too, including muscle cells, egg cells, protozoans, and even plant cells... | {
"Header 1": "Voltage-gated Ion Channels Respond to the Membrane Potential",
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When a neuron is stimulated, the membrane potential of the plasma membrane shifts to a less negative value (that is, toward zero). If this **depolarization** is sufficiently large, it will cause **voltage-gated Na+channels** in the membrane to open transiently at the site. As these channels flicker open, they allow a s... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
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Each spring, *Loligo pealei* migrate to the shallow waters off Cape Cod on the eastern coast of the United States. There they spawn, launching the next generation of squid. But more than just meeting and breeding, these animals provide neuroscientists summering at the Marine Biological Laboratory in Woods Hole, Massach... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "squid reveal secrets of membrane excitability",
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Because the squid axon is so long and wide, an electrode made from a glass capillary tube containing a conducting solution can be thrust down the axis of the isolated axon so that its tip lies deep in the cytoplasm (Figure 12–32A). This setup allowed investigators to measure the voltage difference between the inside an... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Setup for action",
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Once Na+ and K+ had been singled out as critical for an action potential, the question then became: What does each of these ions contribute to the action potential? How permeable is the membrane to each, and how does the membrane permeability change as an action potential sweeps by? Again, the squid giant axon provided... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Channel traffic",
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When an action potential reaches the nerve terminals at the end of an axon, the signal must somehow be relayed to the *target cells* that the terminals contact—usually neurons or muscle cells. The signal is transmitted to the target cells at specialized junctions known as synapses. At most synapses, the plasma membrane... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Voltage-gated Ca2+ Channels in Nerve Terminals Convert an Electrical Signal into a Chemical Signal",
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Neurotransmitters can either excite or inhibit a postsynaptic cell, and it is the character of the receptor that recognizes the neurotransmitter that determines how the postsynaptic cell will respond. The chief receptors for excitatory neurotransmitters, such as *acetylcholine* and *glutamate*, are ligand-gated cation ... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Neurotransmitters Can Be Excitatory or Inhibitory",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Many drugs used in the treatment of insomnia, anxiety, depression, and schizophrenia act by binding to transmitter-gated ion channels in the brain. Sedatives and tranquilizers such as barbiturates, Valium, Ambien, and Restoril, for example, bind to GABA-gated Cl– channels. Their binding makes the channels easier to ope... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Most Psychoactive Drugs Affect Synaptic Signaling by Binding to Neurotransmitter Receptors",
"token_count": 376,
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For a process so critical for animal survival, the mechanism that governs synaptic signaling seems unnecessarily cumbersome, as well as error-prone. For a signal to pass from one neuron to the next, the nerve terminal of the presynaptic cell must convert an electrical signal into a secreted chemical. This chemical sign... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "The Complexity of Synaptic Signaling Enables Us to Think, Act, Learn, and Remember",
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When an inhibitory neurotransmitter such as GABA opens Cl – channels in the plasma membrane of a postsynaptic neuron, why does this make it harder for an excitatory neurotransmitter to excite the neuron?
Figure 12–42 Thousands of synapses form on the cell body and dendrites of a motor neuron in the spinal cord. (A) M... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Question 12–8",
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Photosynthetic green algae use light-gated channels to sense and navigate toward sunlight. In response to blue light, one of these channels—called *channelrhodopsin*—allows Na+ to flow into the cell. This depolarizes the plasma membrane and, ultimately, modulates the beating of the flagella the cell uses to swim. Altho... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Optogenetics Uses Light-gated Ion Channels to Transiently Activate or Inactivate Neurons in Living Animals",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pd... |
Which of the following statements are correct? Explain your answers.
- A. The plasma membrane is highly impermeable to all charged molecules.
- B. Channels have specific binding pockets for the solute molecules they allow to pass.
- C. Transporters allow solutes to cross a membrane at much faster rates than do channe... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Question 12–10",
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The neurotransmitter acetylcholine is made in the cytosol and then transported into synaptic vesicles, where its concentration is more than 100-fold higher than in the cytosol. When synaptic vesicles are isolated from neurons, they can take up additional acetylcholine added to the solution in which they are suspended, ... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Question 12–14",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Name the three ways in which an ion channel can be gated.
#### Question 12–18
One thousand Ca2+ channels open in the plasma membrane of a cell that is 1000 μm3 in size and has a cytosolic Ca2+ concentration of 100 nM. For how long would the channels need to stay open in order for the cytosolic Ca2+ concentration to... | {
"Header 1": "Action Potentials Are Mediated by Voltage-gated Cation Channels",
"Header 3": "Question 12–17",
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As we discussed in Chapter 3, cells require a constant supply of energy to generate and maintain the biological order that allows them to grow, divide, and carry out their day-to-day activities. This energy comes from the chemical-bond energy in food molecules, which thereby serve as fuel for cells.
Perhaps the most ... | {
"Header 1": "How Cells Obtain Energy From Food",
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If a fuel molecule such as glucose were oxidized to CO2 and H2O in a single step—by, for example, the direct application of fire—it would release an amount of energy many times larger than any carrier molecule could capture (Figure 13–1A). Instead, cells use enzymes to carry out the oxidation of sugars in a tightly con... | {
"Header 1": "How Cells Obtain Energy From Food",
"Header 3": "The Breakdown and Utilization of Sugars and Fats",
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(A) Stage 1 mostly occurs outside cells in the mouth and the gut—although intracellular lysosomes can also digest large organic molecules. Stage 2 occurs mainly in the cytosol, except for the final step of conversion of pyruvate to acetyl groups on acetyl CoA, which occurs in the mitochondrial matrix. Stage 3 begins wi... | {
"Header 1": "How Cells Obtain Energy From Food",
"Header 3": "Figure 13–3 The breakdown of food molecules occurs in three stages.",
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The central process in stage 2 of catabolism is the oxidative breakdown of glucose in the sequence of reactions known as glycolysis. Glycolysis produces ATP without the involvement of oxygen. It occurs in the cytosol of most cells, including many anaerobic microorganisms that thrive in the absence of oxygen. Glycolysis... | {
"Header 1": "How Cells Obtain Energy From Food",
"Header 3": "Glycolysis Extracts Energy from the Splitting of Sugar",
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For most animal and plant cells, glycolysis is only a prelude to the third and final stage of the breakdown of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phosphorylation, a process that requires the consumption of oxygen. However, for many anaerobic microorganisms, which ca... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
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(A) When inadequate oxygen is present, for example, in a muscle cell undergoing vigorous contraction, the pyruvate produced by glycolysis is converted to lactate in the cytosol. This reaction restores the NAD+ consumed in step 6 of glycolysis, but the whole pathway yields much less energy overall than if the pyruvate w... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"token_count": 696,
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The "paddle wheel" analogy in Chapter 3 explained how cells harvest useful energy from the oxidation of organic molecules by coupling an energetically unfavorable reaction to an energetically favorable one (see Figure 3–30). Here, we take a closer look at a key pair of glycolytic reactions that demonstrate how enzymes—... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Glycolytic Enzymes Couple Oxidation to Energy Storage in **Activated Carriers**",
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Arsenate (AsO4 3–) is chemically very similar to phosphate (PO4 3–) and is used as an alternative substrate by many phosphate-requiring enzymes. In contrast to phosphate, however, an anhydride bond between arsenate and carbon is very quickly hydrolyzed nonenzymatically in water. Knowing this, suggest why arsenate is a ... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Question 13–2",
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In aerobic metabolism in eukaryotic cells, the pyruvate produced by glycolysis is actively pumped into the mitochondrial matrix (see Figure 13–3). There, it is rapidly decarboxylated by a giant complex of three enzymes, called the *pyruvate dehydrogenase complex.* The products of pyruvate decarboxylation are CO2 (a was... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Several Organic Molecules Are Converted to Acetyl CoA in the Mitochondrial Matrix",
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The citric acid cycle accounts for about two-thirds of the total oxidation of carbon compounds in most cells, and its major end products are CO2 and high-energy electrons in the form of NADH. The CO2 is released as a waste product, while the high-energy electrons from NADH are passed to the electron-transport chain in ... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "The Citric Acid Cycle Generates NADH by Oxidizing Acetyl Groups to CO2",
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Catabolic reactions, such as those of glycolysis and the citric acid cycle, produce both energy for the cell and the building blocks from which many other organic molecules are made. So far, we have emphasized energy production rather than the provision of starting materials for biosynthesis. But many of the intermedia... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Many Biosynthetic Pathways Begin with Glycolysis or the Citric Acid Cycle",
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Looking at the chemistry detailed in Panel 13–2 (pp. 434–435), why do you suppose it is useful to link the acetyl group first to another, larger carbon skeleton, oxaloacetate, before completely oxidizing both carbons to CO2?
Figure 13–14 Glycolysis and the citric acid cycle provide the precursors needed for cells to ... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Question 13–4",
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By the early 1930s, Krebs and other investigators had discovered that a select set of small organic molecules are oxidized extraordinarily rapidly in various types of tissue preparations—slices of kidney or liver, or suspensions of minced pigeon muscle. Because these reactions were seen to depend on the presence of oxy... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Minced tissues, curious catalysis",
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Many of the clues that Krebs used to work out the citric acid cycle came from experiments using malonate—a poisonous compound that specifically inhibits the enzyme succinate dehydrogenase, which converts E to F. Malonate closely resembles succinate (E) in its structure (Figure 13–16), and it serves as a competitive inh... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "A poison suggests a cycle",
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The cycle of reactions that Krebs proposed clearly explained how the addition of small amounts of any of the intermediates A through H could cause the large increase in the uptake of O2 that had been observed. Pyruvate is abundant in minced tissues, being readily produced by glycolysis (see Figure 13–4), using glucose ... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Explaining the mysterious stimulatory effects",
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We now return briefly to the final stage in the oxidation of food molecules: oxidative phosphorylation. It is in this stage that the chemical energy captured by the activated carriers produced during glycolysis and the citric acid cycle is used to generate ATP. During oxidative phosphorylation, NADH and FADH2 transfer ... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Electron Transport Drives the Synthesis of the Majority of the ATP in Most Cells",
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A cell is an intricate chemical machine, and our discussion of metabolism—with a focus on glycolysis and the citric acid cycle—has considered only a tiny fraction of the many enzymatic reactions that can take place in a cell at any time (Figure 13–20). For all these pathways to work together smoothly, as is required to... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Regulation of Metabolism",
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All the reactions shown in Figure 13–20 occur in a cell that is less than 0.1 mm in diameter, and each step requires a different enzyme. To add to the complexity, the same substrate is often a part of many different pathways. Pyruvate, for example, is a substrate for half a dozen or more different enzymes, each of whic... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Catabolic and Anabolic Reactions Are Organized and Regulated",
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Animals need an ample supply of glucose. Active muscles need glucose to power their contraction, and brain cells depend almost completely on glucose for energy. During periods of fasting or intense physical exercise, the body's glucose reserves get used up faster than they can be replenished from food. One way to incre... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Feedback Regulation Allows Cells to Switch from Glucose Breakdown to Glucose Synthesis",
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"source_pdf": "datasets... |
As we have seen, gluconeogenesis is a costly process, requiring substantial amounts of energy from the hydrolysis of ATP and GTP. During periods when food is scarce, this expensive way of producing glucose is suppressed if alternatives are available. Thus fasting cells can mobilize glucose that has been stored in the f... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Cells Store Food Molecules in Special Reservoirs to Prepare for Periods of Need",
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- • Food molecules are broken down in successive steps, in which energy is captured in the form of activated carriers such as ATP and NADH.
- • In plants and animals, these catabolic reactions occur in different cell compartments: glycolysis in the cytosol, the citric acid cycle in the mitochondrial matrix, and oxidati... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Essential Concepts",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
... |
acetyl CoA GDP, GTP ADP, ATP gluconeogenesis anabolic pathways glucose catabolism glycogen cell respiration glycolysis citric acid cycle NAD+, NADH
FAD , FADH2 pyruvate
fermentation
electron-transport chain oxidative phosphorylation
fat starch
#### **QUESTIONS**
#### QUESTION 13-8
The oxidation of sugar m... | {
"Header 1": "Fermentations Can Produce ATP in the Absence of Oxygen",
"Header 2": "Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation.",
"Header 3": "Key terms",
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"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The fundamental need to generate energy efficiently has had a profound influence on the history of life on Earth. Much of the structure, function, and evolution of cells and organisms can be related to their need for energy. With no oxygen in the atmosphere, it is thought that the earliest cells may have produced ATP b... | {
"Header 1": "Energy Generation in Mitochondria and Chloroplasts",
"token_count": 607,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The main chemical energy currency in cells is ATP (see Figure 3–32). Small amounts of ATP are generated during glycolysis in the cytosol of all cells (discussed in Chapter 13). But for the majority of cells, most of their ATP is produced by *oxidative phosphorylation*. The generation of ATP by oxidative phosphorylation... | {
"Header 1": "Cells Obtain Most of Their Energy by a Membrane-based Mechanism",
"token_count": 593,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The membrane-based, chemiosmotic mechanism for making ATP arose very early in life's history. The exact same type of ATP-generating processes occur in the plasma membrane of modern bacteria and archaea. Apparently, the mechanism was so successful that its essential features have been retained in the long evolutionary j... | {
"Header 1": "Chemiosmotic Coupling is an Ancient Process, Preserved in Present-Day Cells",
"token_count": 997,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Mitochondria are present in nearly all eukaryotic cells, where they produce the bulk of the cell's ATP. Without mitochondria, eukaryotes would have to rely on the relatively inefficient process of glycolysis for all of their ATP production. When glucose is converted to pyruvate by glycolysis in the cytosol, the net res... | {
"Header 1": "Chemiosmotic Coupling is an Ancient Process, Preserved in Present-Day Cells",
"Header 3": "Mitochondria and Oxidative Phosphorylation",
"token_count": 343,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Isolated mitochondria are generally similar in size and shape to their bacterial ancestors. Although they are no longer capable of living independently, mitochondria are remarkably adaptable and can adjust their location, shape, and number to suit the needs of the cell. In some cells, mitochondria remain fixed in one l... | {
"Header 1": "Chemiosmotic Coupling is an Ancient Process, Preserved in Present-Day Cells",
"Header 3": "Mitochondria Can Change Their Shape, Location, and Number to Suit a Cell's Needs",
"token_count": 567,
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An individual mitochondrion is bounded by two highly specialized membranes—one surrounding the other. These membranes, called the outer and inner mitochondrial membranes, create two mitochondrial compartments: a large internal space called the matrix and a much narrower *intermembrane space* (Figure 14–8). When purifie... | {
"Header 1": "Chemiosmotic Coupling is an Ancient Process, Preserved in Present-Day Cells",
"Header 3": "A Mitochondrion Contains an Outer Membrane, an Inner Membrane, and Two Internal Compartments",
"token_count": 497,
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... |
The generation of ATP is powered by the flow of electrons that are derived from the burning of carbohydrates, fats, and other foodstuffs during glycolysis and the citric acid cycle (discussed in Chapter 13). These high-energy electrons are provided by activated carriers generated during these two stages of catabolism, ... | {
"Header 1": "Chemiosmotic Coupling is an Ancient Process, Preserved in Present-Day Cells",
"Header 3": "The Citric Acid Cycle Generates the High-Energy Electrons Required for ATP Production",
"token_count": 708,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The chemiosmotic generation of energy begins when the activated carriers NADH and FADH2 donate their high-energy electrons to the electron-transport chain in the inner mitochondrial membrane, becoming oxidized to NAD+ and FAD in the process (see Figure 14-10). The electrons are quickly passed along the chain to molecul... | {
"Header 1": "The Movement of Electrons is Coupled to the Pumping of Protons",
"token_count": 737,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The electron-transport chain—or *respiratory chain*—that carries out oxidative phosphorylation is present in many copies in the inner mitochondrial membrane. Each chain contains over 40 proteins, grouped into three large **respiratory enzyme complexes**. These complexes each contain multiple individual proteins, includ... | {
"Header 1": "The Movement of Electrons is Coupled to the Pumping of Protons",
"Header 3": "Protons Are Pumped Across the Inner Mitochondrial Membrane by Proteins in the Electron-Transport Chain",
"token_count": 715,
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Without a mechanism for harnessing the energy released by the energetically favorable transfer of electrons from NADH to O<sub>2</sub>, this energy would simply be liberated as heat. Cells are able to recover much of this energy because the three respiratory enzyme complexes in the electron-transport chain use it to pu... | {
"Header 1": "The Movement of Electrons is Coupled to the Pumping of Protons",
"Header 3": "Proton Pumping Produces a Steep Electrochemical Proton Gradient Across the Inner Mitochondrial Membrane",
"token_count": 464,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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If protons in the intermembrane space were allowed simply to flow back into the mitochondrial matrix, the energy stored in the electrochemical proton gradient would be lost as heat. Such a seemingly wasteful process allows hibernating bears to stay warm, as we discuss further in How We Know (pp. 462–463). In most cells... | {
"Header 1": "The Movement of Electrons is Coupled to the Pumping of Protons",
"Header 3": "ATP Synthase Uses the Energy Stored in the Electrochemical Proton Gradient to Produce ATP",
"token_count": 555,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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(A) The multisubunit protein is composed of a stationary head, called the F<sub>1</sub> ATPase, and a rotating portion called $F_0$ . Both $F_1$ and $F_0$ are formed from multiple subunits. Driven by the electrochemical proton gradient, the F<sub>0</sub> part of the protein—which consists of the transmembrane H<su... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"token_count": 645,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The synthesis of ATP is not the only process driven by the electrochemical proton gradient in mitochondria. Many small, charged molecules, such as pyruvate, ADP, and inorganic phosphate (Pi), are imported into the mitochondrial matrix from the cytosol, while others, such as ATP, must be transported in the opposite dire... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Coupled Transport Across the Inner Mitochondrial Membrane Is Also Driven by the Electrochemical Proton Gradient",
"token_count": 35... |
The remarkable properties that allow ATP synthase to run in either direction allow the interconversion of energy stored in the H+ gradient and energy stored in ATP to proceed in either direction. (A) If ATP synthase making ATP can be likened to a water-driven turbine producing electricity, what would be an appropriate ... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Question 14–4",
"token_count": 546,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
The oxidation of sugars to produce ATP may seem unnecessarily complex. Surely the process could be accomplished more directly—perhaps by eliminating the citric acid cycle or some of the steps in the respiratory chain. Such simplification would certainly make the chemistry easier for students to learn—but it would be ba... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Cell Respiration Is Amazingly Efficient",
"token_count": 874,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell... |
For many years, biochemists struggled to understand why electrontransport chains had to be embedded in membranes to function in ATP production. The puzzle was essentially solved in the 1960s, when it was discovered that transmembrane proton gradients drive the process. The concept of chemiosmotic coupling was so novel,... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Molecular Mechanisms of Electron Transport and Proton Pumping",
"token_count": 202,
"source_pdf": "datasets/websources/biochem/Al... |
Although protons resemble other positive ions such as Na+ and K+ in the way they move across membranes, in some respects they are unique. Hydrogen atoms are by far the most abundant atom in living organisms: they are plentiful not only in all carbon-containing biological molecules but also in the water molecules that s... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Protons Are Readily Moved by the Transfer of Electrons",
"token_count": 255,
"source_pdf": "datasets/websources/biochem/Alberts_-... |
In the 1950s, many researchers believed that the oxidative phosphorylation that takes place in mitochondria generates ATP via a mechanism similar to that used in glycolysis. During glycolysis, ATP is produced when a molecule of ADP receives a phosphate group directly from a "high-energy" intermediate. Such substratelev... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Imaginary intermediates",
"token_count": 234,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed... |
It wasn't until 1961 that Peter Mitchell suggested that the "high-energy intermediate" his colleagues were seeking was, in fact, the electrochemical proton gradient generated by the electron-transport system. His proposal, dubbed the chemiosmotic hypothesis, stated that the energy of an electrochemical proton gradient ... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Harnessing the force",
"token_count": 541,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.... |
If disrupting the electrochemical proton gradient across the mitochondrial inner membrane terminates ATP synthesis, then, conversely, generating an artificial proton gradient should stimulate ATP synthesis. Again, this is exactly what happens. When a proton gradient is imposed artificially by lowering the pH on the out... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Artificial ATP generation",
"token_count": 1043,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th... |
The proteins of the respiratory chain guide the electrons so that they move sequentially from one enzyme complex to another—with no short circuits that skip a complex. Each electron transfer is an oxidation–reduction reaction: as described in Chapter 3, the molecule or atom donating the electron becomes oxidized, while... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "The Redox Potential Is a Measure of Electron Affinities",
"token_count": 893,
"source_pdf": "datasets/websources/biochem/Alberts_... |
The amount of energy that can be released by an electron transfer can be determined by comparing the redox potentials of the molecules involved. Again, let's look at the transfer of electrons from NADH and to O2. As shown in Panel 14–1, a 1:1 mixture of NADH and NAD+ has a redox potential of –320 mV, indicating that NA... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Electron Transfers Release Large Amounts of Energy",
"token_count": 422,
"source_pdf": "datasets/websources/biochem/Alberts_-_Ess... |
Each of the three respiratory enzyme complexes includes metal atoms that are tightly bound to the proteins. Once an electron has been donated to a respiratory complex, it moves within the complex by skipping from one embedded metal ion to another with a greater affinity for electrons.
#### HOW REDOX POTENTIALS ARE ME... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 3": "Metals Tightly Bound to Proteins Form Versatile Electron Carriers",
"token_count": 704,
"source_pdf": "datasets/websources/bioche... |
To determine the energy change for an electron transfer, the $\Delta G^\circ$ of the reaction (kcal/mole) is calculated as follows:
$\Delta G^{\circ} = -n(0.023) \Delta E'_0$ , where n is the number of electrons transferred across a redox potential change of $\Delta E'_0$ millivolts (mV), and
$$\Delta E_0' = E_... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 2": "CALCULATION OF $\\Delta G^{\\circ}$ FROM REDOX POTENTIALS",
"token_count": 1101,
"source_pdf": "datasets/websources/biochem/Alber... |
(A) Ribbon structure shows the position of the heme group (red) associated with cytochrome c (green). (B) The porphyrin ring of the heme group (light red) is attached covalently to side chains in the protein. The heme groups of different cytochromes have different electron affinities because they differ slightly in str... | {
"Header 1": "Figure 14–16 ATP synthase acts like a motor to convert the energy of protons flowing down their electrochemical gradient to chemical-bond energy in ATP.",
"Header 2": "CALCULATION OF $\\Delta G^{\\circ}$ FROM REDOX POTENTIALS",
"Header 3": "Figure 14–25 The iron in a heme group can serve as an elec... |
**Cytochrome** *c* **oxidase**, the final electron carrier in the respiratory chain, has the highest redox potential of all. This protein complex removes electrons from cytochrome *c*, thereby oxidizing it—hence the name "cytochrome *c* oxidase." These electrons are then handed off to $O_2$ to produce $H_2O$ . In to... | {
"Header 1": "Cytochrome c Oxidase Catalyzes the Reduction of Molecular Oxygen",
"token_count": 1320,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Chloroplasts are larger than mitochondria, but both are organized along structurally similar principles. Chloroplasts have a highly permeable outer membrane and a much less permeable inner membrane, in which various membrane transport proteins are embedded. Together, these two membranes—and the narrow, intermembrane sp... | {
"Header 1": "Chloroplasts Resemble Mitochondria but Have an Extra Compartment—the Thylakoid",
"token_count": 438,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The chemistry carried out by photosynthesis can be summarized in one simple equation:
light energy +
$$CO_2$$
+ $H_2O \rightarrow sugars + O_2$ + heat energy
On its surface, the equation accurately represents the process by which light energy drives the production of sugars from CO2. But this superficial accounti... | {
"Header 1": "Chloroplasts Resemble Mitochondria but Have an Extra Compartment—the Thylakoid",
"Header 3": "Photosynthesis Generates—Then Consumes—ATP and NADPH",
"token_count": 958,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Visible light is a form of electromagnetic radiation composed of many wavelengths, ranging from violet (wavelength 400 nm) to deep red (700 nm). Most chlorophylls best absorb light in the blue and red wavelengths (**Figure 14–30**). Because these pigments absorb green light poorly, plants look green to us: the green li... | {
"Header 1": "Chloroplasts Resemble Mitochondria but Have an Extra Compartment—the Thylakoid",
"Header 3": "Chlorophyll Molecules Absorb the Energy of Sunlight",
"token_count": 296,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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In the thylakoid membrane of plants and the plasma membrane of photosynthetic bacteria, chlorophyll molecules are held in large multiprotein complexes called **photosystems**. Each photosystem consists of a set of *antenna complexes*, which capture light energy, and a *reaction center*, which converts that light energy... | {
"Header 1": "Excited Chlorophyll Molecules Funnel Energy into a Reaction Center",
"token_count": 549,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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Photosynthesis is ultimately a biosynthetic process, and to build organic molecules from $CO_2$ , a plant cell requires a huge input of energy, in the form of ATP, and a very large amount of reducing power, in the form of the activated carrier NADPH (see Figure 3–34). To generate both ATP and NADPH, plant cells—and fr... | {
"Header 1": "A Pair of Photosystems Cooperate to Generate Both ATP and NADPH",
"token_count": 968,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The scheme that we have thus far described for photosynthesis has ignored a major chemical conundrum. When a mobile electron carrier removes an electron from a reaction center (whether in photosystem I or photosystem II), it leaves behind a positively charged chlorophyll special pair (see Figure 14–33). To reset the sy... | {
"Header 1": "A Pair of Photosystems Cooperate to Generate Both ATP and NADPH",
"Header 3": "Oxygen Is Generated by a Water-Splitting Complex Associated with Photosystem II",
"token_count": 426,
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We have seen that photosystem II receives electrons from water. But where does photosystem I get the electrons it needs to reset its special pair? It gets them from photosystem II: the chlorophyll special pair in photosystem I serves as the final electron acceptor for the electron-transport chain that carries electrons... | {
"Header 1": "A Pair of Photosystems Cooperate to Generate Both ATP and NADPH",
"Header 3": "The Special Pair in Photosystem I Receives its Electrons from Photosystem II",
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The light reactions of photosynthesis generate ATP and NADPH in the chloroplast stroma, as we have just seen. But the inner membrane of the chloroplast is impermeable to both of these compounds, which means that they cannot be exported directly to the cytosol. To provide energy and reducing power for the rest of the ce... | {
"Header 1": "A Pair of Photosystems Cooperate to Generate Both ATP and NADPH",
"Header 3": "Carbon Fixation Uses ATP and NADPH to Convert CO2 into Sugars",
"token_count": 2041,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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energetically favorable reaction. Carbon fixation is energetically favorable because a continuous supply of the energy-rich ribulose 1,5-bisphosphate is fed into it. As this compound is consumed—by the addition of $CO_2$ (see Figure 14–39)—it must be replenished. The energy and reducing power needed to regenerate r... | {
"Header 1": "A Pair of Photosystems Cooperate to Generate Both ATP and NADPH",
"Header 3": "Carbon Fixation Uses ATP and NADPH to Convert CO2 into Sugars",
"token_count": 673,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The glyceraldehyde 3-phosphate generated by carbon fixation in the chloroplast stroma can be used in a number of ways, depending on the needs of the plant. During periods of excess photosynthetic activity, much of it is retained in the chloroplast stroma and converted to *starch*. Like glycogen in animal cells, starch ... | {
"Header 1": "Sugars Generated by Carbon Fixation Can Be Stored As Starch or Consumed to Produce ATP",
"token_count": 606,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
As we mentioned earlier, the first living cells on Earth—both prokaryotes and primitive eukaryotes—may have consumed geochemically produced organic molecules and generated ATP by fermentation. Because oxygen was not yet present in the atmosphere, such anaerobic fermentation reactions would have dumped organic acids—suc... | {
"Header 1": "THE EVOLUTION OF ENERGY-GENERATING SYSTEMS",
"Header 3": "Oxidative Phosphorylation Evolved in Stages",
"token_count": 695,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The major evolutionary breakthrough in energy metabolism, however, was almost certainly the formation of photochemical reaction centers that could use the energy of sunlight to produce molecules such as NADH. It is thought that this development occurred early in the process of evolution—more than 3 billion years ago, i... | {
"Header 1": "THE EVOLUTION OF ENERGY-GENERATING SYSTEMS",
"Header 3": "Photosynthetic Bacteria Made Even Fewer Demands on Their Environment",
"token_count": 747,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The conditions today that most resemble those under which cells are thought to have lived 3.5–3.8 billion years ago may be those near deepocean hydrothermal vents. These vents represent places where the Earth's molten mantle is breaking through the overlying crust, expanding the width of the ocean floor. Indeed, the mo... | {
"Header 1": "The Lifestyle of *Methanococcus* Suggests That Chemiosmotic Coupling Is an Ancient Process",
"token_count": 460,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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With the evolution of photosynthesis in prokaryotes more than 3 billion years ago, organisms would have no longer depended on preformed organic chemicals. They could have then made their own organic molecules from CO<sub>2</sub>. The delay of more than a billion years between the appearance of bacteria that split water... | {
"Header 1": "The Lifestyle of *Methanococcus* Suggests That Chemiosmotic Coupling Is an Ancient Process",
"Header 2": "Figure 14–45 Oxygen entered Earth's atmosphere billions of years ago.",
"token_count": 492,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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- • Mitochondria, chloroplasts, and many prokaryotes generate energy by a membrane-based mechanism known as chemiosmotic coupling, which involves using an electrochemical proton gradient to drive the synthesis of ATP.
- • Mitochondria produce most of an animal cell's ATP, using energy derived from oxidation of sugars a... | {
"Header 1": "The Lifestyle of *Methanococcus* Suggests That Chemiosmotic Coupling Is an Ancient Process",
"Header 2": "Figure 14–45 Oxygen entered Earth's atmosphere billions of years ago.",
"Header 3": "Essential Concepts",
"token_count": 574,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_... |
matrix
#### Question 14–11
Which of the following statements are correct? Explain your answers.
- A. After an electron has been removed by light, the affinity for electrons of the positively charged chlorophyll in the reaction center of the first photosystem (photosystem II) is even greater than the electron affi... | {
"Header 1": "The Lifestyle of *Methanococcus* Suggests That Chemiosmotic Coupling Is an Ancient Process",
"Header 2": "Figure 14–45 Oxygen entered Earth's atmosphere billions of years ago.",
"Header 3": "Questions",
"token_count": 2033,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Bio... |
At any one time, a typical eukaryotic cell carries out thousands of different chemical reactions, many of which are mutually incompatible. One series of reactions makes glucose, for example, while another breaks it down; some enzymes synthesize peptide bonds, whereas others hydrolyze them, and so on. Indeed, if the cel... | {
"Header 1": "Intracellular Compartments and Protein Transport",
"token_count": 822,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
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The major membrane-enclosed organelles of an animal cell are illustrated in Figure 15–2, and their functions are summarized in Table 15–1. These organelles are surrounded by the *cytosol*, which is enclosed by the plasma membrane. The *nucleus* is generally the most prominent organelle in eukaryotic cells. It is surrou... | {
"Header 1": "Intracellular Compartments and Protein Transport",
"Header 3": "Eukaryotic Cells Contain a Basic Set of Membraneenclosed Organelles",
"token_count": 1628,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
In trying to understand the relationships between the different compartments of a modern eukaryotic cell, it is helpful to consider how they evolved. The compartments probably evolved in stages. The precursors of the first eukaryotic cells are thought to have been simple microorganisms, resembling bacteria, which had a... | {
"Header 1": "Intracellular Compartments and Protein Transport",
"Header 3": "Membrane-enclosed Organelles Evolved in Different Ways",
"token_count": 970,
"source_pdf": "datasets/websources/biochem/Alberts_-_Essential_Cell_Biology__4th_ed._.pdf"
} |
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