Biology Campbell 12 ed-115-414-99-121.pdf

9 Cellular Respiration and Fermentation KEY CONCEPTS 9.1 Catabolic pathways yield energy by oxidizing organic fuels p. 165 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate p. 170 9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxida...

9 Cellular Respiration and Fermentation KEY CONCEPTS 9.1 Catabolic pathways yield energy by oxidizing organic fuels p. 165 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate p. 170 9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules p. 171 9.4 During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis p. 174 9.5 Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen p. 179 9.6 Glycolysis and the citric acid cycle connect to many other metabolic pathways p. 182 Study Tip Make a visual study guide: Draw a cell with a large mitochondrion, labeling the parts of the mitochondrion. As you go through the Mitochondrion chapter, add key reactions for each stage of cellular respiration, linking the stages together. Label the carbon molecule(s) with the most energy and the carbon molecule(s) with the least energy. Your cell can be a simple sketch, as shown here. Figure 9.1 This hoary marmot (Marmota caligata) obtains energy for its cells by feeding on plants. In the process of cellular respiration, mitochondria in the cells of animals, plants, and other organisms break down organic molecules, generating ATP and waste products: carbon dioxide, water, and heat. Note that energy flows one way, but chemicals are recycled. How is the chemical energy stored in food used to generate ATP, the molecule that drives most cellular work? Light energy Photosynthesis Organic + O2 molecules CO2 + H2O Go to Mastering Biology generates For Students (in eText and Study Area) • Get Ready for Chapter 9 • BioFlix® Animation: Cellular Respiration • Figure 9.12 Walkthrough: Free-Energy Change During Electron Transport used in Cellular respiration in mitochondria For Instructors to Assign (in Item Library) • BioFlix® Tutorial: Glycolysis • BioFlix® Tutorial: Cellular Respiration: Inputs and Outputs Ready-to-Go Teaching Module (in Instructor Resources) • Oxidative Phosphorylation (Concept 9.4) generates used in breaks down organic molecules,generating Plant cell ATP Heat 164 powers most cellular work Animal cell CONCEPT 9.1 Catabolic pathways yield energy by oxidizing organic fuels Living cells require transfusions of energy from outside sources to perform their many tasks—for example, assembling polymers, pumping substances across membranes, moving, and reproducing. The outside source of energy is food, and the energy stored in the organic molecules of food ultimately comes from the sun. As shown in Figure 9.1, energy flows into an ecosystem as sunlight and exits as heat; in contrast, the chemical elements essential to life are recycled. Photosynthesis generates oxygen, as well as organic molecules used by the mitochondria of eukaryotes as fuel for cellular respiration. Respiration breaks this fuel down, using oxygen (O2) and generating ATP. The waste products of this type of respiration, carbon dioxide (CO2) and water (H2O), are the raw materials for photosynthesis. Mastering Biology Animation: Energy Flow and Chemical Recycling Let’s consider how cells harvest the chemical energy stored in organic molecules and use it to generate ATP, the molecule that drives most cellular work. Metabolic pathways that release stored energy by breaking down complex molecules are called catabolic pathways (see Concept 8.1). Transfer of electrons from food molecules (like glucose) to other molecules plays a major role in these pathways. In this section, we consider these processes, which are central to cellular respiration. Catabolic Pathways and Production of ATP Organic compounds possess potential energy as a result of the arrangement of electrons in the bonds between their atoms. Compounds that can participate in exergonic reactions can act as fuels. Through the activity of enzymes (see Concept 8.4), a cell systematically degrades complex organic molecules that are rich in potential energy to simpler waste products that have less energy. Some of the energy taken out of chemical storage can be used to do work; the rest is dissipated as heat. One catabolic process, fermentation, is a partial degradation of sugars or other organic fuel that occurs without the use of oxygen. However, the most efficient catabolic pathway is aerobic respiration, in which oxygen is consumed as a reactant along with the organic fuel (aerobic is from the Greek aer, air, and bios, life). The cells of most eukaryotic and many prokaryotic organisms can carry out aerobic respiration. Some prokaryotes use substances other than oxygen as reactants in a similar process that harvests chemical energy without oxygen; this process is called anaerobic respiration (the prefix an- means “without”). Technically, the term cellular respiration includes both aerobic and anaerobic processes. However, it originated as a synonym for aerobic respiration because of the relationship of that process to organismal respiration, in which an animal breathes in oxygen. Thus, cellular respiration is often used to refer to the aerobic process, a practice we follow in most of this chapter. Although very different in mechanism, aerobic respiration is in principle similar to the combustion of gasoline in an automobile engine after oxygen is mixed with the fuel (hydrocarbons). Food provides the fuel for respiration, and the exhaust is carbon dioxide and water. The overall process can be summarized as follows: Organic Carbon + Oxygen S + Water + Energy compounds dioxide Carbohydrates, fats, and proteins from food can all be processed and consumed as fuel. In animal diets, a major source of carbohydrates is starch, a storage polysaccharide that can be broken down into glucose (C6H12O6) subunits. Here, we will learn the steps of cellular respiration by tracking the degradation of the sugar glucose: C6H12O6 + 6 O2 S 6 CO2 + 6 H2O + Energy (ATP + heat) Mastering Biology BioFlix® Animation: Introduction to Cellular Respiration This breakdown of glucose is exergonic, having a freeenergy change of -686 kcal (-2,870 kJ) per mole of glucose decomposed (∆G = - 686 kcal/mol). Recall that a negative ∆G1 ∆G 6 02 indicates that the products of the chemical process store less energy than the reactants and that the reaction can happen spontaneously—in other words, without an input of energy (see Concept 8.2). Catabolic pathways do not directly move flagella, pump solutes across membranes, polymerize monomers, or perform other cellular work. Catabolism is linked to work by a chemical drive shaft—ATP (see Concept 8.3). To keep working, the cell must regenerate its supply of ATP from ADP and P i (see Figure 8.12). To understand how cellular respiration ~ accomplishes this, let’s examine the fundamental chemical processes known as oxidation and reduction. Redox Reactions: Oxidation and Reduction How do the catabolic pathways that decompose glucose and other organic fuels yield energy? The answer is based on the transfer of electrons during the chemical reactions. The relocation of electrons releases energy stored in organic molecules, and this energy ultimately is used to synthesize ATP. The Principle of Redox In many chemical reactions, there is a transfer of one or more electrons (e - ) from one reactant to another. These electron transfers are called oxidation-reduction reactions, or redox reactions for short. In a redox reaction, the loss of electrons CHAPTER 9 Cellular Respiration and Fermentation 165 from one substance is called oxidation, and the addition of electrons to another substance is known as reduction. (Note that adding electrons is called reduction; adding negatively charged electrons to an atom reduces the amount of positive charge of that atom.) To take a simple, nonbiological example, consider the reaction between the elements sodium (Na) and chlorine (Cl) that forms table salt: . Figure 9.2 Methane combustion as an energy-yielding redox reaction. The reaction releases energy to the surroundings because the electrons lose potential energy when they end up being shared unequally, spending more time near electronegative atoms such as oxygen. Reactants 1 Cl– becomes reduced (gains electron) We could generalize a redox reaction this way: becomes oxidized Xe – 1 Y X 1 Ye – becomes reduced In the generalized reaction, substance Xe -, the electron donor, is called the reducing agent; it reduces Y, which accepts the donated electron. Substance Y, the electron acceptor, is the oxidizing agent; it oxidizes Xe - by removing its electron. Because an electron transfer requires both an electron donor and an acceptor, oxidation and reduction always go hand in hand. Not all redox reactions involve the complete transfer of electrons from one substance to another; some change the degree of electron sharing in covalent bonds. Methane combustion, shown in Figure 9.2, is an example. The covalent electrons in methane are shared nearly equally between the bonded atoms because carbon and hydrogen have about the same affinity for valence electrons; they are about equally electronegative (see Concept 2.3). But when methane reacts with O2, forming CO2, electrons end up shared less equally between the carbon atom and its new covalent partners, the oxygen atoms, which are very electronegative. In effect, the carbon atom has partially “lost” its shared electrons; thus, methane has been oxidized. Now let’s examine the fate of the reactant O2. The two atoms of O2 share their electrons equally. But after the reaction with methane, when each O atom is bonded to two H atoms in H2O, the electrons of those covalent bonds spend more time near the oxygen (see Figure 9.2). In effect, each O atom has partially “gained” electrons, so the oxygen molecule (O2) has been reduced. Because the O atom is so electronegative, O2 is one of the most powerful of all oxidizing agents. Energy must be added to pull an electron away from an atom, just as energy is required to push a ball uphill. The more electronegative the atom (the stronger its pull on electrons), the more energy is required to take an electron away from it. An electron loses potential energy when it shifts from a less electronegative atom toward a more electronegative one, just as a ball loses potential energy when it rolls downhill. A redox reaction that moves electrons closer to an O atom, such as the 166 UNIT TWO The Cell 2 O2 CO2 + H C Energy + 2 H2O becomes reduced H Na+ Cl 1 + CH4 becomes oxidized (loses electron) Na Products becomes oxidized H O O O C O H O H H Methane (reducing agent) Oxygen (oxidizing agent) Carbon dioxide Water VISUAL SKILLS Is the carbon atom oxidized or reduced during this reaction? Explain. Mastering Biology Animation: Redox Reactions burning (oxidation) of methane, therefore releases chemical energy that can be put to work. Oxidation of Organic Fuel Molecules During Cellular Respiration The oxidation of methane by O2 is the main combustion reaction that occurs at the burner of a gas stove. The combustion of gasoline in an automobile engine is also a redox reaction; the energy released pushes the pistons. But the energy-yielding redox process of greatest interest to biologists is respiration: the oxidation of glucose and other molecules in food. Examine again the summary equation for cellular respiration, but this time think of it as a redox process: becomes oxidized C6H12O6 1 6 O2 6 CO2 1 6 H2O 1 Energy becomes reduced As in the combustion of methane or gasoline, the fuel (glucose) is oxidized and O2 is reduced. The electrons lose potential energy along the way, and energy is released. In general, organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of “hilltop” electrons, whose energy may be released as these electrons “fall” down an energy gradient during their transfer to oxygen. The summary equation for respiration indicates that hydrogen is transferred from glucose to the O atoms in O2. But the important point, not visible in the summary equation, is that the energy state of the electron changes as hydrogen (with its electron) is transferred to oxygen. In respiration, the oxidation of glucose transfers electrons to a lower energy state, liberating energy that becomes available for ATP synthesis. So, in general, we see fuels with multiple C ¬ H bonds oxidized into products with multiple C ¬ O bonds. The main energy-yielding foods—carbohydrates and fats— are reservoirs of electrons associated with hydrogen, often in the form of C ¬ H bonds. Only the barrier of activation energy holds back the flood of electrons to a lower energy state (see Figure 8.13). Without this barrier, a food substance like glucose would combine almost instantaneously with O2. If we supply the activation energy by igniting glucose, it burns in air, releasing 686 kcal (2,870 kJ) of heat per mole of glucose (about 180 g). Body temperature is not high enough to initiate burning, of course. Instead, if you swallow some glucose, enzymes in your cells will lower the barrier of activation energy, allowing the sugar to be oxidized in a series of steps. Stepwise Energy Harvest via NAD∙ and the Electron Transport Chain If energy is released from a fuel all at once, it cannot be harnessed efficiently for constructive work. For example, if a gasoline tank explodes, it cannot drive a car very far. Cellular respiration does not oxidize glucose (or any other organic fuel) in a single explosive step either. Rather, glucose is broken down in a series of steps, each one catalyzed by an enzyme. At key steps, electrons are stripped from the glucose. As is often the case in oxidation reactions, each electron travels with a proton—thus, as a hydrogen atom. The hydrogen atoms are not transferred directly to O2, but instead are usually passed first to an electron carrier, a coenzyme called nicotinamide adenine dinucleotide, a derivative of the vitamin niacin. This coenzyme is well suited as an electron carrier because it can cycle easily between its oxidized form, NAD1, and its reduced form, NADH. As an electron acceptor, NAD + functions as an oxidizing agent during respiration. How does NAD + trap electrons from glucose and the other organic molecules in food? Enzymes called dehydrogenases remove a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (glucose, in the preceding example), thereby oxidizing it. The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD + , forming NADH (Figure 9.3). The other proton is released as a hydrogen ion (H + ) into the surrounding solution: H C OH 1 NAD+ 2 e– + H+ NAD+ C CH2 O O P O– O O P O O N+ Nicotinamide (oxidized form) H O + 2H (from food) Reduction of NAD+ Oxidation of NADH H H OH CH2 N N H H HO OH NH2 N Nicotinamide (reduced form) + H+ VISUAL SKILLS Describe the structural differences between the oxidized form and the reduced form of nicotinamide. NH2 N H O O C H HO – NH2 H+ NADH Dehydrogenase O C O 1 NADH 1 H+ By receiving 2 negatively charged electrons but only 1 positively charged proton, the nicotinamide portion of NAD + has its charge neutralized when NAD + is reduced to NADH. The name NADH shows the hydrogen that has been received in the reaction. NAD + is the most versatile electron acceptor in cellular respiration and functions in several of the redox steps during the breakdown of glucose. Electrons lose very little of their potential energy when they are transferred from glucose to NAD + . Each NADH molecule formed during respiration represents stored energy that can be tapped to make ATP when the electrons complete their “fall” down an energy gradient from NADH to O2. How do electrons that are extracted from glucose and stored as potential energy in NADH finally reach oxygen? It will help to compare the redox chemistry of cellular respiration to a much simpler reaction: the reaction between hydrogen and oxygen to form water (Figure 9.4a). Mix H2 and O2, provide a spark for activation energy, and the gases combine explosively. In fact, combustion of liquid H2 and O2 is harnessed to help power the rocket engines that boost satellites into orbit and launch spacecraft. The explosion represents a release of energy as the electrons of hydrogen “fall” closer to the electronegative oxygen atoms. Cellular respiration also brings hydrogen and oxygen together to form water, but there are two important differences. First, in cellular respiration, the hydrogen that reacts with oxygen 2 e– + 2 H+ H Dehydrogenase N H m Figure 9.3 NAD∙ as an electron shuttle. The full name for NAD + , nicotinamide adenine dinucleotide, describes its structure—the molecule consists of two nucleotides joined together at their phosphate groups (shown in yellow). (Nicotinamide is a nitrogenous base, although not one that is present in DNA or RNA; see Figure 5.23.) The enzymatic transfer of 2 electrons and 1 proton (H+ ) from an organic molecule in food to NAD + reduces the NAD + to NADH: Most of the electrons removed from food are transferred initially to NAD + , forming NADH. CHAPTER 9 Cellular Respiration and Fermentation 167 is derived from organic molecules rather than H2. Second, instead of occurring in one explosive reaction, respiration uses an electron transport chain to break the fall of electrons to oxygen into several energy-releasing steps (Figure 9.4b). An electron transport chain consists of a number of molecules, mostly proteins, built into the inner membrane of the mitochondria of eukaryotic cells (and the plasma membrane of respiring prokaryotes). Electrons removed from glucose are shuttled by NADH to the “top,” higherenergy end of the chain. At the “bottom,” lower-energy end, O2 captures these electrons along with hydrogen nuclei (H + ), forming water. (Anaerobically respiring prokaryotes have an electron acceptor at the end of the chain that is different from O2.) Electron transfer from NADH to oxygen is an exergonic reaction with a free-energy change of -53 kcal/mol (-222 kJ/mol). Instead of this energy being released and wasted in a single explosive step, electrons cascade down the chain from one carrier molecule to the next in a series of redox reactions, losing a small amount of energy with each step until they finally reach oxygen, the terminal electron acceptor, which has a very great affinity for electrons. Each “downhill” carrier has a greater affinity for electrons than, and is thus capable of accepting electrons from (oxidizing), its “uphill” neighbor, with O2 at the bottom of the chain. Therefore, the electrons transferred from glucose to NAD +, reducing it to NADH, fall down an energy gradient in the electron transport chain to a far more stable location in an electronegative oxygen atom from O2. Put another way, O2 pulls electrons down the chain in an energy-yielding tumble analogous to gravity pulling objects downhill. In summary, during cellular respiration, most electrons travel the following “downhill” route: glucose S NADH S electron transport chain S oxygen. Later in this chapter, you will learn more about how the cell uses the energy released from this exergonic electron fall to regenerate its supply of ATP. For now, having covered the basic redox mechanisms of cellular respiration, let’s look at the entire process by which energy is harvested from organic fuels. The Stages of Cellular Respiration: A Preview The harvesting of energy from glucose by cellular respiration is a cumulative function of three metabolic stages. We list them here along with a color-coding scheme we will use throughout the chapter to help you keep track of the big picture: 1. GLYCOLYSIS (color-coded blue throughout the chapter) 2. PYRUVATE OXIDATION (light orange) and the CITRIC ACID CYCLE (dark orange) 3. OXIDATIVE PHOSPHORYLATION: Electron transport and chemiosmosis (purple) Free energy, G Free energy, G t spor tran tron ain ch Elec Biochemists usually reserve the term cellular respiration for stages 2 and 3 together. In this text, however, we include glycolysis as a part of cellular respiration because most respiring cells deriving energy from glucose use glycolysis to produce the starting material for the citric acid cycle. As diagrammed in Figure 9.5, glycolysis and then pyruvate oxidation . Figure 9.4 An introduction to electron transport chains. and the citric acid cycle are the catabolic (a) Uncontrolled reaction. (b) Cellular respiration. In cellular respiration, the pathways that break down glucose and The one-step exergonic reaction same reaction occurs in stages: An electron other organic fuels. Glycolysis, which of hydrogen with oxygen to transport chain breaks the “fall” of electrons in this form water releases a large reaction into a series of smaller steps and stores occurs in the cytosol, begins the degradaamount of energy in the form some of the released energy in a form that can be tion process by breaking glucose into two of heat and light: an explosion. used to make ATP. (The rest of the energy is released as heat.) molecules of a compound called pyruvate. In eukaryotes, pyruvate enters the mito1 + /2 O2 H2 + 1/2 O2 chondrion and is oxidized to a compound 2H (from food via NADH) called acetyl CoA, which enters the citric Controlled acid cycle. There, the breakdown of release of glucose to carbon dioxide is completed. + – 2H + 2e energy for (In prokaryotes, these processes take place synthesis of ATP in the cytosol.) Thus, the carbon dioxide ATP produced by respiration represents fragExplosive ATP ments of oxidized organic molecules. release of Some of the steps of glycolysis and heat and light ATP energy the citric acid cycle are redox reactions in which dehydrogenases transfer electrons 2 e– from substrates to NAD + or the related 12 O 2 + 2H electron carrier FAD, forming NADH or FADH2. (You’ll learn more about FAD and H 2O H2O FADH2 later.) In the third stage of respiration, the electron transport chain accepts 168 UNIT TWO The Cell c Figure 9.5 An overview of cellular respiration. During glycolysis, each glucose molecule is broken down into two molecules of pyruvate. In eukaryotic cells, as shown here, the pyruvate enters the mitochondrion. There it is oxidized to acetyl CoA, which will be further oxidized to CO2 in the citric acid cycle. The electron carriers NADH and FADH2 transfer electrons derived from glucose to electron transport chains. During oxidative phosphorylation, electron transport chains convert the chemical energy to a form used for ATP synthesis in the process called chemiosmosis. (During earlier steps of cellular respiration, a few molecules of ATP are synthesized in a process called substrate-level phosphorylation.) To visualize these processes in their cellular context, see Figure 6.32b. Mastering Biology Animation: Overview of Cellular Respiration GLYCOLYSIS Glucose Pyruvate CYTOSOL PYRUVATE OXIDATION CITRIC ACID CYCLE Acetyl CoA OXIDATIVE PHOSPHORYLATION (Electron transport and chemiosmosis) MITOCHONDRION ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation electrons from NADH or FADH2 generated during the first two stages and passes these electrons down the chain. At the end of the chain, the electrons are combined with molecular oxygen (O2) and hydrogen ions (H +), forming water (see Figure 9.4b). The energy released at each step of the chain is stored in a form the mitochondrion (or prokaryotic cell) can use to make ATP from ADP. This mode of ATP synthesis is called oxidative phosphorylation because it is powered by the redox reactions of the electron transport chain. In eukaryotic cells, the inner membrane of the mitochondrion is the site of electron transport and another process called chemiosmosis, together making up oxidative phosphorylation. (In prokaryotes, these processes take place in the plasma membrane.) Oxidative phosphorylation accounts for almost 90% of the ATP generated by respiration. A smaller amount of ATP is formed directly in a few reactions of glycolysis and the citric acid cycle by a mechanism called substrate-level phosphorylation (Figure 9.6). This mode of ATP synthesis . Figure 9.6 Substrate-level phosphorylation. Some ATP is made by direct transfer of a phosphate group from an organic substrate to ADP by an enzyme. (For examples in glycolysis, see Figure 9.8, steps 7 and 10.) Enzyme Enzyme Electrons carried via NADH and FADH2 Electrons carried via NADH ADP occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation. “Substrate molecule” here refers to an organic molecule generated as an intermediate during the catabolism of glucose. You’ll see examples of substrate-level phosphorylation later in the chapter, in both glycolysis and the citric acid cycle. You can think of the whole process this way: When you withdraw a relatively large sum of money from an ATM, it is not delivered to you in a single bill of a large denomination. Instead, the machine dispenses a number of smaller-denomination bills that you can spend more easily. This is analogous to ATP production during cellular respiration. For each molecule of glucose degraded to CO2 and H2O by respiration, the cell makes up to about 32 molecules of ATP, each with 7.3 kcal/mol of free energy. Respiration cashes in the large denomination of energy banked in a single molecule of glucose (686 kcal/mol under standard conditions) for the small change of many molecules of ATP, which is more practical for the cell to spend on its work. This preview has introduced you to how glycolysis, the citric acid cycle, and oxidative phosphorylation fit into the process of cellular respiration so you can keep the big picture in mind as you take a closer look at each of these three stages of respiration. As you read about the chemical reactions, remember that each reaction is catalyzed by a specific enzyme, some of which are shown in Figure 6.32b. CONCEPT CHECK 9.1 P ATP Substrate Product MAKE CONNECTIONS Review Figure 8.9. In the reaction shown above, is the potential energy higher for the reactants or for the products? Explain. 1. Compare and contrast aerobic and anaerobic respiration, including the processes involved. 2. WHAT IF? If the following redox reaction occurred, which compounds would be oxidized? Reduced? C 4H6O5 + NAD + S C 4H4O5 + NADH + H+ For suggested answers, see Appendix A. CHAPTER 9 Cellular Respiration and Fermentation 169 CONCEPT . Figure 9.7 The inputs and outputs of glycolysis. 9.2 Glycolysis harvests chemical energy by oxidizing glucose to pyruvate Mastering Biology Animation: Glycolysis GLYCOLYSIS The word glycolysis means “sugar splitting,” and that is exactly what happens during this pathway. Glucose, a sixcarbon sugar, is split into two three-carbon sugars. These smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.) As summarized in Figure 9.7, glycolysis can be divided into two phases: the energy investment phase and the energy payoff phase. During the energy investment phase, the cell actually spends ATP. This investment is repaid with interest during the energy payoff phase, when ATP is produced by substrate-level phosphorylation and NAD + is reduced to NADH by electrons released from the oxidation of glucose. The net energy yield from glycolysis, per glucose molecule, is 2 ATP plus 2 NADH. The ten steps of the glycolytic pathway are shown in Figure 9.8. All of the carbon originally present in glucose is accounted for in the two molecules of pyruvate; no carbon is released as CO2 during glycolysis. Glycolysis occurs whether or not O2 is present. However, if O2 is present, the chemical energy stored in pyruvate and NADH can be extracted by pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation. GLYCOLYSIS OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE PYRUVATE OXIDATION GLYCOLYSIS: OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE PYRUVATE OXIDATION ATP Energy Investment Phase Glucose 2 ATP 2 ADP + 2 P used Energy Payoff Phase 4 ADP + 4 P 4 2 NAD+ + 4 e– + 4 H+ formed ATP 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Inputs and Outputs Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 NAD+ + 4 e– + 4 2 ATP H+ 2 NADH + 2 H+ . Figure 9.8 The steps of glycolysis. Glycolysis, a source of ATP and NADH, takes place in the cytosol. Two of the enzymes (in steps 1 and 3 ) are shown in Figure 6.32b. GLYCOLYSIS: Energy Investment Phase WHAT IF? What would happen if you removed the dihydroxyacetone ATP phosphate generated in step 4 as fast as it was produced? Glyceraldehyde 3-phosphate (G3P) Glucose CH2OH O H H H OH H OH HO H ADP CH2O H H OH HO Hexokinase OH 1 H Hexokinase transfers a phosphate group from ATP to glucose, making it more chemically reactive. The charged phosphate also traps the sugar in the cell. 170 UNIT TWO Fructose ATP 6-phosphate Glucose 6-phosphate ATP The Cell P O H CH2O H OH OH Phosphoglucoisomerase 2 Glucose 6phosphate is converted to fructose 6-phosphate. O P CH2OH H HO HO H H OH HC Fructose 1,6-bisphosphate ADP Phosphofructokinase P OCH2 H HO HO H H 3 Phosphofructokinase transfers a phosphate group from ATP to the opposite end of the sugar, investing a second molecule of ATP. This is a key step for regulation of glycolysis. CH2O O OH O CHOH CH2O P P Isomerase Aldolase 4 5 Dihydroxyacetone phosphate (DHAP) CH2O P Aldolase cleaves the sugar CH2OH molecule into two different Conversion between DHAP three-carbon and G3P: This reaction sugars. never reaches equilibrium; G3P is used in the next step as fast as it forms. C O . Figure 9.9 Oxidation of pyruvate to acetyl CoA, the step before the citric acid cycle. Pyruvate enters the mitochondrion through a transport protein and is processed by a complex of several enzymes known as pyruvate dehydrogenase (shown in the computer-generated image based on cryo-EMs). This complex catalyzes the three numbered steps, which are described in the text. The CO2 molecule will diffuse out of the cell. The NADH will be used in oxidative phosphorylation. The acetyl group of acetyl CoA will enter the citric acid cycle. (Coenzyme A is abbreviated S-CoA when it is attached to a molecule, emphasizing its sulfur atom, S.) CONCEPT CHECK 9.2 1. VISUAL SKILLS During the redox reaction in glycolysis (see step 6 in Figure 9.8), which one of the molecules acts as the oxidizing agent? The reducing agent? For suggested answers, see Appendix A. CONCEPT 9.3 After pyruvate is oxidized, the citric acid cycle completes the energy-yielding oxidation of organic molecules PYRUVATE OXIDATION GLYCOLYSIS OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE 10 nm Pyruvate dehydrogenase Glycolysis releases less than a quarter of the chemical energy in glucose that can be harvested by cells; most of the energy remains stockpiled in the two molecules of pyruvate. When O2 is present, the pyruvate in eukaryotic cells enters a mitochondrion, where the oxidation of glucose is completed. In aerobically respiring prokaryotic cells, this process occurs in the cytosol. (Later in the chapter, we’ll examine other fates for pyruvate—for example, when O2 is unavailable or in a prokaryote that is unable to use O2.) MITOCHONDRION CYTOSOL Oxidation of Pyruvate to Acetyl CoA 3 1 C O C O S-CoA C Pyruvate dehydrogenase NAD + O CH3 2 CH3 Pyruvate Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A, or acetyl CoA (Figure 9.9). This step, linking glycolysis and the citric acid cycle, is carried out by a multienzyme complex that catalyzes three reactions: 1 Pyruvate’s carboxyl group Coenzyme A CO2 O– NADH + H + Acetyl CoA Transport protein Mastering Biology BioFlix® Animation: Acetyl CoA The energy payoff phase occurs after glucose is split into two three-carbon sugars. Thus, the coefficient 2 precedes all molecules in this phase. GLYCOLYSIS: Energy Payoff Phase 2 NAD + 2 ADP + 2 H+ 2 Triose phosphate dehydrogenase 6 ATP 2 2 NADH P OC CH2O P 1,3-Bisphosphoglycerate Two sequential reactions: (1) G3P is oxidized by the transfer of electrons to NAD+, forming NADH. (2) Using energy from this exergonic redox reaction, a phosphate group is attached to the oxidized substrate, making a high-energy product. Phosphoglycerokinase 7 2 O– C CHOH 2 Pi 2 O 2 2 ADP 2 H 2O O C CHOH CH2 O P 3-Phosphoglycerate The phosphate group is transferred to ADP (substrate-level phosphorylation) in an exergonic reaction. The carbonyl group of G3P has been oxidized to the carboxyl group (—COO–) of an organic acid (3-phosphoglycerate). H Phosphoglyceromutase 8 2 O– P CO CH2OH 2-Phosphoglycerate This enzyme relocates the remaining phosphate group. Enolase 9 2 O– C O ATP O P CO Pyruvate kinase CH2 Phosphoenolpyruvate (PEP) Enolase causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP), a compound with a very high potential energy. 10 O– C O C O CH3 Pyruvate The phosphate group is transferred from PEP to ADP (a second example of substrate-level phosphorylation), forming pyruvate. Mastering Biology BioFlix® Animation: Glycolysis CHAPTER 9 Cellular Respiration and Fermentation 171 ( ¬ COO-), already somewhat oxidized and thus carrying little chemical energy, is now fully oxidized and given off as a molecule of CO2. This is the first step in which CO2 is released during respiration. 2 Next, the remaining two-carbon fragment is oxidized and the electrons transferred to NAD +, storing energy in the form of NADH. 3 Finally, coenzyme A (CoA), a sulfur-containing compound derived from a B vitamin, is attached via its sulfur atom to the two-carbon intermediate, forming acetyl CoA. Acetyl CoA has a high potential energy, which is used to transfer the acetyl group to a molecule in the citric acid cycle, a reaction that is therefore highly exergonic. The Citric Acid Cycle The citric acid cycle functions as a metabolic furnace that further oxidizes organic fuel derived from pyruvate. Figure 9.10 summarizes the inputs and outputs as . Figure 9.10 An overview of pyruvate oxidation and the citric acid cycle. The inputs and outputs per pyruvate molecule are shown with a focus on the carbon atoms involved. To calculate on a per-glucose basis, multiply by 2 because each glucose molecule is split during glycolysis into two pyruvate molecules. CYTOSOL Pyruvate (from glycolysis, 2 molecules per glucose) C C PYRUVATE OXIDATION GLYCOLYSIS C OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE ATP C C C PYRUVATE OXIDATION C CO2 NAD+ NADH + H+ CoA Acetyl CoA C CoA C NADH + H+ CoA NAD + CITRIC ACID CYCLE 2 CO2 2 NADH FAD ADP + P i + 2 H+ ATP Mastering Biology Animation: The Citric Acid Cycle 172 UNIT TWO C 2 NAD+ FADH2 MITOCHONDRION C The Cell pyruvate is broken down to three CO2 molecules, including the molecule of CO2 released during the conversion of pyruvate to acetyl CoA. The cycle generates 1 ATP per turn by substrate-level phosphorylation, but most of the chemical energy is transferred to NAD + and FAD during the redox reactions. The reduced coenzymes, NADH and FADH2, shuttle their cargo of high-energy electrons into the electron transport chain. The citric acid cycle is also called the tricarboxylic acid cycle or the Krebs cycle, the latter honoring Hans Krebs. Krebs was the German-British scientist largely responsible for working out the pathway in the 1930s. Now let’s look at the citric acid cycle in more detail. The cycle has eight steps, each catalyzed by a specific enzyme. You can see in Figure 9.11 that for each turn of the citric acid cycle, two carbons (red) enter in the relatively reduced form of an acetyl group (step 1 ), and two different carbons (blue) leave in the completely oxidized form of CO2 molecules (steps 3 and 4 ). The acetyl group of acetyl CoA joins the cycle by combining with the compound oxaloacetate, forming citrate (step 1 ). Citrate is the ionized form of citric acid, for which the cycle is named. The next seven steps decompose the citrate back to oxaloacetate. It is this regeneration of oxaloacetate that makes the process a cycle. Referring to Figure 9.11, we can tally the energy-rich molecules produced by the citric acid cycle. For each acetyl group entering the cycle, 3 NAD + are reduced to NADH (steps 3 , 4 , and 8 ). In step 6 , electrons are transferred not to NAD + , but to FAD, which accepts 2 electrons and 2 protons to become FADH2. In many animal tissue cells, the reaction in step 5 produces a guanosine triphosphate (GTP) molecule by substrate-level phosphorylation. GTP is a molecule similar to ATP in its structure and cellular function. This GTP may be used to make an ATP molecule (as shown) or directly power work in the cell. In the cells of plants, bacteria, and some animal tissues, step 5 forms an ATP molecule directly by substrate-level phosphorylation. The output from step 5 represents the only ATP generated during the citric acid cycle. Recall that each glucose gives rise to two molecules of acetyl CoA that enter the cycle. Because the numbers noted earlier are obtained from a single acetyl group entering the pathway, the total yield per glucose from the citric acid cycle turns out to be doubled, or 6 NADH, 2 FADH2, and the equivalent of 2 ATP. Most of the ATP produced by respiration is generated later, from oxidative phosphorylation, when the NADH and FADH2 produced by the citric acid cycle and earlier steps relay the electrons extracted from food to the electron transport chain. In the process, they supply the necessary energy for the phosphorylation of ADP to ATP. We will explore this process in the next section. each other.) Notice that the carbon atoms that enter the cycle from acetyl CoA do not leave the cycle in the same turn. They remain in the cycle, occupying a different location in the molecules on their next turn, after another acetyl group is added. Therefore, the oxaloacetate regenerated at step 8 is made up of different carbon atoms each time around. Carboxylic acids are represented in . Figure 9.11 A closer look at the citric acid cycle. In the chemical structures, red type traces the fate of the two carbon atoms that enter the cycle via acetyl CoA (step 1 ), and blue type indicates the two carbons that exit the cycle as CO2 in steps 3 and 4 . (The red type goes only through step 5 because the succinate molecule is symmetrical; the two ends cannot be distinguished from GLYCOLYSIS PYRUVATE OXIDATION their ionized forms, as ¬ COO–, because the ionized forms prevail at the pH within the mitochondrion. In eukaryotic cells, all the citric acid cycle enzymes are located in the mitochondrial matrix except for the enzyme that catalyzes step 6 , which resides in the inner mitochondrial membrane. (The enzyme that catalyzes step 3 , isocitrate dehydrogenase, is shown in Figure 6.32b.) OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE ATP 1 Acetyl CoA (from oxidation of pyruvate) adds its two-carbon acetyl group to oxaloacetate, producing citrate. S-CoA C O CH3 Acetyl CoA 2 Citrate is converted to its isomer, isocitrate, by removal of one water molecule and addition of another. 8 The substrate is oxidized, reducing NAD+ to NADH and regenerating oxaloacetate. CoA-SH NADH + H+ O COO 8 COO CH H2O COO– – CH2 Oxaloacetate HO COO– COO– C CH2 – COO Malate CH2 CH2 2 H2O – HO COO– CITRIC ACID CYCLE 7 NADH + H+ 3 CO2 COO– CH COO– Fumarate CoA-SH 6 CoA-SH COO– CH2 FADH 2 FAD C 4 CH2 C Succinate Pi GTP GDP NAD + CH2 COO– O COO– COO– 5 CH2 CO2 O S-CoA Succinyl CoA NADH + H+ ADP ATP Mastering Biology BioFlix® Animation: The Citric Acid Cycle 3 Isocitrate is oxidized, reducing NAD+ to NADH. Then the resulting compound loses a CO2 molecule. c-Ketoglutarate CH2 CH2 COO– hydrogens are transferred to FAD, forming FADH2 and oxidizing succinate. CH Citrate Isocitrate NAD + HC 6 Two COO– HC COO– 7 Addition of a water molecule rearranges bonds in the substrate. 1 CH2 NAD + HO COO– C 5 CoA is displaced by a phosphate group, which is transferred to GDP, forming GTP, a molecule with functions similar to ATP. GTP can also be used, as shown, to generate ATP. CHAPTER 9 4 Another CO2 is lost, and the resulting compound is oxidized, reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an unstable bond. Cellular Respiration and Fermentation 173 CONCEPT CHECK 9.3 1. VISUAL SKILLS In the citric acid cycle shown in Figure 9.11, what molecules capture energy from the redox reactions? How is ATP produced? 2. What processes in your cells produce the CO2 that you exhale? 3. VISUAL SKILLS The conversions shown in Figure 9.9 and step 4 of Figure 9.11 are each catalyzed by a large multienzyme complex. What similarities are there in the reactions that occur in these two cases? . Figure 9.12 Free-energy change during electron transport. The overall energy drop (∆G) for electrons traveling from NADH to oxygen is 53 kcal/mol, but this “fall” is broken up into a series of smaller steps by the electron transport chain. (An oxygen atom is represented here as 1⁄2 O2 to show that O2 is reduced, not individual oxygen atoms.) To view these proteins in their cellular context, see Figure 6.32b. Mastering Biology Figure Walkthrough GLYCOLYSIS For suggested answers, see Appendix A. CONCEPT 9.4 ATP During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis The Pathway of Electron Transport The electron transport chain is a collection of molecules embedded in the inner membrane of the mitochondrion in eukaryotic cells. (In prokaryotes, these molecules reside in the plasma membrane.) The folding of the inner membrane to form cristae increases its surface area, providing space for thousands of copies of each component of the electron transport chain in a mitochondrion. Structure fits function: The infolded membrane with its concentration of electron carrier molecules is well-suited for the series of sequential redox reactions that take place along the electron transport chain. Most components of the chain are proteins, which exist in multiprotein complexes numbered I through IV. Tightly bound to these proteins are prosthetic groups, nonprotein components such as cofactors and coenzymes essential for the catalytic functions of certain enzymes. Figure 9.12 shows the sequence of electron carriers in the electron transport chain and the drop in free energy as electrons travel down the chain. During this electron transport, electron carriers alternate between reduced and oxidized states as they accept and then donate electrons. Each component UNIT TWO The Cell NADH 50 2 e– NADH has the lowest affinity for electrons. NAD+ FADH2 40 2 e– FAD Fe•S II I FMN Fe•S Free energy (G) relative to O2 (kcal/mol) Our main objective in this chapter is to learn how cells harvest the energy of glucose and other nutrients in food to make ATP. But the metabolic components of respiration we have examined so far, glycolysis and the citric acid cycle, produce only 4 ATP molecules per glucose molecule, all by substratelevel phosphorylation: 2 net ATP from glycolysis and 2 ATP from the citric acid cycle. At this point, molecules of NADH (and FADH2) account for most of the energy extracted from each glucose molecule. These electron escorts link glycolysis and the citric acid cycle to the machinery of oxidative phosphorylation, which uses energy released by the electron transport chain to power ATP synthesis. In this section, you will learn first how the electron transport chain works and then how electron flow down the chain is coupled to ATP synthesis. 174 OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE PYRUVATE OXIDATION Q III Cyt b 30 Complexes I-IV each consist of multiple proteins with electron carriers. Fe•S Cyt c1 IV Cyt c 20 10 0 Electron transport chain Electrons (from NADH or FADH2) move from an electron carrier with a lower affinity for electrons to an electron carrier down the chain with a greater affinity for electrons, releasing free energy. Cyt a Cyt a3 2 e– The last electron carrier (Cyt a3) passes its electrons to an O in O2, which is very electronegative. 2 H+ + 1 2 O2 O2 has the highest affinity for electrons. VISUAL SKILLS Compare the position of the electrons in NADH (see Figure 9.3) at the top of the chain with that in H2O, at the bottom. Describe why the electrons in H2O have less potential energy, using the term electronegativity. H2O of the chain becomes reduced when it accepts electrons from its “uphill” neighbor, which has a lower affinity for electrons. It then returns to its oxidized form as it passes electrons to its “downhill” neighbor, which has a higher affinity for electrons. Now let’s take a closer look at the electrons as they drop in energy level, passing through the components of the electron transport chain in Figure 9.12. We’ll first look at the passage of electrons through complex I in some detail as an illustration of the general principles involved in electron transport. Electrons acquired from glucose by NAD + during glycolysis and the citric acid cycle are transferred from NADH to the first molecule of the electron transport chain in complex I. This molecule is a flavoprotein, so named because it has a prosthetic group called flavin mononucleotide (FMN). In the next redox reaction, the flavoprotein returns to its oxidized form as it passes electrons to an iron-sulfur protein (Fe # S in complex I), one of a family of proteins with both iron and sulfur tightly bound. The ironsulfur protein then passes the electrons to a compound called ubiquinone (Q in Figure 9.12). This electron carrier is a small hydrophobic molecule, the only member of the electron transport chain that is not a protein. Ubiquinone is individually mobile within the membrane rather than residing in a particular complex. (Another name for ubiquinone is coenzyme Q, or CoQ; you may have seen it sold as a nutritional supplement.) Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochromes. Their prosthetic group, called a heme group, has an iron atom that accepts and donates electrons. (The heme group in a cytochrome is similar to the heme group in hemoglobin, the protein of red blood cells, except that the iron in hemoglobin carries oxygen, not electrons.) The electron transport chain has several types of cytochromes, each named “cyt” with a letter and number to distinguish it as a different protein with a slightly different electron-carrying heme group. The last cytochrome of the chain, cyt a3, passes its electrons to oxygen (in O2), which is very electronegative. Each O also picks up a pair of hydrogen ions (protons) from the aqueous solution, neutralizing the -2 charge of the added electrons and forming water. Another source of electrons for the electron transport chain is FADH2, the other reduced product of the citric acid cycle. Notice in Figure 9.12 that FADH2 adds its electrons from within complex II, at a lower energy level than NADH does. Consequently, although NADH and FADH2 each donate an equivalent number of electrons (2) for oxygen reduction, the electron transport chain provides about one-third less energy for ATP synthesis when the electron donor is FADH2 rather than NADH. We’ll see why in the next section. The electron transport chain makes no ATP directly. Instead, it eases the fall of electrons from food to oxygen, breaking a large free-energy drop into a series of smaller steps that release energy in manageable amounts, step by step. How does the mitochondrion (or the plasma membrane in prokaryotes) couple this electron transport and energy release to ATP synthesis? The answer is a mechanism called chemiosmosis. Chemiosmosis: The Energy-Coupling Mechanism Populating the inner membrane of the mitochondrion or the prokaryotic plasma membrane are many copies of a protein complex called ATP synthase, the enzyme that makes ATP from ADP and inorganic phosphate (Figure 9.13). ATP synthase works like an ion pump running in reverse. Ion pumps usually use ATP as an energy source to transport ions against their gradients. Enzymes can catalyze a reaction in either direction, depending on the ΔG for the reaction, which is affected by the local concentrations of reactants and products (see Concepts 8.2 and 8.3). Under the conditions of cellular respiration, rather than hydrolyzing ATP to pump protons against their concentration gradient, ATP synthase uses the energy of an existing ion gradient to power ATP synthesis. The power source for ATP synthase is a difference in the concentration of H + (a pH difference) on opposite sides of the inner mitochondrial membrane. This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is . Figure 9.13 ATP synthase, a molecular mill. Multiple ATP synthases reside in eukaryotic mitochondrial and chloroplast membranes and in prokaryotic plasma membranes. (See Figure 6.32b and c.) Inner mitochondrial membrane Intermembrane space Mitochondrial matrix 1 H+ ions flowing INTERMEMBRANE SPACE H+ ions down their gradient enter a channel in a stator, which is anchored in the membrane. Stator Rotor 2 H+ ions enter binding sites within a rotor, changing the shape of each subunit so that the rotor spins within the membrane. 3 Each H+ ion makes one complete turn before leaving the rotor and passing through a second channel in the stator into the mitochondrial matrix. Internal rod 4 Spinning of the rotor causes an internal rod to spin as well. This rod extends like a stalk into the knob below it, which is held stationary by part of the stator. Catalytic knob ADP + Pi ATP MITOCHONDRIAL MATRIX 5 Turning of the rod activates catalytic sites in the knob that produce ATP from ADP and P i . Mastering Biology BioFlix® Animation: ATP Synthase Animation: Rotating ATP Synthase CHAPTER 9 Cellular Respiration and Fermentation 175 called chemiosmosis (from the Greek osmos, push). The word osmosis was previously used in discussing water transport, but here it refers to the flow of H + across a membrane. From studying the structure of ATP synthase, scientists have learned how the flow of H + through this enzyme powers ATP generation. ATP synthase is a multisubunit complex with four main parts, each made up of multiple polypeptides (see Figure 9.13). Protons move one by one into binding sites on the rotor, causing it to spin in a way that catalyzes ATP production P i. The flow of protons thus behaves somewhat from ADP and ~ like a rushing stream that turns a waterwheel. ATP synthase is the smallest molecular rotary motor known in nature. . Figure 9.14 Chemiosmosis couples the electron transport chain to ATP synthesis. 1 NADH and FADH2 shuttle high-energy electrons extracted from food during glycolysis and the citric acid cycle into an electron transport chain built into the inner mitochondrial membrane. (See Figure 6.32b.) The gold arrows trace the transport of electrons, which are finally passed to a terminal acceptor (O2, in the case of aerobic respiration) at the “downhill” end of the How does the inner mitochondrial membrane (or the prokaryotic plasma membrane) generate and maintain the H + gradient that drives ATP synthesis by the ATP synthase protein complex? Establishing the H + gradient is a major function of the electron transport chain, which is shown in its mitochondrial location in Figure 9.14. The chain is an energy converter that uses the exergonic flow of electrons from NADH and FADH2 to pump H + across the membrane, from the mitochondrial matrix into the intermembrane space. The H + has a tendency to move back across the membrane, diffusing down its gradient. And the ATP synthases are the only sites that provide a route through the membrane for H +. As we described previously, the passage chain, forming water. Most of the electron carriers of the chain are grouped into four complexes (I–IV). Two mobile carriers, ubiquinone (Q) and cytochrome c (Cyt c), move rapidly, ferrying electrons between the large complexes. As the complexes shuttle electrons, they pump protons from the mitochondrial matrix into the intermembrane space. FADH2 deposits its electrons via complex II and so results in fewer protons being pumped into the intermembrane space than occurs with NADH. Chemical energy originally harvested from food is transformed into a proton-motive force, a gradient of H+ across the membrane. 2 During chemiosmosis, the protons flow back down their gradient via ATP synthase, which is built into the membrane nearby. The ATP synthase harnesses the proton-motive force to phosphorylate ADP, forming ATP. Together, electron transport and chemiosmosis make up oxidative phosphorylation. Intermembrane space Inner mitochondrial membrane Mitochondrial matrix GLYCOLYSIS PYRUVATE OXIDATION CITRIC ACID CYCLE OXIDATIVE PHOSPHORYLATION ATP H+ Intermembrane space H+ H+ Protein complex of electron carriers H+ H+ H+ Cyt c III I II FADH2 FAD 2 H+ + 1 2 O2 NAD+ H2O ADP + P i (carrying electrons from food) Mitochondrial matrix H+ ATP synthase IV Q NADH H+ H+ H+ H+ H+ H+ Inner mitochondrial membrane H+ ATP H+ 1 Electron transport chain Electron transport and pumping of protons (H+), which create an H+ gradient across the membrane 2 Chemiosmosis ATP synthesis powered by the flow of H+ back across the membrane Oxidative phosphorylation WHAT IF? If complex IV were nonfunctional, could chemiosmosis produce any ATP, and if so, how would the rate of synthesis differ? 176 UNIT TWO The Cell Mastering Biology Animation: Electron Transport BioFlix® Animation: Electron Transport of H + through ATP synthase uses the exergonic flow of H + to drive the phosphorylation of ADP. Thus, the energy stored in an H + gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis. At this point, you may be wondering how the electron transport chain pumps hydrogen ions into the intermembrane space. Researchers have found that certain members of the electron transport chain accept and release protons (H +) along with electrons. (The aqueous solutions inside and surrounding the cell are a ready source of H +.) At certain steps along the chain, electron transfers cause H + to be taken up and released into the surrounding solution. In eukaryotic cells, the electron carriers are spatially arranged in the inner mitochondrial membrane in such a way that H + is accepted from the mitochondrial matrix and deposited in the intermembrane space (see Figure 9.14). The H + gradient that results is referred to as a proton-motive force, emphasizing the capacity of the gradient to perform work. The force drives H + back across the membrane through the H + channels provided by ATP synthases. In general terms, chemiosmosis is an energy-coupling mechanism that uses energy stored in the form of an H + gradient across a membrane to drive cellular work. In mitochondria, the energy for gradient formation comes from exergonic redox reactions along the electron transport chain, and ATP synthesis is the work performed. But chemiosmosis also occurs elsewhere and in other variations. Chloroplasts use chemiosmosis to generate ATP during photosynthesis; in these organelles, light (rather than chemical energy) drives both electron flow down an electron transport chain and the resulting H + gradient formation. Prokaryotes, as already mentioned, generate H + gradients across their plasma membranes. They then tap the proton-motive force not only to make ATP inside the cell but also to rotate their flagella and to pump nutrients and waste products across the membrane. Because of its central importance to energy conversions in prokaryotes and eukaryotes, chemiosmosis has helped unify the study of bioenergetics. Peter Mitchell was awarded the Nobel Prize in 1978 for originally proposing the chemiosmotic model. An Accounting of ATP Production by Cellular Respiration In the last few sections, we have looked rather closely at the key processes of cellular respiration. Now let’s take a step back and remind ourselves of its overall function: harvesting the energy of glucose for ATP synthesis. During respiration, most energy flows in this sequence: glucose S NADH S electron transport chain S proton-motive force S ATP. We can do some bookkeeping to calculate the ATP profit when cellular respiration oxidizes a molecule of glucose to six molecules of carbon dioxide. The three main departments of this metabolic enterprise are glycolysis, pyruvate oxidation and the citric acid cycle, and the electron transport chain, which drives oxidative phosphorylation. Figure 9.15 gives a detailed accounting of the ATP yield for . Figure 9.15 ATP yield per molecule of glucose at each stage of cellular respiration. CYTOSOL Electron shuttles span membrane 2 NADH GLYCOLYSIS Glucose 2 Pyruvate MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 6 NADH PYRUVATE OXIDATION OXIDATIVE PHOSPHORYLATION CITRIC ACID CYCLE 2 Acetyl CoA (Electron transport and chemiosmosis) + 2 ATP + 2 ATP by substrate-level phosphorylation by substrate-level phosphorylation Maximum per glucose: + about 26 or 28 ATP by oxidative phosphorylation, depending on which shuttle transports electrons from NADH in cytosol About 30 or 32 ATP VISUAL SKILLS After reading the discussion in the text, explain exactly how the total of 26 or 28 ATP from oxidative phosphorylation was calculated (see the yellow bar). 2 FADH2 Mastering Biology Animation: ATP Yield from Cellular Respiration CHAPTER 9 Cellular Respiration and Fermentation 177 each glucose molecule that is oxidized. The tally adds the 4 ATP produced directly by substrate-level phosphorylation during glycolysis and the citric acid cycle to the many more molecules of ATP generated by oxidative phosphorylation. Each NADH that transfers a pair of electrons from glucose to the electron transport chain contributes enough to the protonmotive force to generate a maximum of about 3 ATP. Why are the numbers in Figure 9.15 inexact? There are three reasons we cannot state an exact number of ATP molecules generated by the breakdown of one molecule of glucose. First, phosphorylation and the redox reactions are not directly coupled to each other, so the ratio of the number of NADH molecules to the number of ATP molecules is not a whole number. We know that 1 NADH results in 10 H + being transported out across the inner mitochondrial membrane, but the exact number of H + that must reenter the mitochondrial matrix via ATP synthase to generate 1 ATP has long been debated. Based on experimental data, however, most biochemists now agree that the most accurate number is 4 H +. Therefore, a single molecule of NADH generates enough proton-motive force for the synthesis of 2.5 ATP. The citric acid cycle also supplies electrons to the electron transport chain via FADH2, but since its electrons enter later in the chain, each molecule of this electron carrier is responsible for transport of only enough H + for the synthesis of 1.5 ATP. These numbers also t