Chapter 7 Summary

Core Concepts Summary

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7.1 Cellular respiration is a series of catabolic reactions that convert the energy in fuel molecules into ATP.

During cellular respiration, sugar molecules like glucose are broken down in the presence of oxygen to produce carbon dioxide and water. page 132

Cellular respiration releases energy because the potential energy of the reactants is greater than that of the products. page 132

ATP is generated in two ways during cellular respiration: substrate-level phosphorylation and oxidative phosphorylation. page 133

Cellular respiration is an oxidation–reduction reaction. page 133

In oxidation–reduction reactions, electrons are transferred from one molecule to another. Oxidation is the loss of electrons, and reduction is the gain of electrons. page 133

Electron carriers transfer electrons to an electron transport chain, which harnesses the energy of these electrons to generate ATP. page 133

Cellular respiration is a four-stage process that includes (1) glycolysis; (2) pyruvate oxidation; (3) the citric acid cycle; and (4) oxidative phosphorylation. page 135

7.2 Glycolysis is the partial oxidation of glucose and results in the production of pyruvate, as well as ATP and reduced electron carriers.

Glycolysis takes place in the cytoplasm. page 135

Glycolysis is a series of 10 reactions in which glucose is oxidized to pyruvate. page 137

Glycolysis consists of preparatory, cleavage, and payoff phases. page 137

For each molecule of glucose broken down during glycolysis, a net gain of two molecules of ATP and two molecules of NADH is produced. page 137

The synthesis of ATP in glycolysis results from the direct transfer of a phosphate group from a substrate to ADP, a process called substrate-level phosphorylation. page 137

7.3 Pyruvate is oxidized to acetyl-CoA, connecting glycolysis to the citric acid cycle.

The conversion of pyruvate to acetyl-CoA results in the production of one molecule of NADH and one molecule of carbon dioxide. page 137

Pyruvate oxidation occurs in the mitochondrial matrix. page 137

7.4 The citric acid cycle results in the complete oxidation of fuel molecules and the generation of ATP and reduced electron carriers.

The citric acid cycle takes place in the mitochondrial matrix. page 138

The acetyl group of acetyl-CoA is completely oxidized in the citric acid cycle. page 138

The citric acid cycle is a cycle because the acetyl group of acetyl-CoA combines with oxaloacetate, and then a series of reactions regenerates oxaloacetate. page 138

A complete turn of the citric acid cycle results in the production of one molecule of GTP (which is converted to ATP), three molecules of NADH, and one molecule of FADH2. page 138

Citric acid cycle intermediates are starting points for the synthesis of many different organic molecules. page 140

7.5 The electron transport chain transfers electrons from electron carriers to oxygen, using the energy to pump protons and synthesize ATP by oxidative phosphorylation.

NADH and FADH2 donate electrons to the electron transport chain. page 140

In the electron transport chain, electrons move from one redox couple to the next. page 140

The electron transport chain is made up of four complexes. Complexes I and II accept electrons from NADH and FADH2, respectively. The electrons are transferred from these two complexes to coenzyme Q. page 140

Reduced coenzyme Q transfers electrons to complex III and cytochrome c transfers electrons to complex IV. Complex IV reduces oxygen to water. page 142

The transfer of electrons through the electron transport chain is coupled with the movement of protons across the inner mitochondrial membrane into the intermembrane space. page 142

The buildup of protons in the intermembrane space results in a proton electrochemical gradient, which stores potential energy. page 142

The movement of protons back into the mitochondrial matrix through the Fo subunit of ATP synthase is coupled with the formation of ATP, a reaction catalyzed by the F1 subunit of ATP synthase. page 142

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7.6 Glucose can be broken down in the absence of oxygen by fermentation, producing a modest amount of ATP.

Pyruvate, the end product of glycolysis, is processed differently in the presence and the absence of oxygen. page 145

In the absence of oxygen, pyruvate enters one of several fermentation pathways. page 145

In lactic acid fermentation, pyruvate is reduced to lactic acid. page 145

In ethanol fermentation, pyruvate is converted to acetaldehyde, which is reduced to ethanol. page 145

During fermentation, NADH is oxidized to NAD+, allowing glycolysis to proceed. page 145

Glycolysis and fermentation are ancient biochemical pathways and were likely used in the common ancestor of all organisms living today. page 146

7.7 Metabolic pathways are integrated, allowing control of the energy level of cells.

Excess glucose molecules are linked together and stored in polymers called glycogen (in animals) and starch (in plants). page 147

Other monosaccharides derived from the digestion of dietary carbohydrates are converted into intermediates of glycolysis. page 147

Fatty acids contained in triacylglycerols are an important form of energy storage in cells. The breakdown of fatty acids is called β-oxidation. page 148

Phosphofructokinase-1 controls a key step in glycolysis. It has many allosteric activators, including ADP and AMP, and allosteric inhibitors, including ATP and citrate. page 149

The ATP in muscle cells used to power exercise is generated by lactic acid fermentation, aerobic respiration, and β-oxidation. page 150

Self-Assessment

  1. Name and describe the four major stages of cellular respiration.

    Self-Assessment 1 Answer

    Cellular respiration is a series of chemical reactions that convert the energy stored in fuel molecules into a chemical form that can readily be used by cells. Cellular respiration occurs in four stages: (1) Glycolysis: Glucose is partially broken down and a modest amount of energy (in the form of ATP and reduced electron carriers) is released. (2) Pyruvate oxidation: Pyruvate (the breakdown product of glucose from stage 1) is converted to acetyl-coenzyme A, and carbon dioxide and electron carriers are produced. (3) Citric acid cycle: Acetyl-CoA is broken down and carbon dioxide, ATP, and reduced electron carriers are produced. (4) Oxidative phosphorylation: In these reactions, electron carriers generated in stages 1–3 donate their electrons to an electron-transport chain. This chain transfers electrons along a series of membrane-associated proteins to a final electron acceptor and in the process harnesses the energy of the electrons to produce a large amount of ATP. In aerobic respiration, oxygen is the final electron acceptor, so it is consumed and water is produced.

  2. Explain what an oxidation–reduction reaction is and why the breakdown of glucose in the presence of oxygen to produce carbon dioxide and water is an example of an oxidation–reduction reaction.

    Self-Assessment 2 Answer

    Oxidation‒reduction reactions are used to store or release chemical energy. Oxidation is the loss of electrons and reduction is the gain of electrons. This gain and loss always happens in a single reaction in which electrons are transferred from one molecule to another. In many reactions, electrons are not completely transferred between molecules. Instead, there is a change in electron density around an atom. This happens in the breakdown of glucose in the presence of oxygen to produce carbon dioxide and water. The carbon atoms in glucose are oxidized because they go from sharing electrons equally in the carbon‒carbon bonds to partially losing electrons in the carbon‒oxygen bonds of the carbon dioxide molecule. The opposite is true for oxygen, which is reduced in the same reaction. The oxygen atoms go from sharing electrons equally to partially gaining electrons when water is formed.

  3. Describe two different ways in which ATP is generated in cellular respiration.

    Self-Assessment 3 Answer

    ATP is generated by substrate-level phosphorylation and oxidative phosphorylation. In substrate-level phosphorylation, a phosphorylated organic molecule directly transfers a phosphate group to ADP. This pathway produces only a small amount of the total ATP generated in the process of cellular respiration. In contrast, most of the ATP generated in cellular respiration is produced through oxidative phosphorylation (stage 4 of cellular respiration). In these reactions, ATP is generated indirectly through the reduction of electron carriers, the transfer of electrons from electron carriers to the electron-transport chain, and the subsequent synthesis of ATP from ADP and inorganic phosphate.

  4. Write the overall chemical equation for glycolysis, noting the starting and ending products and highlighting the energy-storing molecules that are produced.

    Self-Assessment 4 Answer

    The starting product is glucose and the end product is pyruvate. Energy-storing molecules that are produced are ATP and NADH.

    Glucose + 2NAD+ + 2ADP + 2Pi → 2 pyruvate + 2ATP + 2NADH + 2H+ + 2H2O

  5. Describe two different metabolic pathways that pyruvate can enter.

    Self-Assessment 5 Answer

    In the first pathway, pyruvate is converted to acetyl-CoA, which is the starting substrate for the citric acid cycle. During the citric acid cycle, the chemical energy in the bonds of acetyl-CoA is transferred to ATP by substrate-level phosphorylation and to the electron carriers NADH and FADH2. The second pathway is fermentation, a reaction that happens without oxygen. There are many fermentation pathways but all rely on oxidation of NADH to NAD+ when pyruvate or a derivative of pyruvate is reduced. Two major fermentation pathways are lactic acid fermentation and ethanol fermentation. In the lactic acid pathway, electrons from NADH are transferred to pyruvate to produce lactic acid and NAD+. In the ethanol fermentation pathway, pyruvate releases carbon dioxide to form acetaldehyde, and electrons from NADH are transferred to the molecule to produce ethanol and NAD+.

  6. Name the products of the citric acid cycle.

    Self-Assessment 6 Answer

    In two turns of the citric acid cycle (one for each acetyl-CoA), 2 ATP, 6 NADH, and 2 FADH2 are produced. Carbon dioxide and oxaloacetate are also produced.

  7. Describe how the movement of electrons along the electron transport chain leads to the generation of a proton gradient.

    Self-Assessment 7 Answer

    The movement of electrons along the electron-transport chain in the inner mitochondrial membrane is coupled to the transfer of protons through several enzyme complexes and electron carriers. Electrons donated by NADH enter through complex I, and electrons donated by FADH2 enter through complex II. From complexes I and II, coenzyme Q (CoQ) picks up electrons and transfers them to complex III. Complex III donates electrons to cytochrome c, which in turn transfers them to complex IV, which then donates them to the final electron acceptor, oxygen. As the electrons pass through the complexes, protons are pumped into the intermembrane space. This creates a concentration and charge gradient, providing a source of potential energy that is then used to drive the synthesis of ATP. See Fig. 7.10.

  8. Describe how a proton gradient is used to generate ATP.

    Self-Assessment 8 Answer

    The protons accumulated in the intermembrane space cannot passively diffuse across the membrane, so they diffuse through a transport channel called ATP synthase. This enzyme is composed of two subunits: Fo (the channel through which protons flow) and F1 (the catalytic unit that synthesizes ATP). Proton flow through the channel causes it to rotate, which converts the energy of the proton gradient into mechanical rotational energy (kinetic energy). The rotation of the Fo subunit leads to rotation of the F1 subunit. Rotation causes conformational changes in the F1 subunit that allow it to catalyze the synthesis of ATP from ADP and Pi.

  9. Explain how muscle tissue generates ATP during short-term and long-term exercise.

    Self-Assessment 9 Answer

    Muscle tissue generates ATP during short-term exercise by converting stored glycogen to glucose. Glucose is rapidly broken down anaerobically to pyruvate, which then feeds into the lactic acid fermentation pathway. During long-term exercise, the liver releases glucose into the blood, which is taken up by muscle cells and oxidized to produce ATP. In addition, adipose tissue releases fatty acids that are also taken up by muscle cells and broken down by β-oxidation. These processes are slower to convert glucose and other molecules to energy; however, the end result is the production of more ATP than the fermentation pathway can produce.