Chapter Summary

• As a material is oxidized, the electrons it loses are transferred to another material, which is thereby reduced. Such oxidation–reduction, or redox, reactions transfer large amounts of energy. Review Figure 9.2

• The coenzyme NAD⁺ is a key electron carrier in biological redox reactions. It exists in two forms, one oxidized (NAD⁺) and the other reduced (NADH). Review Focus: Key Figure 9.3

• Glycolysis does not use O₂. Under aerobic conditions, cellular respiration continues the process of breaking down glucose. Under anaerobic conditions, fermentation occurs. Review Figures 9.1, 9.4, Activities 9.1, 9.2

• The pathways of cellular respiration after glycolysis are pyruvate oxidation, the citric acid cycle, and electron transport/ATP synthesis.

• Glycolysis consists of ten enzyme-catalyzed reactions that occur in the cell cytoplasm. Two pyruvate molecules are produced for each partially oxidized molecule of glucose, providing the starting material for both cellular respiration and fermentation. Review Figure 9.5

  191

• Pyruvate oxidation follows glycolysis and links glycolysis to the citric acid cycle. This pathway converts pyruvate into acetyl CoA.

• Acetyl CoA is the starting point of the citric acid cycle. It reacts with oxaloacetate to produce citrate. A series of eight enzyme-catalyzed reactions oxidize citrate and regenerate oxaloacetate, continuing the cycle. Review Figure 9.6, Activity 9.3

• Oxidation of electron carriers in the presence of O₂ releases energy that can be used to form ATP in a process called oxidative phosphorylation.

• The NADH and FADH₂ produced in glycolysis, pyruvate oxidation, and the citric acid cycle are oxidized by the respiratory chain, regenerating NAD⁺ and FAD. Oxygen (O₂) is the final acceptor of electrons and protons, forming water (H₂O). Review Figure 9.7, Activity 9.4

• The respiratory chain not only transports electrons, but also transfers protons across the inner mitochondrial membrane, creating the proton-motive force.

• Protons driven by the proton-motive force can return to the mitochondrial matrix via ATP synthase, a molecular motor that couples this movement of protons to the synthesis of ATP. This process is called chemiosmosis. Review Figure 9.8, Activity 9.5

• There is considerable experimental evidence for chemiosmosis. Review Figures 9.9, 9.10, Animations 9.1, 9.2

• In the absence of O₂ in most organisms, glycolysis is followed by fermentation. Together, these pathways partially oxidize pyruvate and generate end products such as lactic acid or ethanol. In the process, NAD⁺ is regenerated from NADH so that glycolysis can continue, thus generating a small amount of ATP. Review Figure 9.11

• For each molecule of glucose used, glycolysis plus fermentation yields two molecules of ATP. In contrast, glycolysis operating with pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation yields up to 32 molecules of ATP per molecule of glucose. Review Figure 9.12, Activity 9.6

• The catabolic pathways for the breakdown of carbohydrates, fats, and proteins feed into the energy-harvesting metabolic pathways. Review Figure 9.13

• Anabolic pathways use intermediate components of the energy-harvesting pathways to synthesize fats, amino acids, and other essential building blocks.

• The formation of glucose from intermediates of glycolysis and the citric acid cycle is called gluconeogenesis.

• The rates of glycolysis and the citric acid cycle are controlled by allosteric regulation and by the diversion of excess acetyl CoA into fatty acid synthesis. Key regulated enzymes include phosphofructokinase, citrate synthase, isocitrate dehydrogenase, and fatty acid synthase. See Figure 9.16, Activity 9.7