Chapter 6: Respiration

Glycolysis yields a modest net gain of two ATP molecules through substrate-level phosphorylation and two NADH electron carriers, leaving most of the energy still locked within pyruvate. Under aerobic conditions, pyruvate molecules cross into the mitochondrial matrix, where they undergo oxidative decarboxylation to form acetyl coenzyme A, which then enters the citric acid cycle, also called the Krebs cycle or tricarboxylic acid cycle. During each complete cycle turn, the acetyl group is fully oxidized to carbon dioxide while generating three NADH, one FADH₂, and one ATP per turn, with oxaloacetate continuously regenerated to accept new acetyl groups. The chapter emphasizes that the electron transport chain, a series of membrane-bound protein complexes including cytochromes, flavoproteins, iron-sulfur clusters, and coenzyme Q situated in the inner mitochondrial membrane, captures the high-energy electrons from NADH and FADH₂. As electrons cascade toward oxygen, the terminal electron acceptor, the energy released pumps protons across the membrane into the intermembrane space, establishing an electrochemical gradient that drives ATP synthase and produces approximately thirty-four of the thirty-six total ATP per glucose through chemiosmotic coupling, a mechanism elucidated by Peter Mitchell's groundbreaking theory. The chapter contrasts aerobic respiration with anaerobic fermentation, where pyruvate is converted to ethanol and carbon dioxide in plants and yeasts or to lactate in animals, regenerating NAD⁺ but supplying only two ATP. Additionally, the chapter demonstrates that respiration serves as the central metabolic hub by oxidizing alternative substrates including fatty acids through beta oxidation and deaminated amino acids, which feed into the cycle as acetyl CoA or direct citric acid cycle intermediates, linking catabolism to biosynthesis and illustrating the approximately thirty-eight percent efficiency of energy capture from glucose's free energy, with surplus energy released as heat.