Chapter 7: Cellular Respiration and Fermentation

Unlike the rapid, wasteful combustion of fuel, cellular respiration releases this energy gradually through controlled enzymatic steps, maximizing the capture of free energy in the form of ATP. The chapter systematically presents the three major stages of aerobic respiration: glycolysis occurs in the cytoplasm and cleaves glucose into two pyruvate molecules while generating a small net yield of ATP and NADH; the pyruvate oxidation and citric acid cycle in the mitochondrial matrix completely oxidize the carbon skeleton of glucose while producing additional ATP, NADH, and FADH2; and oxidative phosphorylation harnesses the reducing power of electron carriers to drive the synthesis of the vast majority of cellular ATP. The electron transport chain establishes a proton gradient across the inner mitochondrial membrane through stepwise electron transfer, and this electrochemical gradient—the proton-motive force—provides the energy for ATP synthase to phosphorylate ADP. The chapter clarifies why aerobic respiration generates approximately 32 ATP per glucose molecule, vastly exceeding the yield of anaerobic pathways. When oxygen is unavailable, cells employ alternative strategies: some microorganisms use anaerobic respiration with alternate electron acceptors, while others rely on fermentation to regenerate NAD+ and maintain glycolytic flux, producing either ethanol or lactate depending on the organism. The chapter concludes by integrating respiration into broader metabolism, showing how carbohydrates, lipids, and proteins feed into common catabolic pathways and how many of these same reactions provide building blocks for biosynthesis and cellular growth.