Summary

Mechanisms of ATP Production and  Their Implications

• Aerobic catabolism occurs in four steps: (1) glycolysis, (2) the Krebs cycle, (3) electron transport, and (4) oxidative phosphorylation. It can oxidize carbohydrates, lipids, or proteins to produce ATP. The maximum net yield of ATP in carbohydrate oxidation is currently estimated to be 29 ATP molecules per glucose molecule.
• Anaerobic glycolysis is a redox-balanced process by which ATP can be made without O₂ in certain tissues. Whether a tissue can employ anaerobic glycolysis depends on the amount and type of its lactate dehydrogenase (LDH). Anaerobic glycolysis typically can use only carbohydrate fuel. It releases only about 7% of the energy of glucose. Thus it produces just two ATP molecules per glucose molecule, and the lactic acid produced is itself an energy-rich compound.
• Vertebrates retain the lactic acid that they produce. The metabolism of lactic acid requires O₂, either to oxidize the lactic acid via the Krebs cycle—thereby producing ATP—or to convert lactic acid to glycogen or glucose (gluconeogenesis)—a process that uses ATP.
• Phosphagens in muscle cells serve as temporary stores of high-energy phosphate bonds, which they can transfer to ADP to make ATP anaerobically.

The Interplay of Aerobic and Anaerobic Catabolism during Exercise

• Behavior and biochemistry are linked during physical activity because attributes of performance depend on how the ATP for muscular effort is synthesized.
• Submaximal forms of exercise can be supported entirely (except during transition phases) by aerobic catabolism using O₂ taken in from the environment by breathing. From the viewpoint of ATP supply and demand, submaximal forms of exercise can thus be sustained indefinitely.
• Supramaximal forms of exercise in vertebrates, crustaceans, and some other animals require a continuing input of ATP from anaerobic glycolysis. The steady use of anaerobic glycolysis—manifested by a steady accumulation of lactic acid—eventually causes metabolic self-termination of the exercise.
• In vertebrates, metabolic transitions occur at the start and the end of even light submaximal exercise. An oxygen deficit occurs at the start, and excess postexercise oxygen consumption (EPOC) occurs at the end. The oxygen deficit is a consequence of the fact that the breathing and circulatory systems increase O₂ delivery gradually, not stepwise, at the start of exercise.
• As the duration of all-out exertion increases, ATP must increasingly be supplied by steady-state aerobic catabolism, rather than by nonsteady-state mechanisms that can produce ATP exceptionally rapidly but cannot produce a great deal of it. The pace of all-out exertion therefore declines as duration increases.
• Closely related species, and even individuals within one species, often differ greatly in their emphasis on aerobic and anaerobic mechanisms of producing ATP for exercise. These metabolic differences help explain differences in exercise performance.

Responses to Impaired O₂ Influx from the Environment

• Many of the animals that are adapted to living without O₂ undergo metabolic depression when deprived of O₂. Metabolic depression can be so profound as to lower an animal’s metabolic rate to less than 5% of the usual rate, thereby greatly reducing the rate at which ATP must be supplied by catabolic mechanisms.
• Invertebrate anaerobes deprived of O₂ produce ATP by means of a diversity of complex anaerobic catabolic pathways that generate end products such as acetic acid, succinic acid, and propionic acid. The invertebrates commonly excrete these organic products during anoxia as a way of avoiding end-product accumulation in their bodies.
• Virtually all vertebrates use simple anaerobic glycolysis to produce ATP in tissues deprived of O₂, and vertebrates invariably retain lactic acid in their bodies, setting the stage for potential metabolic self-limitation. Usually when vertebrates experience anoxia, it is strictly regional; whereas some tissues become anoxic, others—most notably the CNS—retain an O₂ supply. Only a few vertebrates can tolerate total-body anoxia.
• Turtles capable of total-body anoxia employ anaerobic glycolysis to make ATP. A key part of their strategy for survival is a metabolic depression of the CNS sufficiently profound to produce a comatose state.
• Goldfish and crucian carp undergoing total-body anoxia remain alert. They have the unusual ability to convert lactic acid to ethanol, which they can excrete, thereby preventing end-product accumulation in their bodies.