Chapter 7: Energy Metabolism

Energy Metabolism begins with an in-depth discussion of how ATP is produced under both oxygen-rich and oxygen-limited conditions, emphasizing the biochemical differences between oxidative phosphorylation and anaerobic glycolysis. The chapter outlines the advantages and limitations of each pathway, noting that aerobic metabolism is more efficient (producing up to 29 ATP per glucose), while anaerobic metabolism provides rapid ATP but yields much less energy. The authors introduce key concepts such as oxygen debt, metabolic end products like lactate, and how various animals have evolved unique adaptations to cope with hypoxia or anoxia. Examples include goldfish and crucian carp that convert lactate into ethanol to avoid acid buildup, and turtles that buffer lactic acid in their shells during long-term anoxia. The chapter also explores metabolic rate and how it can be quantified through direct and indirect calorimetry. Oxygen consumption and respiratory quotient (RQ) are presented as critical tools for evaluating aerobic metabolism. Further sections examine the use of carbohydrates, lipids, and proteins as fuels, highlighting how the choice of fuel affects both ATP yield and endurance. The role of mitochondria in determining aerobic scope and performance is addressed, with examples from flight muscles in insects and thermogenic tissues in mammals. The authors emphasize that metabolic strategy is tightly coupled to an animal’s ecological context, such as the need for burst activity versus sustained energy output. Finally, the chapter explores how animals shift between aerobic and anaerobic metabolism during exercise, diving, and environmental stress, illustrating the physiological flexibility that supports survival in diverse conditions.