Chapter 6: Nutrition, Feeding, and Digestion

Nutrition, Feeding, and Digestion begins with a clear explanation of ATP as the universal energy currency of the cell, describing how its hydrolysis releases energy that drives essential cellular processes. The chapter then outlines the three main stages of cellular respiration: glycolysis, the Krebs cycle (citric acid cycle), and oxidative phosphorylation. Glycolysis, which occurs in the cytosol, breaks down glucose into pyruvate, producing small amounts of ATP and NADH. The Krebs cycle, located in the mitochondrial matrix, further oxidizes pyruvate-derived molecules and generates high-energy electron carriers. These carriers feed into the electron transport chain in the mitochondrial inner membrane, where a proton gradient powers ATP synthesis via chemiosmosis and ATP synthase. The importance of oxygen as the terminal electron acceptor is emphasized, and the consequences of anaerobic metabolism—such as lactic acid production and reduced ATP yield—are discussed in detail. The chapter also covers the roles of key enzymes, redox reactions, and coenzymes (NAD⁺, FAD) in energy transformation. In addition to carbohydrate metabolism, the authors explain how fats and proteins are catabolized for energy, integrating these pathways into a unified model of metabolic flexibility. The regulation of cellular respiration is explored through feedback mechanisms that adjust enzyme activity based on energy demand. The chapter concludes by tying cellular metabolism to whole-animal physiology, emphasizing that understanding energy generation at the molecular level is vital for interpreting animal performance, endurance, and survival across diverse environments.