Chapter 23: Metabolism, Nutrition, and Energetics

Metabolism encompasses both catabolic pathways that break down organic molecules to release energy and anabolic pathways that construct new molecules from available substrates. Cells maintain an internal nutrient pool of amino acids, lipids, and carbohydrates distributed through the bloodstream to support ongoing metabolic demands. Carbohydrate metabolism centers on glucose oxidation through three integrated stages: glycolysis in the cytoplasm converts glucose to pyruvate and yields two ATP molecules; the citric acid cycle oxidizes acetyl-CoA in the mitochondria and generates electron carriers for downstream energy capture; the electron transport chain transfers electrons through membrane-bound proteins and establishes a proton gradient that drives ATP synthase to produce approximately 30 to 32 ATP molecules per glucose. Lipid metabolism involves emulsification by bile salts, absorption into chylomicrons, and hepatic repackaging into lipoproteins for distribution or storage; fatty acid oxidation via beta-oxidation generates acetyl-CoA for energy production. Protein catabolism through deamination removes amino groups from excess amino acids, allowing the liver to convert resulting ammonia into urea for renal excretion. The body alternates between an absorptive state following meals when insulin promotes glucose uptake and storage, and a postabsorptive state when counterregulatory hormones mobilize stored fuels to maintain blood glucose homeostasis. Nutritional requirements include adequate energy substrates, essential amino acids and fatty acids, and both fat-soluble and water-soluble vitamins that support metabolic function. Metabolic disorders including eating disorders, obesity, phenylketonuria, and ketoacidosis represent pathological disruptions of normal nutrient processing. Basal metabolic rate reflects minimum resting energy expenditure and is regulated by hypothalamic feeding and satiety centers responding to hormones such as ghrelin and leptin. Thermoregulation maintains body temperature through heat gain via shivering thermogenesis and heat loss through radiation, conduction, convection, and evaporation, coordinated by hypothalamic temperature sensors and autonomic responses.