Catabolism

Catabolism refers to the set of metabolic pathways that break down complex molecules into simpler ones, releasing energy and providing building blocks for cellular functions. Through hydrolysis and oxidation, large biomolecules such as carbohydrates, proteins, and fats are degraded into smaller units (sugars, amino acids, and fatty acids). The energy released is captured in ATP and in reducing equivalents like NADH and FADH2, which fuel other cellular processes.

Major catabolic pathways include carbohydrate, lipid, and protein breakdown. Carbohydrate catabolism occurs in stages: glycolysis in the cytosol, pyruvate oxidation in mitochondria, the citric acid cycle, and oxidative phosphorylation. Lipid catabolism proceeds via beta-oxidation in mitochondria to produce acetyl-CoA, which enters the citric acid cycle. Protein catabolism involves proteolysis to amino acids, which can be deaminated and used for energy or as substrates for biosynthesis. In the absence of oxygen, anaerobic pathways such as lactate fermentation (in animals) or ethanol fermentation (in yeast) provide ATP.

Energy yield and end products vary by pathway and condition. Most ATP is generated through oxidative phosphorylation, driven by NADH and FADH2 produced in earlier steps. For glucose, glycolysis yields a net 2 ATP and 2 NADH; subsequent TCA cycle and oxidative phosphorylation generate substantial additional ATP. Complete catabolism in aerobic conditions ultimately yields carbon dioxide and water.

Regulation and physiological role: catabolism is tightly controlled by enzymes responding to the cellular energy state. Hormones such as glucagon, adrenaline, and cortisol promote catabolic fluxes during fasting or stress, while insulin tends to favor anabolic storage. Catabolic processes supply energy during fasting, exercise, and stress and provide carbon skeletons for biosynthetic needs. Dysregulation can contribute to muscle wasting and metabolic disorders.