Metabolism: Definition, Anabolic and Catabolic Pathways, and Energy Transformation

Metabolism (verb: metabolize) is the sum of all enzyme-catalyzed chemical reactions in a living organism that convert substrates into products, interconvert energy forms, and sustain life.

Two Arms

Catabolism (breakdown)

Exergonic. Complex molecules → simpler ones + ATP + reducing equivalents (NADH, FADH₂). Example: glycolysis, β-oxidation, Krebs cycle, oxidative phosphorylation.

Anabolism (biosynthesis)

Endergonic. Simple precursors → macromolecules (proteins, lipids, nucleic acids, glycogen) consuming ATP/NADPH. Example: gluconeogenesis, lipogenesis, protein synthesis.

Regulation

• Allosteric control (ATP/AMP, citrate, acetyl-CoA).
• Hormonal (insulin → anabolic; glucagon/epinephrine/cortisol → catabolic).
• Transcriptional (PPARs, SREBPs, mTOR).

Metabolic dysregulation underlies diabetes, obesity, cancer, inborn errors.