The citric acid cycle, also known as the Krebs cycle, is the central metabolic pathway by which cells extract energy from food. Operating in the mitochondria, it oxidises fragments derived from carbohydrates, fats, and proteins, releasing carbon dioxide and capturing energy in carrier molecules that drive ATP production. Hans Krebs elucidated the cycle in the 1930s, earning a Nobel Prize. Every bite of food ultimately funnels through this pathway to become usable energy, which is why its enzymes need B vitamins as cofactors and why metabolic poisons targeting it are so deadly. It is the biochemical crossroads of nutrition.