In biochemistry, respiration refers to the series of biochemical oxidations in which organic molecules are converted to carbon dioxide and water while the chemical energy thus obtained is trapped in a form useful to the cell. Biochemical respiration occurs in both plant and animal cells. Carbohydrates, amino acids, and fatty acids—the organic fuel molecules of the cell—can be converted to acetyl CoA, a derivative of acetic acid and coenzyme A.
Acetyl CoA then enters a series of reactions in the mitochondria, organelles in the cell's cytoplasm. The series of reactions, known as the Krebs cycle, converts the acetic acid portion of acetyl CoA to carbon dioxide, protons, and hydride ions, the latter usually as part of the coenzyme NADH. This molecule is oxidized back to NAD when it donates the hydride ion to the series of enzymes known as the electron transport chain. In a process called oxidative phosphorylation, each electron transport enzyme is in turn reduced (receives the hydride ion), then oxidized (donates a hydride ion to the next enzyme in the series), and the chemical energy liberated in this series of reactions is coupled to the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and phosphoric acid.
ATP, the cell's form of energy storage and supply, furnishes the chemical energy needed for muscle contraction, protein synthesis, active transport of substances across membranes, and electrical impulses. At the end of the electron transport chain, a hydride ion is donated to an atom of oxygen; this pair, together with a proton from the surrounding solution, forms a molecule of water. Thus, in the overall process of cellular respiration, the fuel molecules are converted to carbon dioxide and water while the chemical energy gained is trapped in a useful form as ATP.