Formation of ATP

Maintenance of life requires a continuous supply of energy, and respiration is the process that fulfils this continuous need. Respiration is the catabolic process by which a complex organic substrate is oxidised down to simple end-products, releasing biological energy that the cell traps in the bonds of ATP (adenosine triphosphate) - the cell's immediate, spendable energy currency.

The synthesis of ATP from ADP and inorganic phosphate is called phosphorylation. Three distinct kinds of phosphorylation occur in nature, depending on where the energy for forming that new phosphate bond comes from:

1. Photophosphorylation - ATP is formed using light energy trapped by chlorophyll, during the light reactions of photosynthesis (already covered in the photosynthesis chapter).
2. Substrate-level phosphorylation - a phosphate group is transferred directly from a high-energy intermediate substrate onto ADP, without any electron-transport chain being involved. It occurs in the cytoplasm of the cell (during glycolysis) and in the matrix of the mitochondria (during the Krebs cycle).
3. Oxidative phosphorylation - ADP is phosphorylated using the energy released when the reduced coenzymes NADH+H⁺ and FADH₂ are oxidised. This happens only on the inner mitochondrial membrane.

Whenever a cell needs energy for any metabolic activity, ATP is hydrolysed back to ADP and inorganic phosphate, and this hydrolysis releases the stored energy for the cell to use. The rest of this chapter traces exactly how respiration builds up this ATP supply, first without oxygen (anaerobic respiration) and then, far more productively, with it (aerobic respiration).