Aerobic respiration involves four stages:
- In glycolysis, glucose is broken down in the cytoplasm to produce 2 molecules of pyruvate and 2 molecules of NADH, with a net production of 2 ATP by substrate-level phosphorylation. Both pyruvate and NADH enter mitochondria
- The link reaction, in the mitochondrial matrix, converts pyruvate to acetyl coenzyme A, producing CO2 and more NADH
- Acetyl coenzyme A is taken up by the Krebs cycle (also in mitochondrial matrix), which generates (2x) CO2, (3x) NADH, (1x) FADH2, and (1x) ATP by substrate-level phosphorylation.
- The reduced coenzymes (NADH and FADH2) supply electrons to the electron transport chain (within the inner mitochondrial membrane) in which O2 is the final electron acceptor. The transfer of electrons along carriers at successuvely lower energy levels is coupled with the synthesis of ATP, a process called oxidative phosphorylation. The coenzymes are reoxidised and so NAD+ (and FAD) are available to the dehydrogenase reactions of glycolysis, the link reaction and the Krebs cycle
Respiration is the process by which organic molecules (mainly carbohydrates and lipids) are broken down; the energy released is used to produce ATP.
Anaerobic respiration occurs without the use of oxygen. It involves glycolysis with additional reactions (producing lactate in animals and bacteria, and ethanol in plants and fungi) to regenerate NAD+, so allowing continued production of ATP (for short periods)
Carbohydrate, lipid and protein can all be respired aerobically, with lipid yielding more energy per unit mass than carbohydrate. Only carbohydrate (glucose) can be respired anaerobically.
The respiratory quotient (RQ) can be used to determine the type of respiration and respiratory substrate.