How Does Aerobic Respiration Work?

Aerobic respiration transfers energy from glucose to ATP through glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. Oxygen is needed for the final stage, where it allows the electron transport chain to keep working.

Glycolysis

Glycolysis takes place in the cytoplasm. ATP is used to phosphorylate glucose, which is split into triose phosphate molecules. These are oxidised to form pyruvate. Some ATP is produced, giving a net gain of ATP, and NAD is reduced. Glycolysis itself does not require oxygen, even when it is part of aerobic respiration.

Link reaction and Krebs cycle

Pyruvate moves into the mitochondrial matrix. In the link reaction, it is converted to acetate, releasing carbon dioxide and reducing NAD. The acetate combines with coenzyme A to form acetylcoenzyme A.

In the Krebs cycle, acetylcoenzyme A combines with a four-carbon compound to form a six-carbon compound. A series of reactions releases carbon dioxide, produces some ATP and reduces more coenzymes, including NAD and FAD. The four-carbon compound is regenerated so the cycle can continue. Reduced coenzymes carry electrons to the final stage.

Oxidative phosphorylation

This takes place at the inner mitochondrial membrane. Electrons from reduced NAD and reduced FAD pass along an electron transport chain. The energy released moves protons across the membrane, creating a proton gradient. Protons then flow back through ATP synthase, which makes ATP from ADP and inorganic phosphate. Oxygen accepts electrons and protons at the end of the chain, forming water.

This movement of protons down their gradient through ATP synthase is called chemiosmosis. It produces most of the ATP made during aerobic respiration; glycolysis and the Krebs cycle produce smaller amounts of ATP directly.

Exam mistake to avoid: oxygen is not needed for glycolysis directly. Its role as the final electron acceptor lets oxidative phosphorylation continue and allows reduced coenzymes to be reused.