Cellular respiration is the metabolic pathway that converts glucose into ATP, the energy currency of the cell. Two distinct forms exist: anaerobic, which bypasses oxygen, and aerobic, which relies on it.
When oxygen supply lags behind demand—such as during intense exercise—cells shift to anaerobic pathways, producing lactate and a modest ATP yield. The accumulation of lactate and oxygen deficit leads to muscle fatigue and heavy breathing.
Aerobic respiration proceeds in three coordinated stages, each building on the previous to extract maximal energy from one glucose molecule.
Glycolysis occurs in the cytosol and does not require oxygen. It breaks glucose into two pyruvate molecules, yielding a net gain of two ATP and two NADH.
Pyruvate enters mitochondria, where it is converted to Acetyl‑CoA and enters the Krebs cycle. Each turn produces one ATP (or GTP), three NADH, and one FADH₂, while releasing two molecules of CO₂. For one glucose, the cycle runs twice, adding eight NADH and two FADH₂ to the pool.
High‑energy electrons from NADH and FADH₂ travel through the inner mitochondrial membrane. Oxygen acts as the final electron acceptor, combining with hydrogen to form water. The electron flow powers a proton gradient that drives ATP synthase, generating the bulk of cellular ATP—about 30–32 molecules per glucose.
Without oxygen, the electron transport chain stalls, halting ATP synthesis and forcing the cell to revert to glycolysis and fermentation. Thus, oxygen is indispensable for the complete oxidation of glucose and the efficient production of ATP.
