Glucose molecules (green) drift into the cell and are drawn toward the mitochondria (orange), where they are broken down to release energy as ATP (yellow bursts). When oxygen is plentiful, mitochondria run aerobic respiration, extracting the full ~32 ATP per glucose via the Krebs cycle and electron transport chain. When oxygen availability drops, the cell falls back on anaerobic fermentation — a fast but wasteful pathway that yields only 2 ATP per glucose and produces lactate (pink), which accumulates as an oxygen debt.
aerobic: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + ~32 ATP
anaerobic: C6H12O6 → 2 lactate + 2 ATP
ATP rate = demand · [ f_O2·32 + (1−f_O2)·2 ]
- Activity level — how much energy the cell currently demands; higher activity pulls glucose in and drives ATP synthesis faster, but also outpaces oxygen delivery sooner.
- Oxygen availability — the fraction of metabolism that can run aerobically; below 100% part of the demand is met anaerobically instead.
- Glucose supply — how much fuel is available to be broken down; starving the cell of glucose throttles ATP output regardless of oxygen.
- Lactate buildup — rises whenever anaerobic fermentation is active and slowly clears when the cell is mostly aerobic again, mirroring real oxygen debt after intense exertion.
Real-world relevance: this is why muscles burn during a sprint (anaerobic, lactate builds up) but not during a jog (aerobic, oxygen keeps pace) — the same trade-off every aerobic organism balances between speed and efficiency of energy production.