7.5—Lactate after anaerobic respiration

When oxygen is limited, the electron transport chain and Krebs cycle cannot continue normally. Reduced NAD transfers hydrogen to pyruvate, forming lactate and regenerating NAD for glycolysis.

Anaerobic metabolism yields only the small net ATP gain from glycolysis, but it can maintain short-term ATP production. Lactate can later be oxidised to pyruvate or converted to glucose when oxygen is available.

After intense exercise, deeper breathing supplies extra oxygen for processing lactate; the increased ventilation is not evidence that anaerobic respiration itself produced more ATP.

Lactate formation does not make oxygen irrelevant and does not create a large ATP yield. Separate immediate fermentation from later oxygen debt recovery.