anaerobic metabolism

Meaning: Glycolysis in the cytoplasm converts glucose to 2 pyruvate, with a net gain of 2 ATP by substrate-level phosphorylation and 2 NADH. Without O₂ the electron transport chain stalls, NADH is not reoxidized, and NAD⁺ runs short. Fermentation solves this by reducing pyruvate (to lactate in animal cells, or to ethanol and CO₂ in yeast), which oxidizes NADH back to NAD⁺. The yield is 2 ATP per glucose, far below aerobic respiration. A second sense: some prokaryotes carry out anaerobic respiration, using an electron transport chain with a terminal acceptor other than O₂, such as sulfate or nitrate.

Equation: glucose + 2 ADP + 2 Pi → 2 lactate + 2 ATP, where each coefficient is in mol, ADP is adenosine diphosphate, Pi is inorganic phosphate, and ATP is adenosine triphosphate.

On the MCAT: Expect passages on intense muscle exercise, red blood cells (no mitochondria), or yeast fermentation. Typical predictions: lactate rises, pH falls, and glycolytic rate increases to make up for low ATP yield. The Cori cycle returns lactate to the liver for gluconeogenesis.

Common confusion: Fermentation itself does not make the ATP. Its role is regenerating NAD⁺, and the ATP comes from glycolysis. Also, fermentation is not the same as anaerobic respiration, which uses an electron transport chain.