7.2—Glycolysis in aerobic and anaerobic respiration

Glycolysis takes place in the cytoplasm and does not require oxygen. Glucose is phosphorylated and split into triose phosphate; oxidation then produces two pyruvate, four ATP gross (two net) and two reduced NAD.

The initial ATP investment makes glucose more reactive. Reduced NAD carries hydrogen to later stages when oxygen is available, while pyruvate can enter fermentation when it is not.

Two ATP are used at the start and four are formed later, so the net gain is two ATP per glucose, not four.

Glycolysis is not the complete aerobic pathway and does not require mitochondria. A reduced-NAD molecule is not the same thing as ATP.