Cengage Microbiology Complete Practice Test 2026

The key idea is how much energy is actually produced when glycolysis runs on its own under anaerobic conditions. In glycolysis, a glucose molecule is split to form two pyruvate molecules. The process uses two ATP at the start (investment phase) but later yields four ATP, giving a net gain of two ATP per glucose. It also generates two NADH molecules.

If respiration can’t occur because there’s no oxygen, fermentation steps in to keep glycolysis going by regenerating NAD+. This allows glycolysis to continue producing ATP, but the fermentation process itself does not make extra ATP beyond what glycolysis already provided. So the total ATP per glucose under these conditions is still two.

That’s why the correct description is that glycolysis nets two ATP per glucose, and fermentation yields two ATP per glucose when respiration is unavailable. The other options overstate the ATP from glycolysis or the ATP produced by fermentation, which isn’t correct.