AIChE Chemical Engineering Jeopardy Practice Exam

In glycolysis, energy is spent upfront and then harvested later, so you end up with a small net gain of ATP per glucose. Specifically, two ATP are consumed in the early steps to activate glucose and prepare it for breakdown. Later, the payoff steps generate ATP through substrate-level phosphorylation as two glyceraldehyde-3-phosphate molecules are converted through the pathway, yielding four ATP in total. Subtract the two ATP that were spent from the four produced, and the net gain is two ATP per glucose. This is the amount available for the cell after glycolysis completes. (Glycolysis also makes two NADH per glucose, which can be used to produce more ATP in the presence of oxygen.)