Glycolysis is the first stage of cellular respiration and the one every cell shares. It takes one six-carbon glucose and, in ten enzyme-catalysed steps in the cytosol, splits it into two three-carbon pyruvate molecules. No oxygen is needed, which is why it runs in a muscle cell out of breath and in a yeast cell making wine.
The pathway spends before it earns. The first stage phosphorylates glucose twice, costing two ATP, which traps the sugar in the cell and destabilises it enough to break. Aldolase then splits the six-carbon sugar into two three-carbon halves. In the payoff stage each half is oxidised, handing electrons to NAD+ to make NADH, and gives up two phosphates to ADP by substrate-level phosphorylation.
The ledger lands on a net gain of two ATP and two NADH per glucose, with two pyruvate left over. Most of the energy is still in the pyruvate, bound for the Krebs cycle if oxygen is present and for fermentation if not. Three steps are effectively irreversible, and one of them, phosphofructokinase, is where the cell decides how fast the whole pathway runs.
Where the two pyruvate go — and all three answers are the same question, where does NAD⁺ come from?