Glycolysis

Glucose is a stable molecule. It will not just fall apart into useful pieces on its own, so the cell spends 2 ATP upfront to attach phosphate groups to it. This investment phase strains the bonds and destabilizes the 6-carbon sugar, splitting it into two 3-carbon molecules that are primed to react. Nothing comes for free in chemistry: you pay energy to make a reaction fast enough to matter.

Once split, each 3-carbon molecule goes through a payoff phase. Enzymes strip off electrons and a proton, and NAD+ catches them to become NADH. This happens because the 3-carbon intermediate is slightly oxidized at this step, and NAD+ is built to grab high-energy electrons whenever a substrate loses them. Each 3-carbon piece also gets phosphorylated using inorganic phosphate, then transfers that phosphate directly to ADP, making ATP by substrate-level phosphorylation. Since there are two 3-carbon molecules per glucose, every payoff step happens twice. Count it up: 4 ATP made minus 2 ATP spent gives a net of 2 ATP, plus 2 NADH and 2 pyruvate, all from one glucose.

Glycolysis runs in the cytosol, not the mitochondrion, and none of its ten steps use oxygen as a reactant. That is why fermentation and anaerobic cells can still run glycolysis for energy. But the small net yield is also why glycolysis is just the opening act: the pyruvate and NADH it produces feed into the mitochondrion next, where the Krebs cycle and oxidative phosphorylation squeeze out far more ATP, provided oxygen is around to accept electrons at the end of the chain.