glycolysis
/ gly-KOL-uh-sis /
Before a cell can extract most of the energy from a sugar, it has to break the sugar into manageable halves, a bit like cutting a long loaf in two before sharing it out. Glycolysis is that first cut. It is the opening stage of breaking down glucose, and it happens in the watery interior of the cell, the cytosol, with no oxygen required.
The word means "sugar splitting." In glycolysis, one six-carbon glucose molecule is broken, through about ten enzyme-catalyzed steps, into two three-carbon molecules called pyruvate. The cell first spends two ATP to get the process going (an investment), then earns four ATP back, for a net gain of two ATP. It also loads electrons onto two NAD+ carriers, producing two NADH. So the modest but real payoff of glycolysis is two ATP and two NADH per glucose, plus two pyruvate to hand on.
Glycolysis is ancient and universal: essentially every living cell does it, which hints that it evolved before oxygen was common on Earth. It is fast and oxygen-independent, making it the emergency energy source when oxygen runs short, such as in a hard-sprinting muscle. But it captures only a small slice of glucose's energy; the much larger payoff waits in the mitochondrial stages that follow, if oxygen is available.
A muscle in an all-out sprint runs glycolysis at full tilt for quick ATP, faster than oxygen can be delivered, which is why it can keep working briefly even when breathless.
Glycolysis splits one glucose into two pyruvate, netting 2 ATP and 2 NADH, without oxygen.
Glycolysis itself needs no oxygen and yields only a small net of 2 ATP; the large energy payoff comes only later, in the oxygen-using mitochondrial stages.