Glycolysis

Glycolysis is the first stage in the breakdown of glucose and the main route by which cells get energy from sugar. It takes place in the cytoplasm of nearly all cells, prokaryotic and eukaryotic, and does not use oxygen. In ten enzyme-controlled steps, one six-carbon glucose molecule is converted into two three-carbon molecules of pyruvate.

The first half is an investment phase: two ATP are spent to attach phosphate groups to the sugar, which is then split in two. The enzyme phosphofructokinase controls the pace of this phase and is slowed when ATP is plentiful. In the payoff phase, the three-carbon molecules are oxidized, yielding four ATP by direct phosphate transfer and two NADH, an electron carrier. The net gain per glucose is therefore two ATP, two NADH and two pyruvate.

What happens next depends on oxygen. With oxygen, pyruvate enters the mitochondria and feeds the citric acid cycle; without it, fermentation converts pyruvate to lactate or ethanol to regenerate the NAD+ that glycolysis needs to keep running. Mature red blood cells, which have no mitochondria, rely on glycolysis alone for their ATP.