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Biology · Glossary

What is Glycolysis?

Also known as: substrate-level phosphorylation

Definition 15.3 University Biology — Year 1 · Chapter 15 — Cellular Respiration and Fermentation

Glycolysis is a sequence of ten enzyme reactions in the cytosol that converts one glucose (C6) into two pyruvate (C3). In the investment phase two ATP are spent to phosphorylate the sugar (hexokinase, then phosphofructokinase, PFK) and the six-carbon diphosphate is split into two three-carbon phosphates. In the payoff phase each triose is oxidised by NAD+\mathrm{NAD^+} (the only oxidation of glycolysis) and its phosphates are transferred to ADP by substrate-level phosphorylation — a phosphate handed directly from a high-energy intermediate to ADP, with no membrane or oxygen involved. The balance:

glucose+2 NAD++2 ADP+2 Pi→2 pyruvate+2 NADH+2 H++2 ATP+2 H2O.\text{glucose} + 2\,\mathrm{NAD^+} + 2\,\mathrm{ADP} + 2\,\mathrm{P_i} \to 2\,\text{pyruvate} + 2\,\mathrm{NADH} + 2\,\mathrm{H^+} + 2\,\mathrm{ATP} + 2\,\mathrm{H_2O} .

It needs no oxygen; it is the oldest and most universal pathway of metabolism.

Glycolysis in outline. Two ATP are spent to build a six-carbon diphosphate, which splits into two trioses; their oxidation and two substrate-level phosphorylations each return four ATP and two NADH. Phosphofructokinase is the pathway’s control valve.
Glycolysis in outline. Two ATP are spent to build a six-carbon diphosphate, which splits into two trioses; their oxidation and two substrate-level phosphorylations each return four ATP and two NADH. Phosphofructokinase is the pathway’s control valve.
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