12.2.3—Pyruvate and the link reaction

Glycolysis is the cytoplasmic stage in which one six-carbon glucose molecule is phosphorylated, split into two three-carbon routes and oxidised to form two pyruvate molecules. It gives a small net ATP gain and reduces coenzyme NAD.

1. Set the location and starting molecule: Glycolysis occurs in the cell cytoplasm and begins with one glucose molecule. It can supply ATP without requiring mitochondria, although later aerobic processing uses the pyruvate.
2. Investment: Transfer phosphate groups from ATP to glucose-derived material. This investment makes the substrate more reactive for the pathway; it is not the final ATP yield.
3. Lysis: The phosphorylated six-carbon intermediate is split into two three-carbon triose-phosphate molecules, creating two parallel routes.
4. Oxidation: Each three-carbon route loses hydrogen, which is accepted by NAD to form reduced NAD. This stores transferable reducing power rather than making ATP directly.
5. Recovery: Phosphate groups are transferred from the pathway intermediates to ADP, producing more ATP by substrate-linked phosphorylation. Because the recovery exceeds the investment, glycolysis has a small net ATP gain per glucose.
6. Output and boundary: The two three-carbon routes finish as two pyruvate molecules. Pyruvate may be processed in the next stage when conditions allow; the link reaction and Krebs cycle are separate cards.

Investment → recovery: early ATP use raises the reactivity of the glucose pathway; splitting doubles the three-carbon route; oxidation reduces NAD; and later phosphate transfer returns more ATP than was invested. The pathway therefore converts one glucose into two pyruvate while producing immediately usable ATP and reducing power.

Glycolysis is not the link reaction or the Krebs cycle, does not occur in the mitochondrial matrix, and does not directly release carbon dioxide. Do not treat the ATP used in the investment phase as a net loss or replace the pathway with a fixed numerical yield beyond the syllabus-supported net-gain idea.