Glycolysis

The name comes from Greek: glycos (sugar) and lysis (splitting). Gustav Embden, Otto Meyerhof and J. Parnas worked out the scheme, so it is also called the EMP pathway.

Glycolysis takes place in the cytoplasm and occurs in all living organisms; in anaerobic organisms it is the only process of respiration. One glucose is partly oxidised to two molecules of pyruvic acid through a chain of ten enzyme-controlled reactions.

In plants the glucose comes from sucrose, the end product of photosynthesis, or from stored carbohydrate. Invertase splits sucrose into glucose and fructose, and both enter glycolysis.

Hexokinase phosphorylates glucose to glucose-6-phosphate, which isomerises to fructose-6-phosphate; from here glucose and fructose follow the same steps.

ATP is used at two steps: glucose to glucose-6-phosphate, and fructose-6-phosphate to fructose-1,6-bisphosphate.

Fructose-1,6-bisphosphate (6C) splits into two 3-carbon triose phosphates: dihydroxyacetone phosphate and 3-phosphoglyceraldehyde (PGAL).

NADH + H⁺ is formed at one step only: PGAL is oxidised, taking up inorganic phosphate, to 1,3-bisphosphoglycerate (BPGA); two redox equivalents (two hydrogen atoms) pass from PGAL to NAD⁺.

ATP is made at two steps: BPGA to 3-phosphoglyceric acid (PGA), and phosphoenolpyruvate (PEP) to pyruvic acid. As each happens once per triose and there are two trioses, 4 ATP are made per glucose.

Per glucose: 2 ATP used, 4 ATP made directly, so a net gain of 2 ATP, plus 2 NADH + H⁺ and 2 pyruvic acid. What happens to pyruvate next depends on what the cell needs.