All living organisms contain cells that derive energy by breaking down nutrients, like glucose, during the process of respiration. The energy thus released is trapped in the form of ATP. It takes place in both aerobic as well as anaerobic organisms as it doesn’t involve oxygen. It involves the cellular degradation of glucose to pyruvate. Glycolysis occurs in cytoplasm of the cell, it involves two phases – Preparatory and Pay-off phase.
The preparatory phase starts with the phosphorylation of glucose using ATP to form glucose-6-phosphate. It isomerises to form fructose-6-phosphate. It is then phosphorylated to form fructose-1, 6-bisphosphate. It splits into two molecules of 3-carbon 3 phosphoglyceraldehyde, and DHAP. As DHAP can’t be utilised directly, it isomerises into PGAL. Thus, two molecules of PGAL are formed from one molecule of fructose-1, 6- bisphosphate.
The pay-off phase starts with the phosphorylation of PGAL using inorganic phosphate to form DPGA. The two redox-equivalents removed from PGAL are transferred to NAD+ to form NADH2. The DPGA thus formed undergoes dephosphorylation to form PGA. This is an energy yielding step to form ATP molecules. As the inorganic phosphate is directly transferred from a substrate to the ADP molecule to form ATP, it is called substrate-level phosphorylation. The phosphate group in 3-phosphoglycerate is relocated to form 2-phosphoglycerate, which is then converted into PEP. The final step is the energy yielding reaction involving dephosphorylation of PEP to form 3-carbon pyruvate. The phosphate released from the PEP molecule is captured by ADP to form ATP molecule. This ends the process of glycolysis.
Of the ten reactions, the three involving glucose, fructose-6-phosphate and phosphoenolpyruvate as substrates are irreversible, while the others are reversible. As two molecules of PGAL are formed from fructose-1, 6- bisphosphate, the reactions in the pay-off phase will occur two times - once for each PGAL. The preparatory phase utilises 2 ATP molecules to break down glucose, while the pay-off phase yields 4 ATP molecules and 2 NADH2 molecules. Thus, the net gain from a single molecule of glucose in the glycolysis process is 2 ATP and 2 NADH2. The key product of glycolysis is pyruvate. It can be utilised in different ways based on the availability of oxygen. In the presence of oxygen, it is utilised by aerobic organisms in Kreb’s cycle for complete oxidation. In the absence of oxygen, it is utilised by anaerobic organisms in lactic acid or alcohol fermentation.