Some more on Glycolysis

ATP can be made in 3 ways :

  • substrate level phosphorylation (animals- glycolysis)
  • oxidative phosphorylation (animals- mitochondria and electron transport chain)
  • photophosphorylation (plants)

First Phase Enzymes

  1. Hexokinase
  2. Phosphohexose isomerase
  3. Phospho-fructokinase-1 (PFK-1)
  4. Aldose
  5. Triose phosphate isomerase

Hexokinase and PFK-1 are both enzymes for the irreversible reactions of the first phase. Their reactions are energetically favorable due to a large negative change of  free energy(ΔG)

Second Phase Enzymes

       6. Glyceraldehye 3-phosphate dehydrogenase

       7. Phosphoglycerate kinase

       8. Phosphoglycerate mutase

       9. Enolase

     10. Pyruvate kinase

The most energetically favorable reaction in glycolysis is reaction 10. between phosphoenolpyruvate to pyruvate using pyruvate kinase. The reaction is irreversible due to ΔG being very negative.

ΔG is close to zero when the reaction is reversible.



  • First metabolic pathway to give any living organism energy
  • used in every cell- universal
  • organelles are not needed for the process to take place, can occur in both prokaryotes and eukaryotes
  • occurs in the cytosol
  • 10 enzymes in the glycolysis procees, divided into 2 phases, the energy investment phase and the energy generation phase


Glucose enters glycolysis to form 2 mols of pyruvate. When oxygen and is available the pyruvate continues into the TCA cycle. Both mitochondria and oxygen much be present for this passage to occur.

However, when no oxygen is available, the pyruvate either forms 2 mols of carbon dioxide and ethanol through fermentation or 2 mols of lactic acid(lactate) which is also through fermentation. Lactate regenerates NAD+ when pyruvate is converted. red blood cells, rich in oxygen but lacking mitochondria use fermentation.

Process of glycolysis

5 enzyme reactions consisting of 2 irreversible reactions and 3 reversible reactions make up the Energy Investment Phase.

5 enzyme reactions consisting of 1 irreversible reactions and 4 reversible reactions make up the Energy Generation Phase.

Net gain of this reaction: 2 ATP and 2 NADH