GLYCOLYSIS (glykys = sweet ; lysis = split/breakdown)

Other name: Embden Meyerhof Parnas pathway (EMP pathway)


A very important pathway in the body, glycolysis is the breakdown of sugar which is glucose (a molecule containing 6 carbons – hexose) into 2 pyruvate molecules, each containing 3 carbon molecules. This process releases energy for utilization by the body in the form of adenosine triphosphate (ATP) through a sequence of enzyme reactions. Glycolysis is a catabolic pathway, i.e a pathway which involves the breakdown of larger complexes through oxidative reactions. Catabolic pathways release energy and are exogenic in nature. Glycolysis is a very important part of the metabolism of glucose and takes place in aerobic as well as anaerobic organisms and does not require molecular oxygen. This takes place in the cytosol of the cell.

Glycolysis is carried out in a sequential 10 step reaction, which are enzyme catalysed. It is represented in the following manner:

C6H12O6 + 2ADP + 2Pi + 2NAD+   →   2C3H4O3 + 2H2O + 2ATP + 2NADH + 2H+

Thus, one molecule of glucose in the presence of phosphate and adenosine diphosphate gives two 3 carbon molecules of pyruvate, along with releasing water and energy in the form of ATP.

Enzymes involved in glycolysis

Each step of the glycolysis pathway requires the presence of an enzyme to continue the process. These enzymes include:

1) Hexokinase

2) Phosphohexose isomerase

3) Phosphofructokinase 1

4) Aldolase

5) Phosphotriose isomerase / Triose – P – isomerase

6) Glyceraldehyde 3-phosphate dehydrogenase

7) Phosphoglycerate kinase

8) Phosphoglycerate mutase

9) Enolase

10) Pyruvate kinase

Glycolysis takes place in two steps:

  1. Preparatory phase (energy invested)
  • This phase comprises steps 1-5 of the glycolysis pathway. 
  • It is called the preparatory phase as glucose is prepared for the conversion to pyruvate by the cleaving of the hexose chain – ringed structure to form a linear structure. 
  • Energy is invested in this phase in the form of 2 ATP molecules which helps to convert glucose into 2 three carbon sugar phosphates known as Glyceraldehyde-3-phosphate, which is the final product of the preparatory phase.
Preparatory phase of glycolysis
  1. Payoff phase (energy is released)
  • This phase is steps 5-10 of the glycolysis pathway
  • This phase is known as the payoff phase as energy is released in the form of 2 ATP molecules as glyceraldehyde-3-phosphate converts to 2 moles of pyruvate.
  • This is the final phase of glycolysis and consists of intermediates and there is a net gain of the energy-rich molecules ATP and NADH.
Payoff Phase of glycolysis



  1. Pyruvate is oxidised from glucose
  2. NAD+ is reduced to NADH
  3. Phosphorylation of ADP into ATP


Pyruvate formed has different fates in the body, depending on the organism and also the metabolic fate, pyruvate has 3 different paths:


The enzymes are the most important factors that help carry the pathway forward and thus play an important role in regulation of glycolysis.

The most important of these are the 3 enzymes which carry out irreversible kinase reactions:

  1. Hexokinase/glucokinase
  2. Phosphofructokinase
  3. Pyruvate kinase
Regulation of glycolysis

Enzymes are regulated by the following biological mechanisms:

1. Gene Expression

2. Allostery

3. Protein-protein interaction (PPI)

4. Post translational modification (PTM)

5. Localization


In conclusion, Glycolysis is an extremely important pathway which is essential for many organisms for the formation and utilisation of energy (ATP). Pyruvate formed is utilised in many future pathways in the body.

2 thoughts on “GLYCOLYSIS

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