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.


Metabolism: an overview and types


  • About
  • Catabolism
  • Anabolism
  • Metabolic pathways


  • Metabolism is the collection of chemical reactions takes place to sustain life of an organism.
  • The main purposes of metabolism is to convert food to energy to run cellular activity; to convert food to building blocks for nucleic acids, lipids, protein and carbohydrates; and to remove metabolic waste.
  • The metabolic reactions which are enzyme catalysed are responsible for the growth and reproduction of organism, to maintain their structure and interact with environment.
  • Metabolic reaction is of two types one is catabolic reaction, means the breaking down of compounds and another is anabolic reaction- the building up of the compounds. The catabolic reaction liberates energy and the anabolic reaction uses energy.
  • Metabolic pathways include the steps through which one chemical is transformed into another and each step is facilitating by an enzyme.
  • Enzymes are the key component of the metabolic reaction; they act as catalyst- allows the reaction to proceed more rapidly.


Metabolic reaction is of two types:

  1. Catabolism
  2. Anabolism


  • In catabolism the compound through the set of chemical reaction is broken down into simpler compound or molecules.
  • This is achieved by breaking down and oxidizing food molecules.
  • Catabolism is responsible to provide energy for working of the cell and component needed for the anabolic processes which build molecules.
  • The nature of these catabolic reactions based on the source of energy and carbon which is differ from organism to organism.

The chief metabolic processes in a cell are:

  • Glycolysis
  • Pentose-phosphate pathway
  • Entner-doudoroff pathway
  • Tricarboxylic acid cycle
  • Fermentation
  • Glyoxylate cycle
  • Lipid hydrolysis
  • Protein hydrolysis


  • Anabolism is the set of constructive reactions which used energy released by the catabolic pathway to synthesize complex molecules.
  • The complex molecule construct cellular structure step by step, make up from small and simple precursor.
  • The biomolecules are necessary for the growth and reproduction, some biomolecule serve as the central metabolic intermediates.
  • Some organisms can synthesis all the necessary organic compound like autotrophs. They can be grown on simple media. On the other hand, the organisms which cannot synthesize organic compounds from atmosphere are known as fastidious organisms.
  • Following anabolic process takes place in organism:
  • Synthesis of glucose, lipids, amino acid and protein, nucleic acids
  • Synthesis of other growth factors like vitamins, hormones etc.

Metabolic process:


  • In the glycolysis process glucose and other sugar are partially oxidized to the smaller molecule i.e. pyruvate
  • Embden-Myerhof pathway, pentose phosphate pathway and Entner-Doudroff pathway are the three routes for the conversion of sugar into pyruvate.
  • It is anaerobic process in which organism obtain energy in the absence of oxygen, also called anerobic fermentation.

Tricarboxylic acid pathway:

  • Given by H. A. kerbs in 1973
  • Also known as citric acid cycle. Because citric acid is the first product of the kerb cycle which is as known as TCA cycle as the citric acid has three carboxylic group.

Glyoxyalte cycle:

  • It is anaplerotic reaction which means one product of a cycle is taken up by the other cycle
  • Oxaloacetate is taken from TCA cycle and used for carbon source from the amino acid synthesis.

Pentose phosphate pathway:

  • It is an alternative pathway for the sugar degradation.
  • Its main function is to generate power in the form of NADH in extramitochondrial cytoplasm and the second function is to convert hexoses into pentose for the synthesis of the nucleic acids. The third function is complete degradation of pentose.