Glycolysis is derived from the Greek words (glykys = sweet and lysis = splitting).

It is a universal catabolic pathway in the living cells.     Glycolysis can be defined as the sequence of reactions for the breakdown of Glucose (6-carbon molecule) to two molecules of pyruvic acid (3-carbon molecule) under aerobic conditions; or lactate under anaerobic conditions along with the production of small amount of energy.    

This pathway was described by Embden, Meyerhof and Parnas. Hence, it is also called as Embden-Meyerhof pathway (EM pathway).  


Site of Glycolysis

Glycolysis takes place in the cytoplasm of virtually all the cells of the body.

Types of Glycolysis

There are two types of glycolysis.

  • Aerobic Glycolysis: From the word aerobic, meaning with the presence of oxygen. It occurs when oxygen is sufficient. Final product is pyruvate along with the production of Eight ATP molecules.
  • Anaerobic Glycolysis: This type of glycolysis takes place in the absence of oxygen. Final product is lactate along with the production of two ATP molecules.

Steps of Glycolysis

Glycolysis is a lengthy process and made possible by a total of 11 enzymes. There are two phases of the glycolytic pathway

  1. Preparatory phase
  2. Payoff phase.

Glucose is converted to pyruvate in 10 steps by glycolysis.

The glycolytic patway can be divided into two phases:

Preparatory Phase/Glucose Activation Phase

  • This phase is also called glucose activation phase. In the preparatory phase of glycolysis, two molecules of ATP are invested and the hexose chain is cleaved into two triose phosphates.
  • During this, phosphorylation of glucose and it’s conversion to glyceraldehyde-3-phosphate take place. The steps 1, 2, 3, 4 and 5 together are called as the preparatory phase.

Payoff Phase/Energy Extraction Phase

  • This phase is also called energy extraction phase. During this phase, conversion of glyceraldehyde-3-phophate to pyruvate and the coupled formation of ATP take place.
  • Because Glucose is split to yield two molecules of D-Glyceraldehyde-3-phosphate, each step in the payoff phase occurs twice per molecule of glucose. The steps after 5 constitute payoff phase.

Step 1 : Uptake and Phosphorylation of Glucose

  • Glucose is phosphorylated to form glucose-6-phosphate.
  • Glucose forms glucose-6-phosphate through phosphorylation using glucokinase (an enzyme in the liver) and hexokinase (non-specific liver enzyme) and extrahepatic tissue as catalysts. Such enzymes break down ATP into ADP and add Pi to the glucose.
  • Hexokinase is a key glycolytic enzyme. Hexokinase catalyses a regulatory step in glycolysis that is irreversible.
  • However, for hexokinase’s actions to takes place, it needs Mg2+.

Step 2 : Isomerization of Glucose-6-Phsphate to Fructose-6-Phosphate

  • Glucose-6-phosphate is isomerised to fructose-6-phosphate by phosphohexose isomerase.
  • For the reaction to take place, it needs the help of aldose-ketose isomerization using a catalyst phosphohexose isomerase. It causes glucopyranose ring’s opening to a linear structure changing the structure of the furanose ring of fructose-6-phosphate.

Step 3 : Phosphorylation of F-6-P to Fructose 1,6-Biphosphate

  • Fructose-6-phosphate is further phosphorylated to fructose 1,6-bisphosphate.
  • The enzyme is phosphofructokinase-1. It catalyses the transfer of a phosphate group from ATP to fructose-6-phosphate.
  • The reaction is irreversible.
  • One ATP is utilised for phosphorylation.
  • Phosphofructokinase-1 is the key enzyme in glycolysis which regulates breakdown of glucose.

Step 4 : Cleavage of Fructose 1,6-Biphosphate

  • The 6 carbon fructose-1,6-bisphosphate is cleaved into two 3 carbon units; one glyceraldehyde-3-phosphate (GAP) and another molecule of dihydroxy acetone phosphate (DHAP).
  • The enzyme which catalyses the reaction is aldolase. Since the backward reaction is an aldol condensation, the enzyme is called aldolase.
  • The reaction is reversible.

Step 5 : Interconversion of the Triose Phosphates

  • GAP is on the direct pathway of glycolysis, whereas DHAP is not. Hence Triose-phosphate isomerase converts DHAP into GAP useful for generating ATP. Thus net result
  • is that glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate.
  • This reaction is rapid and reversible.

Step 6 : Oxidative phosphorylation of GAP to 1,3-Bisphosphoglycerate

  • The first step in the payoff phase is the oxidation of glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate.
  • This reaction is catalyzed by glyceraldehyde 3-phosphate dehydrogenase.
  • It is the energy-yielding reaction. Reactions of this type in which an aldehyde group is oxidised to an acid are accompanied by liberation of large amounts of potentially useful energy. During this reaction, NAD+ is reduced to NADH.
  • This is a reversible reaction.

Step 7 : Conversion of 1,3-Biphosphoglycerate to 3-Phosphoglycerate

  • The enzyme phosphoglycerate kinase transfers the high-energy phosphoryl group from the carboxyl group of 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.
  • This is a unique example where ATP can be produced at substrate level without participating in electron transport chain. This type of reaction where ATP is formed at substrate level is called as Substrate level phosphorylation.

Step 8 : Conversion of 3-Phosphoglycerate to 2-Phosphoglycerate

  • 3-phospho glycerate is isomerized to 2-phospho glycerate by shifting the phosphate group from 3rd to 2nd carbon atom.
  • The enzyme is phosphogluco mutase.
  • This is a readily reversible reaction.
  • Mg2+ is essential for this reaction.

Step 9 : Dehydration of 2-Phosphoglycerate to Phosphoenolpyruvate

  • 2-phosphoglycerate is converted to phosphoenol pyruvate by the enzyme enolase.
  • One water molecule is removed.
  • A high energy phosphate bond is produced. The reaction is reversible.
  • Enolase requires Mg++.

Step 10 : Conversion of Phosphoenol Pyruvate to Pyruvate

  • Phosphoenol pyruvate (PEP) is dephosphorylated to pyruvate, by pyruvate kinase.
  • First PEP is made into a transient intermediary of enol pyruvate; which is spontaneously isomerized into keto pyruvate, the stable form of pyruvate.
  • One mole of ATP is generated during this reaction. This is again an example of substrate level phosphorylation.
  • The pyruvate kinase is a key glycolytic enzyme. This step is irreversible.

Additional Step in Anaerobic Condition

When animal tissues cannot be supplied with sufficient oxygen to support aerobic oxidation of the pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by the reduction of pyruvate to lactate.

Some tissues and cell types (such as erythrocytes, which have no mitochondria and thus cannot oxidize pyruvate to CO2) produce lactate from glucose even under aerobic conditions.

The reduction of pyruvate is catalyzed by lactate dehydrogenase.


Net energy (ATP) yield per molecule of Glucose in Glycolysis

Energy Yield in Aerobic Glycolysis

StepEnzymeSourceNo. of ATP
6Glyceraldehyde-3- phosphate dehydrogenaseNADH(+3) x 2 = +6
7Phosphoglycerate kinaseATP(+1) x 2 = +2
10Pyruvate kinaseATP(+1) x 2 = +2
Net Yield  8 ATPs

Energy Yield in Anaerobic Glycolysis

StepEnzymeSourceNo. of ATP Formed/consumed
7Phosphoglycerate kinaseATP(+1) x 2 = +2
10Pyruvate kinaseATP(+1) x 2 = +2
Net Yield  2 ATPs

Significance of the Glycolysis Pathway

  1. Glycolysis is the only pathway that is takes place in all the cells of the body.
  2. Glycolysis is the only source of energy in erythrocytes.
  3. When performing physically-demanding tasks, muscle tissues may experience an insufficient supply of oxygen, the anaerobic glycolysis serves as the primary energy source for the muscles.
  4. The glycolytic pathway may be considered as the preliminary step before complete oxidation.
  5. It provides carbon skeletons for non-essential amino acid synthesis including the glycerol portion of fat.
  6. The majority of glycolytic pathway reactions are reversible, which is essential for gluconeogenesis or the formation of new glucose.

Frequently Asked Questions

Q 1. How many ATP are produced in glycolysis ?

At the end of the glycolysis process, a total of two (2) ATP is produced.

Q 2. Where does glycolysis occur?

It occurs in the cell’s cytoplasm.

Q 3. What is the end product of glycolysis?

The end products of glycolysis are two ATP, two NADH, and two pyruvates.

Q 4. Is glycolysis aerobic or anaerobic?

The glycolysis process itself is anaerobic, but after finishing the glycolysis process, the cell will continue respiration, which can move in the direction of aerobic or anaerobic. The choice primarily depends on the circumstances of the cell.

Q 5. What is the function of glycolysis?

It is the pathway of all cells in the body.
It is the main source of energy for the red blood cells.
It supplies the cells ample level of oxygen when performing strenuous activities.
It gives carbon skeletons for non-essential amino acid synthesis.
It is vital for the formation of new glucose.

Q 6. Does glycolysis occur in all cells?

Glycolysis occurs in both eukaryotic and prokaryotic cells.

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  1. This is a very clear description of glycolysis. It helped us to understand and memorize the steps of glycolysis very easily. Thank you very much.

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