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Glycolysis- Pathway, Energetics and Significance

Cellular respiration begins with glycolysis. During fermentation, the cells take small amounts of ATP in the absence of oxygen.

Glycolysis pathway and energetics

Cellular respiration begins with glycolysis. During fermentation, the cells take small amounts of ATP in the absence of oxygen. During glycolysis, glucose is divided into two molecules with three carbons each known as pyruvates so that it can be converted into energy. A significant portion of organisms alive today rely on glycolysis as their primary metabolic pathway, one that evolved long ago. Organisms use glycolysis for cellular respiration. However, glycolysis doesn't require oxygen, and it is a pathway that is also present in many anaerobic organisms.

There are two main phases of glycolysis in the cytosol of a cell: the energy-requiring phase, outlined above the dotted lines in the image below, and the energy-releasing phase, outlined below.

Energy requiring phase

In this phase, two phosphate groups are attached where the glucose molecule gets rearranged and reorganized. As a result of the phosphate groups, fructose-1,6-bisphosphate (now known as fructose-1,6-bisphosphate) becomes unstable and splits in half, forming two phosphate-containing three-carbon sugars. Within this process, phosphates are derived from the ATP molecules and hence two ATP molecules are used because of it. In the process of breaking down an unstable sugar, three-carbon sugars are formed. The following step can only be entered by one compound-glyceraldehyde-3-phosphate. DHAP, however, can easily be converted into favorable sugar, so both sugars are converted in the end.

Energy releasing phase

Each carbon atom in a sugar molecule is converted into another carbon atom in another sugar molecule, pyruvate, by a series of reactions. Two ATP molecules are produced and one NADH molecule is produced in these reactions. In all, two ATP molecules are created and two NADH molecules are generated during this phase since it takes place twice.

A different enzyme catalyzes each reaction in glycolysis. phosphofructokinase, the enzyme that catalyzes the creation of fructose-1,6-bisphosphate, is essential to the regulation of glycolysis. Phosphofructokinase increases or decreases glycolysis according to the energy requirements of the cell. In general, glycolysis produces two molecules of pyruvate from one six-carbon glucose molecule. These two molecules of NADH and four molecules of ATP (two ATP molecules produced - two ATP molecules used up) are the net products.

There are three stages in the glycolysis pathway:
Step 1
Through the action of the enzyme hexokinase, glucose is added to a phosphate group in the cytoplasm.
A phosphate group is transferred from ATP to glucose, resulting in glucose-6-phosphate.

Step 2
There is the conversion of glucose-6-phosphate into fructose, 6-phosphate and for this, an enzyme called phosphoglucomutase is responsible.

Step 3
In this process, with the help of adding phosphate group, there is an enzyme called phosphofructokinase enzyme that converts fructose 6-phosphate into fructose 1.6-biphosphate.

Step 4
By being converted to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate, aldolase can convert fructose 1,6-bisphosphate.

Step 5
As fructose 6-phosphate is converted to fructose 1,6-bisphosphate by the phosphofructokinase enzyme, the phosphate group is added.

Step 6
This step produces two reactions:
  • This is accomplished by glyceraldehyde 3-phosphate dehydrogenase, which transforms glyceraldehyde phosphate into nicotinamide adenine dinucleotide.
  • Phosphate is added to oxidized glyceraldehyde phosphate by glyceraldehyde 3-phosphate dehydrogenase to form 1,3-bisphosphoglycerate.
Step 7

ATP is formed from 1,3-bisphosphoglycerate by phosphoglycerokinase, which transfers the phosphate from ADP to phosphoglycerate. During the process of this reaction, two molecules of phosphoglycerate and ATP are produced.

Step 8
In both phosphoglycerate molecules, the phosphate is relocated to the second carbon by the enzyme phosphoglyceromutase to yield two molecules of 2-phosphoglycerate.

Step 9
The enzyme enolase converts 2-phosphoglycerate into phosphoenolpyruvate by removing a water molecule.

Step 10
Pyruvate kinase converts phosphoenolpyruvate to pyruvate and ATP by transferring phosphate to ADP. A pyruvate molecule and an ATP molecule are the end products of this process.


Eukaryotes and prokaryotes both undergo glycolysis. While there are different metabolic processes in the body, glycolysis is the most important one since it produces the intermediates needed for the other metabolic processes in the body. Organisms without mitochondria carry out glycolysis in the cytosol, which is an important process. Gluconeogenesis, fermentation, etc. all use pyruvate as a step intermediate in glycolysis. During the second stage of glycolysis, two molecules of glyceraldehyde 3-phosphate are produced from one glucose molecule, and this, two molecules of pyruvate are created to complete glycolysis. Glyceraldehyde 3-phosphate compounds are considered in the calculation of glycolysis energy.

When there is no oxygen, when there is insufficient oxygen, and if there is a high demand for energy in the muscles, then anaerobic glycolysis occurs. Lactic acid fermentation provides the RBCs with energy since they lack mitochondria. The lens of the eye is another place where anaerobic respiration occurs.

Within anaerobic glycolysis, the following two processes take place:

Lactic acid fermentation - During lactate fermentation, lactate is transformed into pyruvate by an enzyme called lactate dehydrogenase in the absence of oxygen in the muscles.

Ethanol fermentation - Fermentation of ethanol using glucose instead of pyruvate: In this process, glucose is converted into ethanol rather than pyruvate.

We can therefore conclude that anaerobic respiration results in lactic acid or ethanol accompanied by ATP molecules.


  • A common metabolic intermediate, glucose-6-phosphate, is required for various metabolic reactions, such as glycogen synthesis and HMP synthesis.
  • The synthesis of glucosamine requires fructose-6-phosphate.
  • Triose, such as glyceraldehyde - 3 - P, is used in the HMP pathway to synthesize pentose.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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