Question

What is glycolysis? Explain it with a neat labeled diagram. Explain its uses.

Answer 1

Glycolysis (Embden-Meyerhof pathway): The oxidation of glucose to pyruvate and lactate. It is the principal route for glucose metabolism for the production of ATP molecules.

Complete answer:
Glycolysis is defined as the sequence of reactions converting glucose (or glycogen) to pyruvate or lactate, with the production of ATP. Glycolysis takes place in all the cells of the body. The enzymes of this pathway are present in the cytosol (cytoplasm) of the cell. Glycolysis occurs in the absence of oxygen (anaerobic conditions) or in the presence of oxygen (aerobic conditions). Lactate is the end product under anaerobic conditions. In the aerobic condition, pyruvate is formed, which is then oxidized to carbon dioxide and water.
The reaction of Glycolysis:
1. Glucose is phosphorylated (addition of phosphate group) to glucose 6-phosphate by enzyme hexokinase. This is an irreversible reaction, dependent on ATP and Mg ion.
2. Glucose 6-phosphate undergoes isomerization to give fructose 6-phosphate in the presence of the enzyme phosphohexose isomerase and Mg ion.
3. Fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate by enzyme phosphofructokinase (PFK). This is an irreversible step in glycolysis.
4. The six-carbon fructose 1,6- bisphosphate is split (hence the name glycolysis) into 2 three-carbon compounds, glyceraldehyde 3-phosphate, and dihydroxyacetone phosphate by the enzyme aldolase.
5. The enzyme phosphotriose isomerase catalyzes the interconversion of glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. This interconversion is reversible. So, Two molecules of glyceraldehyde 3-phosphate are obtained from one molecule of glucose.
6. Enzyme glyceraldehyde 3-phosphate dehydrogenase converts glyceraldehyde 3-phosphate to 1,3-bisphosphoglycerate. This step is important as it is involved in the formation of NADH + H+.
7. The enzyme phosphoglycerate kinase acts on a 1,3-bisphosphoglycerate molecule resulting in the formation of 3-phosphoglycerate and synthesis of ATP.
8. 3-Phosphoglycerate molecule is converted to a 2-phosphoglycerate molecule by the enzyme phosphoglycerate mutase. This is an isomerization reaction.
9. The high energy compound phosphoenolpyruvate is generated from 2-phosphoglycerate by the enzyme enolase.
10.The enzyme pyruvate kinase catalyzes the transfer of high energy phosphate from phosphoenolpyruvate to ADP, leading to the formation of ATP.
Glucose (6C)

Glucose-6-phosphate (6C)

Fructose-6-phosphate (6C)

Fructose1, 6-bisphosphate (6C)

                                   Glyceraldehyde-3-phosphate Dihydroxy acetone phosphate   


       (3C) (3C)

1,3 bisphosphoglyceric acid (3C)

3-phosphoglyceric acid (3C)

 2-phosphoglycerate (3C)







   phosphoenolpyruvate (3C)







        Pyruvic acid (3C) 

Uses: Glycolysis is a major pathway for ATP synthesis in tissues lacking mitochondria, e.g. erythrocytes, cornea, lens, etc.
Glycolysis is very essential for the brain which is dependent only on glucose for energy.
Glycolysis is a central metabolic pathway with many of its intermediates being precursors to other pathways.
The occurrence of glycolysis is very essential in skeletal muscle during strenuous exercise where oxygen supply is very limited (anoxic conditions).

Note: Production of ATP in glycolysis: Under anaerobic conditions, 2 ATP are synthesized while, under aerobic conditions, 8 ATP is synthesized.
In aerobic conditions there is a production of 4 ATPs in the seventh and tenth step of glycolysis. 2 ATPs are used in the first 2 steps resulting in a net formation of 2 ATPs. In the presence of mitochondria and oxygen, the NADH produced in the sixth step of glycolysis can participate in the shuttle pathways for the synthesis of ATP. 3 ATPs are generated from each molecule of NADH.
Net ATP production: 2+6 = 8 ATPs.