Question

Explain why all the steps of glycolysis, beginning with the substrate glyceraldehyde-3-phosphate, occur twice.

Answer 1

Glycolysis is also known as EMBDEN-MEYERHOF PATHWAY. It is the pathway in which glucose is split into two 3-carbon pyruvate molecules under aerobic conditions; or lactate under anaerobic conditions, along with production of a small quantity
of energy. It is the only pathway that is taking place in all the cells of the body. Glycolysis is the only source of energy in erythrocytes. In strenuous exercise, when muscle tissue lacks enough oxygen, anaerobic glycolysis forms the major source of energy for muscles.

The steps of Glycolysis are:-

Step 1 -

Glucose is phosphorylated to glucose-6- phosphate & the enzyme is hexokinase, which splits the ATP into ADP, and the Pi is added on to the glucose. The energy released by the hydrolysis of ATP is utilized for the forward reaction.

Step 2-

Glucose-6-phosphate is isomerised to fructose-6- phosphate by phosphohexose isomerase

Step 3-

Fructose-6-phosphate is further phosphorylated to fructose1,6-bisphosphate & the enzyme is phosphofructokinase.

Step 4-

The fructose-1,6-bisphosphate is a 6 carbon compound & is cleaved into two 3 carbon units namely one glyceraldehyde-3-
phosphate and another molecule of dihydroxy acetone phosphate (DHAP) & the enzyme is called aldolase. Immediately the Dihydroxy acetone phosphate is isomerised to glyceraldehyde-3-phosphate by the enzyme phosphotriose isomerase. As a result of this, we can say that the glucose is now cleaved into 2 molecules of glyceraldehyde-3-phosphate. So one glucose molecule is cleaved in to two molecules of glyceraldehyde-3-phosphate. This is the reason by which from step 4 on wards all the steps/reactions happen twice.