Glycolysis is the initial stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of cells and involves the breakdown of glucose into pyruvate. In the process, ATP molecules are produced.

Net Gain of ATP Molecules by Glycolysis

The net gain of ATP molecules by glycolysis is two. This occurs through the following steps:

1. Energy Investment Phase

In this phase, two ATP molecules are used to convert glucose into fructose-1,6-bisphosphate. This reaction is catalyzed by the enzyme phosphofructokinase.

Glucose + 2ATP → Fructose-1,6-bisphosphate + 2ADP

2. Cleavage Phase

In this phase, fructose-1,6-bisphosphate is cleaved into two molecules of glyceraldehyde-3-phosphate. This reaction is catalyzed by the enzyme aldolase.

Fructose-1,6-bisphosphate → 2 Glyceraldehyde-3-phosphate

3. Energy Liberation Phase

In this phase, each molecule of glyceraldehyde-3-phosphate is converted into pyruvate. This reaction involves the production of four ATP molecules and two NADH molecules.

2 Glyceraldehyde-3-phosphate + 4ADP + 2Pi + 2NAD+ → 2 Pyruvate + 4ATP + 2NADH + 2H+

Overall, the net gain of ATP molecules by glycolysis is two. This is because two ATP molecules are used in the energy investment phase, and four ATP molecules are produced in the energy liberation phase, resulting in a net gain of two ATP molecules.