Question

How many ATP molecules are formed during substrate level phosphorylation?

Answer 1

Substrate-level phosphorylation is a process in which a phosphate group is directly transferred from a high-energy substrate molecule to ADP, forming ATP. This process occurs in two main stages of cellular respiration: glycolysis and the citric acid cycle (Krebs cycle).

1. Glycolysis:

In glycolysis, glucose is broken down into two molecules of pyruvate, and during this process, 2 ATP molecules are produced through substrate-level phosphorylation. Specifically, ATP is generated in the following steps:

• 1,3-Bisphosphoglycerate transfers a phosphate group to ADP, forming ATP.
• Phosphoenolpyruvate (PEP) also transfers a phosphate group to ADP to form another ATP.

2. Citric Acid Cycle:

In the citric acid cycle, which occurs in the mitochondria, one molecule of glucose (which has been converted into 2 molecules of pyruvate) yields 2 rounds of the cycle. Each round of the cycle produces 1 ATP molecule through substrate-level phosphorylation. Therefore, 2 ATP molecules are produced for each glucose molecule that enters the cycle.

Thus, a total of 4 ATP molecules are formed in the citric acid cycle (2 ATP per cycle, and there are 2 cycles per glucose molecule). When you add the 2 ATP molecules from glycolysis, you get a total of 6 ATP molecules produced through substrate-level phosphorylation.

Summary: A total of 6 ATP molecules are formed during substrate-level phosphorylation – 2 ATP molecules in glycolysis and 4 ATP molecules in the citric acid cycle.