**Question:** Substrate level phosphorylation in glycolysis is seen in -

**Core Concept:** Substrate level phosphorylation is a process where a phosphate group is directly added to a substrate molecule, resulting in an increase in the molecule's energy content. In the context of glycolysis, this process refers to the addition of a phosphate group to an intermediate product during the process, resulting in an increase in its energy content.

**Why the Correct Answer is Right:** Substrate level phosphorylation occurs in glycolysis due to the involvement of enzyme phosphofructokinase (PFK), which phosphorylates fructose-6-phosphate (F6P) to form fructose-6,6-bisphosphate, a high-energy compound. This process is crucial for the energetic benefit of glycolysis, as it increases the phosphate group's energy content and allows for the efficient release of energy during the later steps of glycolysis and cellular respiration.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because substrate level phosphorylation does not occur in the first step of glycolysis, which involves the conversion of glucose-6-phosphate to glucose-6-phosphate dehydrogenase (G6PDH) reaction.

B. This option is incorrect because substrate level phosphorylation does not occur in the second step of glycolysis, which involves the conversion of glyceraldehyde-3-phosphate (GAP) to 1,3-bisphosphoglycerate (1,3-BPG) by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) reaction.

C. This option is incorrect because substrate level phosphorylation occurs in glycolysis, specifically in the conversion of F6P to fructose-6,6-bisphosphate by PFK, as explained above.

D. This option is incorrect because substrate level phosphorylation does not occur in the last step of glycolysis, which involves the conversion of 1,3-BPG to 2,3-bisphosphoglycerate (2,3-BPG) by aconitase enzyme.

**Core Concept:** In glycolysis, substrate level phosphorylation occurs due to the involvement of specific enzymes that catalyze reactions and increase the phosphate group's energy content. This process is crucial for the efficient release of energy in later steps of glycolysis and cellular respiration.

**Why Each Wrong Option is Incorrect:**

A. The G6PDH reaction is essential for the production of NADH and FADH2, which play a crucial role in the electron transport chain and ATP synthesis. However, it does not involve substrate level phosphorylation.

B. The PFK reaction is incorrect as it is the first step in glycolysis where substrate level phosphorylation occurs, specifically in the conversion of F6P to fructose-6,6-bisphosphate.

C. The aconitase enzyme is involved in the conversion of 1,3-BPG to 2,3-BPG, which is essential for the production of NADH and FADH2, but not substrate level phosphorylation.

D. The last step in glycolysis is the conversion of 1,3-BPG to 2,3