Question

Substrate level phosphorylation results in the production ofATP molecules. During which of the following processes does substrate level phosphorylation occur?
I. Glycolysis
II. Krebs Cycle
III. Electron Transport Chain

A. I only
B. II only
C. III only
D. I and II only
E. II and III only

Answer 1

Hint: Substrate Level Phosphorylation is the process of production of ATP (Adenosine triphosphate) by the direct physical addition of a phosphate group to ADP (Adenosine diphosphate) during aerobic respiration.

Complete answer:
The process of aerobic respiration comprises of a sequence of reactions and cycles namely:
- Glycolysis
- Link reaction/Intermediate reaction where Acetyl Coenzyme A is formed
- Krebs cycle
- Electron Transport Chain
- Oxidative Phosphorylation

Now looking at the question, we need to focus on the production of ATP molecules, which takes place in two different steps- Glycolysis and Krebs Cycle.

> Glycolysis: As we can see the whole process of glycolysis in the given diagram, there are two steps where ATP is produced.
- When 1,3 - bisphosphoglycerate is converted into 3 - phosphoglycerate.
- When Phosphoenolpyruvate is converted into pyruvate.

> Krebs Cycle: 2 molecules of ATP are produced during every conversion of \[Succinyl - CoA\] into Succinate.
Thus, ATP is produced during Glycolysis in the cytoplasm under anaerobic conditions and during Krebs Cycle in the matrix of mitochondria.
There is no direct production of ATP in the process of the Electron Transport chain.

Hence, the correct answer is option (D).

Note: As the question asserts only on the substrate level phosphorylation to produce ATP, we will only take into account the direct production of ATP in the whole respiration. We will not take into account indirect production like $FAD{H_2}$, $NADH$, or GTP.