NEET 2020 Zoology Question 85 Solution

Substrate level phosphorylation is the direct synthesis of ATP/GTP from ADP/GDP coupled to the cleavage of a high-energy metabolic intermediate, occurring independently of the electron transport chain and ATP synthase.

During the conversion of Succinyl-CoA to succinic acid, a molecule of GTP is synthesized by substrate level phosphorylation: $$\text{Succinyl-CoA} + \text{GDP} + \text{P}_i \xrightarrow{\text{Succinyl-CoA synthetase}} \text{Succinate} + \text{CoA-SH} + \text{GTP}$$ In a coupled reaction, GTP transfers its phosphate to ADP, yielding ATP.

In ONE turn of the citric acid cycle (for each Acetyl-CoA entering the cycle), there are: - 3 steps of NADH formation - 1 step of FADH2 formation - Exactly 1 step of substrate-level phosphorylation (GTP/ATP).

Per molecule of glucose (which yields 2 molecules of Acetyl-CoA), there are two turns and two substrate-level phosphorylations. But the question explicitly specifies "in ONE turn of citric acid cycle", which is exactly One.