With reference to glucose metabolism and cellular respiration, consider the following statements:

1. The conversion of succinate to fumarate in the TCA cycle is unique because it is the only step catalysed by an enzyme embedded in the inner mitochondrial membrane, and it directly links the TCA cycle to the electron transport chain.
2. Oxidative phosphorylation is the only stage of cellular respiration where oxygen is directly consumed, and all the ATP generated in this stage is produced by substrate-level phosphorylation.
3. In glycolysis, the enzyme phosphofructokinase-1 (PFK-1) is allosterically inhibited by high concentrations of ATP and citrate, making it a key regulatory point for both glycolysis and the TCA cycle.

Which of the statements given above are correct?

Show answer and explanation

Correct answer: (b) Statements 1 and 3 only

Explanation

Statement 1 is correct: succinate dehydrogenase, which turns succinate into fumarate, is the only Krebs cycle enzyme bound to the inner mitochondrial membrane. It is also complex II of the electron transport chain, so it links the two.
Statement 2 is wrong: oxygen is used as the final electron acceptor in oxidative phosphorylation, but the ATP made there comes from chemiosmosis (ATP synthase driven by a proton gradient), not from substrate-level phosphorylation. Substrate-level phosphorylation happens in glycolysis and the Krebs cycle.
Statement 3 is correct: phosphofructokinase-1 is a key control point of glycolysis. High levels of ATP and citrate (from the Krebs cycle) inhibit it, slowing glycolysis when the cell has enough energy.

So statements 1 and 3 are correct.