In chemiosmotic phosphorylation, what is the most direct source of energy that is used to convert ADP + Pi + ATP?

A) energy released as electrons flow through the electron transport system

B) energy released from substrate-level phosphorylation

C) energy released from ATP synthase pumping hydrogen ions against their concentration gradient

D) energy released from movement of protons through ATP synthase

E) No external source of energy is required because the reaction is exergonic.

The correct answer will be option-D.

Explanation:

During the electron transport chain, an electrochemical gradient is established due to the movement of electrons.

The high number of protons gets accumulated in the intermembrane space of the mitochondria whereas less number of protons present in the matrix.

The protons are pushed from the outer membrane to matrix through ATP synthase where the movement of ATP causes synthase to spin and bind ADP and Pi to produce ATP.

Thus, option-D is the correct answer.