Questions: ATP Synthesis and Oxidative Phosphorylation

Question 1

What directly powers the mechanical rotation of ATP synthase's rotor subunit during oxidative phosphorylation?

• A. Electron flow through cytochrome c
• B. Hydrolysis of NADH in the mitochondrial matrix
• C. Protons flowing down their electrochemical gradient through the Fo subunit
• D. Direct phosphoryl transfer from FADH₂ to ADP

Answer: (C)

Protons (H⁺) flow from the intermembrane space (where the ETC has accumulated them) back into the matrix through the Fo subunit of ATP synthase, moving down their electrochemical gradient. This flow drives the physical rotation of the c-ring rotor. The rotational energy is transmitted to the F1 subunit, where conformational changes in the β subunits catalyze ADP + Pi → ATP. Electrons do not pass through ATP synthase; their flow through the ETC creates the gradient that then drives the synthase.

Question 2

The proton gradient that drives ATP synthesis is purely a chemical (concentration/pH) gradient — the electrical component across the inner mitochondrial membrane does not contribute to driving ATP synthesis.

• T. True
• F. False

Answer: False

The driving force is an electrochemical gradient, also called the proton-motive force. It has two components: a pH gradient (ΔpH — the matrix is more basic than the intermembrane space) and a membrane potential (Δψ — the matrix is electrically negative relative to the intermembrane space). Both components contribute energy to drive H⁺ through ATP synthase. In mitochondria, the electrical component (Δψ) actually contributes more to the total proton-motive force than the pH gradient.

Question 3

Why is the process called 'oxidative' phosphorylation when ATP synthase itself does not carry out any oxidation reactions?

Model answer: The 'oxidative' refers to the oxidation of NADH and FADH₂ by the electron transport chain, not to ATP synthase itself. These oxidation reactions release electrons that are passed through the ETC to oxygen, and this electron flow powers the pumping of protons that drives ATP synthase. Phosphorylation (ADP → ATP) is coupled to — and dependent on — that upstream oxidation, hence the combined term.

This naming often confuses students who focus only on the ATP synthase step. The full process is a two-part coupled system: (1) oxidation of reduced coenzymes by the ETC creates the proton gradient, and (2) phosphorylation of ADP uses that gradient's energy. 'Oxidative phosphorylation' names the entire coupled process, distinguishing it from substrate-level phosphorylation (where ATP is made directly without a gradient, as in glycolysis).