Higher Biology 2025, Question 7: the electron transport chain, ATP synthase and NAD

01Start with the stem and the diagram

Do not skip to part (a). Both the introductory sentence and the diagram tell you immediately that this question is about the electron transport chain stage of aerobic respiration, and that the blanks in the diagram will be filled in as the question goes on.

02Part (a): be specific about the location

This is a straightforward state question, but specificity matters: the answer is the inner mitochondrial membrane, not "the mitochondria".

03Filling in the blanks

For part (ii), the diagram shows an enzyme that allows hydrogen ions back across the membrane and produces ATP. That is ATP synthase, and it is the only enzyme you learn about in connection with the ETC, which is a useful sense-check when an unfamiliar diagram is making you panic.

For part (iii), use the fact that oxygen is the final acceptor of electrons and hydrogen ions, forming water. Watch where you place each answer: electrons and hydrogen ions are added to oxygen, so oxygen is molecule Y and water is molecule Z.

04The role of NAD

This is a describe question worth two marks, so you need two statements. Note the word role: you are being asked the purpose of NAD, not simply what happens to it.

NAD carries hydrogen ions and electrons, combining with them to form NADH. It carries them from the citric acid cycle in the central matrix and the glycolysis stage in the cytoplasm to the electron transport chain on the inner mitochondrial membrane.

Two locations or processes are enough for the second mark, as long as one of them is the electron transport chain.

05The mutation question: work backwards

Fewer than 25% of candidates answered this one correctly in 2025. The command word is suggest, so you are applying what you know to new information.

ATP is produced when hydrogen ions flow through ATP synthase down their concentration gradient. If ATP production is reduced, fewer hydrogen ions must be passing through ATP synthase, that is mark one.

Now link it back to the mutated carrier proteins. Fewer hydrogen ions crossing back means a shallower gradient, which means the carrier proteins are not pumping as they should. Any of these earns mark two: fewer electrons are passed along the chain to release energy; less energy is generated to actively transport hydrogen ions; fewer hydrogen ions are actively transported across the membrane.

The marking instructions give these two parts in the opposite order, but working backwards from the stated effect is far more reliable when the question is built on a process you already know.