Higher Biology 2025, Question 6: metabolic pathways, induced fit and feedback inhibition

01Read the diagram before you read the question

The diagram in Question 6 is fully explained by the text above it. Isoleucine is the final product of the pathway, but the question directs you to the first stage: threonine is the substrate, enzyme 1 is the catalyst, and intermediate A is the product.

Thirty seconds mapping substrate, enzyme and product onto the diagram makes every part of this question shorter.

02Part (a): catabolic or anabolic?

The word "breakdown" in the stem is the giveaway. Catabolic reactions break larger molecules down into smaller ones, so the answer is catabolic.

03Describing induced fit for the mark

The examiner splits this into two parts, and so should your answer.

The first part is a describe command word, which needs a statement, not a single word. Threonine is the substrate of enzyme 1, so the two are specific to one another; when threonine binds, an induced fit is achieved. What you must actually write is that the active site changes shape so that it better fits the threonine molecule.

Writing "induced fit" on its own earns nothing. Describe what induced fit does to the active site.

The second part asks how this increases the rate of reaction. Stating that the activation energy of the reaction is lowered earns the second mark.

Being confident about what is and is not on the course specification helps here. Trying to explain the molecular mechanism of how induced fit lowers activation energy is a good way to run out of room and miss mark two.

04Spotting feedback inhibition

Three words in the stem, concentration, high level, and blocked, all point to feedback inhibition, and isoleucine is the end product of the pathway. Because this is a describe question, you need a full statement: isoleucine reaches a critical concentration, then binds to and inhibits an earlier enzyme in the pathway.

The second part uses suggest, so you are applying knowledge to a new situation. The advantage is conservation of energy and resources. The marking instructions also accept prevention of overproduction of isoleucine.

05The part most candidates dropped

According to the 2025 course report, fewer than 25% of candidates answered the final part correctly. The trap is momentum: having just written about feedback inhibition, it is easy to skim the next stem and write about it again. Feedback inhibition would decrease the rate of reaction, so it cannot be the answer here.

If you feel like you are repeating yourself from question to question, you have almost certainly misread one of them.

06What to take into your next past paper

Map the diagram first, split describe questions into as many statements as there are marks, and treat any question that seems to want the answer you just gave as a signal to reread the stem.