Enzymes: why high temperature stops an enzyme working, mark by mark

THE QUESTION

3 marks, and each one is for a different sentence

One word decides the first mark, and a very common answer uses the wrong one. The other two marks are for saying what changes and what that stops.

WHAT EARNS THE MARKS

The sentence each mark is for

1. Mark 1 of 3At a high temperature the enzyme is denatured.The scheme wants this word. An enzyme is a protein, so it cannot be killed and it does not die. Denatured is the term for what heat does to it.
2. Mark 2 of 3The shape of the active site changes.This says what denaturing means. The active site is the part of the enzyme the substrate fits into, and heat changes its shape.
3. Mark 3 of 3The substrate no longer fits into the active site, so no enzyme-substrate complex forms.The last mark is for the consequence. Naming the active site is one idea. Saying the substrate can no longer bind to it is a second, and the scheme pays for each.

WHAT LOSES THEM

Answers that sound right and score less

• A student writesThe heat kills the enzyme.Enzymes are not alive. Killed and dies are rejected in mark schemes for exactly this reason, and the first mark goes with them.
• A student writesThe enzyme is destroyed.It is too vague to earn the mark for denatured, and it says nothing about the active site.
• A student writesThe enzyme changes shape, so it cannot work.Cannot work restates the question. The mark is for the substrate no longer fitting the active site.
• A student writesThe substrate changes shape.It is the enzyme that changes. The substrate is the same molecule it was before.

THE REST OF THE CHAPTER

The same pattern on other enzyme questions

Low temperature and high temperature are marked as opposites, and mixing them up is the other common way to lose marks in this chapter.

• Low temperatureThe enzyme is inactive and not denatured. The molecules have little kinetic energy, so the enzyme and substrate collide less often. Warming it restores the activity.
• High temperatureThe enzyme is denatured. The change to the active site is permanent, so cooling it does not restore the activity.
• The wrong pHThe active site also changes shape at a pH far from the optimum, so the substrate no longer fits.