Enzymes (temperature and pH) – IGCSE Biology exam questions with mark scheme

1Grade 4–56 marks

The graph shows the effect of temperature on the rate of a reaction controlled by an enzyme.

(a) State the optimum temperature for this enzyme. [1]

(b) Explain why the rate of reaction increases between 10 °C and 40 °C. [2]

(a) 40 °C (unit needed)(1)
(b) (enzyme and substrate) molecules have more kinetic energy / move faster(1)
(b) more (successful) collisions / more enzyme–substrate complexes formed (per second)(1)
(c) enzyme is denatured; reject 'enzyme killed' / 'enzyme dies'(1)
(c) (shape of) active site changes(1)
(c) substrate no longer fits / is no longer complementary so fewer enzyme–substrate complexes form(1)
Answer: (a) 40 °C (b) more kinetic energy, more collisions (c) denatured, active site changes shape, substrate no longer fits
Examiner tip: Never write that the enzyme is 'killed' — enzymes are proteins, not living things, and this is rejected.

2Grade 8–9Triple only6 marks

Design an investigation to find out how pH affects the rate at which the enzyme amylase breaks down starch.

Include experimental details in your answer. [6]

use a range of (at least three) pH values, set using buffer solutions, e.g. pH 4, 6, 8; allow range of pH values set by adding acid / alkali; accept check pH with a pH meter / universal indicator (C — what is changed)(1)
control the quantity of enzyme / substrate: same volume or concentration of amylase and/or starch (one correctly controlled quantity is enough); accept same source / batch of amylase (O / S1)(1)
keep the temperature constant (S2), e.g. using a water bath at 35–40 °C; a stated constant temperature without naming the water bath is enough; accept equilibrate solutions in the water bath before mixing(1)
every 30 s / at regular intervals remove a drop of the mixture and add to iodine (solution) on a spotting tile (M1); allow take samples after a fixed time and test with Benedict's (heat) for reducing sugar(1)
record the time taken until the iodine stays orange-brown / no longer turns blue-black (all starch digested) (M2); accept brown / yellow / orange for the end colour; accept rate = 1 ÷ time; accept measure colour change with a colorimeter; allow compare Benedict's colour / amount of precipitate after the fixed time(1)
repeat at each pH and calculate a mean (R / M); if not awarded, accept any other valid detail not already credited, e.g. control with boiled amylase / water instead of amylase; add buffer to the starch (or enzyme) before mixing; plot rate (1 ÷ time) against pH(1)
Answer: Vary pH with buffers; fixed volumes/concentrations of amylase and starch; water bath; sample into iodine at intervals; time until no blue-black; repeat and mean.
Examiner tip: Many answers forget to say HOW the end point is detected: sampling into iodine on a spotting tile and timing until it no longer turns blue-black are each worth a separate mark.

Still losing marks on this topic?

Book a free introductory session and we'll find exactly which step is costing you.

Book a free intro