Free Enzymes questions for IGCSE Biology (0610), part of Biological molecules & enzymes. Each question has the answer and the marking points an examiner would look for.
An enzyme solution is boiled for 5 minutes and then cooled to its optimum temperature. It is mixed with its substrate. What happens?
A No reaction takes place because the active site has changed shape permanently.
B The reaction is slower because there are fewer substrate molecules.
C The reaction is faster because the enzyme molecules have more kinetic energy.
D The reaction takes place at the optimum rate because the enzyme has cooled.
Show answer
Answer: A
Question 2Multiple choice · 1 mark
What is an enzyme?
A a carbohydrate that speeds up reactions
B a mineral needed for growth
C a fat that stores energy
D a protein that functions as a biological catalyst
Show answer
Answer: D
Question 3Structured · 5 marksA* challenge
An enzyme in the stomach works best at pH 2. An enzyme in the small intestine works best at pH 8.
(a)
Explain why the stomach enzyme does not work in the small intestine.
[2]
(b)
Describe how you could investigate the effect of pH on an enzyme.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
wrong pH, denatured
the pH is not optimum B1 the enzyme is denatured/active site shape changes B1
2
(b)
buffers, same temperature, measure rate
use buffer solutions of different pH B1 keep temperature and concentrations constant B1 measure the rate (e.g. time for starch to disappear with iodine) B1
3
Question 4Structured · 14 marks
A student investigates the effect of temperature on the activity of amylase. She mixes amylase with starch suspension and takes a drop every 30 seconds to test with iodine solution.
temperature / °C
10
20
30
40
50
60
time for starch to disappear / s
1110
570
300
150
780
not digested
Table 4.1
(a)
State the colour of iodine solution in the presence of starch, and when starch is absent.
[2]
(b)
Name the product of starch digestion by amylase.
[1]
(c)
Calculate the rate of reaction at 30 °C, using rate = 1000 / time.
[1]
(d)
Describe the effect of temperature on the activity of amylase shown by the results.
[3]
(e)
Explain why the enzyme does not digest starch at high temperatures.
[3]
(f)
Explain why the rate is low at 10 °C.
[2]
(g)
Suggest how the student could find the optimum temperature more precisely.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
blue-black; brown/orange
blue-black with starch B1 stays brown / orange without B1
2
(b)
maltose
maltose B1
1
(c)
3.33 s⁻¹
1000 / 300 = 3.33 B1
1
(d)
activity increases up to about 40 °C, then decreases; no activity at the highest temperatures
Any 3 of: activity increases from 10 °C to an optimum at about 40 °C; activity decreases above the optimum; no digestion at 60 °C; data quote with units B3
3
(e)
enzyme denatured; active site changes shape; substrate no longer fits
the enzyme is denatured B1 the shape of the active site changes B1 starch no longer fits / is not complementary, so no enzyme–substrate complexes form B1
3
(f)
less kinetic energy, fewer collisions, fewer enzyme–substrate complexes
molecules have less kinetic energy B1 fewer successful collisions between enzyme and substrate / fewer enzyme–substrate complexes B1
2
(g)
repeat at smaller intervals around the optimum, e.g. every 2 °C
Any 2 of: use smaller temperature intervals around the optimum; e.g. every 1–2 °C between 30 and 50 °C; repeat and calculate means B2
2
Question 5Structured · 5 marks
Amylase digests starch.
(a)
Describe how iodine solution can show when starch has been digested.
[2]
(b)
State three variables to control in an investigation into the effect of temperature on amylase.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
samples onto spotting tile; no blue-black
take samples at intervals onto a spotting tile with iodine B1 when it no longer turns blue-black, starch has gone B1
2
(b)
pH; volumes; concentrations
pH B1 volume and concentration of starch B1 volume and concentration of amylase B1
3
Question 6Structured · 13 marks
A student investigates the effect of pH on the activity of pepsin.
(a)
Define the term enzyme.
[2]
(b)
State where pepsin is produced in the body and the substrate it digests.
[2]
(c)
Sketch a graph, with labelled axes, to show how the rate of reaction of pepsin changes with pH.
Fig. 6.1
[2]
(d)
Explain why enzyme activity falls at pH values far from the optimum.
[3]
(e)
Explain why an enzyme only catalyses one reaction.
[2]
(f)
Describe how the student could use buffer solutions to carry out the investigation fairly.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
protein that acts as a biological catalyst
a protein B1 that functions as a biological catalyst (speeds up reactions without being changed) B1
2
(b)
stomach; protein
stomach B1 protein B1
2
(c)
peak at pH 2; falls on either side
axes labelled rate and pH, curve rising to a single peak at about pH 2 B1 falling on both sides of the optimum B1
2
(d)
bonds holding active site change; shape changes; substrate no longer fits (denatured)
the bonds holding the protein’s shape are disrupted B1 the active site changes shape B1 the substrate no longer fits so fewer enzyme–substrate complexes form (denatured) B1
3
(e)
active site complementary in shape to one substrate
the active site has a specific shape B1 that is complementary to only one substrate B1
2
(f)
same volume of buffer at range of pHs; keep temperature, enzyme and substrate concentrations constant
add equal volumes of buffer solutions of different pH B1 keep temperature, volumes and concentrations of enzyme and substrate the same B1
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Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in