Question 1Multiple choice · 1 markA* challenge
Which row shows the optimum pH of two digestive enzymes?
| pepsin (stomach protease) | salivary amylase | |
|---|---|---|
| A | about 2 | about 11 |
| B | about 7 | about 7 |
| C | about 7 | about 2 |
| D | about 2 | about 7 |
Show answer
Answer: D
10 free IGCSE Biology Biological Molecules & Enzymes questions written in the style of Cambridge Biology papers: 1 multiple-choice and 9 structured, each with its answer and mark scheme. 4 of them are A* challenge questions, built to catch the mistakes strong students still make.
This topic covers Biological molecules, Food tests, Enzymes. Work through the questions below, then generate a full Biological Molecules & Enzymes paper with a fresh set every time.
Generate a full Biological Molecules & Enzymes paper →Which row shows the optimum pH of two digestive enzymes?
| pepsin (stomach protease) | salivary amylase | |
|---|---|---|
| A | about 2 | about 11 |
| B | about 7 | about 7 |
| C | about 7 | about 2 |
| D | about 2 | about 7 |
Answer: D
The activity of an enzyme changes with temperature.
Explain why the rate increases from 10 °C to 35 °C.
Explain why the rate decreases sharply above 45 °C.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | more kinetic energy, more collisions molecules have more kinetic energy B1 more frequent effective collisions between enzyme and substrate B1 | 2 |
| (b) | denaturation, active site shape changes the enzyme is denatured B1 the shape of the active site changes B1 the substrate no longer fits B1 | 3 |
DNA carries genetic information.
State which bases pair together in DNA.
State the name of the shape of a DNA molecule.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | A–T and C–G adenine with thymine B1 cytosine with guanine B1 | 2 |
| (b) | double helix double helix B1 | 1 |
DNA is a large molecule found in the nucleus.
Describe the structure of DNA.
Explain why the order of bases is important.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | double helix; bases paired A–T, C–G two strands coiled into a double helix B1 each strand has bases B1 bases pair A with T and C with G B1 | 3 |
| (b) | codes for amino acid sequence the sequence of bases codes for the sequence of amino acids B1 in a protein B1 | 2 |
A student mixes starch solution with amylase at different temperatures. Every 10 s a drop of the mixture is tested with iodine solution. The time taken for the starch to disappear is recorded in Table 5.1.
| temperature / °C | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|
| time for starch to disappear / s | 300 | 150 | 80 | 140 | 600 |
State the colour of iodine solution when starch is present.
The rate of reaction can be calculated as 1000 ÷ time. Calculate the rate at 40 °C.
Explain why the rate at 30 °C is lower than the rate at 40 °C.
Explain the result at 60 °C.
The student concludes that the optimum temperature of amylase is 40 °C. Suggest how she could find the optimum temperature more precisely.
Suggest one source of error in the method, and how it could be reduced.
Name the product of starch digestion by amylase.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | blue-black blue-black B1 | 1 |
| (b) | 12.5 1000 ÷ 80 = 12.5 B1 | 1 |
| (c) | less kinetic energy, fewer collisions molecules have less kinetic energy B1 fewer (successful) collisions between enzyme and substrate / fewer enzyme–substrate complexes formed B1 | 2 |
| (d) | enzyme denatured; active site shape changes; starch no longer fits most amylase molecules are denatured B1 the shape of the active site changes B1 starch fits fewer active sites, so it is broken down only very slowly / not at all B1 | 3 |
| (e) | smaller intervals 35–45 °C; repeats test more temperatures between 30 °C and 50 °C / at smaller intervals (e.g. every 2 °C) B1 repeat each temperature and calculate a mean B1 | 2 |
| (f) | testing every 10 s too coarse; colour judgement; mix not at temperature source of error, e.g. tested only every 10 s / solutions not at the test temperature before mixing / judging the colour is subjective B1 matching improvement, e.g. test more often / pre-incubate both solutions / compare with a colour standard B1 | 2 |
| (g) | maltose maltose B1 | 1 |
Amylase digests starch.
Describe how iodine solution can show when starch has been digested.
State three variables to control in an investigation into the effect of temperature on amylase.
| 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 |
A student tests a food sample for starch, reducing sugar, protein and fat.
Describe the test for reducing sugar, including the positive result.
Describe the test for protein, including the positive result.
Describe the test for starch.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | Benedict’s, heat, brick red add Benedict’s solution B1 heat in a water bath B1 blue to brick red/orange B1 | 3 |
| (b) | biuret, purple add biuret reagent B1 blue to purple/violet B1 | 2 |
| (c) | iodine, blue-black add iodine solution B1 orange-brown to blue-black B1 | 2 |
Water is important to organisms.
State two roles of water as a solvent in organisms.
| Part | Answer and marking points | Marks |
|---|---|---|
| digestion; excretion; transport Any 2 of: in digestion; transport of substances (e.g. in blood plasma/xylem); excretion of urea in urine B2 | 2 |
A student investigates the effect of pH on the activity of pepsin.
Define the term enzyme.
State where pepsin is produced in the body and the substrate it digests.
Sketch a graph, with labelled axes, to show how the rate of reaction of pepsin changes with pH.
Explain why enzyme activity falls at pH values far from the optimum.
Explain why an enzyme only catalyses one reaction.
Describe how the student could use buffer solutions to carry out the investigation fairly.
| 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 | 2 |
A student tests a sample of crisps.
Describe the result if the crisps contain fat, using the ethanol emulsion test.
Explain why a control test with water is carried out alongside each food test.
State a safety precaution for the ethanol emulsion test.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | cloudy white emulsion a cloudy white emulsion forms B1 | 1 |
| (b) | compare colour to compare the colour and show the reagent alone does not give a positive result B1 | 1 |
| (c) | no naked flames keep ethanol away from flames (it is flammable) B1 | 1 |
In the 0610 syllabus, Biological Molecules & Enzymes covers Biological molecules, Food tests, Enzymes.
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