Enzymes: Biology 9700 (Cambridge International AS & A Level)
Syllabus 3.1, 3.2 · Strand 1 Cells and biological molecules
- Questions
- 10
- Total marks
- 66
- Tier mix
- 10 Core
0 of 10 questions completed
Syllabus coverage
- 3.1 2 questions completed
- 3.2 8 questions completed
Enzymes are the biological catalysts that make life’s chemistry fast enough to sustain it, and this topic (syllabus 3.1 and 3.2) explains both how they work and what changes their activity. Each enzyme is a globular protein with an active site whose shape is complementary to its substrate. When substrate binds, an enzyme–substrate complex forms and the activation energy of the reaction is lowered, so the reaction proceeds much more quickly. Two models describe this fit: the lock-and-key model, in which the active site is already the right shape, and the induced-fit model, in which the site moulds itself slightly around the substrate as binding occurs.
The practical heart of the topic is investigating the factors that alter reaction rate: temperature, pH, enzyme concentration, substrate concentration and the presence of inhibitors. You should be able to explain each trend, including why rate falls once an enzyme is denatured or the substrate becomes limiting. Building on this, the maximum rate, Vmax, is used to derive the Michaelis–Menten constant, Km, which compares how strongly different enzymes bind their substrates. You should also distinguish competitive inhibitors, which compete for the active site, from non-competitive inhibitors, which bind elsewhere, and appreciate why immobilising an enzyme in a bead of alginate brings practical advantages.
The exam-style questions below are original, written to match these objectives, each with a full worked solution so you can check your reasoning step by step.
Question 1
Enzymes are described as biological catalysts because they speed up the rate of intracellular reactions without themselves being permanently changed.
Which statement correctly explains how an enzyme increases the rate of a reaction?
Question 2
A student investigates how temperature affects the activity of catalase, an enzyme extracted from mushroom tissue that catalyses the breakdown of hydrogen peroxide into water and oxygen. At each temperature, the student mixes the same volume and concentration of mushroom extract with the same volume and concentration of hydrogen peroxide solution, and uses a gas syringe to collect the oxygen gas produced in the first two minutes.
| Temperature / °C | 10 | 20 | 30 | 40 | 50 | 60 |
|---|---|---|---|---|---|---|
| Volume of oxygen collected in 2 minutes / cm3 | 4 | 9 | 15 | 23 | 6 | 1 |
(a) Describe the pattern shown by the results in the table. [2]
(b) Explain, in terms of the enzyme and substrate molecules, why the volume of oxygen collected increases between 10°C and 40°C. [3]
(c) Explain why the volume of oxygen collected decreases sharply between 40°C and 60°C. [3]
Question 3
A researcher investigates an amylase enzyme that breaks down starch into maltose. In a first series of trials, she keeps the enzyme concentration, temperature and pH constant, and measures the initial rate of reaction at different starch concentrations.
| Starch concentration / % | 0.5 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 |
|---|---|---|---|---|---|---|
| Initial rate of reaction / arbitrary units | 2.1 | 4.0 | 5.8 | 7.5 | 7.6 | 7.6 |
(a) Describe the shape of the graph these results would produce, and explain, in terms of the enzyme's active sites, why the initial rate stops increasing above a starch concentration of about 2.0%. [4]
(b) The researcher repeats the experiment using double the original enzyme concentration, keeping every other condition, including the range of starch concentrations, the same. State and explain the effect this would have on the maximum rate of reaction reached. [2]
(c) In a further trial, the researcher repeats the original experiment (using the original enzyme concentration) but at a pH well below the enzyme's optimum pH. State and explain the effect this would have on the initial rate of reaction at every starch concentration tested. [3]
Question 4
A research group studies an enzyme, enzyme X, and two molecules, inhibitor P and inhibitor Q, each of which reduces the rate at which enzyme X converts its substrate into product. The group measures the initial rate of reaction across a range of substrate concentrations, with no inhibitor present, with inhibitor P present, and with inhibitor Q present. The concentration of enzyme X and of each inhibitor is kept the same throughout.
| Substrate concentration / mmol dm-3 | 1 | 2 | 5 | 10 | 20 | 50 |
|---|---|---|---|---|---|---|
| Rate, no inhibitor / arbitrary units | 10 | 18 | 30 | 38 | 42 | 44 |
| Rate, with inhibitor P / arbitrary units | 3 | 6 | 14 | 24 | 36 | 43 |
| Rate, with inhibitor Q / arbitrary units | 5 | 9 | 15 | 19 | 21 | 22 |
(a) Using the data in the table, state, with a reason, which inhibitor is a competitive inhibitor and which is a non-competitive inhibitor. [3]
(b) Explain, in terms of where each type of inhibitor binds on the enzyme, why the effect of inhibitor P becomes much smaller at high substrate concentration, but the effect of inhibitor Q does not. [4]
(c) State whether the Km of enzyme X is increased, decreased or unchanged in the presence of (i) inhibitor P and (ii) inhibitor Q, and explain what this indicates about the substrate concentration needed to reach half of the maximum rate in each case. [3]
Question 5
Enzyme R and enzyme S both catalyse the same reaction but are extracted from different organisms. When each enzyme's rate of reaction is measured across a range of substrate concentrations, enzyme R is found to have a Km of 0.15 mmol dm-3 for the substrate, while enzyme S has a Km of 6.0 mmol dm-3 for the same substrate.
(a) (i) State what is meant by the Km of an enzyme. [2]
(a) (ii) Using the data given, state and explain which enzyme, R or S, has the higher affinity for the substrate. [2]
(b) A food-processing company wants to use enzyme R repeatedly in a continuous industrial process, and decides to immobilise it by trapping it in beads of calcium alginate gel rather than using it free in solution. Describe how enzyme R could be immobilised in this way, and explain two advantages of using the immobilised enzyme rather than the free enzyme in this process. [5]
Question 6
Two models are used to describe how an enzyme's active site fits its substrate: the lock-and-key model and the induced-fit model.
Which statement correctly distinguishes the induced-fit model from the lock-and-key model?
Question 7
A technician measures the initial rate of reaction of two digestive enzymes, salivary amylase and pepsin, across a range of pH values. The substrate concentration, enzyme concentration and temperature are kept the same throughout each enzyme's series of measurements.
| pH | 1 | 2 | 3 | 5 | 7 | 9 |
|---|---|---|---|---|---|---|
| Rate with salivary amylase / arbitrary units | 1 | 2 | 4 | 15 | 26 | 3 |
| Rate with pepsin / arbitrary units | 20 | 27 | 12 | 2 | 0 | 0 |
(a) Describe the pattern shown by the results for salivary amylase across the pH range tested. [2]
(b) Explain, in terms of the enzyme's tertiary structure and active site, why the rate of reaction with salivary amylase is much lower at pH 1 and at pH 9 than it is at pH 7. [3]
(c) Salivary amylase normally acts in the mouth, while pepsin normally acts in the stomach. Using the data in the table, explain how each enzyme's optimum pH suits the environment in which it normally functions. [3]
Question 8
A student investigates how enzyme concentration affects the initial rate of an enzyme-catalysed reaction. The substrate is kept in large excess (a high, non-limiting concentration) throughout, while temperature and pH are held constant. Only the concentration of enzyme is varied.
| Enzyme concentration / % | 0 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|---|
| Initial rate of reaction / arbitrary units | 0 | 8 | 16 | 24 | 32 | 40 |
(a) Describe the relationship between enzyme concentration and initial rate of reaction shown by these results. [2]
(b) Explain, in terms of active sites, why the initial rate of reaction is directly proportional to enzyme concentration while the substrate remains in excess. [3]
(c) Use the data to calculate the initial rate of reaction that would be expected at an enzyme concentration of 6%, assuming the substrate is still in large excess at this concentration. [2]
(d) The student then repeats the experiment using a fixed, limited substrate concentration instead of a large excess, testing the same range of enzyme concentrations. Explain how the graph of initial rate against enzyme concentration would differ from the one obtained in this experiment. [2]
Question 9
An investigation follows a single enzyme-catalysed reaction over time. A fixed concentration of enzyme is mixed with a fixed initial concentration of substrate at time , and the concentration of product formed is measured at intervals. Temperature and pH are kept constant throughout.
| Time / min | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|---|---|---|
| Product formed / mmol dm-3 | 0 | 8 | 14 | 18 | 20.5 | 22 | 23 | 23.8 | 24 |
A tangent is drawn to the curve of product formed against time at ; this tangent passes through the points and .
(a) Explain why the initial rate of this reaction should be found from the tangent to the curve at , rather than from the average rate calculated over the full 10 minutes. [3]
(b) Use the tangent given to calculate the initial rate of reaction, in mmol dm-3 per minute. [2]
(c) Explain, in terms of the enzyme and substrate, why the rate of reaction decreases progressively as time passes, even though temperature and pH remain constant. [3]
(d) Explain why the curve becomes horizontal (product formed no longer increases) after about 8 to 10 minutes. [2]
Question 10
Compared with the same enzyme-catalysed reaction with no inhibitor present, a competitive inhibitor and a non-competitive inhibitor have different effects on Vmax and on Km.
Which option correctly states these effects?