Question 1
Which elements are always present in a protein?
- carbon, hydrogen and oxygen only
- carbon, hydrogen, oxygen and nitrogen
- carbon, oxygen and nitrogen only
- carbon, hydrogen and nitrogen only
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Answer: B (1)
Question 2
Complete the table to show the smaller units that make up each large molecule.
| Large molecule | Smaller units |
|---|---|
| Starch | |
| Protein | |
| Lipid |
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- starch: (simple) sugars / glucose (1)
- protein: amino acids (1)
- lipid: fatty acids (1)
- lipid: glycerol (1)
Question 3
A student is given a food sample. Describe how the student could test the sample for (a) protein and (b) starch. Include the result for a positive test in each case.
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- (a) add biuret reagent (1)
- turns from blue to purple / lilac (1) allow violet
- (b) add iodine solution (1)
- turns from orange-brown / yellow-brown to blue-black (1)
Question 4
The table shows the rate of an enzyme-controlled reaction at different temperatures. (a) State the optimum temperature for this enzyme. (b) Explain why the rate is lower at 50 °C than at 40 °C.
| Temperature in °C | Rate in arbitrary units |
|---|---|
| 10 | 2 |
| 20 | 5 |
| 30 | 9 |
| 40 | 12 |
| 50 | 3 |
| 60 | 0 |
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- (a) 40 °C (1)
- (b) the enzyme is denatured (1)
- the shape of the active site changes (1)
- the substrate no longer fits / is no longer complementary to the active site (1)
Question 5
A student investigates the effect of temperature on the time taken for amylase to break down starch. Describe how the student could carry out the investigation and how they would know when the starch had been broken down.
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- mix amylase and starch (solution) in a test tube at a set temperature, using a water bath (1)
- repeat at several different temperatures (1)
- keep pH / volumes / concentrations the same (1)
- take samples at regular intervals and add to iodine solution; starch is gone when iodine stays orange-brown / does not turn blue-black (1) allow time recorded for this
Question 6
Biology only (what this means)
Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels meanA student investigates how pH affects the activity of amylase. (a) State the independent variable. (b) State the dependent variable. (c) Suggest how the student could keep the temperature the same in every test.
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- (a) pH (1)
- (b) time taken for the starch to be broken down / rate of reaction (1)
- (c) use a water bath / thermostatically controlled water bath (1)
Question 7
Explain how changes in temperature and changes in pH affect the activity of an enzyme.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, detailed explanation covering both temperature and pH, including the optimum, the increase in rate and the denaturing of the enzyme with the change in shape of the active site. |
| 2 | 3–4 | Explains the effect of one factor in detail, or gives a partial explanation of both, with some correct use of terms such as active site or denatured. |
| 1 | 1–2 | Gives simple statements, for example that temperature or pH changes the rate, with little or no explanation. |
Indicative content
- as temperature increases the rate increases because particles have more kinetic energy and collide more often
- there is an optimum temperature where the rate is highest
- above the optimum the shape of the active site changes
- the substrate no longer fits so the enzyme is denatured and the rate falls
- each enzyme has an optimum pH where the rate is highest
- pH higher or lower than the optimum reduces the rate
- extreme pH changes the shape of the active site and denatures the enzyme