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Exam questions · Biology · Enzymes and Digestion

Investigating the Effect of pH on Amylase

  • 10 exam questions
  • 29 marks
  • 10 quick checks
  1. 1 Name [1 mark]

    Name the solution used to show when all of the starch has been digested.

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    Model answer

    Iodine solution.

    Mark scheme

    • Iodine (solution) — 1 mark
  2. 2 Explain [2 marks]

    Figure 1 shows the apparatus used to investigate amylase. Explain the purpose of the water bath and of the spotting tile.

    A water bath, test tube, thermometer, dropping pipette, spotting tile and stopwatch, with no labels.
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    Model answer

    The water bath keeps the temperature constant, so that temperature does not affect the rate. The spotting tile holds drops of iodine, so that samples of the mixture can be tested.

    Mark scheme

    • Water bath keeps the temperature constant — 1 mark
    • Spotting tile holds drops of iodine for testing samples — 1 mark
  3. 3 Explain [2 marks]

    A buffer solution is added to the mixture of starch and amylase. Explain why.

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    Model answer

    The buffer keeps the pH constant. This means that the pH stays at the value being tested, so only the pH changes.

    Mark scheme

    • Keeps the pH constant — 1 mark
    • At the value being tested / so pH is the only variable — 1 mark
  4. 4 State [3 marks]

    In an investigation into the effect of pH on the rate of reaction of amylase, state the independent variable, the dependent variable and one control variable.

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    Model answer

    The independent variable is the pH. The dependent variable is the time taken for the starch to be digested. A control variable is the temperature (or the volume or concentration of the amylase or starch).

    Mark scheme

    • Independent variable: pH — 1 mark
    • Dependent variable: time for the starch to be digested / rate — 1 mark
    • Control variable: e.g. temperature, volume or concentration — 1 mark
  5. 5 Calculate [2 marks]

    At pH 7 the mean time taken for the starch to be digested was 60 seconds. Use rate = 1000 ÷ time to calculate the rate. Give your answer to one decimal place.

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    Model answer

    1000 ÷ 60 = 16.7.

    Mark scheme

    • 1000 ÷ 60 — 1 mark
    • 16.7 — 1 mark
  6. 6 Calculate [3 marks]

    Figure 2 shows the results of the investigation. (a) Use Figure 2 to give the optimum pH. (b) The rate at pH 8 was 8.0. Use rate = 1000 ÷ time to calculate the time taken at pH 8.

    A graph of rate against pH for amylase with five plotted points and a curve that peaks at pH 7.
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    Model answer

    (a) pH 7. (b) Time = 1000 ÷ 8.0 = 125 seconds.

    Mark scheme

    • (a) pH 7 — 1 mark
    • (b) 1000 ÷ 8.0 — 1 mark
    • 125 seconds — 1 mark
  7. 7 Describe [3 marks]

    Describe the effect of pH on the rate of reaction shown in Figure 2.

    A graph of rate against pH for amylase with five plotted points and a curve that peaks at pH 7.
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    Model answer

    The rate increases as the pH rises from 5 to 7. The rate is highest at pH 7, at about 16.7. Above pH 7 the rate decreases.

    Mark scheme

    • Rate increases from pH 5 to 7 — 1 mark
    • Highest at pH 7 (about 16.7) — 1 mark
    • Decreases above pH 7 — 1 mark
  8. 8 Explain [3 marks]

    Explain why the rate of reaction is lower at pH 9 than at pH 7.

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    Model answer

    pH 9 is not the optimum pH for amylase. The shape of the active site changes and the enzyme is denatured. The starch no longer fits the active site, so fewer enzyme-substrate complexes form.

    Mark scheme

    • pH 9 is away from the optimum — 1 mark
    • The active site changes shape / the enzyme is denatured — 1 mark
    • The starch no longer fits / fewer complexes form — 1 mark
  9. 9 Suggest [4 marks]

    The investigation only used whole pH values from 5 to 9, and judged the colour of the iodine by eye. Suggest two improvements to the method, and explain why each would help.

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    Model answer

    Test more pH values near the optimum, such as 6.5, 7 and 7.5, to find the optimum pH more precisely. Use a colorimeter to measure the colour of the iodine, so that the end point is not decided by eye.

    Mark scheme

    • Test pH values in smaller steps near pH 7 — 1 mark
    • To find the optimum more precisely — 1 mark
    • Use a colorimeter / colour standards — 1 mark
    • So that the end point is not subjective — 1 mark
  10. 10 Describe [6 marks]

    Describe how you would investigate the effect of pH on the rate of reaction of amylase.

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    Model answer

    Put starch, amylase and a buffer of a certain pH into a test tube in a water bath at a set temperature. Start a stopwatch. Every 30 seconds, take a drop of the mixture and add it to a drop of iodine on a spotting tile. When the iodine stays orange-brown, all the starch has been digested, so record the time. Repeat with buffers of different pH, keeping the temperature, volumes and concentrations the same. Calculate the rate as 1000 ÷ time and repeat each pH to find a mean.

    Mark scheme

    • Mix starch, amylase and a buffer — 1 mark
    • In a water bath at a constant temperature — 1 mark
    • Take drops every 30 seconds and add them to iodine — 1 mark
    • Time taken for the iodine to stay orange-brown — 1 mark
    • Repeat with different pH buffers, keeping other variables the same — 1 mark
    • Calculate the rate / repeat to find a mean — 1 mark

Quick check

  1. 1

    What does the colour of iodine solution tell you in this investigation?

    1. AThe pH of the mixture
    2. BWhether starch is still present
    3. CThe temperature
    4. DHow much amylase there is
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    B: Whether starch is still present

    Iodine is blue-black when starch is present and orange-brown when the starch has all been digested.

  2. 2

    What is the independent variable?

    1. AThe time taken
    2. BThe temperature
    3. CThe volume of starch
    4. DThe pH of the buffer
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    D: The pH of the buffer

    The pH is the variable that you change deliberately.

  3. 3

    What is the dependent variable?

    1. AThe time taken for the starch to be digested
    2. BThe pH
    3. CThe type of enzyme
    4. DThe size of the test tube
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    A: The time taken for the starch to be digested

    The time taken for the starch to be digested is what you measure.

  4. 4

    Why is a water bath used in this investigation?

    1. ATo change the pH
    2. BTo make the starch
    3. CTo keep the temperature constant
    4. DTo make the iodine work
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    C: To keep the temperature constant

    It keeps the temperature the same in every test, so that only the pH changes.

  5. 5

    What is the purpose of a buffer?

    1. ATo act as an enzyme
    2. BTo keep the pH constant
    3. CTo heat the mixture
    4. DTo add starch
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    B: To keep the pH constant

    A buffer keeps the pH constant at the value being tested.

  6. 6

    The time taken at one pH is 50 s. What is the rate, using rate = 1000 ÷ time?

    1. A0.05
    2. B5
    3. C50
    4. D20
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    D: 20

    1000 ÷ 50 = 20.

  7. 7

    At which pH was amylase fastest in the example investigation?

    1. ApH 7
    2. BpH 5
    3. CpH 9
    4. DpH 3
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    A: pH 7

    The rate was highest at pH 7, at about 16.7.

  8. 8

    Why does the rate fall at a pH far from the optimum?

    1. AThe starch disappears
    2. BThe enzyme is used up
    3. CThe enzyme is denatured
    4. DThe iodine stops working
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    C: The enzyme is denatured

    The active site changes shape and the enzyme is denatured, so fewer substrate molecules fit.

  9. 9

    Which change would make the results more precise?

    1. AUsing a colder water bath
    2. BSampling every 10 seconds
    3. CUsing less starch each time
    4. DTaking only one sample
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    B: Sampling every 10 seconds

    Sampling every 10 seconds, instead of every 30 seconds, gives a more precise value for the time.

  10. 10

    A student says the rate at pH 9 is lower than at pH 7 because the enzyme is used up. What is the best response?

    1. AEnzymes are not used up; the enzyme is denatured, so the starch does not fit
    2. BThat is correct
    3. CThe enzyme has turned into starch
    4. DThe iodine has used up the enzyme
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    A: Enzymes are not used up; the enzyme is denatured, so the starch does not fit

    Enzymes are not used up. At pH 9 the enzyme is being denatured, so the active site no longer fits the starch.