2.4 Required practical: pH and amylase
7 exam questions worth 17 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Give the independent variable and the dependent variable in this investigation.
Mark scheme
- independent: pH (1)
- dependent: time taken for starch to be broken down / for iodine to stay orange(-brown) (1)
Model answer
The independent variable is the pH. The dependent variable is the time taken for the starch to be broken down.
Why is a buffer solution added to the mixture?
Mark scheme
to keep the pH constant / to set the pH (of the mixture)
Model answer
The buffer keeps the mixture at a fixed pH.
The test tubes of amylase and starch were left in the water bath for 5 minutes before they were mixed.
Explain why.
Mark scheme
- so the solutions reach the temperature of the water bath / 35 °C (1)
- so temperature is controlled / the temperature does not affect the rate / fair test (1)
Model answer
This lets both solutions reach 35 °C before the reaction starts, so the temperature stays the same and does not affect the results.
At pH 6 the iodine stayed orange-brown after 120 seconds.
Calculate the rate of reaction using rate = 1000 / time. Give your answer to 1 decimal place.
Mark scheme
- 1000 ÷ 120 (1)
- = 8.3 (s-1) (1)
Model answer
rate = 1000 ÷ 120 = 8.3 s-1.
At pH 2, the iodine was still blue-black after 10 minutes.
Explain this result.
Mark scheme
- the active site (of amylase) changes shape / amylase is denatured (1)
- so starch is not broken down / starch is still present (1)
Model answer
At pH 2 the shape of the active site of the amylase changes, so the enzyme is denatured. The starch cannot fit, so it is not broken down and the iodine stays blue-black.
Suggest one change to the method that would make the time measurements more accurate. Explain your answer.
Mark scheme
- take samples more often / every 10 seconds (1)
- so the end point is found closer to the true time (1)
- or: use a colorimeter (1) so the colour change is not judged by eye (1)
Model answer
Take a sample every 10 seconds instead of every 30 seconds, so the time when the starch has all gone is known more precisely.
Describe a method to find the optimum pH of amylase.
You should include how you would make it a fair test.
Mark scheme
Level 3 (5–6 marks): a clear, logical method that would give valid results, with control of variables and repeats. Level 2 (3–4 marks): a method with most steps, some control of variables. Level 1 (1–2 marks): some relevant steps.
Indicative content:
- iodine in a spotting tile
- mix amylase, starch and a buffer of known pH
- water bath to keep the temperature constant
- take a drop every 10 or 30 seconds and add it to iodine
- record the time when iodine stays orange-brown / does not turn blue-black
- repeat with buffers of different pH values
- keep volumes and concentrations of amylase and starch the same
- repeat and calculate a mean; calculate rate = 1000 / time
- the pH with the shortest time / highest rate is the optimum
Model answer
Put a drop of iodine in each well of a spotting tile. Put 2 cm3 of amylase, 2 cm3 of starch and 1 cm3 of pH 5 buffer in a water bath at 35 °C for 5 minutes. Mix them and start a stopwatch. Every 30 seconds put a drop of the mixture on the iodine, and record the time when it stays orange-brown. Repeat with buffers of pH 4, 6, 7, 8 and 9. Keep the temperature, volumes and concentrations the same each time. Repeat each pH three times and calculate a mean, then a rate as 1000 ÷ time. The pH with the highest rate is the optimum.
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