Cambridge International A Level Biology Revision Guide

Chapter 3: Enzymes
b
c
The molecule malonic acid, which is shown here, inhibits this reaction.
COOH
It does not bind permanently to the enzyme. Describe how malonic acid
inhibits the enzyme succinate dehydrogenase.
CH 2
[3]
Heavy metals such as lead and mercury bind permanently to –SH groups
COOH
of amino acids present in enzymes. These –SH groups could be in the active
site or elsewhere in the enzyme.
malonic acid
i Name the amino acid which contains –SH groups. [1]
ii Explain the function of –SH groups in proteins and why binding of heavy metals to these groups
would inhibit the activity of an enzyme. [4]
iii What type of inhibition would be caused by the heavy metals? [1]
[Total: 10]
9 You are provided with three solutions: A, B and C. One solution contains the enzyme amylase, one
contains starch and one contains glucose. Starch is the substrate of the enzyme. The product is the sugar
maltose. You are provided with only one reagent, Benedict’s solution, and the usual laboratory apparatus.
a Outline the procedure you would follow to identify the three solutions. [6]
b What type of reaction is catalysed by the enzyme? [1]
[Total: 7]
10 The activity of the enzyme amylase can be measured at a particular temperature by placing a sample into a
Petri dish containing starch-agar (‘a starch-agar plate’). Starch-agar is a jelly containing starch. One or more
‘wells’ (small holes) are cut in the agar jelly with a cork borer, and a sample of the enzyme is placed in each well.
The enzyme molecules then diffuse through the agar and gradually digest any starch in their path. At the end
of the experiment, iodine in potassium iodide solution is poured over the plate. Most of the plate will turn
blue-black as iodine reacts with starch, but a clear ‘halo’ (circle) will be seen around the well where starch has
been digested. Measuring the size of the halo can give an indication of the activity of the enzyme.
A student decided to investigate the rate at which a mammalian amylase is denatured at 60 °C. She heated
different samples of the enzyme in a water bath at 60 °C for 0, 1, 5, 10 and 30 minutes. She then allowed the
samples to cool down to room temperature and placed samples of equal volume in the wells of five starch-agar
plates, one plate for each heating period. She then incubated the plates in an oven at 40 °C for 24 hours.
The results of the student’s experiment are shown on the next page. A diagram of one dish is shown, and the
real size of one halo from each dish is also shown.
a Why did the student cut four wells in each dish rather than just one? [1]
b One dish contained samples from amylase which was not heated (time zero). This is a control dish.
Explain the purpose of this control. [1]
c Explain why the starch-agar plates were incubated at 40 °C and not room temperature. [1]
d Describe what was happening in the dishes during the 24 hours of incubation. [4]
e Why was it important to add the same volume of amylase solution to each well? [1]
f Measure the diameter in mm of the representative halo from each dish. Record the results in a suitable table. [4]
g Only one halo from each dish is shown in the diagrams. In practice there was some variation in the
diameters of the four halos in each dish. How would you allow for this when processing your data? [1]
h Plot a graph to show the effect of length of time at 60 °C on the activity of the enzyme. [5]
i Describe and explain your results. [4]
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