BiologyMad A-Level Biology



ENZYME QUESTIONS

1. The turnover number of an enzyme is defined as the number of substrate molecules converted to product by one molecule of enzyme in one minute. In an experiment carried out at 20°C, the turnover number for an enzyme was found to be 2500 at the start of the experiment but dropped to 1000 after 5 minutes.

(a) (i) Suggest why the turnover number decreased after 5 minutes.

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(2)

(ii) How would you expect the turnover number to differ from 2500 at the start of an identical experiment but carried out at 30°C? Explain your answer.

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(2)

(b) Explain why it would be important to have a control in the experiment at 20°C and at 30°C.

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(1)

(Total 5 marks)

2. The total amount of product formed in an enzyme-controlled reaction was investigated at two different temperatures, 55 °C and 65 °C. The results are shown in the graph.

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(a) (i) Explain how you would calculate the rate of the reaction at 55 °C over the first 2 hours of the investigation.

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(1)

(ii) Explain why the initial rate of this reaction was faster at 65 °C than it was

at 55 °C.

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(3)

(b) Use your knowledge of enzymes to explain the difference in the two curves between 4 and 6 hours.

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(1)

(c) In this investigation, the enzyme and its substrate were mixed in a buffer solution.

What was the purpose of the buffer solution?

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(1)

(Total 6 marks)

3. Read the following passage.

Starch is an important storage carbohydrate in most plants and is found as insoluble granules

in the cytoplasm. It is a polymer of glucose.

Corn starch is a very cheap substance and can be converted to fructose using various enzymes

obtained from microorganisms. The starch suspension is heated to a temperature of 105°C

5 and an enzyme called α-amylase obtained from bacteria is added. It is a thermostable enzyme and it begins to hydrolyse the starch. The temperature is then reduced to 90°C and hydrolysis

continues for 1 - 2 hours. The bonds within the polysaccharide molecule are hydrolysed and

the long chains are broken into smaller chains called dextrins.

The next step involves the conversion of dextrins to glucose and this is carried out by a fungal

10 enzyme called amyloglucosidase. The substrate is adjusted to conditions producing the

maximum rate of reaction, a temperature of 60 0C and a pH of 4.5, before the enzyme is

added. Amyloglucosidase removes glucose, one molecule at a time from the end of the

dextrin molecule.

After concentration, the resulting glucose syrup can be converted into fructose. This process

15 involves glucose isomerase produced by bacteria.

Use information from the passage and your own knowledge to answer the following questions.

(a) Draw a simple flow chart showing the steps in converting starch to glucose. Add to your flowchart the names of the enzymes that control the different steps.

(2)

(b) Explain what is meant by a thermostable enzyme (line 5 – 6).

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(1)

(c) The diagram shows part of a dextrin molecule.

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(i) Draw a similar diagram to show the structure of one of the molecules produced when amyloglucosidase is added to dextrin.

(2)

(ii) Glucose is a reducing sugar. Describe how you would test for the presence of a reducing sugar in a dextrin solution which has been incubated with amyloglucosidase.

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(2)

(d) Explain why α-amylase may be described as a polymer.

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(2)

(e) Describe how and explain why the rate of reaction of amyloglucosidase would vary with temperature.

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(6)

(Total 15 marks)

4. Meat is the muscle tissue of an animal. When animals are slaughtered, biochemical changes occur in the muscle which result in the meat becoming tough. There are several ways of making the meat tender again. These involve partly breaking down the proteins which make up muscle tissue.

(a) One way is to add a protein-digesting enzyme called papain just before freezing the meat. As the meat is thawed and cooked, this enzyme digests the protein. Use your knowledge of enzymes to explain why:

(i) the rate at which the protein is digested increases as the meat warms up in the early stages of cooking;

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(3)

(ii) protein digestion stops once the meat has been heated to 90 °C.

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(3)

(b) Another way is to leave the meat for 10 days before freezing. Explain how lysosomes in muscle cells make the meat tender before it is frozen.

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(2)

(Total 8 marks)

5. (a) The graph shows the energy changes which take place during a chemical reaction.

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(i) Use the graph to explain what is meant by the term activation energy.

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(1)

(ii) Draw a curve on the graph to show the energy changes which would take place if the same chemical reaction were catalysed by an enzyme.

(2)

The flow chart shows the way in which fructose is produced from starch in the food industry.

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(b) Describe a biochemical test which could be used to show that reducing sugars were produced in the first stage of this process.

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(2)

(c) Acid could have been used in place of the α-amylase in the first stage of this process. Suggest why:

(i) acid could have been used;

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(1)

(ii) acid was not used.

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(1)

(d) In the laboratory, the optimal conditions for bacterial α-amylase are a pH of 7 and a temperature of 80°C.

In terms of your knowledge of the way in which enzymes work, explain why the rate of reaction would change if:

(i) the temperature fell by 10°C;

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(2)

(ii) the pH changed substantially.

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(3)

(Total 12 marks)

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