YEAR 12 BIOLOGY WORKSHEET QUESTIONS

TOPIC: ENZYMES

NAME:___________________________________

1. Enzymes are catalysts which catalyse specific reactions by lowering their activation energy.The lock and key and the induced fit models have been used to explain the way in whichenzymes work.

(a) Explain what is meant by activation energy.

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(b) (i) Describe how the lock and key model can be used to explain how an enzymebreaks down a substrate molecule.

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(ii) Describe how the induced fit model differs from the lock and key model ofenzyme action.

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Catalase is an enzyme found in many cells. It catalyses the breakdown of hydrogen peroxide as shown in the equation.

hydrogen peroxide catalase water + oxygen

Cylinders of potato were cut with a cork borer. The cylinders were then sliced into discs of the same thickness and put into a small beaker containing 50 cm3 of hydrogen peroxide. The mass of the beaker and its contents was recorded over a period of 15 minutes. The results are shown in the graph.

1 0 0

9 9

9 8

9 7

9 6

9 50 5 1 0 1 5

T im e/m in ute s

M ass o f co n ten tsof b ea k er as

p ercen tag e o fin itia l m a ss

(c) Explain why the mass of the contents of the beaker fell as the reaction progressed.

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(d) Explain, in terms of collisions between enzyme and substrate molecules, why the rate of the reaction changed over the period of time shown on the graph.

(2)(Total 8 marks)

2. Lactose is a disaccharide found in milk. Many adults are unable to digest lactose and suffer intestinal problems if they drink milk. Milk can be treated with the enzyme lactase and this reduces the amount of lactose present. The diagram shows an industrial reactor used to produce lactose-reduced milk.

L ac tose -red u ce d

m ilkR ea c to r k ep t a t 9 °C

F re sh m ilk

M ilk p assed th ro u g h re ac to r sev era l tim e s

L ac ta se im m o b ilise do n m em b ra ne s

(c) (i) Suggest one advantage of immobilising the lactase used in this reaction.

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(ii) In terms of your knowledge of the way in which enzymes work, explain why it is necessary to pass the milk through the reactor several times to reduce the amount of lactose sufficiently.

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(d) The graph shows the change in lactose concentration during the course of the reaction.

L ac to se asp ercen tag e o f

in itia l co n cen tra tio n

1 0 0

0

R eac tio n s to p p edan d m ilk rem ov e d

Tim e

(i) Explain the change in the rate of reaction with time.

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(ii) Suggest why the reaction is stopped at the time shown on the graph.

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

E n erg y

R ea c tan ts

P rod u c ts

R ea c tio n tim e

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

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(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.

s ta rch

g lu co se

g lu co se an dfru c to se

m o d ified sta rch ,d ex trin s an d

g lu co se

1 0 0 °C

5 5 ° C

5 0 ° C

-am y la se

g lu c oa m y lase

g lu c ose iso m e rase

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(b) In the laboratory, the optimal conditions for bacterial -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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(ii) the pH changed substantially.

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(3)(Total 9 marks)

4. 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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(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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(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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(Total 5 marks)

5. 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.

1 0

8

6

4

2

00 1 2 3 4 5 6 7

To ta lam ou n t

o fp ro d u c tfo rm ed /a rb itra ry

u n its

Tim e / h o u rs

5 5 °C

6 5 °C

(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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(ii) Explain why the initial rate of this reaction was faster at 65 °C than it wasat 55 °C.

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

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(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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(Total 6 marks)

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

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(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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(Total 8 marks)7. (a) When heated, hydrogen peroxide breaks down to water and oxygen.

2H2O2 2H2O + O2

The graph shows the energy changes which take place during this reaction.

XE n erg y

Tim e

(i) What is represented by the part of the curve labelled X?

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(ii) This reaction also takes place in many living cells. Here it is catalysed by the enzyme catalase. Sketch a curve on the graph to show the energy changes which take place when the enzyme is present.

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(iii) When the reaction with catalase is carried out in a test-tube, the test-tube feels warm at the end of the reaction. Use the graph to explain why.

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(b) The turnover number of an enzyme is the number of substrate molecules converted to product per second. The maximum turnover number of catalase is 200 000 molecules per second. Explain why the turnover number falls as the temperature gets lower.

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(Total 7 marks)

8. (a) Amylase is an enzyme which hydrolyses starch to maltose. Some amylase and starch were mixed and the mixture incubated at 37°C until the reaction was complete.

(i) Sketch a curve on the axes below to show the progress of this reaction.

To ta l a m o u n t o fm alto se fo rm ed

Tim e(1)

(ii) Explain why the rate of the reaction decreases as the reaction progresses.

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The effect of temperature on the rate of reaction of an enzyme was investigated. A test tube containing the enzyme and a test tube containing the substrate were incubated separately at each of the temperatures being investigated. After 5 minutes, they were mixed and the rate of reaction was determined. The experiment was repeated but, this time, the enzyme and the substrate were left for 60 minutes before they were mixed. The results of the investigation are shown in the graph.

R ate o freac tio n

In c u b a ted fo r5 m in u tes

In c u b a ted fo r6 0 m in u te s

0 1 0 2 0 3 0 4 0 5 0 60 7 0Tem p era tu re / °C

(b) The enzyme solution used in this investigation was made by dissolving a known mass of enzyme in a buffer solution. Explain why a buffer solution was used.

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(c) (i) Use the graph to describe how incubation time affects the rate of the reaction.

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(ii) The maximum rate of reaction with an incubation time of 60 minutes is less than the maximum rate of reaction with an incubation time of 5 minutes. Explain why.

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(d) Explain how inhibitors affect the rate of enzyme-controlled reactions.

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(Total 15 marks)