Enzymes (OCR AS Biology)

Exam Questions

3 hours33 questions
1a
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1 mark

Amylase is an enzyme which digests starch polymers. It is present in saliva in the mouth.

State why amylase is described as an extracellular enzyme. 

1b
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2 marks

Describe how the structure of amylase allows it to catalyse the digestion of starch. 

1c
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4 marks

Complete the sentences using the most appropriate terms. Your terms can be single words or short phrases.

In the modified model of enzyme activity, the enzyme and substrate interact. This is called the …………… hypothesis. The model suggests that the initial interactions are ……………, but these interactions cause …………… changes in the enzyme’s …………… and the substrate. This maximises the activity of the enzyme.

1d
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2 marks

When food and saliva are swallowed and enter the stomach, amylase can no longer digest starch.

Suggest why amylase activity stops in the stomach. 

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2a
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2 marks

Figure 2.1 shows how temperature affects the rate of an enzyme-catalysed reaction.

ocr-a-2-4e-sq-temperature-enzyme-activity

Figure 2.1

Define the temperature coefficient. 

2b
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3 marks

Outline why enzyme activity is lower when the temperature is lower. 

2c
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3 marks

Outline how denaturation occurs.

2d
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1 mark

The optimum temperature of the enzyme in Figure 2.1 is 35℃.

Suggest why this enzyme is likely to have been derived from a mammal.

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3a
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1 mark

A student investigated the activity of catalase, an enzyme which converts hydrogen peroxide into water and oxygen. Catalase was added to solutions containing hydrogen peroxide mixed with a buffer with varying pH. The reaction releases oxygen, creating a foam layer on the surface of the solution.

To measure the rate of the enzyme-controlled reaction, they measured the height of the foam. Their results are shown in Table 1 below.

Table 1

pH

Height of foam /mm 
1 2 3 Average
2.0 9.0 3.0 9.0 7.0
4.0 4.0 12.0 7.0 7.7
6.0 7.0 11.0 10.0 9.3
8.0 36.0 43.0 30.0 36.3
10.0 2.0 4.0 3.0 3.0

 

Identify the optimum pH for catalase activity. 

3b
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2 marks

List two variables which the student should control in their experiment.

3c
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2 marks

State why the height of the foam is lowest at pH 10.

3d
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4 marks

Describe what happens to catalase at pH 10. 

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4a
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4 marks

Aspirin is an irreversible inhibitor of COX enzymes. COX enzymes are involved in synthesising prostaglandins and thromboxane, which produce inflammation.

Describe the mechanism of action of a competitive inhibitor. 

4b
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2 marks

Outline how competitive inhibitors affect the rate of reaction of an enzyme. 

4c
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2 marks

Outline why aspirin is used for the treatment of menstrual pain.

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5a
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2 marks

Cofactors and coenzymes can play important roles in enzyme activity.

Define the terms in the table below. 

Term Definition
Cofactor  
Coenzyme  
5b
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4 marks

Describe the role of cofactors in the digestion of starch. 

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1a
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1 mark

Icefish live in very cold water.

Icefish contain biological molecules that allow them to tolerate cold temperatures.

Adaptations can be grouped into three general categories.

Which category of adaptation is represented by cold-tolerant molecules?

1b
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2 marks

One example of a cold-tolerant molecule present in icefish is a modified form of the protease enzyme trypsin.

Fig. 3 shows how trypsin is converted from a molecule called trypsinogen. This conversion occurs in the lumen of the small intestine.q3b-paper-3-june-2019-ocr-a-level-biology

Fig. 3

State two conclusions that can be drawn from Fig. 3 about the roles of the molecules and ions that affect how trypsin functions.

1 .....................................................................................
2 ......................................................................................

1c
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9 marks

A group of students investigated the effect of temperature on the activity of two forms of trypsin: human trypsin and icefish trypsin.

Part of their method is shown below:

  • use 10cm3 of 5% trypsin solution for all trials
  • measure enzyme activity at 10, 20, 30, 40 and 50°C for both enzymes
  • carry out each trial in the same pH buffer
  • repeat the experiment 5 times at each temperature
  • measure enzyme activity by recording the area of gelatine on a photographic film that is broken down over a set time period
  • calculate the rate of enzyme activity at each temperature.
(i)
Suggest and explain two improvements that would increase the validity of the students’ investigation.

Improvement .....................................................................................................................
Explanation .......................................................................................................................

Improvement .....................................................................................................................
Explanation .......................................................................................................................
[4]

(ii)
Suggest appropriate units to use to represent the rate of enzyme activity in this investigation.
[1]
(iii)
The students recorded the temperature that produced the fastest reaction rate in each of the five replicates. These results are shown in Table 3.

Replicate Temperature that produced the fastest reaction rate (°C)
Human trypsin Icefish trypsin
1 40 20
2 10 10
3 30 20
4 40 30
5 40 30
Mean = 32.0 22.0
Mode = 40 20 and 30
Median = 40 20

Table 3

One of the students made the following statement:

I think the mean is a more accurate measure than the median or mode for these results.

Evaluate the student’s statement.

 [2]

(iv)
The students wanted to know whether there was a difference between the reaction rates of the two forms of trypsin at 30°C.

They performed a statistical test on the mean of the five replicates for human trypsin and the five replicates for icefish trypsin.

Suggest the most appropriate statistical test for the students to use and explain why this test is appropriate.
 [2]

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2
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8 marks

Liver cells have a high metabolic rate. Hydrogen peroxide is a metabolic product produced in significant quantities in liver cells. It needs to be removed in order to prevent serious damage to the liver cells.

Hydrogen peroxide is detoxified by the enzyme catalase:

                     2H2O2 rightwards arrow2 H2O + O2


Catalase has a very high turnover number. A single catalase molecule can catalyse the breakdown of approximately 6 million hydrogen peroxide molecules every minute. Catalase is found in peroxisomes inside the liver cells. Peroxisomes are organelles surrounded by a single membrane.

The activity of catalase was investigated in a laboratory, using chopped liver tissue and dilute hydrogen peroxide. When the chopped liver was added to the hydrogen peroxide large quantities of froth as bubbles of oxygen were produced in the liquid.

Fig. 17.3
shows the effect of increasing enzyme concentration on the rate of the reaction.

q17c-paper-1-specimen-ocr-a-level-biology

Fig. 17.3

(i)
Identify two variables that would need to be controlled in this laboratory investigation.

1 .........................................................

2 .........................................................

[1]

(ii)
How could you control one of the variables that you identified in (i) in the laboratory investigation?

[1]

(iii)
Using the information given in part (a), deduce why and how catalase activity is regulated inside the liver cells.

[6]

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3a
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2 marks

A researcher investigated the effect of pH on the activity of stomach enzyme pepsin. 

Their results are shown in the Fig.1 below. 

pepsin-rate-of-reaction-at-different-ph-sq

Fig.1

The rate of reaction can be calculated by using the following formula:

reaction space rate space equals space fraction numerator Amount space of space product space produced space left parenthesis straight g space dm to the power of negative 3 end exponent right parenthesis over denominator Time space left parenthesis straight s right parenthesis end fraction

Calculate the rate of reaction at pH 4. Give your answer with the correct units.

3b
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2 marks

Describe the differences between the curves at pH 2 and pH 4.

3c
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1 mark

State why product production at pH 2 does not continue indefinitely but reaches a plateau at around 14.75 g.

3d
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3 marks
(i)

Predict the outcome if the pH were increased to pH 10.

[1]

(ii)

Explain your answer at part (i).

[2]

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4a
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3 marks

Lysozyme is an enzyme found in many secretions of the body, such as tears, saliva and milk. It is able to break down the peptidoglycan cell walls of bacteria and form part of the body's chemical defence system against pathogens.

The diagram in Fig.1 below shows a molecular model of lysozyme.

3-1-fig-1-1Fig.1

With reference to the parts labelled A and B in Fig.1, explain the term 'secondary structure'.

4b
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2 marks

Enzymes, such as lysozyme, are described as being 'biological catalysts'.

Outline what is meant by the term 'biological catalyst'.

4c
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2 marks

State whether lysozyme can be classified as an intracellular or extracellular enzyme and explain your answer.

4d
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3 marks

Lysozyme molecules will eventually stop working and are broken down.

Outline how cells replace the enzymes that are broken down.

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5a
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2 marks

The graph in Fig.1 below represents the change in the amount of energy needed for two reactions (P and Q) to occur over time.

3-1-fig-2-1Fig.1

(i)
Identify which reaction (P or Q) is catalysed by an enzyme.
[1]
(ii)
Explain your answer in part (i).
[1]
5b
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1 mark

The enzyme lactase catalyses the breakdown of the glycosidic bond in lactose.

A student investigated the effect of increasing the concentration of lactose on the rate of activity of lactase.

Five test tubes were set up with each containing 3 cm3 of different concentrations of a lactose solution. The test tubes were placed in a water bath at 38°C for ten minutes. A container with a lactase solution was also put into the water bath.

After ten minutes, 1 cm3 of the lactase solution was added to each test tube. The test tubes were returned to the water bath and kept at 38°C for another ten minutes.

Thereafter, the temperature of the water bath was raised to boiling point before Benedict solution was added to each test tube. The time taken for a colour change was recorded and used to calculate the rate of enzyme activity.

The results are shown in Fig.2

3-1-fig-2-2
Fig.2

Explain why the action of lactase can be described as 'catabolic'.

5c
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1 mark

State what happened to lactase when the temperature in this investigation was raised to boiling point.

5d
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5 marks

Describe and explain the results shown on the graph in Fig.2.

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1a
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4 marks

A team of scientists investigated a species of archaea isolated from hydrothermal vents. The microorganism, Pyrococcus furiosus, is described as a hyperthermophile because it can survive in extremely high temperatures.

The scientists purified É‘-glucosidase from Pyrococcus furiosus, an enzyme essential for the metabolism of carbohydrates. They measured the rate of enzyme activity in a range of temperatures. Their results are shown in Table 1 below.

Table 1

Temperature / ℃ Rate of Reaction / Arbitrary Units
50 0.01
60 0.05
70 0.08
80 0.12
85 0.21
90 0.35
95 0.50
100 0.61
102 0.80
105 0.99
107 1.00
110 0.99
115 0.94

Plot the results from Table 1 above. 

1b
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2 marks

Calculate the temperature coefficient for the reaction between 100 ℃ and 110 ℃. Give your answer to 1 decimal place.

1c
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2 marks

Suggest how this enzyme achieves temperature resistance. 

1d
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2 marks

Studying the enzymes of thermophilic archaea can have many applications for research, industry and medicine.

Suggest two potential uses for temperature-resistant enzymes. 

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2a
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4 marks

A student investigated the activity of pectinase, an enzyme which breaks down pectin, a polysaccharide present in plant cell walls. When pectinase is added to fruit, it catalyses the breakdown of cells, speeding up the production of juice.

The student added 20 g of apple to a beaker with 5 cm3 of pectinase and 5 cm3 of various pH buffers. After incubation for 10 minutes at 60℃, the student measured the volume of juice produced (cm3). Their results are shown in Table 1 below.

Table 1

pH

Volume of juice extracted / cm3
1 2 3 Mean
2 10 11 12 11
4 18 17 19 18
6 9 10 11 10
8 7 8 6 7
10 6 5 4 5

Plot the results of Table 1 above, showing the mean volume of juice extracted against pH level.

2b
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4 marks

Explain why the volume of juice extracted is lower in the pH 6 buffer compared to the pH 4 buffer. 

2c
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4 marks

Describe how an acidic environment impacts pectinase's enzyme structure.

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3a
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5 marks

A student investigated the activity of catalase, an enzyme which catalyses the breakdown of hydrogen peroxide into water and oxygen. The volume of oxygen released in a given length of time can be used as a measure of the rate of reaction.

The student investigated the effect of varying the concentration of substrate on the rate of reaction. Their results are given in Table 1 below.

Table 1

Concentration of hydrogen peroxide

Volume of oxygen gas collected / cm3 

at various time intervals

5s 15s 25s 35s 45s
High 41 65 76 77 77
Medium 13 24 28 28 28
Low 9 17 17 17 17


Interpret the results in Table 1 above.

3b
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2 marks

Suggest two variables the student should control in their experiment. 

3c
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3 marks

Explain how you would dilute a stock solution of 3% hydrogen peroxide with water to prepare three 10 cm3 solutions of varying concentrations as shown in the table 1 below.

Table 1

Solution concentration / %

Instructions 

0.3

 

0.03

 

0.006

 

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4a
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1 mark

Figure 4.1 shows the effect of two inhibitors on the rate of an enzyme-catalysed reaction.

ocr-a-2-4h-sq-inhibitor-graph

Figure 4.1

Identify which curve, A-C, represents an enzyme-catalysed reaction in the presence of a non-competitive inhibitor. 

4b
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5 marks

Describe the effect of non-competitive inhibitors on the activity of an enzyme, with reference to Figure 4.1. 

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5a
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2 marks

A student investigated the activity of amylase in the presence or absence of chloride ions. They recorded the rate of formation of maltose against time. The results are plotted in Figure 5.1.

ocr-a-2-4h-sq-amylase-graph

Figure 5.1


The rate of maltose formation in the first 30 seconds of the experiment, for both curves.

Give your answers in cm3min-1 to 2 decimal places.

5b
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3 marks

Describe, with reference to Figure 5.1, the role of chloride ions in amylase activity. 

5c
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3 marks

Suggest how chloride ions might affect the structure of amylase.

5d
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1 mark

Suggest how the student measured the formation of the product in this experiment.

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