Enzymes (OCR A Level Biology): Exam Questions

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?

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.

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 ......................................................................................

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.

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]

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 2 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.

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]

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

Their results are shown in the Fig.1 below. 

Fig.1

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

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

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

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

(i) Predict the outcome if the pH were increased to pH 10.

[1]

(ii) Explain your answer at part (i).

[2]

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.

Fig.1

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

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

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

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

Lysozyme molecules will eventually stop working and are broken down.

Outline how cells replace the enzymes that are broken down.

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.

Fig.1

(i) Identify which reaction (P or Q) is catalysed by an enzyme.

[1]

(ii) Explain your answer in part (i).

[1]

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

Fig.2

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

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

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

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 ɑ-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

Plot the results from Table 1 above. 

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

Suggest how this enzyme achieves temperature resistance. 

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

Suggest two potential uses for temperature-resistant enzymes. 

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

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

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

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

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

Interpret the results in Table 1 above.

Suggest two variables the student should control in their experiment. 

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

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

Figure 4.1

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

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

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.

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.

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

Suggest how chloride ions might affect the structure of amylase.

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