4: Enzymes and Metabolism - Problems

Question 1

A student carried out an experiment to study the activity of catalase. He added a piece of liver tissue to a boiling tube with hydrogen peroxide solution and collected the oxygen released.

With reference to the functions of the liver, suggest why liver tissue contains a large amount of catalase.

Answer 1

Many metabolic reactions take place in the liver. Therefore, a large amount of hydrogen peroxide may be produced in the liver.
A large amount of catalase is present to catalyse the breakdown of the toxic hydrogen peroxide into water and oxygen.

Question 2

True or false: enzymes are denatured at extreme temperatures.

Answer 2

False

Question 3

Fruits and vegetables like apples, bananas and lettuce turn brown after they are cut or dropped. This is because an enzyme called polyphenol oxidase (PPO) is found in their cells. The enzyme catalyses a reaction, which is the major cause of the browning in fruits and vegetables.

word equation: polyphenolic compounds + oxygen –PPO–> brown pigment

The enzyme is found in the cytoplasm of the cells, while polyphenolic compounds are usually present in the vacuoles. Explain why after dropping an apple, part of the fruit would become brown. (3)

Answer 3

Plant tissues may be damaged by mechanical force upon dropping, the vacuole membrane and cell membrane rupture.
The ruptured vacuole membrane allows the enzyme PPO in the cytoplasm and polyphenolic compounds in the vacuole to come together.
The ruptured cell membrane exposes polyphenolic compounds to another substrate oxygen, (allowing the formation of enzyme-substrate complexes and) producing brown pigment.

Question 4

Fruits and vegetables like apples, bananas and lettuce turn brown after they are cut or dropped. This is because an enzyme called polyphenol oxidase (PPO) is found in their cells. The enzyme catalyses a reaction, which is the major cause of the browning in fruits and vegetables.

word equation: polyphenolic compounds + oxygen –PPO–> brown pigment

Explain how boiling lettuce for one minute after cutting prevents the browning of lettuce. (2)

Answer 4

Boiling brings the PPO to a high temperature that is above its optimum temperature, causing a conformational change in shape of the active site of PPO. The PPO is denatured.
It can no longer bind to the substrate molecules to form enzyme-substrate complexes. Thus it does not catalyse the conversion of polyphenolic compounds into brown pigments; the reaction does not occur.

Question 5

Fruits and vegetables like apples, bananas and lettuce turn brown after they are cut or dropped. This is because an enzyme called polyphenol oxidase (PPO) is found in their cells. The enzyme catalyses a reaction, which is the major cause of the browning in fruits and vegetables.

word equation: polyphenolic compounds + oxygen –PPO–> brown pigment

Explain how putting an apple in the refrigerator after cutting prevents the browning of apple. (2)

Answer 5

At low temperature, PPO becomes inactive.
The chance of PPO and its substrate molecules colliding to form enzyme-substrate complex with each other is low, thus the reaction occurs only at low rate.

Question 6

Fruits and vegetables like apples, bananas and lettuce turn brown after they are cut or dropped. This is because an enzyme called polyphenol oxidase (PPO) is found in their cells. The enzyme catalyses a reaction, which is the major cause of the browning in fruits and vegetables.

word equation: polyphenolic compounds + oxygen –PPO–> brown pigment

Explain how putting food products made from banana in vacuum packs prevents the browning of the food products. (2)

Answer 6

Oxygen is not available in a vacuum pack.
Oxygen is one of the substrates of the reaction. Without oxygen, the enzyme-substrate complex cannot be formed, and the enzymatic reaction cannot occur.

Question 7

(DSE 2015 IB Q7)
Hydrogen peroxide is a by-product of some metabolic reactions inside our body. If it were allowed to accumulate, it would kill our cells. Fortunately, we have an enzyme called catalase which speeds up the breakdown of hydrogen peroxide into water and oxygen. An investigation is carried out to observe the effect of temperature on the activity of catalase. Below are the steps:

1. Extract catalase from an animal organ.
2. Mix 5mL 0.1% hydrogen peroxide solution with 1mL catalase extract.
3. Place the mixture in a water bath set at 0ºC.
4. Measure and record the volume of oxygen gas released in the first 5 minutes.
5. Repeat steps 2 to 4 with the water bath set at 20ºC, 40ºC, and 60ºC.

Suggest an animal organ in which catalase is present in great abundance and from which the enzyme can be obtained. Explain why this organ has so much catalase.

Answer 7

Liver
It is the organ where many metabolic reactions take place; it is the organ for detoxification.

(Many metabolic reactions take place in the liver. Therefore, a large amount of hydrogen peroxide may be produced in the liver.
A large amount of catalase is present to catalyse the breakdown of the toxic hydrogen peroxide into water and oxygen.)

Question 8

(DSE 2015 IB Q7)
Hydrogen peroxide is a by-product of some metabolic reactions inside our body. If it were allowed to accumulate, it would kill our cells. Fortunately, we have an enzyme called catalase which speeds up the breakdown of hydrogen peroxide into water and oxygen. An investigation is carried out to observe the effect of temperature on the activity of catalase. Below are the steps:

1. Extract catalase from an animal organ.
2. Mix 5mL 0.1% hydrogen peroxide solution with 1mL catalase extract.
3. Place the mixture in a water bath set at 0ºC.
4. Measure and record the volume of oxygen gas released in the first 5 minutes.
5. Repeat steps 2 to 4 with the water bath set at 20ºC, 40ºC, and 60ºC.

Suggest and explain the importance of an important procedure that was missed out.

Answer 8

The hydrogen peroxide and catalase solution should be incubated at the set temperatures for 10 minutes before mixing.
This is to ensure that the mixture has reached the set temperature at the beginning of the reaction.

Question 9

Three test tubes (W, X, and Y) are set up to measure the rate of an enzymatic reaction. The enzyme catalyses the following reaction:
substrate P —-> product Q
In each test tube, either the concentration of substrate P or product Q was measured. The graph below shows the results.

State in which of the test tubes is the concentration of substrate P measured. (1)

Answer 9

X and Y

Question 10

Three test tubes (W, X, and Y) are set up to measure the rate of an enzymatic reaction. The enzyme catalyses the following reaction:
substrate P —-> product Q
In each test tube, either the concentration of substrate P or product Q was measured. The graph below shows the results.

Compare and explain the difference in rate of reaction at the 2nd minute and the 20th minute in tube W. (4)

Answer 10

The rate at the 2nd minute is higher than that at the 20th minute since the steepness of the curve was larger at the 2nd minute than the 20th minute.
This is because the concentration of substrate at the 2nd minute is higher than that at the 20th minute.
Therefore, the enzyme molecules collide with the substrate molecules more frequently at the 2nd minute.
The chance of forming enzyme-substrate complexes is higher, thus the rate of reaction is higher.

Question 11

Three test tubes (W, X, and Y) are set up to measure the rate of an enzymatic reaction. The enzyme catalyses the following reaction:
substrate P —-> product Q
In each test tube, either the concentration of substrate P or product Q was measured. The graph below shows the results.

The experimental condition of one of the test tubes is different from the others. Identify the test tube and give one piece of evidence from the graph to support the answer. (2)

Answer 11

X
The sum of the substrate concentration in tube Y and the product concentration in tube W is always 100%, indicating that the experimental conditions of these two tubes are the same.

Question 12

Three test tubes (W, X, and Y) are set up to measure the rate of an enzymatic reaction. The enzyme catalyses the following reaction:
substrate P —-> product Q
In each test tube, either the concentration of substrate P or product Q was measured. The graph below shows the results.

Suggest one possible difference in experimental condition between test tube X and the other test tubes. (2)

Answer 12

Lower temperature

Question 13

Jelly was used to investigate the effect of pH on the activity of a protease. Jelly contains a protein called gelatin; the protease catalyses the breakdown of gelatine into soluble products. Four petri dishes containing 10cm³ buffer solutions at different pH. Four jelly blocks of the same size were cut and one was put into each petri dish. Then, 10cm³ protease solution was added to each petri dish. After leaving the petri dishes at room temperature for two hours, the area of the upper face of each jelly block was measured. The results are shown below.

Describe and explain the effect of pH and the activity of the protease. (4)

Answer 13

When pH increases from 4 to 6, the area of the remaining jelly block decreases. This shows the activity of the protease increases as pH increases from 4 to 6.
When pH increases from 6 to 10, the area of the remaining jelly block increases. This shows the activity of the protease decreases as pH increases from 6 to 10.
The protease denatures when pH is lower or higher than the optimum pH. The substrate can no longer fit into the active site of the protease to form the enzyme-substrate complex. Therefore, the rate of the reaction is greatly lowered.

Question 14

Jelly was used to investigate the effect of pH on the activity of a protease. Jelly contains a protein called gelatin; the protease catalyses the breakdown of gelatine into soluble products. Four petri dishes containing 10cm³ buffer solutions at different pH. Four jelly blocks of the same size were cut and one was put into each petri dish. Then, 10cm³ protease solution was added to each petri dish. After leaving the petri dishes at room temperature for two hours, the area of the upper face of each jelly block was measured. The results are shown below.

Describe and explain the effect of pH and the activity of the protease. (4)

Answer 14

When pH increases from 4 to 6, the area of the remaining jelly block decreases. This shows the activity of the protease increases as pH increases from 4 to 6.
When pH increases from 6 to 10, the area of the remaining jelly block increases. This shows the activity of the protease decreases as pH increases from 6 to 10.
The protease denatures when pH is lower or higher than the optimum pH. The substrate can no longer fit into the active site of the protease to form the enzyme-substrate complex. Therefore, the rate of the reaction is greatly lowered.

Question 15

An investigation was carried out about how pH affects the activity of an enzyme used in the extraction of fruit juice. The table below shows the content of four beakers in the experiment.
The beakers were allowed to stand for 10 minutes. The contents were then filtered and the volumes of juice collected were measured:
Volume of juice collected (W, X, Y, Z) = (10, 19, 11, 11)

Explain why water was added instead of enzyme solution in some of the beakers. (2)

Answer 15

The beakers were the control set-ups with no enzymes added.
Water was added to these beakers to keep the total volume the same, so that the results of different beakers can be compared.

Question 16

An investigation was carried out about how pH affects the activity of an enzyme used in the extraction of fruit juice. The table below shows the content of four beakers in the experiment.
The beakers were allowed to stand for 10 minutes. The contents were then filtered and the volumes of juice collected were measured:
Volume of juice collected (W, X, Y, Z) = (10, 19, 11, 11)

Explain why water was added instead of enzyme solution in some of the beakers. (2)

Answer 16

The beakers were the control set-ups with no enzymes added.
Water was added to these beakers to keep the total volume the same, so that the results of different beakers can be compared.

Question 17

An investigation was carried out about how pH affects the activity of an enzyme used in the extraction of fruit juice. The table below shows the content of four beakers in the experiment.
The beakers were allowed to stand for 10 minutes. The contents were then filtered and the volumes of juice collected were measured:
Volume of juice collected (W, X, Y, Z) = (10, 19, 11, 11)

State two important controlled variables in this investigation. (2)

Answer 17

1. Temperature at which the experiment is conducted
2. The type of fruit used in each beaker

Question 18

An investigation was carried out about how pH affects the activity of an enzyme used in the extraction of fruit juice. The table below shows the content of four beakers in the experiment.
The beakers were allowed to stand for 10 minutes. The contents were then filtered and the volumes of juice collected were measured:
Volume of juice collected (W, X, Y, Z) = (10, 19, 11, 11)

Based on the results of the experiment, what can you conclude about the pH at which the enzyme works better? (2)

Answer 18

Compared with pH 7–8, the enzyme works better at pH 3–4.
At pH 3-4, the amount of juice collected was much greater in the beaker with enzyme added compared with the breaker with no enzyme, while the amount of juice collected showed no significant difference in both beakers at pH 7-8.

Question 19

An investigation was carried out about how pH affects the activity of an enzyme used in the extraction of fruit juice. The table below shows the content of four beakers in the experiment.
The beakers were allowed to stand for 10 minutes. The contents were then filtered and the volumes of juice collected were measured:
Volume of juice collected (W, X, Y, Z) = (10, 19, 11, 11)

Explain how an unsuitable pH affects the activity of an enzyme. (3)

Answer 19

At unsuitable pH, the activity of the enzyme** decreases**.
This is because unsuitable pH causes a conformational change in the active site of the enzyme and makes it denatured.
The substrates can no longer fit into the active site of the enzyme to form the enzyme-substrate complex. The enzyme loses its catalytic ability permanently, thus the rate of reaction is greatly lowered.

Question 20

(DSE Sample IB Q9)
One of the applications of enzymes in everyday life is found in biological washing powders. To study the effectiveness of a biological washing powder, three identical skirts with identical fat stains were washed with the washing powder solutions of the same concentration but at different water temperatures. The investigation and its result are outlined below:

Suggest an explanation why the fat stain can be removed by using the biological washing powder. (3)

Answer 20

The biological washing powder contains lipase,
which catalyses the breakdown of lipids into fatty acids and glyceroI.
Both products are soluble in water and can be removed from the skirt easily through washing.

Question 21

(DSE Sample IB Q9)
One of the applications of enzymes in everyday life is found in biological washing powders. To study the effectiveness of a biological washing powder, three identical skirts with identical fat stains were washed with the washing powder solutions of the same concentration but at different water temperatures. The investigation and its result are outlined below:

State the conclusion from the results of this investigation. (1)

Answer 21

The washing powder is more effective at higher water temperatures.

Question 22

(DSE Sample IB Q9)
One of the applications of enzymes in everyday life is found in biological washing powders. To study the effectiveness of a biological washing powder, three identical skirts with identical fat stains were washed with the washing powder solutions of the same concentration but at different water temperatures. The investigation and its result are outlined below:

Describe how an investigation comparing the effectiveness of the biological washing powder and the ordinary biological washing powder can be carried out. (5)

Answer 22

Use two identical skirts with identical fat stains.
Put the two skirts separately into equal volumes of the two washing powder solutions which contain the same quantity and concentration of washing powder.
The time of washing should be the same and the water temperature used for washing should be kept at 50°C.
Compare the size and intensity of the stains after washing;
the more effective the washing powder, the smaller / lighter the stain.
Repeat the investigation at different water temperatures to see if the effectiveness of different washing powders is affected by water temperature.

Question 23

The graph below shows the activity of three different proteases at different temperatures. The three proteases are added to a biological washing powder to remove different for stains.

It is stated on the packing of the washing powder that the washing powder works best at 35ºC, but for removal of stains like egg stains, a higher temperature can be used.

Explain why the washing powder works best at 35ºC. (1)

Answer 23

Enzymes X, Y and Z are all moderately active at 35 °C.

Question 24

The graph below shows the activity of three different proteases at different temperatures. The three proteases are added to a biological washing powder to remove different for stains.

It is stated on the packing of the washing powder that the washing powder works best at 35ºC, but for removal of stains like egg stains, a higher temperature can be used.

Based on the graph, suggest and explain a temperature that would be the best for the removal of egg stains. (2)

Answer 24

45 °C
45 °C is the optimum temperature of enzyme Z, which is the enzyme for removal of egg stains.

Question 25

The graph below shows the activity of three different proteases at different temperatures. The three proteases are added to a biological washing powder to remove different for stains.

It is stated on the packing of the washing powder that the washing powder works best at 35ºC, but for removal of stains like egg stains, a higher temperature can be used.

It is not recommended to use water of temperature higher than 60ºC when using this biological washing powder to remove protein stains. Explain with reference to the effects of temperature on the structure of enzymes. (2)

Answer 25

At a temperature higher than 60 °C, the activity of all the enzymes in the washing powder becomes 0 arbitrary units.
This is because the high temperature causes a conformational change in the active site of the enzymes and they are denatured.
The substrates can no longer fit into the active site of the enzymes to form enzyme-substrate complexes. The enzymes lose catalytic ability permanently.

Question 26

A glucose test strip is often used to detect the presence of glucose in a urine sample. The test strip is a thin plastic strip with a pad at one end. The pad contains two different enzymes (glucose oxidase and peroxidase) and a colourless compound. These two enzymes catalyse the following reactions:
glucose + oxygen –glucose oxidase–> gluconic acid (colourless) + hydrogen peroxide (colourless)
hydrogen peroxide + colourless compound –perioxidase–> water + coloured compound
If a urine sample contains glucose, the colour of the pad changes.

Explain why the test strip is specific for testing glucose with reference to the properties of enzymes. (2)

Answer 26

The active site of the enzyme glucose oxidase has a specific shape, so the property demonstrated by this enzyme is that enzymes are specific in action.
Only glucose can fit into the active site of glucose oxidase to form an enzyme-substrate complex and produce hydrogen peroxide, which causes the colour change.

Question 27

A glucose test strip is often used to detect the presence of glucose in a urine sample. The test strip is a thin plastic strip with a pad at one end. The pad contains two different enzymes (glucose oxidase and peroxidase) and a colourless compound. These two enzymes catalyse the following reactions:
glucose + oxygen –glucose oxidase–> gluconic acid (colourless) + hydrogen peroxide (colourless)
hydrogen peroxide + colourless compound –perioxidase–> water + coloured compound
If a urine sample contains glucose, the colour of the pad changes.

Suggest the role of peroxidase in the test strip. (2)

Answer 27

The products of the reaction catalysed by glucose oxidase are colourless.
Peroxidase catalyses the reaction between the product and a colourless compound, forming a coloured compound. Thus the result becomes visible.

Question 28

A glucose test strip is often used to detect the presence of glucose in a urine sample. The test strip is a thin plastic strip with a pad at one end. The pad contains two different enzymes (glucose oxidase and peroxidase) and a colourless compound. These two enzymes catalyse the following reactions:
glucose + oxygen –glucose oxidase–> gluconic acid (colourless) + hydrogen peroxide (colourless)
hydrogen peroxide + colourless compound –perioxidase–> water + coloured compound
If a urine sample contains glucose, the colour of the pad changes.

Explain why a refrigerated urine sample must be allowed to return to room temperature before testing. (2)

Answer 28

Enzymes become inactive at low temperatures since the chance of successful collision of enzymes and substrate molecules to form enzyme-substrate complex is low.
The strip may give a false negative result if the refrigerated sample is tested directly.

Question 29

Enzymes are proteins with specific shape. Describe the functions of enzymes in our body and explain why their shape is important in relation to their functions. (7+3)

Answer 29

Enzymes are biological catalysts.
They catalyse metabolic reactions in our body by lowering the activation energy.
Importance of the shape of the enzymes in relation to their functions:
Enzymes bind with substrate molecules to form enzyme-substrate complexes during reaction.
Each enzyme has an active site with a specific shape.
An enzyme only acts on a substrate that can fit into its active site.
Therefore, each enzyme catalyses one type of reaction only.
Factors like high temperatures or extreme pH which can cause a change in shape of the active site can affect the activity of enzymes.