Enzymes (Edexcel GCSE Biology)

The 'lock and key hypothesis' is often used as a model to describe the action of enzymes.

Using this model, explain how enzymes work.

Thousands of unique enzymes exist in nature.

Use your knowledge of proteins and the 'lock and key hypothesis' to suggest how so many unique enzymes can exist in nature.

State what happens when a substrate with a shape that is not complementary to the enzyme tries to enter the active site.

Figure 1 below represents the reactants and products in a chemical reaction and four different enzymes (A, B, C or D).

Figure 1

Explain which enzyme (A, B, C or D) is responsible for catalysing this reaction?

Figure 1 shows the effect of temperature on the rate of an enzyme catalysed reaction.

Figure 1

(i) Describe the effect of temperature at points A, B and C on the rate of the enzyme catalysed reaction.

(3)

(ii) Explain why the rise in temperature as shown in Figure 1, initially increases the rate of an enzyme catalysed reaction

(2)

A student conducted an experiment to investigate how starch is digested by the enzyme amylase. Four identical mixtures of starch and amylase were kept at different temperatures. Figure 2 shows the time taken for the starch to be digested at each temperature. 

Figure 2

Identify the temperature which produces the fastest rate of reaction.

(i) Starch is the substrate of salivary amylase in humans.

 State the initial product of the reaction it catalyses.

(1)

(ii) Amylase is classed as a carbohydrase.

State two other classes of enzymes that are used in human digestion.

(2)

Describe a suitable method to investigate the effect of pH on the time taken for amylase to digest starch.

You don't need to include a risk assessment in your method

Figure 1 illustrates the results of two enzyme controlled reactions involving two different types of enzymes.

Figure 1

From these reactions, what conclusion can be drawn about the function of enzymes?

Crop plants provide a source of energy in the form of carbohydrates such as starch and sugars.

(i) Describe the test to identify starch.

(2)

(ii) The amount of energy in the sugars extracted from crop plants can be measured using the calorimeter shown in Figure 1.

Figure 1

Explain why the calorimeter has a lid.

(2)

(iii) State why it is important to stir the water in the calorimeter.

(1)

A student investigated the effect of enzyme concentration on the mass of product formed in one hour.
Figure 1 shows the results of this investigation. 

Figure 1

Complete the graph by plotting the points and drawing a line to show the trend in the data.
The first three points have been plotted for you.

Describe the effect that enzyme concentration has on the mass of product formed.

The ratio of enzyme concentration to the mass of product formed, using an enzyme concentration of 40 arbitrary units, is 1:1.

Calculate the ratio of enzyme concentration to product formed when the enzyme concentration is 5 arbitrary units.

ratio............................................

The investigation used the enzyme pepsin from the stomach, at a temperature of 37°C and at a pH of 7.

(i) Which statement gives one way to increase the mass of product formed in this investigation?

(1)

☐ A increase the pH
☐ B decrease the temperature 
☐ C decrease the enzyme concentration 
☐ D increase the substrate concentration

(ii) Explain why a temperature of 80°C was not used in this investigation.

(3)

Female glow-worms have an enzyme called luciferase. The glow is produced when this enzyme catalyses a reaction between oxygen and a protein.

A scientist devised a plan to investigate the effect of oxygen concentration on this reaction.

The scientist had:

• five flasks of water each with a different concentration of dissolved oxygen

• a solution of the protein

• a solution of the enzyme.

The first step of this plan is:

Step 1. Add some of the protein solution to each of the five flasks.

(i) Describe the next two steps that should be in this plan to obtain results for this investigation.

(2)

Step 2 ...................................
Step 3 .....................................

(ii) Which procedure would improve the investigation?

(1)

☐ A change the concentration of the protein solution in each flask
☐ B change the volume of the protein solution added to each flask
☐ C keep the concentration of dissolved oxygen the same in each flask 
☐ D keep the volume of each solution the same in each flask

(iii) The enzyme luciferase works best at pH 8.

Explain why the activity of the enzyme decreases at pH 5.

(2)

A student had solutions of four different foods labelled W, X, Y and Z.

Each solution was tested for starch and protein.

The colour of the solutions after the tests are shown in Figure 1.

Figure 1

(i) Which solution contains starch but not protein?

(1)

☐ A solution W
☐ B solution X
☐ C solution Y
☐ D solution Z

(ii) Describe how a solution of food can be tested for reducing sugars.

(2)

Figure 2 shows a calorimeter.

Figure 2

Describe how this calorimeter can be used to find the energy content of 10g of food.

Lactase is an enzyme that breaks down lactose into glucose and galactose.

A student made some alginate beads containing lactase.

The student added 10 beads to 20cm³ of a solution of lactose, as shown in Figure 1.

The student timed how long it took for glucose to be produced.

The experiment was repeated using 15, 20 and 25 beads.

Figure 1

The results are shown in Figure 2.

Figure 2

(i) What is the rate of reaction for 25 beads?

(1)

☐ A 0.008s⁻¹
☐ B 0.04s⁻¹
☐ C 0.21s⁻¹
☐ D 4.8s⁻¹ 

(ii) Explain the conclusion that can be made from these results.

(3)

(iii) Explain why the same volume of lactose solution was used for each test.

(2)

Devise a method to find the optimum temperature for the enzyme lactase.

A calorimeter can be used to measure the energy content of food.

Figure 1 shows a simple calorimeter.

Figure 1

(i) Use words from the box to complete the sentences.

(2)

The stirrer distributes heat evenly in the .............................................................. .

The calorimeter has a .............................................................. to prevent the loss of heat.

(ii) This calorimeter was used to measure the energy content of a piece of cheese.

Figure 2 shows some of the results.

Figure 2

Calculate the energy content of this piece of cheese.

(2)

Use the equation

energy in joules (J) = mass of water  4.2  temperature change

energy content of the piece of cheese = ................................................ J

(iii) The same method was used to calculate the energy content of a different type of cheese.

The results are shown in Figure 3.

Figure 3

Explain one reason why the results are different for this type of cheese.

(2)

Why are enzymes called biological catalysts?

☐ A because they slow down biological processes
☐ B because they speed up biological processes
☐ C because they denature biological processes
☐ D because they stop biological processes

Many cells contain an enzyme called catalase.

Catalase breaks down hydrogen peroxide into water and oxygen.

A scientist investigated the effect of hydrogen peroxide concentration on the time taken to produce 20 cm³ of oxygen.

Figure 1 shows the equipment used.

Figure 1

(i) State how the scientist could control the temperature of the flask.

(1)

(ii) Explain why the temperature should be controlled in this investigation.

(3)

(iii) This investigation used five different concentrations of hydrogen peroxide.

Figure 2 shows the results of this investigation.

Figure 2

Complete the graph by plotting the points and drawing a line to show the trend in the data.

The first point has been plotted for you.

(2)

(iv) Describe the trend shown in the graph.

Use data from the table in Figure 2 to support your answer.

(3)

Figure 1 shows the activity of the enzymes pepsin and trypsin at different pH levels.

Figure 1

(i) Describe the trend in the graph for the enzyme trypsin.

Use data from the graph to support your answer.

(4)

(ii) State the optimum pH for the enzyme pepsin.

(1)

(iii) Pepsin only works effectively in the stomach.

Describe the conditions in the stomach that allow pepsin to work effectively.

(2)

At high pH values the active site of the enzyme pepsin changes shape.

When the active site of the enzyme changes shape, the enzyme is

☐ A specific
☐ B denatured
☐ C digested
☐ D dead

State what is produced when proteins are digested.

Starch is a nutrient in food.

Starch is a source of energy.

Name the enzyme that breaks down starch.

Enzymes from different parts of the digestive system were used to investigate the breakdown of starch.

Figure 1 shows the apparatus used in this investigation.

Figure 1

The colour of the contents of each test tube was recorded every two minutes for a total of ten minutes.

The results are shown in Figure 2.

Figure 2

(i) Give one reason why the contents of both test tubes were blue-black at the beginning of the investigation.

(1)

(ii) Explain the results of this investigation after ten minutes.

(3)

The diagram shows equipment that can be used to measure the energy content of different foods.

Figure 3

Devise a method to compare the energy content of two foods using this equipment.

Include details of how to control the variables.

Chymosin is an enzyme that causes milk to curdle.

When milk curdles the proteins in the milk clump together and become solid.

As part of an investigation, milk was heated in test tubes to different temperatures using a Bunsen burner.

Two drops of chymosin solution were added to each test tube and the time taken for the milk to curdle was recorded.

Figure 1 shows the results.

Figure 1

(i) Which variables need to be kept constant in this investigation?

(1)

☐ A the volume of milk and the time
☐ B the temperature and the time
☐ C the volume of milk and the concentration of chymosin
☐ D the temperature and the concentration of chymosin

(ii) Explain why the time taken for the milk to curdle decreases from 30°C to 40°C.

(2)

(iii) Explain what the expected result would be if two drops of chymosin were added to the milk at 70°C in the test tube.

(2)

(iv) As part of this investigation, test tubes containing only milk were heated to each temperature and no chymosin solution was added.
State why these test tubes containing only milk were used.

(1)

(v) Describe two improvements that could be made to the method of this investigation so that the optimum temperature for chymosin can be found.

(2)

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

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

Higher Tier Only

Chymosin can be produced by genetically modified bacteria.

Figure 2 shows a bacterial cell.

Figure 2

Explain how to genetically modify a bacterial cell to produce chymosin.

A lab was asked to test an unknown food sample to determine which substances were present in the food.

They carried out some simple tests using chemical reagents, Figure 1 shows the results.

Figure 1

(i) The lab concluded that the food sample most likely contained a simple carbohydrate like glucose or fructose.

Explain why the lab came to this conclusion.

(3)

(ii) The lab also tested for the presence of fats/lipids and protein in the food.

Suggest which two simple chemical tests they could have carried out.

(2)

(iii) State what would be a positive result for the presence of protein.

(1)

Figure 2 shows a list of chemical reagents that can be used to test the presence of food substances.

Figure 2

(i) Complete the empty cells in Figure 2 listing the substance being tested and what the expected result would be if positive or negative.

(4)

(ii) The biuret test can be regarded as a semi-quantitative test.

Suggest why this is.

(2)

Figure 3 shows an experimental apparatus to determine the energy content of a peanut.

Figure 3

(i) State the name of the apparatus.

(1)

(ii) Describe how this apparatus can be used to find the energy content of food.

(4)

(iii) During the experiment, 100 ml of water was used and the temperature increased from 20^oC to 40^oC. Using the equation below calculate how much energy was stored in the peanut.

(3)

A group of students carried out the following practical to establish the effect of temperature on protein breakdown:

1. Place a drop of biuret solution into every well in a spotting tile.

2. Put 4 cm³ of 1 % protein solution into a boiling tube.

3. Put 4 cm³ of pepsin solution into a second boiling tube.

4. Put both boiling tubes into a water bath at 20 °C.

5. After 4 minutes add the pepsin solution to the protein solution together in one boiling tube, mixing together with a stirring rod.

6. After 20 seconds add a drop of the protein/pepsin mixture to a drop of biuret solution in one well of the spotting tile.

7. Repeat step 6 until the biuret solution no longer changes colour.

8. Repeat steps 1–7 at 10 °C (using an ice water bath), 30 °C, 40 °C and 50 °C.

The concentration of protein present in the solution can also be measured using a colorimeter.

In a colorimeter, the amount of light that cannot pass through a solution is measured; giving the ‘absorbance’ of a solution. The darker the colour, the higher the absorbance.

Suggest advantages of using the colorimeter method rather than the method used by the students.

The graph below shows a calibration curve of concentration of protein versus absorbance.

An experiment was carried out to investigate the effect of a protease enzyme on a 1 % protein solution.

The absorbance at 30 °C was 0.8 arbitrary units after 20 seconds.

Using the calibration curve, estimate the final concentration of protein in this solution after 20 seconds.

The students found that the concentration of protein in the solution at 10 °C after 1 minute was different to the concentration at 30 °C after 1 minute.

Explain the observation above.

The students repeated the experiment while incubating the protease and 1 % protein solution at 80  °C.

The absorbance was measured at approximately 1.14 arbitrary units.

Suggest a reason for this result.

The protein Rubisco is an enzyme. 

Identify where, in a cell, Rubisco would be made.

Rubisco has a unique 3D shape that enables it to carry out its roles in living organisms. 

Describe how the 3D shape of rubisco is determined.

Extreme hyperthermia is a condition in which human body temperature rises above the normal body temperature of 37 °C. Body temperatures above 40 °C can be life threatening.

Use your knowledge of proteins to suggest why body temperatures above 40 °C can be life threatening.

Sketch a graph to show how the rate of reaction of an enzyme such as Rubisco would change over a range of temperatures from 0 to 60 °C.

Use the axes provided below.

Catalase is an enzyme found in the liver which catalyses the breakdown of the harmful substrate hydrogen peroxide into the products water and oxygen.

Some students investigated the effect of pH on catalase activity. Each experiment was repeated at a different pH value (pH 2, 4, 7, 9 and 11) and was set up as follows:

• Five potato cubes of similar dimensions were used as a source of catalase.

• Each cube was added to 50 cm³ of hydrogen peroxide.

• The volume of oxygen released from this reaction was collected in a measuring cylinder.

• This was used to calculate the initial rate of the reaction in dm³s^-1

Which of the rows in the following table correctly identifies the variables in this experiment?

The results below show the effect of pH 7 and pH 9 on the activity of the enzyme catalase.

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

Calculate the difference in rate of reaction between pH 7 and pH 9 during the first 20 seconds. Give your answers with the correct units.

Describe the differences between the curves at pH 7 and pH 9 in the graph in part (b).

Explain the shape of the curve for pH 7 from around 20 s onwards in the graph in part (b).

Predict the outcome of the experiment described in part (a) if the pH were increased to pH 11.

Explain your answer.