Enzymes (Edexcel GCSE Biology: Combined Science)

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

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.

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?

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.

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

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.

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

(1)

(3)

(2)

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

Why are enzymes called biological catalysts?

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

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)

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

Figure 1

(4)

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

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.

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

(1)

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 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.