EnzymesEdexcel A-Level Biology B: Subtopic test

1. 1
   A student investigates catalase, an enzyme in potato tissue that breaks down hydrogen peroxide into water and oxygen. Using a water bath, she mixes potato extract with hydrogen peroxide solution at a series of temperatures and measures the volume of oxygen released during the first 30 seconds. The rate of oxygen release rises up to 40 °C and then falls sharply at higher temperatures.

   (a)

   Why does the student measure the volume of oxygen released in the first 30 seconds rather than over several minutes?

   [1 mark]

   • AThe oxygen produced denatures the catalase after 30 seconds
   • BAll of the hydrogen peroxide has been used up after 30 seconds
   • CThe substrate concentration is still close to its starting value and little product has built up, so the rate reflects the activity of the enzyme under the chosen conditions
   • DEnzymes can only catalyse a reaction for the first 30 seconds

   (b)

   Which statement correctly explains how catalase increases the rate of decomposition of hydrogen peroxide?

   [1 mark]

   • AIt provides a reaction pathway with a lower activation energy
   • BIt raises the temperature of the hydrogen peroxide molecules
   • CIt is used up as it supplies energy to the reaction
   • DIt increases the activation energy so that fewer molecules react by accident

   (c)

   Explain why the rate of oxygen release falls sharply at temperatures above 40 °C.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A drug company tests two inhibitors, X and Y, on an enzyme from a pathogen. The rate of reaction is measured at a range of substrate concentrations with no inhibitor, with X and with Y. With X, the rate is much lower at low substrate concentrations, but at very high substrate concentrations it reaches the same maximum rate as with no inhibitor. With Y, the rate is lower at every substrate concentration, and the maximum rate is never reached however much substrate is added.

   (a)

   Which row correctly identifies the type of inhibition shown by X and by Y?

   [1 mark]

   • AX is non-competitive and Y is competitive
   • BX is competitive and Y is non-competitive
   • CBoth X and Y are competitive
   • DBoth X and Y are non-competitive

   (b)

   X has a shape very similar to the substrate. Which statement best explains why a very high substrate concentration removes the effect of X?

   [1 mark]

   • ASubstrate molecules denature the molecules of X
   • BSubstrate binds at a different site from X and changes its shape
   • CExtra substrate increases the number of active sites on each enzyme
   • DSubstrate molecules greatly outnumber molecules of X, so almost all active sites are occupied by substrate

   (c)

   Explain how Y reduces the maximum rate of the reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a bacterium, the amino acid S is made from substance P by a pathway of three enzyme-controlled steps catalysed by E1, E2 and E3. E1 catalyses the first step. When the concentration of S inside the cell rises, the rate of E1 falls, even though S has a completely different shape from P. When the concentration of S falls, the activity of E1 recovers.

   (a)

   Explain how a rise in the concentration of S reduces the activity of E1.

   [3 marks]

   (b)

   A mutation alters E1 so that S can no longer bind to it. Predict and explain the consequences for the bacterium.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class investigates how temperature affects the activity of amylase, which hydrolyses starch to maltose. They have starch solution, amylase solution, buffer solution, iodine in potassium iodide solution, a white spotting tile, water baths, thermometers and a stopwatch. The teacher wants the investigation to give valid results and the students to show that they understand how the enzyme works.

   (a)

   Describe how the class could investigate the effect of temperature on the initial rate of this reaction, and how they would process their results.

   [6 marks]

   (b)

   Explain how amylase, a globular protein, catalyses the hydrolysis of starch, and why the rate of reaction is highest at an optimum temperature.

   [6 marks]

   Total for question 4: 12 marks

End of questions