Required Practical 1

Question 1

Give examples of variables that could affect the rate of an enzyme-controlled reaction.

What would you do with the other variables if you were testing the effect of one on the rate of an enzyme-controlled reaction?

Answer 1

• Enzyme concentration/volume.
• Substrate concentration/volume.
• Temperature of solution.
• pH of solution.
• Inhibitor concentration.

Any one of these can be the independent variable and need to be varied (e.g. dilution series of varying concentrations). All others would be control variables so would need to be kept constant.

Question 2

Describe how temperature can be controlled.

Answer 2

• Use a thermostatically controlled water bath.
• Monitor using a thermometer at regular intervals and add hot/cold water if temperature fluctuates.

Question 3

Describe how pH can be controlled.

Answer 3

• Use a buffer solution.
• Monitor using a pH meter at regular intervals.

Question 4

Why were the enzyme and substrate solutions left in the water bath for 10 minutes before mixing?

Answer 4

So solutions equilibrate/reach the temperature of the water bath.

Question 5

Describe a control experiment.

Answer 5

• Use denatured enzymes (e.g. by boiling).
• Everything else same as experiment e.g. same concentration/volume of substrate and enzyme, same type/volume of buffer solution and same temperature.

Question 6

Describe two methods by which the rate of an enzyme-controlled reaction can be measured.

Answer 6

METHOD ONE

• Measure time taken for reaction to reach a set point e.g.
  concentration/volume/mass/colour of substrate or product.
  
  • Rate of reaction = 1/time; example units = s⁻¹.

METHOD TWO

• Measure concentration/volume/mass/colour of substrate or
  product at regular intervals throughout the reactions.
  
  • Plot on a graph with time on the x-axis and whatever is being measured on the y-axis.
  • Draw a tangent at t = 0 (or any other time for rate at a particular point).
  • Initial rate of reaction = change in y / change in x; example units = cm³ s⁻¹.

Question 7

Suggest a safety risk and explain how to reduce this risk.

Answer 7

• Handling enzymes may cause an allergic reaction.
• Avoid contact with skin by wearing gloves and eye protection.

Question 8

Explain why using a colourimeter to measure colour change is better than comparison to colour standards.

Answer 8

• Not subjective.
• More accurate.

Question 9

Explain a procedure that could be used to stop each reaction.

Answer 9

• Boil/add strong acid/alkali to denature enzyme.
• Put in ice to lower kinetic energy so no enzyme-substrate complexes form.
• Add high concentration of inhibitor so no enzyme-substrate complexes form.

Question 10

Describe how processed data can be presented as a graph.

Answer 10

1. Independent variable on x-axis, rate of reaction on y-axis, including units.
2. Linear number sequence on axis, appropriate scale (graph should cover at least half of grid).
3. Plot coordinates accurately as crosses.
4. Join point-to-point with straight lines if cannot be certain of intermediate values OR draw a smooth curve but do not extrapolate.

Question 11

Explain why the rate of reaction decreases over time throughout each experiment.

Answer 11

• Initial rate is highest as substrate concentration not limiting/many enzyme-substrate complexes form.
• Reaction slows as substrate used up and often stops as there is no substrate left.