03.06 Enzyme Action: Enzyme Concentration

Question 1

Explain how enzyme concentration affects the rate of an enzyme-catalyzed reaction when substrate is in excess. (5 marks)

Mark Scheme:

1. Higher enzyme concentration provides more active sites for substrate binding. (1 mark)
2. This increases the frequency of enzyme-substrate collisions, raising the reaction rate. (1 mark)
3. As long as substrate is in excess, the rate of reaction is directly proportional to enzyme concentration. (1 mark)
4. A linear increase is observed in the initial rate of reaction with increasing enzyme concentration. (1 mark)
5. If substrate becomes limiting, further increases in enzyme concentration have no effect on reaction rate. (1 mark)

Question 2

Describe how the initial rate of reaction can be calculated from a reaction graph for catalase activity. (4 marks)

Mark Scheme:

1. Plot a graph of reaction progress: Time (X-axis) vs. Volume of product (Y-axis). (1 mark)
2. Draw a tangent to the curve at the steepest point (near time 0). (1 mark)
3. Calculate the slope of the tangent using:

4. The slope gives the initial rate of reaction, which reflects enzyme activity when substrate is abundant. (1 mark)

Question 3

Outline an experiment to investigate the effect of enzyme concentration on the rate of catalase activity. (6 marks)

Mark Scheme:

1. Prepare celery extract (source of catalase) and dilute it to create different enzyme concentrations. (1 mark)
2. Use the same volume of enzyme solution for each concentration to keep total volume constant. (1 mark)
3. Add a fixed concentration of hydrogen peroxide (H₂O₂) as the substrate to each test tube. (1 mark)
4. Immediately measure the volume of oxygen produced over a fixed time period (e.g., 30 seconds) using a gas syringe. (1 mark)
5. Plot a graph of initial rate of reaction (Y-axis) against enzyme concentration (X-axis). (1 mark)
6. Control variables: Temperature, pH, substrate volume, and substrate concentration. (1 mark)

Question 4

Why does the reaction rate level off over time during an enzyme-catalyzed reaction? (4 marks)

Mark Scheme:

1. At the start, substrate concentration is high, and enzyme active sites are fully occupied, leading to a high reaction rate. (1 mark)
2. Over time, substrate molecules are converted into products, reducing substrate concentration. (1 mark)
3. Fewer substrate molecules are available to bind to active sites, decreasing the reaction rate. (1 mark)
4. The reaction eventually stops when all substrate is depleted. (1 mark)

Question 5

Describe and explain the shape of a graph showing initial rate of reaction vs. enzyme concentration. (5 marks)

Mark Scheme:

1. The graph shows a linear increase in initial rate of reaction with increasing enzyme concentration. (1 mark)
2. This occurs because more enzyme molecules provide more active sites for substrate binding. (1 mark)
3. The linear relationship continues as long as substrate is in excess. (1 mark)
4. If substrate becomes limiting, the graph plateaus as further increases in enzyme concentration have no effect. (1 mark)
5. This demonstrates that reaction rate depends on the availability of both enzyme and substrate. (1 mark)

Question 6

Explain why the initial rate of reaction is used to compare enzyme activity at different concentrations. (4 marks)

Mark Scheme:

1. The initial rate is measured before significant substrate depletion occurs, ensuring consistent substrate concentration. (1 mark)
2. It reflects the maximum enzyme activity for the given concentration. (1 mark)
3. Differences in reaction rate are attributed solely to enzyme concentration, avoiding interference from limiting factors. (1 mark)
4. This makes the initial rate a reliable indicator of enzyme efficiency. (1 mark)

Question 7

What happens to the reaction rate if enzyme concentration is increased but substrate is limited? (3 marks)

Mark Scheme:

1. Initially, increasing enzyme concentration raises the reaction rate as more active sites are available. (1 mark)
2. When substrate becomes limiting, the rate of reaction no longer increases with additional enzyme. (1 mark)
3. All substrate molecules are already being processed, so extra enzymes remain inactive without substrate. (1 mark)

Question 8

Why is it important to keep the total volume of reaction mixtures constant in enzyme concentration experiments? (3 marks)

Mark Scheme:

1. Ensures that the reaction conditions (e.g., substrate concentration) are consistent across all samples. (1 mark)
2. Prevents changes in total volume from affecting the rate of substrate-enzyme collisions. (1 mark)
3. Allows a fair comparison of reaction rates at different enzyme concentrations. (1 mark)

Question 9

Describe how a reaction profile graph changes with increasing enzyme concentration. (5 marks)

Mark Scheme:

1. All profiles start with a steep slope, indicating a high initial reaction rate. (1 mark)
2. Higher enzyme concentrations result in steeper initial slopes due to more active sites. (1 mark)
3. As substrate is consumed, the slope levels off, indicating a reduced reaction rate. (1 mark)
4. The reaction ends when all substrate is converted into products. (1 mark)
5. The time taken for the reaction to complete is shorter at higher enzyme concentrations. (1 mark)

Question 10

What conclusions can be drawn from a graph of initial rate vs. enzyme concentration? (5 marks)

Mark Scheme:

• The reaction rate increases proportionally with enzyme concentration when substrate is in excess. (1 mark)
• The slope of the graph indicates the relationship between enzyme concentration and rate. (1 mark)
• If the graph plateaus, it indicates that substrate has become limiting. (1 mark)
• Beyond the plateau, increasing enzyme concentration has no further effect on the rate. (1 mark)
• This demonstrates the importance of both enzyme and substrate availability in determining reaction rates. (1 mark)