03.07 Enzyme Action: Substrate Concentration

Question 1

Explain how substrate concentration affects the rate of an enzyme-catalyzed reaction at low and high substrate levels. (5 marks)

Mark Scheme:

1. At low substrate concentrations, the reaction rate increases as substrate concentration increases. (1 mark)
2. Explanation: More substrate molecules mean more frequent collisions with enzyme active sites. (1 mark)
3. At high substrate concentrations, the rate levels off and reaches a maximum (Vmax). (1 mark)
4. Explanation: All enzyme active sites are occupied, and the enzyme is working at full capacity. (1 mark)
5. Further increases in substrate concentration do not affect the rate because additional substrate must wait for active sites to become available. (1 mark)

Question 2

Define Vmax and explain its significance in enzyme-catalyzed reactions. (4 marks)

Mark Scheme:

1. Vmax is the maximum rate of an enzyme-catalyzed reaction when all active sites are saturated with substrate. (1 mark)
2. At Vmax, the enzyme is working at its full capacity. (1 mark)
3. It indicates the maximum efficiency of the enzyme under the given conditions. (1 mark)
4. Beyond Vmax, reaction rate is limited by enzyme concentration or the speed of enzyme turnover. (1 mark)

Question 3

Describe the shape of a graph showing reaction rate vs. substrate concentration and explain its features. (6 marks)

Mark Scheme:

1. The graph initially shows a linear increase in reaction rate with increasing substrate concentration. (1 mark)
2. This is because more substrate molecules increase the likelihood of collisions with active sites. (1 mark)
3. The graph then levels off, forming a plateau at Vmax. (1 mark)
4. At Vmax, all enzyme active sites are occupied, and the enzyme is saturated with substrate. (1 mark)
5. Increasing substrate concentration beyond this point does not increase the reaction rate. (1 mark)
6. The plateau illustrates that the rate is now limited by the enzyme’s capacity to process substrate. (1 mark)

Question 4

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

Mark Scheme:

1. Prepare solutions with varying concentrations of hydrogen peroxide (substrate). (1 mark)
2. Use a constant concentration of catalase enzyme in each reaction mixture. (1 mark)
3. Add the enzyme to the substrate solution and measure the volume of oxygen gas produced over a fixed time (e.g., 30 seconds) using a gas syringe. (1 mark)
4. Record the initial rate of reaction for each substrate concentration. (1 mark)
5. Plot a graph of reaction rate (Y-axis) against substrate concentration (X-axis). (1 mark)
6. Control variables: Temperature, pH, and reaction volume to ensure accurate results. (1 mark)

Question 5

Why does the reaction rate plateau at high substrate concentrations in an enzyme-catalyzed reaction? (4 marks)

Mark Scheme:

1. At high substrate concentrations, all enzyme active sites are occupied. (1 mark)
2. The enzyme is saturated and working at its maximum capacity (Vmax). (1 mark)
3. Additional substrate molecules must wait for active sites to become available, so they do not increase the reaction rate. (1 mark)
4. The reaction rate is now limited by the enzyme concentration or turnover rate. (1 mark)

Question 6

Explain the importance of calculating the initial rate of reaction when investigating substrate concentration effects. (5 marks)

Mark Scheme:

1. The initial rate reflects enzyme activity before substrate depletion begins. (1 mark)
2. At the start, substrate concentration is consistent across all samples. (1 mark)
3. Differences in reaction rate are solely due to substrate concentration, avoiding interference from limiting factors. (1 mark)
4. The initial rate ensures a reliable comparison of enzyme efficiency under different conditions. (1 mark)
5. Measuring the rate later in the reaction would lead to variability due to changes in substrate concentration. (1 mark)

Question 7

How does substrate concentration influence enzyme efficiency in practical applications? (5 marks)

Mark Scheme:

1. At low substrate concentrations, reaction rates are low, leading to inefficient use of enzymes. (1 mark)
2. Increasing substrate concentration improves efficiency by ensuring active sites are fully utilized. (1 mark)
3. Maximum efficiency is achieved at Vmax, where all active sites are occupied. (1 mark)
4. Adding more substrate beyond Vmax is wasteful, as it does not increase the reaction rate. (1 mark)
5. In industry, ensuring the substrate concentration is sufficient but not excessive optimizes cost-effectiveness. (1 mark)

Question 8

Describe how the rate of an enzyme-catalyzed reaction changes with substrate concentration and explain why. (6 marks)

Mark Scheme:

1. At low substrate concentrations, the rate increases proportionally with substrate concentration. (1 mark)
2. Explanation: More substrate molecules increase the frequency of enzyme-substrate collisions. (1 mark)
3. As substrate concentration rises further, the increase in rate begins to slow. (1 mark)
4. At high substrate concentrations, the rate plateaus at Vmax. (1 mark)
5. Explanation: Enzyme active sites are fully occupied, and the enzyme is saturated. (1 mark)
6. Additional substrate does not affect the rate because enzymes cannot process more than their turnover capacity. (1 mark)

Question 9

What happens to the reaction rate if substrate concentration increases when enzymes are already saturated? (3 marks)

Mark Scheme:

1. When enzymes are saturated, the reaction rate has reached Vmax and cannot increase further. (1 mark)
2. Additional substrate molecules are unable to bind until active sites become available. (1 mark)
3. The reaction rate remains constant regardless of further increases in substrate concentration. (1 mark)

Question 10

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

Mark Scheme:

• At low substrate concentrations, reaction rate is proportional to substrate concentration, indicating enzyme activity depends on substrate availability. (1 mark)
• As substrate concentration increases, the reaction rate begins to slow, indicating active sites are becoming occupied. (1 mark)
• The rate plateaus at Vmax, showing the enzyme is saturated and working at maximum capacity. (1 mark)
• Beyond Vmax, the rate does not increase with additional substrate, as it is limited by enzyme concentration. (1 mark)
• The graph illustrates the relationship between enzyme efficiency, substrate availability, and saturation point. (1 mark)