2.3 Factors affecting enzyme activity
7 exam questions worth 16 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Pepsin is an enzyme that works in the stomach. What is its optimum pH most likely to be?
Choose one answer.
Correct Not quite: the answer is pH 2
Mark scheme
A: pH 2
Model answer
A: pH 2. The stomach contains hydrochloric acid, so pepsin works best in acidic conditions.
Explain why the rate of an enzyme-controlled reaction increases as the temperature rises from 10 °C to 30 °C.
Mark scheme
- molecules have more (kinetic) energy / move faster (1)
- more (successful) collisions between enzyme and substrate / more enzyme–substrate complexes form (per second) (1)
Model answer
At a higher temperature the enzyme and substrate molecules have more kinetic energy and move faster. They collide more often, so more enzyme–substrate complexes form each second.
Explain why the rate of an enzyme-controlled reaction falls to zero at 60 °C.
Mark scheme
- (bonds break so) the active site changes shape (1)
- the substrate no longer fits (1)
- the enzyme is denatured (1)
Model answer
At 60 °C the bonds holding the enzyme together break and the active site changes shape. The substrate no longer fits into the active site, so the enzyme is denatured and the reaction stops.
The rate of an enzyme reaction increases as the substrate concentration increases, then it stays the same.
Explain why the rate stops increasing.
Mark scheme
- all the active sites are occupied / enzyme is saturated (1)
- (so) the enzyme concentration is now the limiting factor / adding substrate cannot make more complexes form (1)
Model answer
At high substrate concentrations all the active sites are occupied, so the amount of enzyme now limits the rate. Adding more substrate cannot increase the rate.
Catalase breaks down hydrogen peroxide into water and oxygen. In one experiment, 36 cm3 of oxygen was collected in 4 minutes.
Calculate the mean rate of reaction in cm3 per minute.
Mark scheme
- 36 ÷ 4 (1)
- = 9 (cm3 per minute) (1)
Model answer
rate = 36 ÷ 4 = 9 cm3 per minute.
A starch solution took 40 seconds to be completely broken down by amylase.
Calculate the rate of reaction using rate = 1000 / time. Give your answer in s-1.
Mark scheme
- 1000 ÷ 40 (1)
- = 25 (s-1) (1)
Model answer
rate = 1000 ÷ 40 = 25 s-1.
Describe and explain how pH affects the activity of an enzyme such as amylase.
Mark scheme
- activity is highest at the optimum pH / pH 7 for amylase (1)
- activity decreases as pH moves away from the optimum (1)
- (at extreme pH) the shape of the active site changes (1)
- substrate no longer fits / enzyme is denatured (1)
Model answer
Amylase works fastest at its optimum pH, about pH 7. As the pH becomes more acidic or more alkaline, the activity decreases. This is because the pH changes the shape of the active site, so the starch no longer fits and the enzyme is denatured.
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