A level Biology 3.2 Factors that affect enzyme action – Exam style question – Paper 1

Question

Catalase is an enzyme that catalyses the conversion of hydrogen peroxide into water and
oxygen.
Two students investigated the effect of enzyme concentration on the rate of reaction of the
enzyme catalase. The students predicted their results would show the same trend. The graphs
show the rates obtained by each student.

Which statement explains the different trend shown by student 2’s results?
A Student 2 included a competitive inhibitor in the investigation.
B Student 2 performed the investigation at a higher temperature.
C Student 2 performed the investigation at pH6 compared to pH8.
D Student 2 used a lower concentration of substrate in the investigation.

▶️Answer/Explanation

Answer     D

The correct answer is D. Student 2 used a lower concentration of substrate in the investigation.

The rate of reaction of an enzyme-catalyzed reaction is dependent on the concentration of both the enzyme and its substrate. In this case, both students were investigating the effect of enzyme concentration on the rate of reaction of catalase, but they obtained different results.

Student 1’s graph shows that the rate of reaction increases with increasing enzyme concentration, which is expected. However, student 2’s graph shows that the rate of reaction plateaus at a certain enzyme concentration and does not increase further with increasing enzyme concentration.

This difference in trend can be explained by the fact that student 2 used a lower concentration of substrate in the investigation. At low substrate concentrations, the rate of reaction is limited by the availability of substrate molecules, not the concentration of enzyme. Therefore, increasing the concentration of enzyme has no effect on the rate of reaction.

In contrast, student 1 used a higher concentration of substrate in the investigation, which allowed the rate of reaction to increase with increasing enzyme concentration.

Therefore, the correct statement that explains the different trend shown by student 2’s results is:

D. Student 2 used a lower concentration of substrate in the investigation.

Question

An enzyme is completely denatured at 50°C. A fixed concentration of this enzyme is added to a fixed concentration of its substrate. The time taken for completion of the reaction is measured at different temperatures.

Which graph shows the results?

Answer/Explanation

Ans:

A

Question

Why do large increases in the temperature or pH alter enzyme activity?

  1. They change the three-dimensional shape of the enzyme.
  2. They disrupt hydrogen and ionic bonds in the enzyme.
  3. They increase hydrophobic interactions in the enzyme.

A 1 and 2               B 1 and 3                 C 2 and 3                 D 1 only

▶️Answer/Explanation

>Ans:

A

The answer is A. 1 and 2.

The increase in temperature can change the three-dimensional shape of the enzyme, which can lead to denaturation of the enzyme. A change in pH can disrupt ionic bonds, which can also lead to denaturation of the enzyme.

Therefore, the correct statement that explains why large increases in the temperature or pH alter enzyme activity is:

A. They change the three-dimensional shape of the enzyme and disrupt hydrogen and ionic bonds in the enzyme.

Question

The graphs show the rate of reaction of an enzyme-catalysed reaction.

     Which graph shows the effect of increasing the concentration of the substrate at two different concentrations of a competitive inhibitor?

Answer/Explanation

Ans:

B

Graph B, represents the effect of increasing the concentration of the substrate at two different concentrations of a competitive inhibitor.
– The competitive inhibitors competes with substrate for active site of enzyme. Because the inhibitor and substrate are in competition for the active site, increasing substrate concentration increases the likelihood that that substrate will bind and the reaction will progress normally, when the inhibitor is in low concentration.
– However, when inhibitor is in high concentration, increasing substrate concentration will increase the rate of reaction slowly.
– When no inhibitor is added, increasing the substrate concentration indefinitely does not increase the rate of an enzyme-catalyzed reaction beyond a certain point. This point is reached when there are enough substrate molecules to completely fill (saturate) the enzyme’s active sites. A plateau occurs because any additional substrate molecules will simply have to wait around until another enzyme becomes available, so the rate of reaction (amount of product produced per unit time) is limited by the concentration of enzyme.

Question

Which statements about enzyme inhibitors are correct?

  1. Competitive inhibitors may be similar shapes to the substrate.
  2. Competitive inhibitors bind to the active site.
  3. Non-competitive inhibitors alter the shape of the enzyme.
  4. Non-competitive inhibitors bind to the substrate.

A 1, 2 and 3               B 2, 3 and 4                C 1 and 2 only                D 3 and 4 only

▶️Answer/Explanation

Ans:

A

The answer is A.

Competitive inhibitors are molecules that bind to the active site of an enzyme and prevent the substrate from binding. Competitive inhibitors are similar in shape to the substrate, and they compete with the substrate for binding to the active site. Therefore, statements 1 and 2 are correct.

Non-competitive inhibitors are molecules that bind to a site on the enzyme other than the active site, causing a conformational change in the enzyme that alters the shape of the active site. Non-competitive inhibitors do not bind to the substrate, so statement 4 is incorrect. Statement 3 is correct, as non-competitive inhibitors alter the shape of the enzyme.

Therefore, the correct statements about enzyme inhibitors are:

A. 1, 2 and 3.

1. Competitive inhibitors may be similar shapes to the substrate.
2. Competitive inhibitors bind to the active site.
3. Non-competitive inhibitors alter the shape of the enzyme.

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