| Date | May 2013 | Marks available | 1 | Reference code | 13M.1.HL.TZ1.27 |
| Level | Higher level | Paper | Paper 1 | Time zone | Time zone 1 |
| Command term | Question number | 27 | Adapted from | N/A |
Question
The enzyme succinic dehydrogenase catalyses the conversion of succinate to fumarate.
The addition of malonate to the reaction mixture decreases the rate of the reaction. If more succinate is added, the reaction rate will increase. What is the role of malonate in this reaction?
A. End-product inhibitor
B. Non-competitive inhibitor
C. Catalyst
D. Competitive inhibitor
Markscheme
D
Examiners report
Some teachers felt that Question 27 required specific knowledge of malonate as an inhibitor of succinate dehydrogenase. This was an example of a question where one answer is distinctly the best. Although increases in substrate concentration reduce the effect of both competitive and non-competitive inhibitors, the effect is greater with competitive inhibitors. Students seemed to have less of an issue with this question than teachers with two thirds answering it correctly and the discrimination index being high.
