| Date | May 2009 | Marks available | 3 | Reference code | 09M.2.SL.TZ1.3 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 3 | Adapted from | N/A |
Question
Define active site.
Explain enzyme-substrate specificity.
Markscheme
site on surface/portion of the enzyme/protein to which the substrate binds
enzymes fit together with substrates similar to a lock and key;
active site has shape that gives specificity;
enzymes catalyze a reaction with a specific substrate;
example of named enzyme and its substrate;
substrate held precisely in (optimum) position to make/break bonds/carry out reaction / chemical interaction occurs between enzyme and substrate;
Accept these points shown in an annotated drawing.
Examiners report
A few candidates mixed up the location by stating that the active site was on the substrate.
Many answers simply mentioned a compatible fit such as a “lock and key.” More marks could have been gained by also mentioning specificity of shape or any reference to the result of the specificity in terms of catalysis or breaking of bonds to form products. Few candidates gave an example of an enzyme and its substrate.
