| Date | May 2013 | Marks available | 1 | Reference code | 13M.3.SL.TZ1.8 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Identify | Question number | 8 | Adapted from | N/A |
Question
The following image represents a model of ribulose bisphosphate (RuBP) carboxylase (also known as Rubisco) from the green alga Chlamydomonas.
Identify the level of protein structure of the part labelled X.
State the role of ribulose bisphosphate (RuBP) carboxylase in the Calvin cycle.
Explain non-competitive inhibition.
Markscheme
secondary (structure) / α helix
fixes/adds carbon/CO2 to RuBP
a. inhibitor binds to enzyme at different location than active site;
b. this causes a change in the shape/conformational change of active site;
c. thus preventing the substrate from binding to the active site / resulting in a decrease of enzyme activity/speed of reaction;
Examiners report
A relatively small number of candidates answered this option, but those who did generally achieved well.
In C2 (a) (i) most answers were correct.
A relatively small number of candidates answered this option, but those who did generally achieved well.
A comment on a G2 suggested that the examination should have used the abbreviation Rubisco as this is found in many texts rather than ribulose bisphosphate. Although many texts may use this abbreviation, the examination was based on the terminology used in the subject guide. Teachers who do not use the terminology of the guide may disadvantage students, as appears to have been possible with this question.
A relatively small number of candidates answered this option, but those who did generally achieved well.
In C2 (c) in general candidates did know how non-competitive inhibitors work. This is a standard question that revision of past papers would have prepared candidates well for.
