| Date | May 2013 | Marks available | 1 | Reference code | 13M.2.HL.TZ1.3 |
| Level | Higher level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | State | Question number | 3 | Adapted from | N/A |
Question
The following diagram shows the synthesis and regulation of some amino acids.
State the type of inhibition shown in this diagram.
Explain how this type of regulation could affect the synthesis of an amino acid.
Markscheme
end-product/non-competitive/(negative) feedback inhibition
amino acid/end product produced if used up/not enough present;
production stops if amino acid/end product unused/accumulates/in excess;
amino acid/end product changes active site of (first) enzyme of pathway;
(this is an example of) negative feedback;
Examiners report
Almost all candidates gave a correct answer, probably because there was a strong hint in the diagram. End-product or non-competitive inhibition was accepted, or references to feedback.
Candidates found this part of the question quite hard and it exposed a wide variety of misunderstandings of the interactions between enzymes, substrates, active sites, allosteric sites and end-product inhibitors. Many candidates failed to relate their answer to pathways used to synthesise essential metabolites in cells.
