Farmers can use insecticides to remove parasites from their sheep. Some of these insecticides act as inhibitors of the enzyme acetylcholinesterase.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2020). The mark or mark type is shown in brackets.

1. [3] 8(a) any three from: 1 substrate / acetylcholinesterase / Ach, cannot bind (to active site); 2 no/few, enzyme substrate complexes form ; 3 (in competitive inhibition) ref. to inhibitor complementary to active site ; 4 (in competitive inhibition) inhibitor binds to active site / inhibitor blocks active site ; 5 (in non-competitive inhibition) inhibitor binds to, allosteric site / description ; 6 (binding of inhibitor to allosteric site) causes change to, 3-D shape / tertiary structure / active site;
2. [3] 8(b) any three from: 1 acetylcholinesterase / Ach, remains attached to receptors (on post-synaptic membrane) or acetylcholinesterase / Ach, not broken down ; 2 Na+ continues to diffuse into post-synaptic neurone / (voltage-gated) sodium ion channels remain open ; 3 postsynaptic membrane remains depolarised / repolarisation of postsynaptic membrane does not occur ; 4 continuous transmission of action potentials ; 5 AVP; e.g. synaptic fatigue paralysis
3. [1] 8(c) any one from: 1 add compound to, bind to / degrade, the inhibitor (and prevent it binding to the active site of acetylcholinesterase); 2 (use compound to) stimulate production of acetylcholinesterase ;