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Date	May 2021	Marks available	1	Reference code	21M.2.HL.TZ2.2
Level	Higher level	Paper	Paper 2	Time zone	Time zone 2
Command term	Label	Question number	2	Adapted from	N/A

Question

Cholinergic synapses use acetylcholine as their neurotransmitter. They are widespread in the body, passing on signals to muscle cells. These synapses are affected by neonicotinoid pesticides.

The diagrams, which are not drawn to scale, show the synapse between two neurons and a detail of the synaptic cleft.

On the diagrams, label with a letter H the hydrophilic end of a phospholipid.

[1]

a.i.

On the diagrams, label with a letter E a vesicle involved in exocytosis.

[1]

a.ii.

On the diagrams, label with a letter P a location where a neonicotinoid pesticide could bind.

[1]

a.iii.

Outline how depolarization of the membrane of an axon occurs.

[2]

Explain how acetylcholine initiates an action potential in a postsynaptic membrane.

[2]

State the action of the enzyme acetylcholinesterase.

[1]

d.i.

Explain what happens to an enzyme if there is a change of pH.

[3]

d.ii.

Markscheme

hydrophilic end of phospholipid shown ✔

For 2a, i, ii and iii, accept letters written on the structure even if a line is not drawn.

Award the mark if the full word is used to label the structure. Labels must be unambiguous.

a.i.

an exocytic vesicle shown ✔

For 2 a, i, ii and iii, accept letters written on the structure even if a line is not drawn.

Award the mark if the full word is used to label the structure. Labels must be unambiguous.

Accept any of the three intact circles or the exocytosis shown within the box.

a.ii.

any receptor protein on post synaptic membrane marked ✔

For 2 a, i, ii and iii, accept letters written on the structure even if a line is not drawn.

Award the mark if the full word is used to label the structure.

Must unambiguously be pointed to a receptor on the synapse side. Accept labels of acetylcholinesterase.

a.iii.

a. (local depolarization) causes ion / sodium / voltage gated channels to open ✔

b. altering membrane permeability to sodium ions/Na⁺/positive ions / Na⁺ diffuses into the cytoplasm ✔

c. membrane potential (of the axon) changes from negative to positive ✔

d. a threshold potential is reached and an action potential is generated ✔

Do not accept sodium pumps.

Mp b requires evidence that the student recognizes sodium is an ion.

Numerical values indicating change from −70 to +30/+40/+55 can be accepted.

a. acetylcholine binds to the receptor protein ✔

b. (causing a) change in tertiary structure / conformation of protein ✔

c. diffusion/entry of ions / Na⁺ through the receptor/channel protein allowed ✔

breaks down acetylcholine (to acetyl and choline) ✔

d.i.

a. change in pH can cause a change in the tertiary structure of the enzyme
OR
enzyme denatured ✔

b. causing a change in the active site / substrate not able to bind ✔

c. the enzyme will not work as efficiently / decreased rate of reaction ✔

d. (may no longer be) the optimum/optimal pH for enzyme activity ✔

Allow answer in the form of a graph.

d.ii.

Examiners report

There were numerous examples of ambiguous labels, for example, an H without an arrow placed beside the membrane making it unclear whether the student was indicating the hydrophobic or hydrophilic part of the membrane.

a.i.

[N/A]

a.ii.

Both receptor proteins and acetylcholinesterase were accepted as proteins to which acetylcholine can bind. The markscheme did not accept locations other than on the synaptic side.

a.iii.

In many exams, candidates seemed to confuse the voltage gated ion channels with sodium-potassium pumps.

[N/A]

d.i.

[N/A]

d.ii.