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Date November 2020 Marks available 3 Reference code 20N.2.HL.TZ0.2
Level Higher level Paper Paper 2 Time zone TZ0 / no time zone
Command term Explain Question number 2 Adapted from N/A

Question

The diagram shows part of two neurons.

[Source: © International Baccalaureate Organization 2020.]

Outline how the amphipathic properties of phospholipids play a role in membrane structure.

[2]
a.i.

State the role of cholesterol in animal cell membranes.

[1]
a.ii.

Describe what happens to the membranes of an animal cell during mitosis.

[2]
b.

State the name of the structure shown.

[1]
c.i.

X indicates the movement of a structure in the neuron. Explain what events trigger this movement and what happens next.

[3]
c.ii.

Markscheme

a. part hydrophobic/not attracted to water/non-polar AND part hydrophilic/attracted to water/polar;

b. bilayer formed (formed naturally by phospholipids in water);

c. hydrophilic heads/parts face outwards and hydrophobic tails/parts face inwards.

Do not allow water loving/hating in mpa or mpc.

a.i.

a. controls/regulates/reduces fluidity of membrane / prevents crystallization of phospholipids;

b. reduces permeability to some substances.

Do not accept ‘stabilizes membrane’.

a.ii.

a. nuclear membrane breaks down/disappears (in prophase/at start of mitosis);

b. nuclear membrane reforms around two new nuclei (in telophase/at end of mitosis);

c. plasma membrane pulled inwards at equator / cleavage furrow formed;

d. membrane pinches apart to form two cells / cytoplasm divided / cytokinesis.

b.

synapse/synaptic

Allow any answer including either of these terms unless out of context.

c.i.

a. depolarization of pre-synaptic membrane / action potential/nerve impulse arrives;

b. uptake of calcium / calcium ions diffuse in / calcium channels open;

c. structures containing neurotransmitter/vesicles move to/fuse with membrane;

d. neurotransmitter/acetylcholine released by exocytosis into cleft/binds to postsynaptic membrane/receptors;

Must see exocytosis.

c.ii.

Examiners report

Knowledge of the amphipathic properties of phospholipids was generally good.

a.i.

Half of candidates were able to state the role of cholesterol in animal cell membranes.

a.ii.

The mark scheme allowed candidates to score marks either for describing changes to the nuclear membrane or the role that the plasma membrane plays in cytokinesis. The best answers referred to both. Many of the candidates who wrote about the nuclear membrane had rather sketchy understanding of how it breaks up into small vesicles at the end of prophase and then reforms around both daughter nuclei in telophase.

b.

As pointed out on some G2 forms, the question here was rather ambiguous as it wasn’t 100 % clear what structure was referred to, so any answer indicating that the candidate knew the diagram showed a synapse was accepted.

c.i.

This was generally well answered. Some suggested that the vesicle is released into the synaptic cleft, instead of neurotransmitter being released by exocytosis.

c.ii.

Syllabus sections

Core » Topic 6: Human physiology » 6.5 Neurons and synapses
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