A2 2024 — CCEA A2 Biology Practice Questions

Question 1

Describe the events at a cholinergic synapse when an action potential arrives at the presynaptic membrane.

Accepted Answer

The arrival of the action potential depolarises the presynaptic membrane and opens voltage-gated calcium ion channels, so calcium ions diffuse into the presynaptic knob. This causes synaptic vesicles to move to and fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft by exocytosis. Acetylcholine diffuses across the cleft and binds to complementary receptors on the postsynaptic membrane, opening sodium ion channels. Sodium ions enter and depolarise the postsynaptic membrane, and if the threshold is reached a new action potential is generated. Acetylcholinesterase in the cleft then hydrolyses the acetylcholine into choline and ethanoic acid, which are reabsorbed into the presynaptic knob, so the postsynaptic neurone is not continuously stimulated.

Question 2

Explain why transmission across a synapse occurs in one direction only.

Accepted Answer

Acetylcholine is synthesised and stored in vesicles only in the presynaptic knob, so it can only be released from that side of the cleft. The complementary receptors that bind acetylcholine are present only on the postsynaptic membrane. Neurotransmitter therefore diffuses only from the presynaptic to the postsynaptic membrane and can only produce a response there, so transmission is unidirectional.

Question 3

Explain how negative feedback control of blood glucose concentration returns it to normal after a carbohydrate-rich meal, and explain why a person with type 1 diabetes cannot do this.

Accepted Answer

The rise in blood glucose after the meal is detected by the beta cells in the islets of Langerhans in the pancreas, which secrete insulin into the blood. Insulin binds to receptors on the cell surface membranes of liver and muscle cells, causing more glucose transporter proteins to be inserted into the membrane so that glucose uptake increases. Insulin also activates enzymes that convert glucose to glycogen and increases the rate at which glucose is respired. Blood glucose concentration therefore falls back towards the set point, and as it does the beta cells reduce insulin secretion - this is negative feedback, because the response reverses the original change. In type 1 diabetes the beta cells have been destroyed by an autoimmune response, so little or no insulin is secreted; the liver and muscle cells do not take up or store glucose and blood glucose remains high, with glucose appearing in the urine.

A2 2024: CCEA Biology practice questions

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Describe the events at a cholinergic synapse when an action potential arrives at the presynaptic membrane.

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