5. The diagram shows a synapse.
[VISUALIZATION: Diagram of a chemical synapse. Labels: A pointing to a synaptic vesicle, B pointing to the presynaptic terminal membrane. Other labels: Presynaptic neuron terminal, synaptic vesicles containing neurotransmitter, synaptic cleft, postsynaptic membrane with receptors, mitochondria in presynaptic terminal.]
(a) Identify the structure labelled A and state its function. [2]
(b) Outline the sequence of events following the arrival of an action potential at the presynaptic terminal (labelled B) that leads to the release of neurotransmitters by exocytosis. [4]
(c) Explain the role of enzymes in the synaptic cleft in terminating the signal, using acetylcholine as an example. [3]
Question
Identify the structure labelled A and state its function. [2]
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Structure A is a small sac within the presynaptic terminal. What does it contain and what is its purpose?
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A is a synaptic vesicle which stores neurotransmitters.
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Question
Outline the sequence of events following the arrival of an action potential at the presynaptic terminal (labelled B) that leads to the release of neurotransmitters by exocytosis. [4]
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Trace the process from the electrical signal arriving at the terminal (B) to the release of chemicals into the gap. Think about membrane potential changes and vesicle movement.
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Arrival of an action potential depolarizes the presynaptic membrane. This causes synaptic vesicles to move towards and fuse with the presynaptic membrane, releasing neurotransmitters into the synaptic cleft via exocytosis.
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Question
Explain the role of enzymes in the synaptic cleft in terminating the signal, using acetylcholine as an example. [3]
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Consider why the neurotransmitter signal needs to be stopped. What specific enzyme acts on acetylcholine, and what does this achieve?
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Enzymes like acetylcholinesterase in the synaptic cleft break down neurotransmitters (e.g., acetylcholine into choline and acetate). This prevents continuous stimulation of the postsynaptic neuron and allows it to repolarize. Breakdown products can be recycled.
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