2. The image is an electron micrograph showing a synapse.
[VISUALIZATION: Electron micrograph of a chemical synapse. Clearly label: presynaptic neuron terminal, **synaptic vesicle containing neurotransmitter (label one as I)**, synaptic cleft, postsynaptic membrane with receptors, mitochondria within the presynaptic terminal.]
(a) Identify the structure labelled I. [1]
(b) Outline the process by which neurotransmitters are released from the presynaptic neuron. [3]
(c) Explain the role of acetylcholinesterase in the synapse. [2]
Question
Identify the structure labelled I. [1]
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Structure I is a small, membrane-bound sac within the presynaptic terminal, indicated in the visualization description as containing neurotransmitter.
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Synaptic vesicle
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Outline the process by which neurotransmitters are released from the presynaptic neuron. [3]
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The process starts with an electrical signal and involves ion movement and membrane fusion.
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Arrival of action potential at presynaptic terminal causes voltage-gated Ca²⁺ channels to open. Ca²⁺ ions diffuse into the terminal, triggering fusion of synaptic vesicles with the presynaptic membrane and release of neurotransmitters into the synaptic cleft via exocytosis.
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Explain the role of acetylcholinesterase in the synapse. [2]
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Think about what needs to happen to the neurotransmitter signal after it has occurred to allow for future signals.
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Acetylcholinesterase is an enzyme in the synaptic cleft that breaks down acetylcholine into choline and acetate. This terminates the signal and prevents continuous stimulation of the postsynaptic neuron, allowing it to repolarize.
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