A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Synapse and Neuromuscular Junction Transmission — mark scheme explained
The short answer
In A-Level Biology, understanding the detailed structure and function of synapses and neuromuscular junctions is crucial for grasping how organisms respond to changes in their internal and external environments.
The question
Describe the sequence of events in transmission across a cholinergic synapse. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
An action potential reaches the presynaptic terminal, causing voltage-gated calcium channels to open.
- S2
Calcium ions enter the presynaptic terminal, triggering the fusion of synaptic vesicles with the presynaptic membrane and the release of ACh into the synaptic cleft.
- S3
ACh diffuses across the synaptic cleft and binds to nAChRs on the postsynaptic membrane, causing them to open.
- S4
The opening of nAChRs allows sodium ions to enter the postsynaptic neuron, depolarizing it and potentially triggering an action potential if the threshold is reached.
- S5
ACh is rapidly broken down by acetylcholinesterase (AChE) in the synaptic cleft, preventing prolonged stimulation.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
An action potential reaches the presynaptic terminal, causing voltage-gated calcium channels to open.
- S2
Calcium ions enter the presynaptic terminal, triggering the fusion of synaptic vesicles with the presynaptic membrane and the release of ACh into the synaptic cleft.
- S3
ACh diffuses across the synaptic cleft and binds to nAChRs on the postsynaptic membrane, causing them to open.
- S4
The opening of nAChRs allows sodium ions to enter the postsynaptic neuron, depolarizing it and potentially triggering an action potential if the threshold is reached.
- S5
ACh is rapidly broken down by acetylcholinesterase (AChE) in the synaptic cleft, preventing prolonged stimulation.
Final answer: The sequence of events in transmission across a cholinergic synapse includes: 1) An action potential reaches the presynaptic terminal, causing voltage-gated calcium channels to open. 2) Calcium ions enter the presynaptic terminal, triggering the fusion of synaptic vesicles with the presynaptic membrane and the release of ACh into the synaptic cleft. 3) ACh diffuses across the synaptic cleft and binds to nAChRs on the postsynaptic membrane, causing them to open. 4) The opening of nAChRs allows sodium ions to enter the postsynaptic neuron, depolarizing it and potentially triggering an action potential if the threshold is reached. 5) ACh is rapidly broken down by acetylcholinesterase (AChE) in the synaptic cleft, preventing prolonged stimulation.
Common mistakes
- Confusing unidirectionality with bidirectional transmission. — Always remember that synaptic transmission is unidirectional because neurotransmitters are released from the presynaptic terminal and bind to receptors on the postsynaptic membrane.
- Misinterpreting temporal summation as a single action potential causing a large depolarization. — Understand that temporal summation involves the summing of multiple small depolarizations caused by successive action potentials arriving at the presynaptic terminal in quick succession.
- Confusing spatial summation with the effect of a single presynaptic neuron. — Remember that spatial summation occurs when multiple presynaptic neurons release neurotransmitters simultaneously onto the same postsynaptic neuron, combining their effects to reach the threshold for an action potential.
- Failing to explain how inhibitory synapses work using GABA or glycine. — Practice explaining that inhibitory synapses use neurotransmitters like GABA or glycine, which bind to receptors that allow chloride ions to enter the postsynaptic neuron, hyperpolarizing it and making it less likely for an action potential to be generated.
- Not recognizing the key differences between cholinergic synapses and neuromuscular junctions. — Always highlight that in a cholinergic synapse, the postsynaptic target is another neuron, while in a neuromuscular junction, it is a muscle fiber. The action potential in a muscle fiber leads directly to muscle contraction.
Where the marks go
- Full worked solution (all marking points)5 marks