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
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For descriptive questions, marks are typically awarded for accurately identifying key components and steps in a process. For explanatory questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic. For comparative questions, marks are awarded for identifying both similarities and differences. For predictive questions, marks are given for logical reasoning based on known principles.
What the command words demand
- Describe
- Provide a detailed account of the structure or process.
- Explain
- Give reasons for how and why something happens, including relevant principles and relationships.
- Compare
- Identify similarities and differences between two or more concepts or processes.
- Predict
- Use knowledge to anticipate the effects of changes or interventions.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, provide detailed answers, and check your work.
- An action potential reaches the presynaptic terminal, causing voltage-gated calcium channels to open.1 mark
- 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.1 mark
- ACh diffuses across the synaptic cleft and binds to nAChRs on the postsynaptic membrane, causing them to open.1 mark
- 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.1 mark
- ACh is rapidly broken down by acetylcholinesterase (AChE) in the synaptic cleft, preventing prolonged stimulation.1 mark
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.
Work through every step correctly and you earn all 5 marks.
Another worked example
Explain how temporal summation occurs in a synapse.
- Multiple action potentials arrive at the presynaptic terminal in quick succession.1 mark
- Each action potential releases a small amount of neurotransmitter, such as ACh.1 mark
- If these arrivals are close enough together, they can summate to produce a larger depolarization on the postsynaptic membrane.1 mark
- This combined effect can potentially reach the threshold for an action potential.1 mark
Final answer: Temporal summation occurs when multiple action potentials arrive at the presynaptic terminal in quick succession. Each action potential releases a small amount of neurotransmitter, such as ACh. If these arrivals are close enough together, they can summate to produce a larger depolarization on the postsynaptic membrane, potentially reaching the threshold for an action potential.
Work through every step correctly and you earn all 4 marks.
Common mistakes
Confusing unidirectionality with bidirectional transmission.
Why it happens: Students may not fully understand that neurotransmitters are only released from the presynaptic terminal and act on receptors on the postsynaptic membrane, ensuring one-way signal transmission.
Fix: 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.
Why it happens: Students may not realize that multiple action potentials are required for temporal summation to occur.
Fix: 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.
Why it happens: Students may not grasp that multiple presynaptic neurons are involved in spatial summation.
Fix: 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.
Why it happens: Students may not understand the role of chloride ions in hyperpolarizing the postsynaptic membrane.
Fix: 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.
Why it happens: Students may not fully appreciate that the postsynaptic target in a neuromuscular junction is a muscle fiber, leading to muscle contraction.
Fix: 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
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Describe Synaptic Transmission | Sequence the events transmitting a nerve impulse across a cholinergic synapse. | 5 |
| Explain Temporal Summation | Explain how repeated impulses at one synapse combine to trigger a postsynaptic action potential. | 4 |
| Compare Synapse Types | Identify similarities and differences between transmission at cholinergic synapses and neuromuscular junctions. | 5 |
| Predict Synaptic Effect | Predict how blocking acetylcholinesterase changes neurotransmitter breakdown and continued synaptic transmission. | 3 |
| Total across these question types | 17 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.