Biology 9700/41 — October/November 2012
Cambridge A-Level · A Level Structured Questions · worked solutions for every part, with the mark scheme
Topics Inheritance · Selection and Evolution · Control and Coordination · (outdated) Biotechnology · Genetic Technology · (outdated) Aspects of Human Reproduction · +4 more
Section A
Answer all the questions.
Nerve impulses have to cross synapses. The events that enable a nerve impulse to cross a cholinergic synapse are listed in Table 1.1.
The events are not listed in the correct order.
Table 1.1
| event | description of event |
|---|---|
| A | Calcium ions enter presynaptic neurone knob. |
| B | Acetylcholine binds to receptor proteins on postsynaptic membrane. |
| C | Vesicles fuse with presynaptic membrane and release acetylcholine into synaptic cleft. |
| D | Postsynaptic membrane becomes depolarised. |
| E | Nerve impulse reaches presynaptic membrane. |
| F | Acetylcholine diffuses across cleft. |
| G | Receptor proteins change shape, channels open and sodium ions enter postsynaptic neurone. |
| H | Calcium ion channels open in presynaptic membrane. |
| I | Nerve impulse generated in postsynaptic neurone. |
| J | Vesicles of acetylcholine move towards presynaptic membrane. |
Complete Table 1.2 to show the events in the correct order.
Two of the events have been done for you.
Table 1.2
| correct order | letter of stage |
|---|---|
| 1 | E |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
| 6 | F |
| 7 | |
| 8 | |
| 9 | |
| 10 |
Answer
| correct order | letter of stage |
|---|---|
| 1 | E |
| 2 | H |
| 3 | A |
| 4 | J |
| 5 | C |
| 6 | F |
| 7 | B |
| 8 | G |
| 9 | D |
| 10 | I |
E, H, A, J, C, F, B, G, D, I
Background Concept
Synaptic transmission at a cholinergic synapse is the process by which an electrical nerve impulse is passed from one neurone to another across a tiny gap (the synaptic cleft) using a chemical messenger — the neurotransmitter acetylcholine (ACh). This conversion from electrical to chemical and back to electrical signal occurs in a precise sequence of events, each one triggered by the previous one. Calcium ions play the crucial role of coupling electrical activity in the presynaptic terminal to chemical release.
In the presynaptic neurone the sequence is: arrival of the action potential → opening of voltage-gated Ca²⁺ channels → Ca²⁺ influx → vesicle mobilisation → vesicle fusion with the presynaptic membrane (exocytosis) → release of ACh into the cleft. In the postsynaptic neurone, ACh diffuses across the cleft and binds to receptor proteins; these are themselves ligand-gated ion channels that open, allowing Na⁺ to enter, depolarising the membrane and (if threshold is reached) generating a new action potential.
Understanding the Question
The question gives ten events (labelled A to J) involved in synaptic transmission, listed in a deliberately scrambled order. We are asked to place them in the correct order. Two of the events (E in position 1 and F in position 6) are already correctly placed, so positions 2, 3, 4, 5, 7, 8, 9 and 10 must be filled. The four marks are split: two for getting the four events that come before F in the correct order (H, A, J, C), and two for getting the four events that come after F in the correct order (B, G, D, I).
Approach
Think of the process as a logical chain, with each event being the necessary trigger for the next. Start with the very first thing that must happen (the nerve impulse must reach the presynaptic membrane) and work forward. After the impulse arrives, the chain of events that must occur before ACh is released is: open calcium channels → calcium enters → vesicles move → vesicles fuse. Then ACh diffuses across the cleft. On the postsynaptic side: ACh binds to receptors → receptors change shape and channels open → sodium enters → membrane depolarises → new nerve impulse generated.
Step-by-Step Reasoning
- Position 1 (E — given): Nerve impulse reaches presynaptic membrane. Nothing else can occur until an action potential has propagated to the axon terminal.
- Position 2 (H): Calcium ion channels open in the presynaptic membrane. The depolarisation caused by the arriving action potential opens voltage-gated calcium channels.
- Position 3 (A): Calcium ions enter the presynaptic neurone knob. With the channels open, Ca²⁺ flows down its steep concentration gradient from the extracellular fluid into the presynaptic terminal.
- Position 4 (J): Vesicles of acetylcholine move towards the presynaptic membrane. The Ca²⁺ influx triggers the mobilisation of ACh-containing vesicles, which are transported (along cytoskeletal tracks) towards the presynaptic membrane.
- Position 5 (C): Vesicles fuse with the presynaptic membrane and release ACh into the synaptic cleft. The vesicles, having reached the membrane, fuse with it (exocytosis) and release their ACh contents into the cleft.
- Position 6 (F — given): Acetylcholine diffuses across the cleft. ACh moves across the narrow gap by simple diffusion, briefly binding to receptor sites en route.
- Position 7 (B): Acetylcholine binds to receptor proteins on the postsynaptic membrane. Once across, ACh molecules bind to specific receptor proteins (nicotinic ACh receptors) on the postsynaptic membrane.
- Position 8 (G): Receptor proteins change shape, channels open and sodium ions enter the postsynaptic neurone. ACh binding causes a conformational change; the ligand-gated ion channels open and Na⁺ flows in down its electrochemical gradient.
- Position 9 (D): Postsynaptic membrane becomes depolarised. The Na⁺ influx makes the inside of the postsynaptic neurone less negative.
- Position 10 (I): Nerve impulse generated in the postsynaptic neurone. If the depolarisation reaches threshold at the axon hillock, voltage-gated sodium channels open and a new action potential is generated.
Key Takeaways
The cholinergic synapse sequence can be remembered as: AP arrives → Ca²⁺ channels open → Ca²⁺ enters → vesicles move → vesicles fuse → ACh diffuses → ACh binds receptors → Na⁺ channels open → depolarisation → new AP. The entry of Ca²⁺ is the crucial link between electrical activity in the presynaptic neurone and chemical release.
Common Mistakes
- Placing A (Ca²⁺ entering) before H (channels opening) — the channels must be open before Ca²⁺ can enter
- Placing J (vesicle movement) before A (Ca²⁺ entry) — Ca²⁺ entry is the trigger for vesicle movement
- Placing C (vesicle fusion) before J (vesicle movement) — vesicles must first be moved to the membrane before they can fuse with it
- Confusing the order after F: B (receptor binding) must come before G (channel opening) because the binding is what causes the conformational change that opens the channels
- Swapping the order of D and I: the membrane must first be depolarised before a new nerve impulse can be generated
Things to Be Careful About
The wording of event H is "Calcium ion channels open" (the process) and event A is "Calcium ions enter" (the consequence). These must occur in that order. Event G is presented as a single combined step ("Receptor proteins change shape, channels open and sodium ions enter") — do not try to split it into separate sub-events. The mark scheme requires the four events before F (H, A, J, C) to be in the correct relative order, and the four events after F (B, G, D, I) to be in the correct relative order, to earn the four marks.
Synapses have many roles in nervous coordination in mammals.
Explain how synapses ensure one-way transmission of nerve impulses.
Answer
- Vesicles (containing ACh) are found only in the presynaptic neurone, so ACh can only be released from the presynaptic membrane.
- Receptor proteins are found only on the postsynaptic membrane, so ACh can only act on the postsynaptic neurone.
Vesicles of ACh are only in the presynaptic neurone; receptor proteins are only on the postsynaptic membrane.
Background Concept
In a nervous system, information must flow in a controlled direction: from receptors, along sensory neurones, through the central nervous system, along motor neurones, to effectors. At each junction between two neurones (a synapse), this one-way flow is structurally enforced. The synapse is not just a relay — it is a one-way valve.
The polarity of a cholinergic synapse comes from the asymmetric distribution of two key components: the neurotransmitter ACh (packaged in vesicles) and the receptor proteins that respond to ACh. ACh-containing vesicles are located only in the presynaptic neurone terminal, and ACh receptors are located only on the postsynaptic membrane. This structural asymmetry is the anatomical basis of one-way transmission.
Understanding the Question
This is a 2-mark "explain" question. The command word "explain" requires the candidate to give a reason, not just state an observation. The question is asking why synapses allow transmission in only one direction, not how they work. We need to identify the structural features that make the synapse one-way.
Approach
Think about the components of a synapse and which side each is on:
- Vesicles containing ACh: only in the presynaptic neurone
- Receptor proteins for ACh: only on the postsynaptic membrane
This asymmetry means ACh can only be released from one side and only act on the other side, so the signal can only travel in one direction.
Step-by-Step Reasoning
Marking point 1: Vesicles containing ACh are found only in the presynaptic neurone. Because of this, ACh can only be released from the presynaptic membrane (by exocytosis). It cannot be released from the postsynaptic neurone because there are no ACh-containing vesicles there.
Marking point 2: Receptor proteins for ACh are found only on the postsynaptic membrane. Because of this, ACh can only bind to receptors on the postsynaptic neurone. If ACh were present in the synaptic cleft and the signal tried to travel the "wrong way", the presynaptic membrane has no receptors, so the signal would have no effect.
Together, these two structural asymmetries make the synapse a one-way valve: the chemical signal (ACh) can only originate from the presynaptic side and only be detected on the postsynaptic side. Therefore, the nerve impulse can only travel from the presynaptic neurone to the postsynaptic neurone.
Key Takeaways
The structural polarity of the synapse is the basis of one-way transmission. Vesicles of ACh are on the presynaptic side; receptors for ACh are on the postsynaptic side. This means ACh can only be released from the presynaptic neurone and only act on the postsynaptic neurone, forcing the impulse to travel in one direction only.
Common Mistakes
- Saying "the impulse can only go one way" without explaining why (the mark scheme requires the reason, not just the observation)
- Saying "ACh is released into the cleft" without specifying that this is from the presynaptic side
- Confusing the direction (it is presynaptic to postsynaptic, not the other way around)
- Not specifying that the receptors are on the postsynaptic membrane
- Saying "the vesicles prevent backflow" — this is wrong; the directionality comes from where the vesicles and receptors are located, not from any "blocking" action
Things to Be Careful About
The mark scheme emphasises "only" — the vesicles are only in the presynaptic neurone, and the receptors are only on the postsynaptic membrane. The word "only" (or an equivalent such as "exclusively") is important because simply saying "vesicles are in the presynaptic neurone" is not enough — the key point is that they are NOT in the postsynaptic neurone, which is what enforces the one-way direction.
In a learning activity, it is believed that the number of synapses between brain neurones increases.
Suggest the advantages of this increased number of synapses.
Answer
- More interconnections of nerve pathways (allowing more routes for information to flow and to be integrated)
- For memory (the formation of memories is associated with the formation of new synaptic connections)
Alternative second point: a wider range of responses / summation (more subthreshold inputs can be combined at a single postsynaptic neurone).
More interconnections of nerve pathways; for memory (any two distinct advantages).
Background Concept
The human brain contains approximately 86 billion neurones, and each neurone typically forms thousands of synaptic connections with other neurones. The total number of synapses in the brain is therefore astronomical, and the pattern of these connections is not fixed — it changes with experience, learning, and the formation of memories. This property is called synaptic plasticity and is a key feature of the brain's ability to process and store information.
The more synaptic connections a neurone has, the more inputs it can receive and integrate, and the more it can influence other neurones. This is analogous to a computer network: more connections between nodes allow more pathways for information to flow, more possibilities for combining inputs, and a greater capacity for storing information.
Understanding the Question
This is a 2-mark "suggest" question. The command word "suggest" requires the candidate to apply biological knowledge to propose reasonable advantages, not just recall facts. The scenario is: in a learning activity, the number of synapses between brain neurones increases. What are the advantages of this?
Approach
Think about what an increased number of synaptic connections would allow:
- More possible pathways for nerve impulses to travel through the brain
- More possibilities for combining inputs at any one neurone (e.g. summation)
- More capacity for forming and storing memories
- More flexibility in the responses that can be generated
The mark scheme awards marks for any two distinct, biologically reasonable advantages.
Step-by-Step Reasoning
Marking point 1: More interconnections of nerve pathways. With more synapses, there are more routes for nerve impulses to travel through the brain. This allows the brain to integrate information from more sources and form more complex patterns of activity.
Marking point 2: For memory. Increased synaptic connections are believed to underlie the formation of memories. Specific patterns of synaptic activity (or specific strengthened synapses) are thought to correspond to specific memories. So more synapses = more capacity for memory.
Marking point 3 (alternative): A wider range of responses. With more interconnections, the brain can generate a greater variety of behavioural outputs from a given set of inputs, because more combinations of neurones can be activated.
Marking point 4 (AVP — any valid point): e.g. summation. More synapses mean more subthreshold inputs can be combined at a single postsynaptic neurone. If the combined depolarisation reaches threshold, an action potential is generated. This is the cellular basis of decision-making in the brain.
Key Takeaways
Increased synaptic density in the brain is associated with:
- More interconnections of nerve pathways
- Memory formation
- A wider range of behavioural responses
- Greater capacity for information processing (e.g. summation)
This reflects the principle of synaptic plasticity, which is the basis of learning and memory at the cellular level.
Common Mistakes
- Saying "for learning" — the mark scheme explicitly ignores "learning" but credits "memory". The candidate should distinguish between the process (learning) and the outcome (memory)
- Being too vague — e.g. "more synapses means a better brain" is not specific enough
- Repeating the same idea twice in different words — this would only earn one mark, not two
- Saying "more impulses can be conducted" — this is incorrect; the rate of impulse conduction is limited by the axon, not by the number of synapses. More synapses mean more complex information processing, not faster conduction
- Not giving specific biological examples
Things to Be Careful About
The mark scheme specifically ignores "learning" (since the question stem already mentions a learning activity) but credits "memory" (the outcome of learning). So when stating the advantage, use the word "memory", not "learning". The candidate should give two distinct advantages for the two marks — the same idea in different words will only earn one mark.
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