Answers · Nerves and reflexes

Synapses — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. SQ1 · Practice

Model answer. A synapse is a junction between two neurones, where the end of one neurone comes very close to the next, with a tiny gap between them.

A full answer includes

I chose: a junction between two neurones.
I did not choose a bundle of neurones: that is a nerve.

Also accept

a junction between two neurones

Watch for

“A bundle of many neurones.” — A bundle of many neurones is a nerve. A synapse is where two neurones meet.
“The cell body of a neurone.” — The cell body holds the nucleus of one neurone. A synapse is where two neurones meet.
“A short neurone inside the spinal cord.” — A short neurone inside the spinal cord is a relay neurone, a cell. A synapse is the junction where two neurones meet.

2. SQ2 · Practice

Model answer. There are only two neurones, sensory and motor, so they meet at one place: the impulse crosses one synapse, in the spinal cord.

A full answer includes

I chose one.
I did not count the receptor or the muscle: only places where two neurones meet.

Also accept

Watch for

“Two.” — Two synapses is the arc with a relay neurone. Here there are only two neurones, so they meet at one place.
“None.” — Neurones never join directly: wherever one meets the next there is a synapse.
“Three.” — Only junctions between two neurones count here, not the receptor end or the muscle.

3. SQ3 · Practice

Options as numbered on the pupil sheet: 1 — synaptic gap; 2 — vesicle; 3 — receptor proteins; 4 — neurotransmitter molecules.

Model answer. A is a vesicle, a sac inside the end of the first neurone. B are neurotransmitter molecules crossing the gap. C is the synaptic gap. D are the receptor proteins on the next neurone's membrane.

A full answer includes

I matched A to vesicle.
I matched B to neurotransmitter molecules.
I matched C to synaptic gap.
I matched D to receptor proteins.
I did not call D vesicles: vesicles are inside the first neurone.

Also accept

A vesicle; B neurotransmitter; C synaptic gap; D receptor proteins

Watch for

“D are vesicles.” — Vesicles are inside the first neurone (A). The proteins on the next neurone's membrane are receptor proteins.

4. SQ4 · Practice

Options as numbered on the pupil sheet: 1 — Tiny sacs empty their chemical into the narrow space.; 2 — An impulse reaches the end of the first neurone.; 3 — A new impulse sets off along the next neurone.; 4 — The chemical spreads across the space, from many molecules to few.; 5 — The chemical fits into proteins on the next neurone's membrane..

Model answer. An impulse arrives at the end of the first neurone and stimulates the vesicles (the sacs) to release neurotransmitter (the chemical) into the gap. The molecules diffuse across the gap (spread from many to few) and bind with receptor proteins on the next neurone (fit the proteins), which stimulates an impulse in it.

A full answer includes

I put the impulse reaching the end of the first neurone first.
I put the sacs (vesicles) emptying their chemical second.
I put the chemical spreading across the space (diffusion) third.
I put the chemical fitting the proteins (binding) fourth.
I put the new impulse last.
I did not put the fitting before the spreading.

Also accept

arrive; release; diffuse; bind; new impulse

Watch for

“The chemical fits the proteins before it spreads across.” — It has to cross the gap first, by diffusion, before it can reach the receptor proteins.

5. SQ5 · Practice

Model answer. The neurotransmitter molecules diffuse across the gap, from where there are many of them, beside the first neurone, to where there are few, beside the next.

A full answer includes

I chose: by diffusion.
I did not choose the impulse carrying them: the impulse does not cross the gap.

Also accept

by diffusion

Watch for

“Carried by the impulse.” — The impulse does not cross the gap: the chemical diffuses across.
“Carried by the blood.” — There is no blood in the synaptic gap. The molecules diffuse across it.
“By active transport.” — The molecules move from where there are many to where there are few, which needs no energy from the cell: that is diffusion.

6. SQ6 · Practice

Model answer. Only the end of the first neurone has vesicles that release neurotransmitter, and only the next neurone has the receptor proteins it binds with. So the message can only cross from the first neurone to the next.

A full answer includes

I chose: only the first neurone has vesicles, and only the next has receptor proteins.
I did not choose the reversed statement: the neurone that sends must have the vesicles.

Also accept

only the first neurone has vesicles; only the next has receptor proteins

Watch for

“Only the next neurone has vesicles, and only the first has receptor proteins.” — That would make impulses cross from the next neurone back to the first. The neurone that sends has the vesicles.
“The gap is narrow beside the first neurone and wide beside the next one.” — The direction is not set by the width of the gap but by where the vesicles and the receptor proteins are.
“The impulse loses strength as it crosses, so it cannot cross back again.” — A new impulse starts in the next neurone. Nothing crosses back because that side has no vesicles to release neurotransmitter.

7. SQ7 · Practice

Model answer. Along each neurone the message is an electrical impulse. But where two neurones meet, at a synapse, the impulse cannot cross the gap: it makes the first neurone release neurotransmitter, a chemical, which diffuses across and starts a new impulse in the next neurone.

A full answer includes

I wrote that along each neurone the impulse is electrical.
I wrote that at each synapse, a junction between neurones, a chemical (the neurotransmitter) diffuses across the gap.
I wrote that the chemical starts a new impulse in the next neurone.
I did not write that the impulse jumps the gap.

Also accept

electrical along neurones; chemical (neurotransmitter) across synapses

Watch for

“The impulse jumps across the gap like a spark.” — At the gap the message is carried by a chemical, not an electrical spark.

8. SQ8 · Practice

Model answer. The end of the first neurone holds vesicles containing neurotransmitter molecules. Between the two neurones is the synaptic gap. The membrane of the next neurone carries receptor proteins that the neurotransmitter fits.

A full answer includes

I wrote vesicles containing neurotransmitter molecules, in the end of the first neurone.
I wrote a synaptic gap between the two neurones.
I wrote receptor proteins on the membrane of the next neurone.
I did not put the vesicles in the next neurone.

Also accept

vesicles with neurotransmitter; synaptic gap; receptor proteins

Watch for

“The next neurone has the vesicles.” — The vesicles are in the first neurone, the one the impulse arrives along.

9. SQ9 · Practice

Model answer. The impulse stimulates the vesicles to release neurotransmitter molecules into the synaptic gap. The molecules diffuse across the gap and bind with receptor proteins on the next neurone. This stimulates an impulse in the next neurone.

A full answer includes

I wrote that the impulse stimulates the release of neurotransmitter from vesicles into the gap.
I wrote that the neurotransmitter diffuses across the gap.
I wrote that it binds with receptor proteins on the next neurone.
I wrote that an impulse is then stimulated in the next neurone.
I did not write that the neurotransmitter travels along the next neurone.

Also accept

release from vesicles; diffuse across gap; bind receptor proteins; new impulse

Watch for

“The neurotransmitter travels along the next neurone.” — The neurotransmitter only crosses the gap. Along the next neurone the message is a new electrical impulse.

10. SQ10 · Practice

Model answer. The experiment shows that a neurone on its own can carry an impulse either way, so the neurones are not the one-way part. The neurones of the arc are joined by synapses, and synapses ensure that impulses travel in one direction only. So the impulse can only go from the receptor towards the effector.

A full answer includes

I chose: the synapses between the neurones.
I did not choose the neurones themselves: the experiment shows an impulse can travel either way along one.

Also accept

the synapses between the neurones

Watch for

“The neurones themselves, which can only carry impulses one way.” — The experiment shows that an impulse can travel either way along a neurone. The one-way step is at the synapses.
“The receptor, which only starts impulses at its own end.” — The receptor starts the impulses, but an impulse started further along the arc could still spread both ways along a neurone. The synapses stop it going backwards.
“The effector, which pulls the impulses towards itself.” — An effector responds to impulses; it does not pull them. The direction is set at the synapses.

11. SQ11 · Practice

Model answer. A synapse is not a neurone, or any kind of cell: it is a junction between two neurones, where one ends very close to the next with a tiny gap between them. The short neurone in the spinal cord that links the sensory neurone to the motor neurone is a relay neurone. There is a synapse at each end of it: one where the sensory neurone meets it, and one where it meets the motor neurone.

A full answer includes

I wrote that a synapse is not a cell: it is a junction between two neurones.
I wrote that the short neurone the pupil describes is a relay neurone.
I wrote that there is a synapse at each end of the relay neurone.
I did not write that a synapse and a relay neurone are the same thing.

Also accept

a synapse is a junction, not a cell; the short neurone is a relay neurone; a synapse at each end of it

Watch for

“A synapse is the same thing as a relay neurone.” — A relay neurone is a cell. A synapse is the junction where two neurones meet; a relay neurone has one at each end.

12. ST1 · Apply

Model answer. Four neurones in a row meet at three places: sensory to the first relay, first relay to second relay, and second relay to motor. So the impulse crosses three synapses.

Checked automatically

Synapses: 3 (exact value)
- Common wrong answer 4 (no unit): Count the joins, not the neurones: three.

A full answer includes

I counted the places where one neurone meets the next.
I wrote 3.
I did not write 4, the number of neurones.

Also accept

three

Watch for

“Four.” — Four neurones in a row have only three places where one meets the next.

13. ST2 · Apply

Model answer. The impulse arrives, but no neurotransmitter is released into the gap. With nothing to diffuse across and bind with the receptor proteins, no impulse is stimulated in the next neurone, so the message stops at that synapse.

A full answer includes

I wrote that no neurotransmitter enters the gap.
I wrote that nothing diffuses across or binds with the receptor proteins.
I predicted that no impulse is stimulated in the next neurone: the message stops.
I did not write that the impulse jumps across instead.

Also accept

no release; nothing binds; no impulse in next neurone

Watch for

“The impulse jumps across instead.” — An impulse cannot cross the gap on its own; without neurotransmitter the message stops.

14. ST3 · Apply

Model answer. The arriving impulse still makes the vesicles release neurotransmitter, which diffuses across the gap and binds with the receptor proteins. Binding stimulates an impulse in the next neurone. But the neurotransmitter is no longer broken down, so it stays in the gap and keeps binding, and impulses keep being stimulated in the next neurone. The muscle keeps being made to contract.

A full answer includes

I chose: impulses keep being stimulated in the next neurone, after the arriving impulse has passed.
I did not choose "no impulse": the neurotransmitter still binds with the receptor proteins.

Also accept

impulses keep being stimulated in the next neurone

Watch for

“No impulse is stimulated, because the neurotransmitter cannot bind.” — The poison acts on the enzyme, not on the receptor proteins. The neurotransmitter still binds, and binding stimulates an impulse.
“The neurotransmitter travels along the next neurone instead of an impulse.” — The neurotransmitter only crosses the gap. Along the next neurone the message is an electrical impulse.
“Impulses travel back across the synapse into the first neurone.” — Only the first neurone has vesicles, and only the next has receptor proteins, so nothing can cross back. The extra impulses are all in the next neurone.

15. ST4 · Apply

Model answer. (a) The sacs are vesicles containing neurotransmitter, and the proteins are receptor proteins. (b) The neurone with the vesicles is the one that releases neurotransmitter, and the one with the receptor proteins receives it, so impulses cross from the right-hand neurone to the left-hand one, the other way from most diagrams.

A full answer includes

(a) I wrote that the sacs are vesicles containing neurotransmitter.
(a) I wrote that the proteins are receptor proteins.
(b) I wrote from right to left.
(b) I wrote that the right-hand neurone has the vesicles, so it releases neurotransmitter.
(b) I wrote that the neurotransmitter binds with receptor proteins on the left-hand neurone.
(b) I did not write left to right just because diagrams are usually drawn that way.

Also accept

(a) vesicles; receptor proteins; (b) right to left: vesicles side sends

Watch for

“(b) Left to right, because impulses always go left to right.” — The direction is set by the vesicles and receptor proteins, not by which way the page is drawn.