Synapses and Neurotransmitters

1. 1 State [1 mark]

   State what a synapse is.

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   Model answer

   A junction between two neurones, with a tiny gap between them.

   Mark scheme

   • Junction / gap between two neurones — 1 mark
2. 2 Name [2 marks]

   Name the chemical that carries a message across a synapse, and name the process by which it crosses the gap.

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   Model answer

   The chemical is a neurotransmitter. It crosses the gap by diffusion.

   Mark scheme

   • Neurotransmitter — 1 mark
   • Diffusion — 1 mark
3. 3 Describe [3 marks]

   Describe what happens to the neurotransmitter after it crosses the synaptic gap.

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   Model answer

   The neurotransmitter binds to receptor molecules on the membrane of the next neurone. This makes the next neurone produce an electrical impulse. The neurotransmitter is then broken down or taken back up.

   Mark scheme

   • Binds to receptors on the next neurone — 1 mark
   • A new impulse is started — 1 mark
   • It is broken down or reabsorbed — 1 mark
4. 4 Calculate [2 marks]

   An impulse takes 0.02 s to travel 2 m along a neurone. Calculate the speed of the impulse. Give the unit.

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   Model answer

   Speed = 2 ÷ 0.02 = 100 m/s.

   Mark scheme

   • 2 divided by 0.02 — 1 mark
   • 100 m/s — 1 mark
5. 5 Explain [3 marks]

   Explain why impulses can only pass in one direction across a synapse.

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   Model answer

   Only the first neurone has vesicles that contain neurotransmitter, so it is the only one that can release the chemical into the gap. Only the second neurone has receptor molecules for the neurotransmitter on its membrane. This means the signal can pass from the first neurone to the second but not the other way.

   Mark scheme

   • Vesicles are only in the first neurone — 1 mark
   • Receptors are only on the second neurone — 1 mark
   • So the chemical signal can only go one way — 1 mark
6. 6 Explain [6 marks]

   Explain how an electrical impulse in one neurone causes an electrical impulse in the next neurone.

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   Model answer

   When the electrical impulse reaches the end of the first neurone, it causes vesicles to release a neurotransmitter into the synaptic gap. The neurotransmitter diffuses across the gap, which is very narrow. It binds to specific receptor molecules on the membrane of the second neurone. This binding causes the second neurone to produce an electrical impulse. The neurotransmitter is then broken down or taken back up, so that the second neurone is not stimulated all the time.

   Mark scheme

   • Impulse arrives at the end of the first neurone — 1 mark
   • Neurotransmitter released from vesicles — 1 mark
   • It diffuses across the synaptic gap — 1 mark
   • It binds to receptors on the next neurone — 1 mark
   • A new impulse starts in the second neurone — 1 mark
   • The neurotransmitter is broken down or reabsorbed — 1 mark

Quick check

1. 1

   What is the name of the gap between two neurones?

   1. AA synapse
   2. BA dendron
   3. CA vesicle
   4. DA myelin sheath
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   A: A synapse

   The gap between neurones is called the synaptic gap, and the junction is a synapse.

2. 2

   What crosses the synaptic gap?

   1. AAn electrical impulse
   2. BA hormone
   3. CA neurotransmitter
   4. DA receptor cell
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   C: A neurotransmitter

   A chemical called a neurotransmitter crosses the gap. The electrical impulse does not.

3. 3

   How does a neurotransmitter move across the synaptic gap?

   1. AOsmosis
   2. BDiffusion
   3. CActive transport
   4. DMitosis
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   B: Diffusion

   Neurotransmitter molecules move by diffusion from a region of high concentration to a region of low concentration.

4. 4

   Where are neurotransmitters stored?

   1. AIn the receptors
   2. BIn the myelin sheath
   3. CIn the cell body of the second neurone
   4. DIn vesicles
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   D: In vesicles

   They are stored in vesicles in the end of the first neurone, ready to be released.

5. 5

   What do neurotransmitters bind to on the next neurone?

   1. AReceptor molecules
   2. BVesicles
   3. CMitochondria
   4. DMyelin
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   A: Receptor molecules

   They fit into receptor molecules on the membrane of the next neurone, like a key into a lock.

6. 6

   Why do impulses only cross a synapse in one direction?

   1. AThe gap is too wide to cross backwards
   2. BThe second neurone cannot make an impulse
   3. CVesicles are on one side and receptors on the other
   4. DNeurotransmitter is destroyed by the first neurone
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   C: Vesicles are on one side and receptors on the other

   Vesicles are found on one side of the gap and receptors only on the other, so the chemical can only act one way.

7. 7

   An impulse travels 0.6 m in 0.006 s. What is its speed?

   1. A0.01 m/s
   2. B3.6 m/s
   3. C10 m/s
   4. D100 m/s
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   D: 100 m/s

   Speed is distance divided by time: \(0.6 \div 0.006 = 100\) m/s.

8. 8

   Why does the end of the first neurone contain many mitochondria?

   1. ATo make myelin
   2. BTo release energy for making and releasing neurotransmitter
   3. CTo digest the neurotransmitter
   4. DTo store receptors
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   B: To release energy for making and releasing neurotransmitter

   Making and releasing neurotransmitter needs energy, which mitochondria release by respiration.

9. 9

   A poison blocks the receptor molecules at a synapse. What will happen?

   1. AMore neurotransmitter will be released
   2. BThe impulse will pass backwards
   3. CThe message will not be passed to the next neurone
   4. DThe synaptic gap will get wider
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   C: The message will not be passed to the next neurone

   If the neurotransmitter cannot bind to the receptors, no new impulse starts in the next cell, so the message is not passed on.