3.5.4Inquiry Process
Synapses — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 135 of 160
Part 1 of 5Structure of a Synapse
Presynaptic neurone
- Its axon terminal contains mitochondria and vesicles filled with neurotransmitter,
- and has voltage-gated calcium channels in its membrane.
Synaptic cleft
- A narrow fluid-filled gap, about 20 nm wide, separates the two cells, so the impulse cannot cross it electrically.
Postsynaptic membrane
- Contains receptor proteins for the neurotransmitter, and some of these are ligand-gated ion channels.
Neurotransmitter
- A chemical messenger, such as acetylcholine, released from vesicles and diffusing across the cleft to carry the signal.
Mitochondria
- Many mitochondria in the presynaptic terminal supply the ATP needed to make, package and recycle neurotransmitter after every impulse.
| Component | Location | Role |
|---|---|---|
| Vesicles | Presynaptic terminal | Store neurotransmitter |
| Ca²⁺ channels | Presynaptic membrane | Trigger vesicle release |
| Receptors | Postsynaptic membrane | Bind neurotransmitter |
| Enzyme | Cleft | Breaks transmitter down |
Part 2 of 5Functions of Synapses
Functions of Synapses
One-way transmission
- Vesicles are only on the presynaptic side and receptors only on the postsynaptic side, so signals pass in one direction.
Integration
- A neurone may receive input from many synapses at once and decide whether to fire based on their combined effect.
Branching
- One neurone can connect to many others, spreading a signal widely through the nervous system.
Filtering
- Weak or accidental signals may fail to cause enough release, so the synapse screens out some background noise.
Drug action
- Many drugs and toxins work at synapses by mimicking or blocking neurotransmitters,
- which shows how important these junctions are for control.
Part 3 of 5Sequence at a Cholinergic Synapse
1
An impulse reaches the axon terminal and depolarises the presynaptic membrane.
2
Calcium ions enter through voltage-gated channels and cause vesicles to fuse with the membrane.
3
Acetylcholine is released by exocytosis and diffuses across the cleft.
4
It binds receptors on the postsynaptic membrane, opening sodium channels and starting depolarisation there.
Part 4 of 5Worked Example
Question
- The synaptic cleft is 20 nm wide.
- How many cleft widths would fit across 1 mm, and what does this tell you about diffusion distance?
- Convert 1 mm to nanometres: 1 mm = 1 × 10⁻³ m = 1 × 10⁶ nm.
- Number of clefts = 1 × 10⁶ ÷ 20 = 5 × 10⁴.
- So the gap is 50 000 times smaller than a millimetre.
Answer
- 50 000 cleft widths fit into 1 mm;
- the tiny distance means neurotransmitter crosses by diffusion almost instantly.
Part 5 of 5Exam Checklist & Tips
3.5.4 – Key Points for the Exam
- Synapse A junction between two neurones, or a neurone and an effector, where signals pass chemically.
- Synaptic cleft The gap between presynaptic and postsynaptic membranes.
- Neurotransmitter A chemical released from a neurone that transmits a signal across a synapse.
- Vesicle A membrane-bound sac that stores and releases neurotransmitter.
Common Mistakes
- Saying the impulse jumps across the cleft; the signal is carried chemically.
- Thinking transmission can run backwards; only the presynaptic side releases transmitter.
- Forgetting calcium ionsthey trigger vesicle fusion, not sodium.
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