Explain neurotransmitter diffusion and receptor binding
Separate movement across the gap from receiving-site binding.
Section 14.1 · Coordination and response
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall cell membranes, diffusion, sensitivity and the difference between cells, tissues and organs.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What crosses the gap by diffusion in the supplied chemical model?
What you will learn
- Diffusion moves the chemical signal
- Binding links the molecules to the receiving cell
Explain neurotransmitter diffusion and receptor binding
Diffusion describes movement across the gap; binding describes interaction with receptor proteins on the next neurone. Neither is whole-cell movement. Original teaching model.
| Feature | Meaning |
|---|---|
| Release location | Presynaptic vesicles into gap |
| Diffusion | Chemical molecules move across gap |
| Binding | Molecules interact with next-cell receptor proteins |
| Consequence in supplied model | Next electrical impulse after binding |
Diffusion moves the chemical signal
Release from presynaptic vesicles puts neurotransmitter molecules into the synaptic gap. Their diffusion across the gap is the second required event. In the simplified initial model, the released molecules are locally concentrated near the releasing side and spread through the space by molecular movement. They do not require an artery to carry them from one membrane to the other.
Diffusion and binding are different descriptions. Diffusion accounts for molecules reaching the next-cell side; binding describes their interaction with receptor proteins there. The third required event explicitly names binding to receptors on the next neurone. Saying only chemicals move across therefore leaves a receiving step unexplained.
Binding links the molecules to the receiving cell
The supplied receptor model has binding sites compatible with the transmitter molecules. If a problem specifies that the relevant sites cannot bind the transmitter, diffusion can still occur without completion of the pictured receptor-mediated sequence. Follow that given condition rather than assuming every chemical molecule in the gap must generate the normal next-cell impulse.
The teaching schematic does not establish numerical diffusion speed, receptor counts or clinical effects. Its arrow summarises chemical movement across a microscopic space. For the required account, combine release into the gap, diffusion across it, binding on the next neurone and subsequent stimulation there. Keep the electrical impulse travelling along each neurone separate from the molecular events at the junction.
Pause and explain
Which distinguishes the second and third events?
Worked example
Distinguish diffusion across the gap from binding at the next cell.
Show the worked solution
- Diffusion concerns transmitter movement through the space.
- Binding concerns interaction with receiving receptor proteins.
Answer Neurotransmitters diffuse across the gap, then bind to receptor proteins on the next neurone.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain the two distinct chemical events after release and before the next impulse.
Give me a hint
Name movement and receiving interaction.
Compare my reasoning
- Transmitter molecules diffuse across the gap.
- They bind receptor proteins on the next neurone.
Neurotransmitters diffuse across the synaptic gap and bind to receptor proteins on the next neurone.
Look for these in your work
- Name diffusion
- Name next-cell receptor binding
The stated model allows transmitter diffusion but prevents binding to its receptors. Is normal next-cell stimulation established?
Give me a hint
Reaching the membrane is not the same as binding.
Compare my reasoning
- The diffusion event can still occur.
- The necessary binding step is interrupted.
No. Diffusion can occur, but the specified binding step is prevented, so the normal receptor-mediated next impulse is not established in this model.
Look for these in your work
- Allow the stated diffusion
- Identify the interrupted binding
Correct: neurotransmitter must travel through a blood vessel to cross the synaptic gap.
Give me a hint
The route here is the space between adjacent cells.
Compare my reasoning
- Vesicles release molecules into the gap.
- They diffuse directly across that gap to the receiving receptors.
The molecules are released locally and diffuse across the synaptic gap; blood transport is not the required between-cell route.
Look for these in your work
- Use local release and diffusion
- Reject the invented blood route
Common mistakes
- Treating diffusion and binding as interchangeable words.
- Replacing gap diffusion with blood transport.
What can you explain now?
Which distinguishes the second and third events?
Compare with the explanation
Diffusion across gap, then receptor binding
It preserves both required steps.
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Make it stick
Explain why diffusion and receptor binding are two distinct required events.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.