Chemical signals, ligands and receptors
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Question
What is a ligand?
Answer
A **signalling molecule that binds to a receptor** to deliver a message.
Question
What is a receptor?
Answer
A **protein** with a **binding site** complementary to a specific ligand; binding the ligand triggers a response.
Question
What makes a receptor specific to one ligand?
Answer
Its binding site is **complementary in shape AND chemistry** to that ligand (lock-and-key), so only the matching ligand fits.
Question
What is a target cell?
Answer
A cell that **carries the matching receptor** for a signal — so it is the cell that actually responds.
Question
Why do only some cells respond to a signal that reaches them all?
Answer
Only cells with the **matching receptor** can **bind** the ligand and respond; cells without it cannot.
Question
Why can the same signal cause different responses in different cells?
Answer
The response depends on **each cell's own receptor and machinery**, not on the signal itself.
Question
Is ligand–receptor binding permanent?
Answer
No — it is **reversible**, so the response is **temporary** and can be switched off.
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Topic 3.4 hub
Chemical signalling
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