Transmembrane receptors and signal transduction
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Flip to reveal answersWhy can't a hydrophilic ligand cross the cell membrane?
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Question
Why can't a hydrophilic ligand cross the cell membrane?
Answer
It is repelled by the **hydrophobic core** of the phospholipid bilayer, so it can't pass through — it must bind a **surface** receptor.
Question
What kind of receptor does a hydrophilic ligand bind?
Answer
A **transmembrane receptor** (e.g. a **G-protein-coupled receptor, GPCR**) — a protein spanning the membrane with a binding site outside and an end inside.
Question
What is signal transduction?
Answer
**Relaying** a signal received at the cell surface into a **response inside** the cell, without the ligand entering.
Question
What is a second messenger? Give an example.
Answer
A small molecule made **inside** the cell that carries the signal onward and amplifies it — e.g. **cyclic AMP (cAMP)**.
Question
How does the pathway amplify the signal?
Answer
**One** ligand → **many** cAMP molecules → a **cascade** where each enzyme activates many more → a **large** response.
Question
What is the final 'response' in this pathway?
Answer
An **enzyme is switched on**, or a **gene is switched on**, inside the cell.
Question
Ligand vs second messenger — what's the difference?
Answer
The **ligand** (first messenger) stays **outside** and binds the receptor; the **second messenger** (cAMP) is made **inside** and relays the signal onward.
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Topic 3.4 hub
Chemical signalling
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