The big idea: Many chemical signals — peptide hormones, adrenaline — are hydrophilic (water-loving). They cannot cross the cell membrane, because the middle of the phospholipid bilayer is hydrophobic and repels them.
So how does the message get inside? The cell uses a transmembrane receptor — a protein that spans the membrane, with a binding site outside and an end inside.
The ligand binds the outside. The receptor then relays the signal across the membrane and makes a second messenger inside — so the message gets in without the ligand ever entering the cell. This relaying of a signal across the membrane is called signal transduction.
A hydrophilic ligand binds a transmembrane receptor on the cell surface; the activated receptor makes a second messenger (cyclic AMP), which sets off a cascade and a cellular response — the ligand itself never enters the cell.
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- Ligand
- The signalling molecule (e.g. a hormone) that binds to a receptor and carries the message.
- Hydrophilic ligand
- A water-loving signal that cannot cross the hydrophobic membrane, so it must bind a receptor on the cell surface.
- Transmembrane receptor
- A protein that spans the membrane, with a binding site outside the cell and an end inside; it relays the signal across the membrane.
- Signal transduction
- Converting (relaying) a signal received at the cell surface into a response inside the cell.
- Second messenger
- A small molecule made inside the cell (e.g. cyclic AMP / cAMP) that carries the signal onward and amplifies it.
Why the receptor must be on the OUTSIDE: A hydrophilic ligand is repelled by the hydrophobic core of the bilayer, so it physically cannot pass through.
Its receptor therefore has to sit on the cell surface with a binding site facing outwards. Compare a lipophilic (fat-soluble) signal such as a steroid: it can dissolve straight through the membrane and meet a receptor inside — so it needs no transduction. The two routes differ because of where the receptor must sit.
Read the pathway as a chain of cause and effect. A common version uses a G-protein-coupled receptor (GPCR), but the shape is the same for many signals: bind → relay → make a second messenger → cascade → response.
The signal-transduction pathway, step by step
- Reception. The hydrophilic ligand binds the outside site of the transmembrane receptor (e.g. a GPCR).
- Activation. Binding makes the receptor change shape, activating an intracellular relay protein on its inside end.
- Second messenger. The relay switches on an enzyme that makes lots of cyclic AMP (cAMP) — the second messenger — inside the cell.
- Cascade. cAMP activates enzymes, and each activated enzyme activates many of the next — a cascade that amplifies the signal.
- Response. The cascade switches an enzyme on (or switches a gene on), producing the cell's response.
A hydrophilic ligand binds a transmembrane receptor on the cell surface; the activated receptor makes a second messenger (cyclic AMP), which sets off a cascade and a cellular response — the ligand itself never enters the cell.
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Why this AMPLIFIES the signal: The pathway is not one-to-one — it multiplies at every step.
One bound ligand activates a receptor that makes many cAMP molecules. Each cAMP switches on enzymes, and each enzyme activates many of the next.
So a tiny outside signal — a handful of hormone molecules — becomes a huge inside response: millions of product molecules. That multiplication is what lets a low concentration of hormone produce a strong, fast effect.
Amplification: ONE bound ligand makes MANY second-messenger molecules, and each step of the cascade activates many of the next — so a tiny outside signal produces a large inside response.
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A worked example: adrenaline: When you get a fright, adrenaline (a hydrophilic signal) reaches a liver cell.
It binds a transmembrane receptor on the surface → the receptor makes cAMP inside → a cascade of enzymes is switched on → the final enzyme breaks glycogen down into glucose, which is released into the blood for energy.
Notice: adrenaline never enters the cell. The message is carried in by cAMP, and amplified, so even a small amount of adrenaline mobilises a lot of glucose.
Worked example — adrenaline: adrenaline binds a transmembrane receptor on a liver cell → the receptor makes cyclic AMP (cAMP) inside → a cascade activates the enzyme that breaks glycogen down to glucose. Adrenaline stays outside; the message is carried in by cAMP.
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| Stage | What happens | Where |
|---|---|---|
| 1. Reception | The ligand binds the transmembrane receptor's outer site | Cell surface (outside) |
| 2. Activation | The receptor changes shape and activates an intracellular relay | Across the membrane |
| 3. Second messenger | The relay makes many molecules of cyclic AMP (cAMP) | Inside the cell |
| 4. Cascade | cAMP activates enzymes, each activating many more (amplification) | Inside the cell |
| 5. Response | An enzyme is switched on, or a gene is switched on | Inside the cell |
Don't confuse 'second messenger' with the ligand: The ligand (first messenger) is the outside signal — it stays outside and only binds the receptor.
The second messenger (e.g. cAMP) is made inside the cell and carries the signal onward.
Memory hook: the ligand knocks on the door; cAMP is the messenger that runs inside.
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How this is tested: A typical HL item is an Explain / Outline asking how a hydrophilic signal that cannot cross the membrane still produces a response inside the cell. Score points for: ligand binds a transmembrane receptor on the surface → the receptor makes a second messenger (cAMP) inside → a cascade of enzyme activations → a response (enzyme/gene switched on) → the signal is relayed and amplified without the ligand entering the cell.
A 1-mark item may ask why the receptor is on the cell surface (the hydrophilic ligand can't cross the hydrophobic membrane), or to name a second messenger (cyclic AMP).
IB-style question — how a hydrophilic hormone triggers a response without entering the cell
A hydrophilic hormone binds to the outside of a target cell but never enters it, yet it switches on an enzyme inside the cell. Explain how the signal is transmitted across the membrane and amplified. [6]
How to score all six marks
- Why it binds outside. The hormone is hydrophilic, so it cannot cross the hydrophobic membrane; it binds a transmembrane receptor on the cell surface.
- Receptor activated. Binding makes the receptor change shape and activate an intracellular relay (e.g. a G protein).
- Second messenger. The relay causes a second messenger — cyclic AMP (cAMP) — to be made inside the cell.
- Cascade. cAMP sets off a cascade: it activates enzymes, each of which activates many of the next.
- Response. The cascade switches on the target enzyme (or switches on a gene) — the cell's response.
- Relay + amplification. The signal is relayed across the membrane without the ligand entering, and amplified — one ligand → many cAMP → a large response. (Award 1 per distinct point, max 6.)
Final answer
The hormone is hydrophilic so it cannot cross the membrane; it binds a transmembrane receptor on the surface. The receptor changes shape and activates a relay that makes the second messenger cyclic AMP (cAMP) inside the cell. cAMP triggers a cascade of enzyme activations, each step activating many more, which switches the target enzyme (or a gene) on. The signal is relayed across the membrane without the ligand entering, and amplified — one ligand produces a large response.
✓ Why this scores full marks: It explains why the ligand binds outside (hydrophilic → can't cross), names the transmembrane receptor, names the second messenger (cAMP), includes the cascade, states the response (enzyme/gene on), AND nails the two key ideas: the signal is relayed across the membrane without the ligand entering, and it is amplified (one ligand → many second-messenger molecules).
A hydrophilic ligand binds a transmembrane receptor on the cell surface; the activated receptor makes a second messenger (cyclic AMP), which sets off a cascade and a cellular response — the ligand itself never enters the cell.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.