The Relay Inside the Cell
One molecule at the surface becomes thousands of small messengers inside. That multiplication is the point.
Stop 2 of 10 on the Signal Line.
A single peptide landing on a receptor would be a weak event if nothing amplified it. Cells solve this with relays: each step hands the message to many copies of the next molecule.
From receptor to second messenger
Once a GPCR is switched on, it activates its G protein. Depending on the type, that G protein turns on an enzyme that makes a small molecule inside the cell. These small molecules are called second messengers, because the peptide itself was the first.
Two are worth knowing by name:
| Second messenger | Made when | Common role |
|---|---|---|
| cAMP | An enzyme called adenylyl cyclase is switched on | Activates protein kinase A, which alters many other proteins |
| Calcium ions | Stores inside the cell are opened | Triggers release of stored products, muscle contraction, gene activity |
Why amplification matters
One activated receptor can switch on several G proteins. Each G protein can run an enzyme that produces a great many cAMP molecules. A tiny trigger ends up producing a large internal signal.
This explains a feature that confuses newcomers: peptide hormones work at extremely low concentrations in the blood. The surface event is small. The relay inside is not.
The relay can be rewired
Different cell types carry different relay machinery. The same receptor can therefore produce different results depending on which enzymes the cell has on hand. That is one reason a hormone affects a pancreas cell differently from a fat cell.
Switching it off
Relays are built to stop. Enzymes called phosphodiesterases break cAMP down within seconds, and pumps remove calcium. Without these off-switches the cell would stay stuck in the "on" state. How signals are cut short at the receptor itself is the subject of the next stop.
End of this stop
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Education only, not medical advice. See the service notice.