Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Signal transduction converts a signal at the receptor into a response inside the cell, in three stages: reception, transduction, response.
- A ligand (peptide, protein or small molecule) binds the ligand-binding domain of a specific receptor. Binding changes the receptor’s shape, so its intracellular domain changes too and starts transduction.
- Water-soluble ligands → receptors on the surface. Small nonpolar ligands (steroids, thyroid hormones) → receptors in the cytoplasm or nucleus.
- GPCRs (seven passes through the membrane) activate a G protein (GDP → GTP), which activates adenylyl cyclase: ATP → cAMP → protein kinase → targets.
- Ligand-gated channels open or close when the ligand binds; ions flow and the cell responds in milliseconds.
- Kinases add phosphate (from ATP) to proteins; phosphatases remove it. A chain of kinases is a phosphorylation cascade.
- Responses: cell growth, secretion, gene expression, enzyme activity, channel opening or closing.
Key relationships
| Component | Stage | Job |
|---|---|---|
| Ligand (first messenger) | reception | binds a specific receptor |
| Receptor | reception | changes shape when bound; starts transduction |
| G protein | transduction | on with GTP, off with GDP; activates adenylyl cyclase |
| Adenylyl cyclase | transduction | makes many cAMP from ATP (amplifies) |
| cAMP (second messenger) | transduction | diffuses; activates protein kinase |
| Protein kinase | transduction | phosphorylates many proteins (amplifies) |
| Target protein or gene | response | changes cell activity |
| Relationship | Use |
|---|---|
| Overall amplification = product of the factors at each step | e.g. 20 × 100 ÷ 4 × 200 × 1000 = 1 × 10⁸ |
| Step where several molecules activate one | divide; this step lowers the number |
Assumptions behind the numbers
- Amplification models use fixed factors per step; real values vary, and signals are switched off over time (GTP broken down, cAMP removed, phosphatases).
- An experiment showing a molecule is sufficient for a response does not show it is necessary.
Mistakes to avoid
- Saying the hormone enters the cell to act when its receptor is on the surface.
- Calling the hormone the second messenger.
- Confusing the GPCR with the G protein.
- Saying kinases remove phosphates.
- Saying phosphorylation always switches proteins on.
- Multiplying at every step, including steps where several molecules activate one.
Quick self-check
- A small nonpolar hormone: where is its receptor? (Cytoplasm or nucleus; the complex often changes gene expression.)
- Three steps each multiply the signal by 10. Overall amplification? (10 × 10 × 10 = 1000)
- Why does a ligand-gated channel give a faster response than a GPCR pathway? (Binding opens the channel directly; there is no chain of relay molecules.)
Next: practice questions.