Chemical signalling

A **signalling molecule that binds to a receptor** to deliver a message.

A **protein** with a **binding site** complementary to a specific ligand; binding the ligand triggers a response.

Its binding site is **complementary in shape AND chemistry** to that ligand (lock-and-key), so only the matching ligand fits.

A cell that **carries the matching receptor** for a signal — so it is the cell that actually responds.

Only cells with the **matching receptor** can **bind** the ligand and respond; cells without it cannot.

The response depends on **each cell's own receptor and machinery**, not on the signal itself.

No — it is **reversible**, so the response is **temporary** and can be switched off.

**Endocrine** (hormone via blood, long range), **paracrine** (local/nearby cells), **autocrine** (a cell signals itself) and **neurotransmitter** (across a synapse).

A **hormone** is released into the **blood** and carried to **distant** target cells — the longest-range mode.

It is **hydrophilic (water-soluble)**, so it **cannot cross** the phospholipid membrane — its receptor must be on the cell surface, and the signal is then **transduced** inside.

It is **lipid-soluble**, so it **diffuses straight through** the membrane and binds a receptor **inside** the cell.

The **hormone–receptor complex acts in the nucleus**, switching **genes on/off** so different proteins are made.

**Peptide** is faster (seconds–minutes) because it activates existing machinery; **steroid** is slower (hours) because new proteins must be made — but it lasts longer.

It acts like a **peptide** — it is **hydrophilic**, so it binds a **surface receptor** and works by **signal transduction** (not via intracellular gene action).

It is repelled by the **hydrophobic core** of the phospholipid bilayer, so it can't pass through — it must bind a **surface** receptor.

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.

**Relaying** a signal received at the cell surface into a **response inside** the cell, without the ligand entering.

A small molecule made **inside** the cell that carries the signal onward and amplifies it — e.g. **cyclic AMP (cAMP)**.

**One** ligand → **many** cAMP molecules → a **cascade** where each enzyme activates many more → a **large** response.

An **enzyme is switched on**, or a **gene is switched on**, inside the cell.

The **ligand** (first messenger) stays **outside** and binds the receptor; the **second messenger** (cAMP) is made **inside** and relays the signal onward.

**Lipid-soluble** signals — **steroid hormones** and **thyroxine** — because they can diffuse through the plasma membrane.

**Inside** the cell — in the **cytoplasm or nucleus** (not on the surface).

It **diffuses straight through the plasma membrane** (the membrane is lipid, and like dissolves like).

A **transcription factor** — it **binds DNA** and **switches specific genes on or off**.

It **changes which proteins the cell makes** (gene expression) — a **slower but longer-lasting** effect.

Intracellular = **slow to start, long-lasting** (changes gene expression); surface + second messenger = **fast, short-lived**.

**No** — second messengers belong to the **surface-receptor** route; a steroid acts directly on the cell's DNA.

A **chemical signal** released at a **synapse**; it diffuses across the cleft and **binds receptors** on the postsynaptic membrane.

The neurotransmitter **binding its receptor** on the postsynaptic membrane — a neurotransmitter in the cleft does nothing until it binds.

The receptor opens channels that let **positive ions (Na⁺) in** → the membrane **depolarises** → the neuron is **more likely to fire**.

The receptor opens channels that let **Cl⁻ in (or K⁺ out)** → the membrane **hyperpolarises** → the neuron is **less likely to fire**.

The **receptor decides**, not the neurotransmitter — different receptors open different ion channels, so the same signal gives opposite effects.

The neurotransmitter is **removed (re-uptake)** or **broken down by an enzyme**, so the receptors empty and the signal stops — keeping it **brief and controlled**.

A **rising response inhibits further signalling**, so the response stops growing and the system **returns to its set point**.