The big idea: Cells coordinate with each other by releasing chemical signals. The same idea works over very different distances — and biologists name the modes by how far the signal travels:
Endocrine — a hormone is released into the blood and carried to distant target cells (long range).
Paracrine — the signal acts locally on nearby cells (short range).
Autocrine — a cell signals itself (it binds its own receptors).
Neurotransmitter — released across a synapse onto the adjacent cell (very short range).
In every case a signal molecule (ligand) binds a matching receptor — only the distance and the delivery route differ.
| Mode | How far does the signal travel? | How it reaches the target | Example |
|---|---|---|---|
| Endocrine | Long range — anywhere in the body | A hormone is released into the BLOOD and carried to distant target cells | Insulin from the pancreas reaching the liver |
| Paracrine | Short range — neighbouring cells | The signal DIFFUSES locally to nearby cells | A growth factor acting on the cells next to it during healing |
| Autocrine | The cell itself | The cell releases a signal that binds its OWN receptors | An immune cell releasing a signal that drives its own division |
| Neurotransmitter | Very short range — across a synapse | Released into the synaptic gap onto the ADJACENT cell | Acetylcholine crossing a synapse to a muscle fibre |
- Chemical signalling
- Cells communicating by releasing molecules (ligands) that bind receptors on a target cell.
- Endocrine signalling
- A hormone is released into the bloodstream and travels long distances to reach distant target cells.
- Paracrine signalling
- A signal acts locally, diffusing only to neighbouring cells.
- Autocrine signalling
- A cell releases a signal that binds receptors on itself.
- Neurotransmitter signalling
- A signal released across a synapse onto the immediately adjacent cell — the shortest range of all.
- Hormone
- A signalling molecule, released by an endocrine gland, that travels in the blood to its target cells.
One easy way to keep the four straight: Think about who receives the message:
ENDO = the message goes far (into the blood). PARA = to the cell beside it. AUTO = back to itself. NEURO = across a synapse.
Distance shrinks as you go down the list: body-wide → neighbour → self → synaptic gap.
Endocrine hormones split into two chemical classes, and the chemistry of the hormone decides where its receptor has to be. The deciding factor is one property: can the hormone cross the cell membrane?
Remember the membrane is a phospholipid bilayer — a water-loving outside with a fatty (hydrophobic) core. That core is the gatekeeper.
PEPTIDE / protein hormones (e.g. insulin) — surface receptors
- Peptide hormones are made of amino acids, so they are hydrophilic (water-soluble).
- The membrane's fatty core repels them — they cannot cross it.
- So they bind a transmembrane receptor sitting on the cell surface.
- Binding activates the receptor, which relays the message inside the cell — this is signal transduction (often using a second messenger).
- The response is fast (the machinery is already there) but short-lived.
The peptide arm: a hydrophilic hormone (e.g. insulin) cannot cross the membrane, so it binds a TRANSMEMBRANE receptor on the cell surface. The activated receptor relays the message inside via a second messenger — signal transduction — without the hormone ever entering the cell.
Interactive diagram
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STEROID hormones (e.g. oestrogen, testosterone) — receptors inside
- Steroid hormones are made from cholesterol, so they are lipid-soluble (fat-soluble).
- Being fat-soluble, they dissolve straight through the phospholipid membrane.
- Inside, they bind an intracellular receptor (in the cytoplasm or nucleus).
- The hormone–receptor complex acts in the nucleus, switching genes on or off so new proteins are made.
- The response is slower (proteins must be transcribed and translated) but longer-lasting.
The rule to remember: It all comes down to solubility versus the membrane:
Can't cross (hydrophilic peptide) → receptor must be on the surface → signal is transduced across the membrane.
Can cross (lipid-soluble steroid) → receptor can be inside → the hormone acts directly on the genes.
So a peptide hormone shouts through the wall and lets a relay carry the message in; a steroid hormone simply walks through the wall and delivers the message itself.
| Peptide / protein hormone | Steroid hormone | |
|---|---|---|
| Examples | Insulin, glucagon, adrenaline* | Oestrogen, testosterone, cortisol |
| Solubility | HYDROPHILIC (water-soluble) | LIPID-soluble (fat-soluble) |
| Can it cross the membrane? | NO — repelled by the lipid core | YES — dissolves straight through |
| Where is its receptor? | On the cell SURFACE (transmembrane receptor) | INSIDE the cell (intracellular receptor) |
| How it acts | Binds the surface receptor → signal transduction inside the cell | Diffuses in → binds its receptor → the complex acts in the NUCLEUS to switch genes on/off |
| Speed of response | FAST (seconds–minutes) | SLOWER (hours — new proteins must be made) |
| Duration of response | Short-lived | Longer-lasting |
Watch this trap — adrenaline: Adrenaline is not a steroid. It is an amino-acid-derived hydrophilic hormone, so it behaves like the peptide class: it binds a surface receptor and works through signal transduction (a fast 'fight-or-flight' response).
The take-home is the logic, not a memorised list: a water-soluble hormone uses a surface receptor; a fat-soluble one uses an intracellular receptor.
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How this is tested: This HL topic is usually a Compare/Distinguish question (Paper 2): contrast peptide and steroid hormones by solubility, receptor location and mechanism. A clean answer links each property to the next — solubility decides whether the hormone can cross the membrane, which decides where the receptor sits.
You may also be asked to outline the modes of signalling (endocrine, paracrine, autocrine, neurotransmitter) and classify each by distance.
On Paper 1A (multiple choice) expect to identify which hormone class binds a surface receptor, or which acts on genes in the nucleus.
IB-style question — compare peptide and steroid hormones
Compare and contrast how a peptide hormone and a steroid hormone bind to their receptors and act on a target cell. [6]
How to score all six marks
- Solubility (the cause). A peptide hormone is hydrophilic (water-soluble); a steroid hormone is lipid-soluble (fat-soluble).
- Crossing the membrane. The peptide cannot cross the phospholipid membrane; the steroid can diffuse straight through it.
- Receptor location (the consequence). The peptide binds a transmembrane receptor on the cell surface; the steroid binds an intracellular receptor (inside the cell).
- Mechanism — peptide. The surface receptor triggers signal transduction inside the cell (relaying the message, often via a second messenger).
- Mechanism — steroid. The hormone–receptor complex acts in the nucleus to switch genes on/off, changing which proteins are made.
- A shared point + a contrast. Both work by a ligand binding a specific receptor (similarity). But the peptide response is fast and short-lived, while the steroid response is slower and longer-lasting (difference). (Award 1 per distinct point, max 6.)
Final answer
A peptide hormone is hydrophilic, so it cannot cross the membrane and binds a transmembrane (surface) receptor, triggering signal transduction inside the cell — a fast, short-lived response. A steroid hormone is lipid-soluble, so it diffuses through the membrane and binds an intracellular receptor; the complex acts in the nucleus to switch genes on/off — a slower, longer-lasting response. Both rely on a ligand binding a specific receptor.
✓ Why this scores full marks: It does what 'compare and contrast' demands: it gives both similarities (a ligand binds a specific receptor) and differences (surface vs intracellular receptor; transduction vs direct gene action; fast/short vs slow/long).
Crucially it explains the cause — solubility decides whether the hormone crosses the membrane, which decides where the receptor sits. A list of facts without that link reads as memorised and loses the 'compare' marks.
| Feature | Peptide hormone | Steroid hormone |
|---|---|---|
| Solubility | hydrophilic | lipid-soluble |
| Crosses membrane? | no | yes (diffuses through) |
| Receptor | surface (transmembrane) | intracellular |
| Acts by | signal transduction | switching genes on/off in the nucleus |