Types of signalling chemicalsSpec C2.1.3, C2.1.4, C2.1.5
In short
Animals use four functional categories of signalling chemicals: hormones, neurotransmitters, cytokines and calcium ions. Hormones travel in the blood and have distant effects; neurotransmitters diffuse across a synaptic gap and act locally. Hormones may be amines, proteins or steroids, and neurotransmitters include amino acids, peptides, amines and nitric oxide.
| Category | Released by | How it reaches target cells | Effect |
|---|---|---|---|
| Hormones | Endocrine glands, secreted into the blood | Transported by the blood around the whole body | Distant, widespread, slower and longer lasting |
| Neurotransmitters | Neurons, at a presynaptic membrane | Diffuse across a synaptic gap (about 20 nm) | Localized to one postsynaptic cell, rapid and brief |
| Cytokines | Many cell types, especially cells of the immune system | Diffuse to nearby cells in tissue fluid | Mainly local, e.g. regulate immune responses and inflammation |
| Calcium ions (Ca²⁺) | Released inside a cell from stores or entering through channels | Act inside the cell that receives the first signal | Trigger responses such as exocytosis of neurotransmitter or muscle contraction |
The difference between localized and distant effects is clearest when hormones are compared with neurotransmitters. A hormone reaches every cell the blood supplies, but only cells with the matching receptor respond. A neurotransmitter only crosses the narrow gap at one synapse and is removed within milliseconds.
Chemical diversity
| Group | Examples |
|---|---|
| Amine hormones | Epinephrine (adrenaline), norepinephrine (noradrenaline) |
| Protein (peptide) hormones | Insulin, glucagon |
| Steroid hormones | Oestradiol, progesterone, testosterone |
| Amino acid neurotransmitters | Glutamate, glycine, GABA |
| Peptide neurotransmitters | Endorphins |
| Amine neurotransmitters | Dopamine, serotonin, norepinephrine (noradrenaline) |
| Gas | Nitric oxide (NO) |
- Many distinct signals are needed. Each needs its own ligand and receptor, so cells can tell signals apart.
- Different properties suit different routes. Hydrophilic proteins and amines stay outside the cell and bind receptors on the plasma membrane. Hydrophobic steroids pass through the membrane to receptors inside.
- Speed and duration vary. Small molecules can be made and broken down quickly; a gas such as NO diffuses freely through membranes and acts briefly near where it is made.
- One molecule can have more than one role. Norepinephrine is a hormone when the adrenal gland secretes it into the blood and a neurotransmitter when a neuron releases it at a synapse.
The IB guide's list of neurotransmitters says 'nitrous oxide'. It means the gas that acts as a neurotransmitter, which chemists call nitric oxide (NO). Nitrous oxide (N₂O, 'laughing gas') is a different molecule. Write 'nitric oxide (NO)' in answers. If a question says 'nitrous oxide', it means this same gaseous neurotransmitter.
Written and checked against the IB Biology HL specification · Updated October 2026