The cells of your body keep in touch with messenger molecules. A messenger is made in one place and changes what cells do somewhere else.
A hormone is a messenger that travels in the blood. A gland releases it, your blood carries it all round the body in about a minute, and it acts on cells far from where it was made. Insulin, the protein that controls the sugar in your blood, is a hormone made in the pancreas.
A neurotransmitter carries a message from one nerve cell to the next. A nerve cell sends signals along its length as tiny pulses of electricity. Where it meets the next cell, a gap about 20 nm wide separates the two. The signal crosses the gap as molecules: the nerve cell releases a puff of neurotransmitter, and the molecules drift across in a tiny fraction of a second.
Fitting a receptor
Both kinds of messenger pass on their message in the same way. They fit a receptor, a protein in the surface of a cell, the way a medicine fits its target. A messenger that fits switches its receptor on, and the receptor passes a signal into the cell. A cell responds only to the messengers its receptors fit, so a hormone that reaches every cell acts only on the cells that carry its receptor.
Many messengers are small amines. Blood and the liquid around your cells have a pH of about 7.4. There, the nitrogen of an -NH₂ group picks up an H⁺ with its lone pair, as amines do in acid, and carries a positive charge: -NH₃⁺. In the receptors for dopamine, serotonin and adrenaline, that charge pairs with a negative -COO⁻ group on the side group of one of the receptor's amino acids, one of the pulls that holds the messenger in place.
| Kind | Travels | Examples |
|---|---|---|
| hormone | in the blood, all round the body | insulin, adrenaline |
| neurotransmitter | across a gap of about 20 nm to the next cell | dopamine, serotonin |