The endocrine system compared with the nervous system

Last time

Before we start: last time, in Cholinergic synapses, drugs and toxins, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Two ways to send a message

Your body has two long distance messaging systems. One is fast and precise, like a phone call. The other is slower and wider, like a broadcast. Cambridge loves to ask you to compare them, so this lesson sets out what each one is, how hormones work, and where the differences really lie.

The endocrine system

The endocrine system is made of glands that secrete hormones directly into the blood. There are no ducts. The blood carries the hormones all round the body. A hormone is a chemical messenger. It reaches every cell, but only affects target cells that have the right receptors for it. Examples include insulin and glucagon from the pancreas, adrenaline from the adrenal glands, and A D H from the pituitary.

Two kinds of hormone

Peptide hormones, such as insulin, glucagon and A D H, are water soluble. They cannot cross the membrane, so they bind to a receptor on the outside and trigger a second messenger inside. Steroid hormones, such as oestrogen and testosterone, are lipid soluble. They diffuse through the membrane, bind to a receptor in the cytoplasm or nucleus, and the complex acts as a transcription factor. So steroid hormones change which genes are switched on, while peptide hormones change enzyme activity quickly.

A past-paper example

October November 2025, Paper 41, question 5. Complete a table to show features of three cell signalling molecules of the endocrine system: A D H, glucagon and insulin. All three are peptide hormones. All are carried in the blood, bind to receptors on the cell surface membrane, and act on target cells only. None of them enter the cell, so they need a second messenger or an enzyme cascade.