How hormones act on cells

Why does a hormone act only on some cells?

The blood carries insulin to every cell, but only some of them respond. The reason is the receptor. A hormone acts on a target tissue by binding to a specific protein, its hormone receptor, in that tissue only. A receptor fits only one hormone, as a key fits one lock. This is why the receptors are called specific. When the hormone binds, a hormone-receptor complex forms. The complex causes biochemical changes in the target cell, and these regulate the metabolism of the tissue.

The chemical nature of hormones

Hormones belong to four chemical groups.

• Peptide, polypeptide and protein hormones. Examples: insulin, glucagon, the hormones of the pituitary and the hypothalamus.
• Steroids. Examples: cortisol, testosterone, estradiol and progesterone.
• Iodothyronines. The thyroid hormones T4 and T3.
• Amino acid derivatives. Example: adrenaline (epinephrine).

Two kinds of receptors

Receptors in the cell membrane (membrane-bound receptors). The peptide hormones and adrenaline are not able to cross the cell membrane. They bind to a receptor on its outer surface and do not enter the cell. Their message passes in through the receptor, and second messengers carry it on. Second messengers include cyclic AMP, IP3 and calcium ions. They regulate the metabolism of the cell.

A common route is this.

1. The hormone binds to the receptor on the surface.
2. The receptor activates an enzyme in the membrane that makes cyclic AMP (cAMP) from ATP.
3. Cyclic AMP, the second messenger, switches on other enzymes.
4. The enzymes change the work of the cell.

The route amplifies the message. One hormone molecule makes many molecules of cAMP, and each cAMP switches on many enzyme molecules. So a very small amount of hormone has a large effect. This is why hormones are made in trace amounts.

Receptors inside the cell (intracellular receptors). The steroid hormones and the thyroid hormones dissolve in fat, so they pass through the cell membrane. Their receptors are inside the cell, mostly in the nucleus. The hormone-receptor complex acts on the genome: it regulates the expression of genes and the working of the chromosomes. The cell then makes new proteins.

The biochemical actions add up and give the physiological and developmental effects that you see in the body.

• Insulin: a peptide. Receptor in the membrane. Second messengers.
• Glucagon: a peptide. Receptor in the membrane. Cyclic AMP.
• Adrenaline: an amino acid derivative. Receptor in the membrane. Cyclic AMP.
• Cortisol: a steroid. Receptor inside the cell. Change in gene expression.
• Thyroxine: an iodothyronine. Receptor inside the cell. Change in gene expression.
• Testosterone: a steroid. Receptor inside the cell. Change in gene expression.

A membrane route is quick, from seconds to minutes. An intracellular route takes longer, from hours to days, because new proteins must be made.

You should find that the curve rises and then flattens. A receptor can only hold so many hormone molecules, so more and more hormone adds less and less. A receptor with a smaller Kd is occupied at a lower hormone concentration. This is a teaching model of binding, which A-Level and IB use. NCERT gives only the elementary idea.

Try it in Python

The program prints the share of receptors occupied at six hormone concentrations.

Practise with new numbers

Common mistakes

• Saying that a peptide hormone enters the cell. It binds to a receptor in the cell membrane. A second messenger works inside.
• Saying that a steroid hormone binds to a receptor on the surface. Its receptor is inside the cell.
• Saying that cyclic AMP is a hormone. It is a second messenger. The hormone is the first messenger.
• Saying that one hormone fits many receptors. Each receptor is specific to one hormone.
• Calling the thyroid hormones peptides. They are iodothyronines, made from the amino acid tyrosine with iodine added.

Remember these

• A hormone acts through a specific receptor in the target tissue only. Hormone and receptor form a complex.
• Four groups: peptide, polypeptide or protein; steroid; iodothyronine; amino acid derivative.
• Membrane receptors (peptides, adrenaline): second messengers such as cyclic AMP, IP3 and Ca$^{2+}$.
• Intracellular receptors (steroids, thyroid hormones): the complex acts on the genome and regulates gene expression.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What is a hormone receptor, and why can the blood carry a hormone to every cell but only some cells respond?

2. Name the four chemical groups of hormones and give one example of each.

3. Where is the receptor of a peptide hormone, and what carries its message into the cell?

4. How does a steroid hormone change the work of a cell?

5. Why does a hormone made in trace amounts have a large effect when it works through cyclic AMP?

References

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 17, The Endocrine System.
• Alberts B. et al. (2022). Molecular Biology of the Cell, 7th edition, W. W. Norton, the chapter on cell signalling.
• NCERT (2023). Biology, Textbook for Class XI, chapter 19.