What is a hormone?

A hormone is a chemical substance secreted by specialised cells located in an endocrine gland, and into the blood which binds to receptors found on or in target cells, acting to change the activity of the cell. Hormones circulate through the bloodstream and are specific. This means that they are only able to influence cells that have the correct receptor for that hormone. If all receptor molecules are occupied by hormone molecules, saturation occurs; this means that any additional hormone secretions will not produce a greater effect.

What are the two types of hormones?

Image: Ben Whitten (Head of Education)

• Protein/amine hormones

• Steroid hormones

Protein/amine hormones Water-soluble and cannot pass through the cell membrane. These types of hormones work by attaching to receptor molecules found on the cell membrane of a target cell. The formation of a hormone-receptor complex causes a secondary messenger substance to diffuse through the cell and activate a particular enzyme; these hormones cause a response rapidly but are short-lasting. Steroid hormones Lipid-soluble and can pass through the cell membrane. These types of hormones enter a target cell by diffusion through the cell membrane and combining with a receptor protein in the cytoplasm, forming a hormone-receptor complex. The hormone-receptor complex then enters the nucleus, altering gene expression (protein synthesis). These hormones are slow in their effect, but longer-lasting.

How do hormones change cell activity?

Hormones change the functioning of cells via changing the quantity, type or activities of proteins (including enzymes). Hormones may do any of the following:

• Activate genes in the nucleus to produce a particular structural protein or enzyme (turning genes on)

• Alter the shape or structure of an enzyme so that it’s turned on/off

• Alter the rate of production of an enzyme or structural protein

What are synthetic hormones?

Some hormones are produced artificially in a laboratory, named synthetic hormones. hormones are those produced artificially in a lab. They can produce side effects which natural hormones do not produce, as they may have a slightly different molecular structure and the body may react to these differences. For example, levothyroxine is a synthetic hormone to treat hypothyroidism (underactive thyroid meaning low thyroxine levels).