13.13: Hormone

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Steroid hormones. How do they work?

As hormones, they are the messengers of the endocrine system. Obviously they must change something in the cell.

How Hormones Work

Hormones are the messenger molecules of the endocrine system. Endocrine hormones travel throughout the body in the blood. However, each hormone affects only certain cells, called target cells. A target cell is the type of cell on which a hormone has an effect. A target cell is affected by a particular hormone because it has receptor proteins that are specific to that hormone. A hormone travels through the bloodstream until it finds a target cell with a matching receptor it can bind to. When the hormone binds to a receptor, it causes a change within the cell. Exactly how this works depends on whether the hormone is a steroid hormone or a non-steroid hormone.

Steroid Hormones

Steroid hormones are made of lipids, such as phospholipids and cholesterol. They are fat soluble, so they can diffuse across the plasma membrane of target cells and bind with receptors in the cytoplasm of the cell (see Figure below). The steroid hormone and receptor form a complex that moves into the nucleus and influences the expression of genes, essentially acting as a transcription factor. Examples of steroid hormones include cortisol and sex hormones.

Non-Steroid Hormones

Non-steroid hormones are made of amino acids. They are not fat soluble, so they cannot diffuse across the plasma membrane of target cells. Instead, a non-steroid hormone binds to a receptor on the cell membrane (see Figure below). The binding of the hormone triggers an enzyme inside the cell membrane. The enzyme activates another molecule, called the second messenger, which influences processes inside the cell. Most endocrine hormones are non-steroid hormones, including insulin and thyroid hormones.

Summary

Hormones work by binding to protein receptors either inside target cells or on their plasma membranes.
The binding of a steroid hormone forms a hormone-receptor complex that affects gene expression in the nucleus of the target cell.
The binding of a non-steroid hormone activates a second messenger that affects processes within the target cell.

Review

Define hormone.
Compare and contrast how steroid and non-steroid hormones affect target cells.

Image	Reference	Attributions
	[Figure 1]	License: CC BY-NC
	[Figure 2]	Credit: LadyofHats Source: CK-12 Foundation License: CC BY-NC-SA 3.0
	[Figure 3]	Credit: LadyofHats Source: CK-12 Foundation License: CC BY-NC-SA 3.0