Hormones can be divided into two broad groups based on solub

Hormones can be divided into two broad groups based on solubility or insolubility in water. Briefly describe

how water-solubility affects the manner by which hormones in both groups affect their target cells. Be sure to

include a brief description of both mechanisms as part of your answer.

Solution

The glands of the endocrine system secrete hormones directly into the extracellular environment. The hormones then diffuse to the bloodstream via capillaries and are transported to the target cells through the circulatory system. This allows hormones to affect tissues and organsfar from the site of production or to apply systemic effects to the whole body.

Hormones activate target cells by diffusing through the plasma membrane of the target cells (lipid-soluble hormones) to bind a receptor protein within thecytoplasm of the cell, or by binding a specific receptor protein in the cell membrane of the target cell (water-soluble proteins

Steroid hormones are transported through the blood by being bound to carrier proteins—serum proteins that bind them and increase the hormones\' solubility in water. Some examples are sex hormone-binding globulin (SHBG), corticosteroid-binding globulin, and albumin. Most studies say that hormones can only affect cells when they are not bound by serum proteins. In order to be active, steroid hormones must free themselves from their blood-solubilizing proteins and either bind to extracellular receptors, or passively cross the cell membrane and bind to nuclear receptors. This idea is known as the free hormone hypothesis. This idea is shown in Figure 1 to the right.

Water soluble hormones are freely transported within the blood. In order to enter a cell they must bind to a membrane receptor as they cannot diffuse through the lipid bilayer.