Question

Why, if lipophilic hormones cross the cell membrane, do they not immediately diffuse out of the cell?

Answer 1

The two factors acting during the transport of a molecule across cell membrane are: Its gradient across the cell membrane and its diffusibility in the cell membrane. In case of lipophilic hormones they are readily diffusible across the cell membrane due to the predominance of lipids in cell membranes and also because there is a gradient of concentration of hormone which makes it to move into the cell. Blood contains more concentration of hormones compared to the cells. Also there are receptors which bind to the hormones as they enter into the cells and restrict their movement outside the cell. This also facilitates storage of the hormones inside the cell. Binding of hormones to receptors may also initiate the cascade of reactions culminating into the responses produced by the hormones.