Steroid Hormones
Steroid hormones are small and hydrophobic, and
most hormones of this class are derived from cholesterol, including estrogen, progesterone,
testosterone, and cortisol. Since they are small and hydrophobic, they can diffuse through the
cell membrane. These hormones bind steriod hormone receptors after they have diffused into the
cell through the plasma membrane. The hormone bound receptors enter the nucleus and bind to target
regions in genes that regulate transcription, turning the genes on or off. Steroid hormone signals
are changes in gene transcription and protein expression caused by the steroid hormone receptors.
Since steroid hormones freely diffuse through membranes, they are not stored after production.
They are usually secreted at a rate equal to their production. It is the rate of production for
these hormones that is highly regulated, by controlling the activity of the enzymes that produce
the hormones.
This animation (Audio - Important) describes the function of
steroid hormones.
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