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The Endocrine System

Steroid hormones are lipids, usually synthesized from cholesterol, that consist of complex ring structures.
  • nonsteroid hormones include amines, short peptides, and protein hormones; these are related in that they are constructed from amino acids, either by their modification or polymerization.
Hormones are able to act selectively on certain target cells due to the presence or absence of specific protein receptors, which exist either inside or on the surface of the target cell. If a cell contains a receptor for a particular hormone, it will respond to the message; if it lacks that receptor, it will be unaffected by the hormone’s presence. While both classes of hormones function in a fundamentally similar fashion, steroid hormones have a different mechanism of action than nonsteroid hormones due to the chemical differences that exist between the groups. Since steroid hormones are relatively small lipids, they are soluble in the plasma membrane and can pass relatively easily into all of the body’s cells. They will exert their action, however, only when they enter cells that contain the appropriate intracellular receptor, with which they then bind. The hormone-receptor complex thus formed is now able to act directly on the DNA in the nucleus, altering gene expression to cause the production of different cellular products (usually enzymes or other proteins).

The result is some change in the properties, characteristics, or abilities of the cell. Nonsteroid hormones, in general, cannot diffuse across the plasma membrane and therefore must interact with membrane receptors. Receptors of this type are usually transmembrane proteins that contain a hormone-binding domain outside of the cell and another domain inside of the cell that changes its shape when the hormone is bound. It subsequently causes other changes in the cell, usually by activating enzymes through a chemical intermediary. Since the hormone is not causing the changes directly, the intermediary is often referred to as a second messenger (the hormone being the “first messenger”). While many such signal transduction pathways exist, a common one involves cyclic AMP (cAMP) as the second messenger.

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