Hormones are
chemical messages manufactured by
endocrine glands. They are
secreted from a
gland and travel through the
blood to their
target organ or tissue.
There are two major categories of
hormones:
Steroid and non-steroid
Each category of hormone affects target
cells through a different
mode of action. Both modes of action require
protein receptors that are specific to individual
hormones.
Steroid hormones - Intracellular receptor
Steroid hormones combine with
intracellular proteins to direction affect the
function of
cell genes.
Because
steroid hormones are
lipophilic (
hydrophobic) they can diffuse directly through the
cell membrane and enter the cytoplasm.
Once a
steroid hormone has entered the cytoplasm, it joins with an
intracellular receptor protein, forming a
hormone-receptor complex. Each
steroid hormone has a different
intracellular receptor protein.
The
hormone-receptor complex enters the
nucleus through nuclear pores. Once inside the
nucleus, the
hormone-receptor complex either
initiates or
stops transcription of a specific gene or genes.
Nonsteroid hormones - Membrane receptor
Nonsteroid
hormones (
amino-acid derived hormones) activate
signal transduction pathways to create new
cell products.
Because nonsteroid
hormones are
hydrophilic they cannot passively enter the
cell through the
cell membrane. They instead bind to
cell membrane surface receptors embedded in the
plasma membrane. Each nonsteroid hormone has a different cell
membrane surface receptor.
The binding of the hormone starts up a
signal transduction pathway, a "relay"
system of
chemical signals made of
relay molecules.
At the end of the
signal transduction pathway, a final relay molecule activates a
protein (usually an
enzyme) that will carry out the cellular response. The cellular response is often the manufacturing of a product or the breakdown of a molecule.
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1 - Anatomy and Physiology Basics