Endocrine System 2009
The coordination of all organs and tissues in our bodies is very complex and is brought about by
the nervous system and endocrine system.
The nervous system controls the functions of cells,
tissues, and organs by rather close contact, that is, nerves come to cells, tissues, organs and rather
closely interact with them.
On the other hand, the activity of cells, tissues, and organs are also
directed by hormones that come to them from far distances.
Endocrine glands secrete a large
number of hormones into the bloodstream for distribution to specific target cells (really specific
receptor sites on the cell membrane, cytoplasm, or nucleus of these target cells).
are proteins, peptides, steroids, or tyrosine derivatives.
Or more simply, derived from amino
acids (proteins, peptides, biogenic amines) or derived from cholesterol.
Peptide and protein hormones are synthesized in RER, packaged in Golgi, and stored in granules.
Biogenic amines (epinephrine, serotonin, etc.) are made from decarboxylated amino acids
in granules until they are released after the cell receives a stimulus.
Peptide, protein, and amines attach to receptors in the cell membrane of their target.
Steroid hormones (glucocortocoids, sex hormones, and aldosterone) have cholesteryl ester as
Steroids are synthesized on mitochondria and
ER. To make any steroid
hormone, cholesterol must be converted to
. This conversion takes place on the
inner membrane of mitochondria and then pregnenolone is transferred back and forth between
the SER and mitochondria for further transformation until the final steroid hormone is produced).
Steroids are rapidly secreted from the cell.
Steroid hormones enter cells by
facilitated diffusion (they do not bind to a receptor in the target cell’s plasma membrane) and
Peptide and protein hormones are made in cells with an extensive RER and Golgi.
Steroid hormones are made in cells with an extensive SER and abundant mitochondria.
Thyroid hormones (derived from tyrosine) are unique in their synthesis and storage and, like
steroids, they enter cells to bind with
So, most hormones bind to receptors (transmembrane proteins) on the target cells surface.
Steroids and thyroid hormones bind to receptors that are in the cytoplasm or nucleus of the target
Both steroids and thyroid hormones are lipophilic and it was once thought they both
entered cells by diffusion.
It now appears that both require a transporter protein to enter cells