There is enormous diversity in the chemical structure of hormones, but most of them can be divided into three groups:
► Steroid hormones (derivatives of the steroid cholesterol; e.g., testosterone)
► Amines (derivatives of the amino acid tyrosine; e.g., thyroxine)
Most hormones are peptides or proteins. They are water-soluble and therefore easily transported in the blood, but they cannot easily pass through lipid-rich cell membranes. Therefore, peptide and protein hormones are packaged in vesicles in the cells that make them and released by exocytosis.
Steroid hormones are lipid-soluble and can easily dissolve in and pass through cell membranes. Therefore, steroid hormones are not packaged in vesicles; instead, they simply diffuse out of the cells that make them as they are synthesized. Since steroid hormones are not soluble in the blood, they are transported in the blood bound to carrier proteins.
Some amine hormones are water-soluble and others are lipid-soluble; thus, their mode of release differs accordingly.
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