Though hormones are able to reach where ever the blood
circulates, they are only able to act upon specific cells that possessreceptors
specific to that particular hormone.
Such cells are termed target cells (target
tissues, or target organ).
Proteins, peptides and amines are not lipid
soluble thus they cannot pass accross the plasma membranes of cells.
The receptors for such hormones are present on the outside of the
cell membrane. Binding of the hormone with its membrane receptor causes
the production within the cell of a second messenger compound
which then exerts the hormone's actions within the cell.
The second messenger modifies activities within the
cell. These activities could include changing enzyme action, altering
protein synthesis (through messenger RNA) or opening or closing membrane