Hormones are classified into 2 categories depending on their chemical make-up and solubility: water soluble and lipid soluble hormones. Each of these classes of hormones has specific mechanisms for their function that dictate how they affect their target cells.
Water-Soluble Hormones
The water soluble hormone include the peptide and amino-acid hormones such as insulin, epinephrine, HGH, and oxytocin. As their name indicates, these hormones are soluble in water, they unable to pass through the phospholipid bilayer of the plasma membrane and are therefore dependent upon receptor molecules on the surface cell. When a water-soluble hormone binds to a receptor molecule on the surface of the cell, it triggers a reaction inside of the cell. This reaction may change a factor inside of the cell such as the permeability of the membrane or the activation of another molecule. A common reaction is to cause molecules of cyclic adenosine monophosphate (cAMP) to be synthesized from adenosine triphosphate (ATP) present int the cell. cAMP acts as a second messenger within the cell where it binds to a second receptor to change the function of the cell's physiology.
Lipid-Soluble Hormones
Lipid soluble hormones include the steroid hormones such as testosterone, estrogens, glococorticoids, and mineralocorticoids. Because they are soluble in lipids, these hormones are able to pass directly through the phospolipids bilayer of the plasma membrane and bind directly to receptors inside the cell nucleus. Lipid soluble hormones are able to directly control the function of a cell from these receptors, often triggering the transcription of particular genes in the DNA to produce messenger RNA (mRNA) that are used to make proteins that affect the cell's growth and function.
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