Hormones are signaling molecules that are carried via the bloodstream to various parts of the body. They act on target cells and organs to regulate numerous biological processes. Hormones are classified into three main types, based on their chemical structures: steroids, amines and peptides.
Steroid hormones are derived from cholesterol. Like cholesterol, they contain four fused carbon rings with different side chains attached. Steroid hormones include estradiol, released from ovaries, female reproductive organs, and testosterone, released from testes, male reproductive organs.
The second class, amine hormones, are synthesized from a single amino acid, either tyrosine or tryptophan. The hormone epinephrine, derived from the amino acid tyrosine, triggers the fight or flight response during a stressful situation, such as a predator attack. The hormone melatonin, derived from tryptophan, regulates the internal biological clock.
Melatonin levels control the sleep-wake cycle, with low levels stimulating wakefulness during the day and high levels causing drowsiness at night. Lastly, peptide hormones consist of a chain of amino acids. The pancreas produces the peptide hormone insulin.
After a meal, insulin is released by the increase in blood glucose which stimulates cells to absorb excess glucose. Steroid hormones are lipophilic, or lipid soluble, and easily diffuse across the plasma membrane. Amine and peptide hormones are hydrophilic, or lipid insoluble, and cannot cross the plasma membrane of cells, therefore, these hormones bind to surface receptors expressed on their target cells to elicit a response.
