Hormones are divided into three structural classes: proteins and peptides, amino acid derivatives, and lipid derivatives. Proteins and peptides and some amino acid derivatives are water-soluble (hydrophilic), so they are carried dissolved in plasma. Proteins include glycoproteins (such as thyroid-stimulating hormone) and small peptides (such as insulin and glucagon). Amino acid derivatives include the catecholamines epinephrine and norepinephrine. Water-soluble hormones bind to receptors on the surface of cells to exert an effect inside the cell.
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