Explain the mechanism of hormone action when the cell is provided with water

Protein and peptide hormones, catecholamines like epinephrine, and eicosanoids such as prostaglandins find their receptors decorating the plasma membrane of target cells.

Binding of hormone to receptor initiates a series of events which leads to generation of so-called second messengers within the cell (the hormone is the first messenger). The second messengers then trigger a series of molecular interactions that alter the physiologic state of the cell. Another term used to describe this entire process is signal transduction.

Structure of Cell Surface Receptors

Cell surface receptors are integral membrane proteins and, as such, have regions that contribute to three basic domains:

• Extracellular domains: Some of the residues exposed to the outside of the cell interact with and bind the hormone - another term for these regions is the ligand-binding domain.
• Transmembrane domains: Hydrophobic stretches of amino acids are "comfortable" in the lipid bilayer and serve to anchor the receptor in the membrane.
• Cytoplasmic or intracellular domains: Tails or loops of the receptor that are within the cytoplasm react to hormone binding by interacting in some way with other molecules, leading to generation of second messengers. Cytoplasmic residues of the receptor are thus the effector region of the molecule.

Several distinctive variations in receptor structure have been identified. As depicted below, some receptors are simple, single-pass proteins; many growth factor receptors take this form. Others, such as the receptor for insulin, have more than one subunit. Another class, which includes the beta-adrenergic receptor, is threaded through the membrane seven times.