Lipid solubility determines the broad mechanism by which a hormones activate their target cells. Steroid hormones are lipids. Because they are nonpolar, steroid hormones pass directly through the plasma membrane of the target cell. Other classes of hormones, peptide hormones such as insulin and glucagon and catecholamines such as epenephrine, which are large polar molecules, usually do not enter their target cells. Instead they bind specific receptor proteins on the plasma membrane to form a hormone receptor complex. The hormone-receptor complex then activates a second messenger such as InsP3 or cAMP in a process is known as signal coupling.