Some nonpolar signaling molecules, such as the steroid hormones involved in endocrine signaling, don’t need a receptor on the cell surface in order to relay information to the interior of the cell. Since steroids are hydrophobic, they pass easily through the hydrophobic core of the phospholipid bilayer and into the target cell. Once inside, steroid hormones bind to receptor proteins located in the cytosol or in the nucleus to form receptor–steroid complexes (Fig. 9.6b). Steroid–receptor complexes formed in the cytosol enter the nucleus, where they act to control the expression of specific genes. Steroid receptors located in the nucleus are often already bound to DNA and need only to bind their steroid counterpart to turn on gene expression.

There are many examples of steroid hormones, including sex hormones, glucocorticoids (which raise blood glucose levels), and ecdysone (involved in insect molting). However, since much of the information received by cells is transmitted across the plasma membrane through transmembrane receptors, we focus our attention here on the sequence of events that takes place when receptors on the surface of cells bind their ligands.