How Signals Cross Membranes

Signal molecules, such as hormones, travel outside of the cell membrane in order to reach their target cells, which contain signal receptors. The cellular response to a hormone, however, typically occurs inside the target cell. This means that the signal information has to cross the cellular membrane. Many hormones are hydrophilic, or soluble in water. Such molecules cannot diffuse across the cell membrane because the membrane is a hydrophobic bilayer that repels water. For this reason, hydrophilic hormones bind to a cell surface receptor, which is a receptor located on the cell membrane. These receptors present a binding site for the signal molecule on the extracellular side of the membrane. When the hormone binds to the receptor, the shape of the receptor protein changes, and the cytoplasmic side of the protein, the side inside the cell, activates a series of reactions that ends with the desired cellular response. The sequence of steps from the binding of the hormone to the cellular response is called a signaling cascade.

Other signal molecules are either small enough to diffuse through the cell membrane or are hydrophobic and thus soluble in the lipid bilayer. These signal molecules travel inside the cells and bind to intracellular receptors in the cytoplasm. A steroid hormone, for example, is a small hydrophobic hormone that passes through the plasma membrane. Steroid hormones bind to an intracellular receptor, which is a receptor located in the cell cytoplasm. The resulting receptor-ligand complex then effects gene transcription by either binding directly to DNA or by binding to DNA transcription factors. Intracellular signaling molecules can elicit changes at the plasma membrane without altering transcription rates. Finally, some hormones, despite being hydrophobic, bind to cell surface receptors. The most common of these hormones are the prostaglandins, which are a group of lipid compounds found in animal and human tissues. Prostaglandins can change the cellular responses to other hormones and affect a variety of cellular processes, such as inflammation, blood clotting, and smooth muscle cell contraction. Prostaglandins are rapidly broken down by extracellular enzymes, so the cells that produce prostaglandins secrete them continuously.

Note that many steroids also bind to cell surface receptors. The cellular response to the steroid signal depends upon the receptor the hormone binds to. Different receptor-ligand complexes provoke different cellular responses, even if the ligand in the different complexes is the same.