There are several different kinds of receptors. Most signaling molecules act on target cells by binding to receptors in the plasma membrane, but some signaling molecules enter target cells and bind to intracellular receptors in the cytoplasm. Four different types of receptors are shown here. We will study each type of receptor and its function. First let's examine the G protein-coupled receptor, or GPCR. Many different types of signaling molecules, such as neurotransmitters and many hormones, act through G protein-coupled receptors. This kind of receptor spans the plasma membrane, and as you might guess, it works through a protein called the G protein. When the signaling molecule binds to the receptor, the receptor becomes activated. It is now able to bind to an inactive G protein, causing a GTP to displace the GDP on the G protein. Now the G protein is activated. The activated G protein then binds to an enzyme and alters its shape and activity. Once activated, the enzyme can trigger the next step, leading to a cellular response. Then the G protein hydrolyzes its GTP and reverts to its inactive form, ready to respond to another activated G protein-coupled receptor. Next, let's review the function of receptor tyrosine kinases, or RTKs. Growth factors signal cells to grow and divide by acting through receptor tyrosine kinases. Binding of signaling molecules causes two RTK monomers to associate closely with each other and form a dimer. Dimerization activates the tyrosine kinase region of each monomer. Each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine in the tail of the other monomer. The fully activated receptor tyrosine kinase is now recognized by specific relay proteins inside the cell. One tyrosine kinase receptor may activate several different relay proteins at once, triggering several different responses within the cell. Now let's move to the ion channel receptor. Some ion channel receptors are ligand-gated ion channels. When a signaling molecule binds as a ligand to the ligand-gated ion channel, the gate region of the channel changes shape. In the example shown here, the channel opens, allowing the flow of specific ions. The change in ion concentration inside the cell may directly affect the activity of the cell in some way. For example, ligand-gated ion channels are involved when one neuron signals another neuron via a neurotransmitter. Some signaling molecules act to close certain ligand-gated ion channels, blocking the flow of specific ions. Finally, let's examine the intracellular receptor, which may be found in the cytoplasm or nucleus of target cells. To reach such a receptor, a signaling molecule must be able to pass through the target cell's plasma membrane. Thyroid hormones and steroid hormones like testosterone and estrogen are able to cross the plasma membrane and bind to intracellular receptors. The activated receptor triggers a change in the cell. For example, in many cases a hormone-receptor complex enters the nucleus and binds to specific genes, turning them on or off.