Receptor proteins are specialized molecules located on the surface of cells that play a crucial role in communication between cells. They bind to specific substances, such as hormones or neurotransmitters, which trigger a response within the cell. This process is essential for various biological functions, including growth, immune responses, and metabolism. When a receptor protein binds to its target molecule, it undergoes a change in shape, which activates a series of cellular processes. This interaction is vital for maintaining homeostasis and allowing cells to respond to their environment. Examples of receptor proteins include insulin receptors and neurotransmitter receptors.