A cell surface receptor is a protein located on the outer membrane of a cell that binds to specific molecules, known as ligands. These ligands can include hormones, neurotransmitters, or other signaling molecules. When a ligand attaches to a receptor, it triggers a response inside the cell, allowing it to communicate with its environment and respond to various signals. Cell surface receptors play a crucial role in many biological processes, including cell communication, immune response, and metabolism. They help cells detect changes in their surroundings and initiate appropriate actions, such as growth, movement, or secretion of substances.