What are the functions of cell surface receptors?

Cell surface receptors are responsible for most of the signaling in multicellular organisms. Ion channel-linked receptors are responsible for modulating the transduction of chemical signals across the cell membrane in response to the chemical messenger binding. This receptor regulates the opening or closing of ion channels which permit ions such as potassium, chloride, or sodium to move across the plasma membrane. These receptors also play a crucial role in stimulating the cellular response to toxins. Additionally, the contraction of cardiac, smooth muscle, and skeletal cells, the release of hormones, and the activation of T-cells are also regulated through ion channels. Enzyme-linked receptors are usually transmembrane proteins that act as enzymes. These receptors have an extracellular binding site for chemical signaling and an intracellular domain. The intracellular domain is regulated by the binding of an extracellular ligand, and therefore these receptors are also known as catalytic receptors. An example  of an enzyme-linked receptor are receptor tyrosine kinases. GPCRs are the largest cell surface receptors which are responsible for activating GTP binding proteins, which then activate either an enzyme in the cell membrane or an ion channel. GPCRs are also responsible for targeting numerous signaling pathways including sight, taste, smell, and pain sensations. The function of GPCRs is to couple ligand binding in the extracellular region to G-protein activation in the intracellular region. Examples of GPCRs include the acetylcholine receptor, and metabotropic glutamate receptors.