What do each class of cell surface receptors do?

The three classes of cell surface receptors are: G-protein-linked receptors, enzyme-linked receptors, and ion channel receptors. 

• Ion channel receptors play a key role in translating chemical signals across the cell membrane. These receptors control the opening or closing of ion channels, enabling the movement of ions like Na+ or K+ across the cell's outer membrane. This mechanism relies on a specific part of the receptor that spans the cell membrane. The binding of the ligand induces a structural change in the receptor, facilitating the passage of specific ions. 
• GPCRs are a type of cell surface receptors which initiate a signaling cascade through interaction with a G-protein, influencing cellular functions. As an example, GPCRs are vital for signaling pathways including those involved in sensory perception such as taste, sight, smell, and pain sensations. Ligand binding induces a conformational change, activating the G protein. The GDP is replaced by GTP on the alpha subunit, leading to dissociation of the G-protein, with the beta-gamma dimer and the activated alpha-GTP acting to initiate the signal cascade. In the absence of a ligand, the alpha subunit is bound to the transmembrane receptor along with GDP, while the beta and gamma subunits remain attached. Enzymes and second messengers such as adenylate cyclase, cyclic AMP, and phospholipase C are commonly activated in this cascade. GPCRs can have both activating and inhibiting effects and are involved in various functions in multicellular organisms, including growth, clotting and endocrine signaling. 
• Enzyme linked receptors possess a catalytic site on the intracellular domain. Typically, upon ligand binding, these receptors undergo dimerization. As a result, they activate the catalytic site and initiate enzymatic activity. Receptor tyrosine kinase is the most prevalent type among enzyme-linked receptors, with other examples including tyrosine-kinase associated receptors, receptor serine/threonine kinase, receptor tyrosine phosphatases, and receptor guanylyl cyclases. Receptor tyrosine kinases activate the kinase enzyme and are able to phosphorylate tyrosine residues. This activation typically triggers signaling cascades involved in cell growth, differentiation, and survival. Receptor serine/threonine kinase play crucial roles in shaping embryonic development, controlling apoptosis, regulating cell growth, and impacting various disease processes (e.g. atherosclerosis). Receptor guanylyl cyclases generate the intracellular signaling molecule cyclic GMP (cGMP). They do this by facilitating the transformation of GTP to cGMP and pyrophosphate.