Which of the following is a cell surface receptor?

(a) Enzyme-linked receptors

(b) G protein-linked receptors

(c) Ion-channel linked receptors

(d) All of the above

The correct answer is (d) All of the above.

Cell surface receptors are proteins embedded in the plasma membrane that bind to extracellular signaling molecules (ligands) and convert this external signal into an intracellular one. The three options listed are the main classes of these receptors.

Main Classes of Cell Surface Receptors

• G protein-linked receptors (GPCRs): This is the largest family of cell surface receptors. They have a characteristic structure that spans the cell membrane seven times. When a ligand binds, the receptor activates an intracellular G protein, which then initiates a signaling cascade.
• Enzyme-linked receptors: These receptors have an intracellular domain that either acts as an enzyme itself or is directly associated with an enzyme. When a ligand binds to the extracellular domain, the enzyme activity is stimulated. The most common type is the receptor tyrosine kinase (RTK).
• Ion-channel linked receptors: Also known as ligand-gated ion channels, these receptors act as a "gate" in the plasma membrane. The binding of a ligand causes the channel to open or close, allowing or blocking the flow of specific ions (like Na⁺ or Ca²⁺) into or out of the cell.