The most important class of receptors is formed by

• A. Lipids
• B. Proteins
• C. Nucleic acids
• D. Carbohydrates

Answer: (B)

Receptors recognize signals and convert them into cellular responses, and this capability comes from proteins. Protein receptors—such as GPCRs, receptor tyrosine kinases, ligand-gated ion channels, and intracellular (nuclear) receptors—offer the diverse shapes, binding pockets, and the ability to undergo conformational changes needed to bind specific ligands with high affinity and transmit that information into the cell. While lipids, nucleic acids, and carbohydrates play roles in signaling and recognition, they do not serve as the primary binding and transduction units across the wide range of signaling pathways the body uses. Thus, the most important class of receptors is formed by proteins.