Protein receptors are located in which locations and support which tissues?

• A. cell membranes; tissues
• B. nucleus; organs
• C. cytoplasm; bones
• D. mitochondria; ligaments

Answer: (A)

Protein receptors are specialized proteins that detect signals from outside the cell and trigger responses inside. The most common location for these receptors is the cell membrane, because many signaling molecules (like hormones or neurotransmitters) cannot pass through the lipid bilayer on their own. When a signal binds to a membrane receptor, it starts a cascade of intracellular events that alter cell behavior, metabolism, or gene expression. This signaling guides how cells work together within tissues, coordinating growth, repair, and function across the tissue.

There are indeed intracellular receptors for certain ligands, but the typical and most widespread situation is membrane-bound receptors on cells that make up tissues. The other locations listed (nucleus, cytoplasm, mitochondria) are not the standard sites for the general receptor-mediated signaling that supports tissue-level function.