GPCRs are best described as which of the following?

• A. They function as ion channels directly
• B. They lack transmembrane domains
• C. They are the most abundant class of receptors in humans
• D. They act as enzyme receptors that phosphorylate substrates

Answer: (C)

GPCRs are seven‑pass transmembrane receptors that signal by activating heterotrimeric G proteins to regulate downstream second messengers. This setup makes them the most abundant and diverse receptor family in humans, found across virtually all tissues and involved in many physiological processes. They are not ion channels themselves—ion channels are separate receptors that form pores. They do have transmembrane domains, specifically seven, which is a hallmark of GPCRs. They are also not enzyme receptors that phosphorylate substrates themselves; that role belongs to receptor tyrosine kinases and similar enzymes. So the best description is that GPCRs represent the most abundant class of receptors in humans.