Which of the following lists are examples of real receptors?

• A. Receptors for hormones, neurotransmitters, growth factors, and cytokines.
• B. Ion channels that mediate ion flow.
• C. Receptors that bind only exogenous ligands.
• D. Receptors that metabolize drugs.

Answer: (D)

Receptors are proteins that bind signaling molecules such as hormones, neurotransmitters, growth factors, and cytokines and convert that binding into a cellular response. These classic binding partners illustrate real receptors because they recognize endogenous mediators and trigger downstream signaling. Ion channels can function as receptors when they are ligand-gated, but not all ion channels fit the broader receptor role, which is about initiating signaling cascades rather than just allowing ion flow. Receptors that bind only exogenous ligands miss the key idea that many receptors respond to endogenous signals. Receptors do not metabolize drugs—the enzymes that metabolize drugs handle that task. So the set that lists receptors for hormones, neurotransmitters, growth factors, and cytokines best represents real receptors.