Real receptors are examples of which categories?

• A. Gases like oxygen and carbon dioxide only.
• B. Light photons only.
• C. Hormones, neurotransmitters, growth factors, and cytokines.
• D. Vitamins only.

Answer: (C)

Receptors are proteins that detect specific signaling molecules and trigger a cellular response. The most common endogenous ligands that engage real receptors fall into the categories of hormones, neurotransmitters, growth factors, and cytokines. Hormones like insulin bind their receptors to regulate metabolism; neurotransmitters such as acetylcholine act on neuronal receptors to transmit signals; growth factors like EGF bind receptors to control cell growth and differentiation; cytokines such as interleukins bind immune cell receptors to modulate immune responses.

Gases (like oxygen or carbon dioxide) and photons (light) are sensed by specialized systems (gas or photoreceptors) and don’t represent the broad, typical ligand classes for most receptors. Vitamins can act as signaling molecules in certain contexts (for example, some vitamins interact with nuclear receptors), but they don’t define the general categories used to classify most receptor signaling.