Which of the following is an example of an ionotropic receptor?

• A. A ligand-gated ion channel
• B. G-protein coupled receptor
• C. Receptor tyrosine kinase
• D. Intracellular hormone receptor

Answer: (A)

Ionotropic receptors are ligand-gated ion channels that open a pore directly in response to ligand binding, allowing ions to flow across the membrane and producing a fast change in membrane potential. This direct coupling of binding to channel opening underlies rapid synaptic transmission, on the order of milliseconds. An example is a ligand-gated ion channel such as the nicotinic acetylcholine receptor, which opens its pore to Na+ (and other ions) when acetylcholine binds.

The other receptor types operate differently. G-protein coupled receptors are metabotropic; they activate intracellular signaling cascades via G proteins rather than forming an ion pore themselves. Receptor tyrosine kinases are enzyme-linked receptors that propagate signals through phosphorylation events, not immediate ion flow. Intracellular receptors bind lipophilic hormones and regulate gene transcription rather than opening an ion channel.