Differentiate ionotropic and metabotropic receptors.

• A. Ionotropic use second messengers; metabotropic are direct ion flow.
• B. Ionotropic are ligand-gated ion channels causing fast responses; metabotropic are G-protein-coupled receptors producing slower second-messenger effects.
• C. Ionotropic are GPCRs; metabotropic are ion channels.
• D. Both are ion channels; one is slower.

Answer: (B)

The distinction hinges on how the receptor transduces the signal. Ionotropic receptors are ligand-gated ion channels: when the neurotransmitter binds, the channel opens and ions rush through, producing a rapid change in membrane potential and fast synaptic responses. Metabotropic receptors are G-protein-coupled receptors: binding activates G proteins, which then set off intracellular cascades with second messengers like cAMP or IP3/DAG. This produces slower, longer-lasting effects by modulating other channels or cellular processes rather than creating an immediate ion flow.

The other statements mix up these roles. Ionotropic receptors do not rely on second messengers; metabotropic receptors are not direct ion channels. And not all receptors are ion channels—metabotropic receptors are GPCRs, not ion channels.