What activates gustatory receptors for bitter taste?

• A. Bitter molecules binding to G-protein coupled receptors.
• B. Salty compounds opening voltage-gated sodium channels.
• C. Sour taste molecules binding to GPCRs.
• D. Sweet molecules binding to potassium channels.

Answer: (A)

Bitter taste detection relies on bitter molecules binding to specific GPCRs on taste receptor cells. These receptors (T2Rs) are G-protein coupled, so ligand binding activates a G protein like gustducin, triggering a phospholipase C–mediated cascade. That leads to IP3 production and Ca2+ release, which opens TRPM5 channels, depolarizing the cell and causing the release of ATP to signal the gustatory nerves. This GPCR-driven transduction is why bitter stimuli are sensed through receptor proteins rather than simple ion channels.

For context, salty tastes come from Na+ entering through epithelial sodium channels, causing depolarization, while sour involves protons modulating ion channels. Sweet tasting also uses GPCRs (T1R2/3) but via a different receptor family, not potassium channels as the option suggests.