Sour taste receptors are activated primarily by which of the following?

• A. H+ ions
• B. Sugar molecules
• C. Na+ ions
• D. Light

Answer: (A)

Sour taste signals the presence of acids, which bring hydrogen ions (H+) to the taste buds. These protons interact with proton-sensitive ion channels in the taste receptor cells, causing depolarization (often by blocking certain K+ channels). The depolarization opens voltage-gated calcium channels, leading to neurotransmitter release and activation of the gustatory nerves. That is why acidic substances taste sour.

Sugar molecules trigger sweet taste through GPCRs (not H+), Na+ ions produce salty taste by entering cells through sodium channels, and light is not a chemical stimulus for taste.