Why does lemon juice taste sour?

Answer: Acids release hydrogen ions that activate sour-taste cells

Peel oils activate bitter-taste cells, which produce the sour taste — Bitter-taste cells do not produce the sour taste of lemon juice. Sour and bitter are different taste qualities; the juice’s acids stimulate the sour-sensing pathway.

Having little sugar makes sweet-taste cells send a sour signal — Sweet-taste cells do not switch to sending a sour signal when sugar is scarce. Less sweetness can make existing sourness more noticeable, but unsweetened water is not sour merely because it lacks sugar.

Acids release hydrogen ions that activate sour-taste cells ✓ — Acids in lemon juice, especially citric acid, release hydrogen ions that stimulate sour-sensing taste cells. Sugar can balance how sour a drink seems, but the acid supplies the sour stimulus.