Taste buds are structures on our tongues that hold taste cells, which give rise to the sense of taste. Each taste bud holds fifty to one-hundred cells, and taste buds may hold cells for all of the five kinds of taste. The classic diagram showing what taste each region of the tongue is sensitive to is, therefore, incorrect (or at least overgeneralized). Nerve bundles come out of the base of each taste bud, and each individual taste cell connects to a single nerve.

The five types of cells found on taste buds are:
Salty: these cells react with substances which release sodium ions (Na+), and are thus salty.
Bitter: there are around eighty different genes (with corresponding proteins) that sense different bitter molecules.
Sour: these react with protons (H+) released by acidic -- therefore sour -- substances.
Sweet: another that relies on proteins to determine whether a given molecule is sweet.
Umami: cells triggered by the salts of glutamic acid, notably MSG and salts found in seaweed.