Tannins are molecules made by plants - they are complex polyphenols built from several phenol molecules. A phenol is made from a hexagon-shaped carbon ring with one or more hydroxyl group (OH) attached to it (see adjacent diagram). They are generally water-soluble molecules and they can combine with proteins, cellulose, pectins etc. Tannins are generally stored in the vacuole of plant cells (as are various oils, resins, crystals of calcium oxalate etc). Collectively, these materials are sometimes referred to as ergastic substances.

Tannins are molecules made by plants - they are complex polyphenols built from several phenol molecules.   A phenol is  made from a hexagon-shaped carbon ring with one or more hydroxyl group (OH) attached to it (see adjacent diagram).  They are generally water-soluble molecules and they can combine with proteins, cellulose, pectins etc.  Tannins are generally stored in the vacuole of plant cells (as are various oils, resins, crystals of calcium oxalate etc).  Collectively, these materials are sometimes referred to as ergastic substances.