Lignins, tannins and related compounds

These higher plant components are characterized by phenolic (hydroxy-aromatic) structures. Such structures, which derive originally from monosaccharides, are common in plant but not animal tissue. 

The types of structural elements in lignin that are likely to be formed from the three precursor phenolic 

Tannins are widespread in nature but are quantitatively and geochemically less important than lignin. They are important in making plants less palatable to herbivores and, when extracted by steeping plant material, particularly bark and leaves, in water, they can be used for tanning leather. The structural units of tannins are polyhydroxy aromatic acids, such as gallic acid and 

Polyhydroxyflavonol units have also been recognized in some tannins. These units are directly related to the 

Co-A ester ofp-coumaric acid (Fig. 2.29), as well as via an acetate pathway. Flavonoids are often found as glycosides (i.e. they are complexed with sugars in a similar way to tannins) and appear to offer protection against UV radiation and microbial attack, as well as providing the colour in petals and autumn leaves. 

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