Phenolic compounds are produced almost universally by plants and can be formed by three known pathways: the shikimic acid pathway, the malonate-acetate pathway, and the isoprenoid pathway. The shikimic acid (or phe-nylpropanoid) pathway yields three aromatic amino acids that are important precursors of plant phenolics - trypto-phan, tyrosine, and phenylalanine. Phenylalanine is modified progressively to hydroxycinnamic acids such as cinnamic acid, salicylic acid, ^-hydroxybenzoic acid, ^-coumaric acid, caffeic acid, sinapic acid, and ferulic acid. Coumarins (derivatives of ^-coumaric acid) are synthesized primarily in leaves but can be found at high concentrations in fruits, stems, and roots.
The malonate-acetate (or polyketide) pathway is similar to fatty acid synthesis: it involves sequential additions of malonyl-CoA to create a polyketide chain, which then undergoes cyclization to form a phenolic ring structure.
Malonyl-CoA, acetyl-CoA, and ^-coumaric acid yield chalcone, which can undergo various modifications, allowing production of a diversity of flavanones, flavones, flaven-3-ols, anthocyanidins, and isoflavones.
Some phenolics are formed by combining products from both the shikimic and malonate-acetate pathways. About 8000 types of plant phenolics are known; about 3000 of these are flavonoids. Plant phenolics range from simple one-ring phenols to lignin, a huge polymeric structure composed of phenylpropanoid units cross-linked to each other through heterogeneous chemical bonds. Lignin, after cellulose, is the most abundant organic material on Earth; it decomposes slowly. The slow rate of lignin decomposition by fungi, actinomycetes, and bacteria is thought to be due to the complexity of its bonds and cross-linkages, and because it has a relatively low nitrogen content. Hydrolyzable and condensed tannins are produced from flavanones: the tannins contain free phenolic groups and generally are soluble in water. Tannins are present in both gymnosperms and angios-perms, but within angiosperms they are more prevalent in dicotyledons than in monocotyledons.
Even lower plants such as ferns, algae, and lichens (fungal-algal systems) are known to produce phenolics, but only one animal - a marine sponge - is thought to have this capability. Because larger, more-complex phenolic compounds are relatively recalcitrant to degradation by microbes, phenolics occur in association with fossil fuels such as coal, shale oil, and petroleum. In oil production fields, produced water (i.e., water pumped up along with petroleum) often contains elevated levels of phenolics. Produced water from the extraction of methane from coal-bed water also contains phenolics. The aromatic and phenolic compounds in produced water originating from oil and coal-bed methane extraction account for much of the water's acute toxicity.
Important classes of phenolics produced by plants are shown in Table 2. This list is generic only, as thousands of different types of phenolics have been reported.
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