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Secondary Compounds
Plants produce and amazing variety of organic compounds over and above their everyday components and metabolites, which are called primary compounds. The primary compounds, carbohydrates, lipids, nucleotides and peptides are shared by all living organisms and are central to life processes. The secondary compounds are derived from everyday components, but are not central to metabolism, hence their name. Often it can be difficult to identify their function; do they help protect against disease or herbivores or environmental stress, or are they just metabolic junk?

Plants produce other kinds of compounds in addition to the phenolics and terpenoids described below. Particularly important (and diverse) are the alkaloids which are nitrogen containing cyclic compounds. This class includes many plant poisons and drugs: caffeine, nicotine, atropine, quinine, cocaine etc.


Terpenoids are built from five-carbon isoprene units and so can also be called "isoprenoids". Molecules with multiples of 5 carbons and particularly with single carbon side chains are likely to belong to this group, although it may be difficult to see the isoprene units because of cyclization. Many spices and fragrances contain terpenoids.

More complex terpenoids include the sterols. These are present in cell membranes and affect their fluidity. A number of animal hormones are sterol derivatives and recently a plant sterol, brassinolide has been discovered to affect plant development. When apples and other plant foods are labelled "cholesterol free" this should be no surprise as plant sterols are generally different from animal sterols. However compounds related to oestrogen are found in the Fabaceae so maybe we should worry about eating beans!

The carotenoids are a large family of yellow to orange pigments built up from eight isoprene units. Carotenes are C-40 hydrocarbons; xanthophylls are carotenoids with hydroxyl or other oxygen-containing groups. Xanthophylls are found in many yellow fruits and flowers and in autumn leaves.

Phenolics are a huge and diverse group of aromatic compounds (containing benzene rings) usually with hydroxyl groups. Phenol itself is the simplest member of the class, although it is not found in plants. Many plant phenolics have three carbon side chains and are called "phenyl-propanoids". Hydroxy-benzoic acid is one of the simpler plant phenols, whereas coumarin is a simple phenyl-propanoid found in some grasses.

More complex phenyl propanoids include the flavonoids (C6-C3-C6). The pigments in red, blue or purple flowers are usually flavonoids called anthocyanins. An anthocyanin is made up of an anthocyanidin with one or more sugar molecules attached.

The flavonoids also give a bitter taste to many plants and plant extracts (fruit juices) and form part of a very complex group, the tannins. Tannins are extracted from tree bark and get their name from their use in the tanning of leather (they are also present in tea).

Probably the most ubiquitous of the phenyl-propanoid derivatives is lignin. This is built up by the random polymerization of C6-C3 units and strengthens cell walls, particularly in the xylem of woody plants. It is a huge molecule that is interwoven with, and chemically bonded to cellulose. All kinds of linkages occur in lignin and it is not possible to show a complete structure.

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Copyright © Michael Knee
The Ohio State University
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