'Every plant is a chemical factory for complex substances which exceeds any human capability. In their poisons, antibiotic agents, prickles and foul tastes, they developed defences against attack long before human stockades and pesticides.'
- Anthony Huxley, Green Inheritance, 1984
Many of the chemicals produced by plants are linked to the ingenious strategies that plants have developed to help them flourish and survive.
Plants can’t run away from their enemies, be they animals or bacteria. Some of their defences include the thick, insulating bark of many trees, and the vicious thorns on roses. But what is it that makes the stem hairs on stinging nettles produce a rash. Why do some plants produce saps that have an extremely bitter taste; and why do others produce antibacterial substances?
Most plants can be regarded as complex chemical factories, since an astonishing array of compounds has evolved within them over millions of years. Scientists divide these compounds into two categories:
Primary Metabolites are found in all plant cells. These include sugars (carbohydrates), fats, oils and proteins, which are involved in the fundamental biochemical reactions common to all life. However, we’re not interested in primary metabolites here. We’re more interested in the second category.
Secondary Metabolites tend to be more specialised, and are usually peculiar to only one plant or species. Their biological function is not always obvious, but they are not formed without a reason. They are important for the survival and propagation of the plant.
While some secondary metabolites are designed to attract creatures that can pollinate their flowers or distribute their seeds, others protect the plant from the sun’s radiation, or serve as ‘chemical signals’ that enable the plant to respond to ‘environmental clues’. Others are defensive compounds, designed to deter or kill disease-causing organisms, potential predators or competitors.
Oddly enough, it’s among the plant chemicals that are generally poisonous to mammals, that so many of our medicines are found. Examples of such drugs in common use today include morphine and digitalis. These are both secondary metabolites, and still isolated from plant sources.
There are three major categories of secondary metabolite: alkaloids, phenols and terpenoids.
Alkaloids are an important group of plant chemicals, of which nearly 10 000 have now been isolated. Many are extremely poisonous to humans but a number, like morphine, atropine and cocaine, are widely used in medicine. One alkaloid that is a powerful insecticide is nicotine.
Phenols include the tannins, which are large molecules produced by almost all plants. Their ecological role is not fully understood, but some tannic acids interfere with the digestive processes of insects, and it’s possible that they also inhibit microbial growth. Their astringent taste is repellent to insects and higher animals alike.
Terpenoids are the largest class of secondary metabolites: over 22 000 have been described. They include compounds called steroids, which, like alkaloids, are particularly useful in medicine. Steroids are complex compounds that all have the same basic structure. Slight structural modification leads to different compounds with different properties, such as male and female sex hormones.
Resins and latexes are also mixtures of terpenoids and other complex substances, and they too have a protective function in the plants that produce them. The resin in the hemp plant (Cannabis sativa) helps protect its vulnerable parts from drying out in hot weather.
Latexes are fluids, which many plant anatomists believe are the by-products of chemical reactions in the plant. Latexes are secreted into special cells to stop them interfering with normal cell functions, but they may well deter predators too. Perhaps the best known plant latex is natural rubber from Hevea brasiliensis.
collecting latex from a rubber tree
Probably the most familiar terpenoids are the essential oils, many of which are responsible for the distinctive tastes and smells of plants (like those used in perfumery or as herbs and spices). Examples are pinene, limonene and camphene. Unlike ‘vegetable oils’, which are mostly extracted from seeds and which are usually food sources for the germinating embryo, essential oils are found in any or all parts of the plant. Because they diffuse readily into the air, they are sometimes called volatile oils.
While many essential oils found in flowers are there to attract the animals or insects that will pollinate them, those found in leaves are there to stop insects eating them, and to prevent infestation by micro-organisms.
Rosemary oil, which we extracted on Capraia, is a mixture of complex chemicals. Although a number of other essential oils are better suited as insect repellents (notably geraniol produced by Pelargonium species), we could not find them on Capraia, so we chose rosemary (Rosmarinus officinalis) which has some insecticidal properties. Many essential oils are useful in medicine because of their important antiseptic, antibacterial, antibiotic or other properties.
HOW TO EXTRACT COMPOUNDS FROM THE PLANT
With such a large number of different plant compounds, it’s not surprising that a variety of methods has to be used to extract them. The most appropriate method depends, among other things, on the part of the plant to be processed and the nature of the compound to be extracted. We’ll look at just two methods:
Essential oils are generally extracted by steam distillation, which involves passing steam through the plant material. As this happens, the essential oils vaporise and are carried away in the steam. As the vapour cools, it condenses and separates into water with a layer of the essential oil on top. This was the method we used to extract the rosemary oil on Capraia.
Solvent extraction, the process we used to extract the antiseptic oil from myrtle (Myrtus communis) is used industrially to produce not essential oils, but highly concentrated perfume materials known as absolutes. In this process, the plant material is immersed in a solvent such as alcohol or petroleum ether until the essence is dissolved in the solvent. The solvent is then evaporated off to give the absolute.
Book 3 of ST240, Our Chemical Environment, The Open University, 1995. ISBN 0 7492 5143 3.
Huxley A., Green Inheritance, Collins, 1984 ISBN 0 0027 2614 9.
Price S., The Aromatherapy Workbook, Thorsons, 1998 ISBN 0 7225 2645 8.
Raven P.H. et al., Biology of Plants, 6th edn., W. H. Freeman, 1999 ISBN 1 5725 9041 6. (See also the W. H. Freeman website)
FURTHER GENERAL READING
Here are some books and articles that you may want to try and get hold of:
Barrow J. D., The Artful Universe, Oxford University Press, 1995 ISBN 0 1985 3996 7.
A quite remarkable book that will change the way you view the world. Extremely accessible.
Burton et al., Chemical Storylines, G. Heinemann Educational Publishers, 1994 ISBN 0 435 63106 3.
Part of the Salters Advanced Chemistry course, which explores the frontiers of research and the applications of contemporary chemistry. For A level and other science courses aimed at 16 to 19-year olds.
Fraser A. and Gilchrist I., Starting Science (Book 1), Oxford University Press, 1998 ISBN 0 19 914235 1.
Part of an integrated science course for the National Curriculum Key Stage 3 and Scottish Environmental Studies (science) for S1 and S2.
Northedge A. et al., The Sciences Good Study Guide, The Open University, 1997 ISBN 0 7492 3411 3.
Indispensable for students of science, technology, mathematics and engineering. Packed with practical exercises and activities, all aimed at making studying more enjoyable and rewarding. Lots of hints and tips for those returning to study.
Selinger B., Chemistry in the Marketplace, 5th edn., Harcourt Brace, 1998 ISBN 0 7295 3300 X.
An excellent and informative reference source for all kinds of real-life applications of chemistry. Explores the world of chemistry that surrounds us in our daily lives, explained in terms that everyone can understand. ‘Makes chemistry come alive.’
PS547 Chemistry for Science Teachers course materials, The Open University, 1992
A course designed for use by science teachers from a wide variety of backgrounds, with varying experience of teaching science. A familiarity with some basic science (perhaps physics or biology) is assumed, but little understanding of chemistry is required. The mathematical understanding needed for the course is not great.