Plant Chemistry: An Introduction. Plant Metabolites

Plant chemistry, also known as phytochemistry, is the study of the chemical compounds in plants. All living things on earth are made of chemicals. In fact, the chemical composition of plants and people is very similar. Both consist of common elements, including carbon, hydrogen, oxygen, and nitrogen. Chlorophyll — the green pigment essential to photosynthesis, the process plants use to transform water and sunlight into food — has been called “green blood” because its chemical makeup is similar to the hemoglobin in human blood. A molecule of chlorophyll and a molecule of hemoglobin each consist of carbon, nitrogen, and oxygen atoms, but in different proportions. There are other important chemical differences between hemoglobin and chlorophyll, but the basic similarities are striking.


Plants produce a variety of chemical compounds, called metabolites, as part of their normal life processes. Some of these compounds allow plants to store energy in the form of sugar, for example, while others, such as toxic compounds, help plants defend against diseases or predators. After all, plants cannot run away from insects and other animals trying to eat them! Some compounds might offer the plant a sort of evolutionary “fitness,” allowing it to survive and even thrive under conditions such as drought or an increase in average temperature that could wipe out other plant species.

There are two types of plant metabolites: primary and secondary. Primary plant metabolites include carbohydrates, lipids, proteins, and nucleic acids. These are compounds essential not only to the life of the plant but also to human health and nutrition. Plants use pieces of primary metabolites as building blocks to produce secondary metabolites. Secondary metabolites include terpenes, phenols, alkaloids, and their glycoside derivatives. These are largely responsible for the characteristic aromas, flavors, colors, nutritional values, and medicinal actions of the herbs and spices we use as foods, medicines, dyes, perfumes, and other products, from disinfectants to poisons. These properties also serve a vital purpose for plant life. For example, secondary metabolites give a flower its fragrance and color, both of which attract the pollinators the plant needs in order to set seed and produce the next generation of the species. The dividing line between primary and secondary metabolites is not always clear, and the two are integrally linked. Learning basic information about both primary and secondary metabolites will help you understand how they impact the properties and effects of herbs.

Primary Metabolites: Carbohydrates

Carbohydrates are essential to life and are part of the makeup of all living creatures. They provide animals, including humans, with energy and fiber and are the basic building blocks for all other plant chemicals.

Carbohydrates are composed primarily of sugars (saccharides) whose molecules are arranged in particular ways. Monosaccharides, for example, contain 1 sugar unit (mono means “one”), while polysaccharides contain 10 or more sugar units linked together (poly means “many”). Oligosaccharides contain between 2 and 10 sugar units (oligo means “few”). Glucose and fructose are two of the most common monosaccharides found in plants. Sucrose, found in sugarcane, is a disaccharide (di means “two”) formed by a link between the glucose and fructose.

Cellulose, the main component of plant cell walls and the most abundant organic compound on earth, is a homopolysaccharide — a compound made of chains of a single type of monosaccharide. Other important homopolysaccharides include starch, fructans, and inulins. These important dietary substances are commonly called complex carbohydrates.

Many other categories and subcategories of carbohydrates are found in plants and fungi. For example, myco-polysaccharides form the cell walls of mushrooms and are rich in fibrous carbohydrate substances called ß-D-glucans (pronounced beta-D-glucans), which have been studied for their immune-stimulating effects. Other ß-glucans are found in grains, including oats. They are an important source of soluble fiber.

Gums and mucilages — two other types of carbohydrates with various uses as food and herbal medicines — also consist largely of mono-saccharides. The term gum generally is understood to mean a sticky plant substance, such as gum arabic, made from black catechu (Acacia catechu). Mucilages are slippery substances used in herbal medicine to coat and soothe irritated or inflamed tissues (such as a sore throat). Marshmallow (Althaea officinalis), psyllium (Plantago ovata), and comfrey (Symphytum officinale) are all rich in slippery mucilage. Another important mucilage is carrageenan, derived from the Irish moss (Chondrus crispus), a seaweed, and from other species. Carrageenan is commonly used as a thickening agent in commercial food products.


Plants of the same species or even cultivar can vary in their plant chemistry, depending on their growing conditions and time of harvest. The compounds in St. John’s wort (Hypericum perforatum) thought to be responsible for its therapeutic activity can vary by as much as 50-fold in concentration, depending upon the season of harvest. Plant compounds can even vary on a daily basis. The chemical compound eugenol, when measured as a total concentration in the essential oil produced from a species of wild basil (Ocimum gratissimum), varied from 98 percent for a plant harvested at 12 a.m. to 11 percent for a plant harvested at 5 p.m. Researchers must consider these variations in concentration levels when testing an herb or herbal product in the lab.


Primary Metabolites: Lipids

Lipids — more commonly known as fats — are a major component of membranes in both plants and animals; they’re also found in various hormones, as well as in vitamins E and A. They serve as reservoirs of energy to fuel essential cell functions and, like carbohydrates, are building blocks for a range of secondary plant metabolites.

Among the most important plant lipids for human health and nutrition are fatty acids. Unlike the fats found in animal products, plant fats are rich in unsaturated fatty acids, which research shows are critical for heart health. The human body is capable of producing almost all fatty acid structures; however, some fatty acids cannot be manufactured by your body and therefore must be supplied through your diet. These fatty acids are of critical importance to human health and are called essential fatty acids.

The two essential fatty acids most closely linked to human health are omega-3 and omega-6. Although fish oils from tuna, mackerel, herring, and sardines may be the best sources of omega-3s, flaxseed (Linum usitatissimum) and the seeds of hemp (Cannabis sativa) contain omega-3 alphalinolenic acid, providing valuable nonanimal sources of essential fatty acids.

Primary Metabolites: Lipids

The seeds of flax (Linum usitatissimum) contain omega-3 fatty acids, as well as fiber and lignans — all important for health.

Primary Metabolites: Proteins

Proteins are large molecules with different but very important functions. Besides serving as structural components of cells and tissues, they also regulate biochemical processes in both plants and animals. In fact, every chemical reaction that occurs in living cells is controlled by a special type of protein called an enzyme.

Proteins are composed of hundreds of units called amino acids. There are about 20 different types of amino acids, and most plants can synthesize those necessary for survival. This isn’t the case for animals, which can synthesize only a few amino acids. For this reason, all animals — including humans — must obtain the missing, essential amino acids through their diets.

Vegetable proteins can be found in beans, nuts, and seeds, including peanuts (Arachis hypogaea), cashews (Anacardium occidentale), almonds (Prunus dulcis), walnuts (Juglans regia), sesame seeds (Sesamum indicum), sunflower seeds (Helianthus annuus), and soy beans (Glycine max).

Secondary Metabolites: Terpenes

Terpenes comprise the largest group of secondary plant metabolites. Thousands of different terpene compounds are found in a wide variety of plant species, and many appear to have important functions for the plants that produce them. For example, some give off aromas that lure pollinators or deter predators.

Terpene-rich volatile (essential) oils have great importance in herbal medicine and cooking. These aromatic compounds are responsible for the fragrances and flavors of kitchen favorites such as thyme (Thymus spp.), ginger (Zingiber officinale), peppermint (Mentha × piperita), and peel from citrus (Citrus spp.). These plants are not only tasty and aromatic, but they also have valuable antispasmodic, antimicrobial, and carminative (digestion-enhancing) effects.

Terpenes give us many other valuable medicinal compounds as well, including bitters, anti-inflammatory agents, expectorants, and sedatives. Limonene, a monoterpene found in citrus peel as well as mint, dill, and caraway, has been studied for potential cancer-preventive effects. Another important group of terpenes are the carotenoids — orange plant pigments found in oranges, peppers, and carrots — that the body converts into vitamin A.

Secondary Metabolites: Phenols

This group of plant chemicals includes thousands of different compounds that share one common chemical characteristic: All contain at least one phenol group. This is probably the largest group of plant secondary metabolites, and its compounds are widespread in nature. Plant phenols range from very simple structures to highly complex ones, such as tannins and lignins.

Secondary Metabolites: Phenols

Green teas are rich in antioxidant polyphenols, powerful plant chemical compounds that are thought to protect your body against cell damage that can lead to cancer and heart disease.


The term antioxidant seems to show up in nearly every health food claim and on every label — but just what does it mean? Antioxidant compounds defend your body against the effects of harmful chemicals called free radicals. Free radicals are unstable compounds that are products of oxidation in your body. Plant antioxidants are composed of a broad variety of substances that include phenols and some terpenes.

The human body produces free radicals in response to airborne pollutants such as cigarette smoke, stress, recreational drugs, food additives, and many other things. Free radicals can wreak havoc in your body, damaging cells and contributing to accelerated aging and a host of health problems, including serious conditions such as heart disease.

Antioxidants “scavenge” or “quench” free radicals to protect the human body against these harmful effects. Antioxidants are found in plants, especially fruits, vegetables, and herbs. Herbs that have particularly potent antioxidant actions include green tea (Camellia sinensis), milk thistle (Silybum marianum), turmeric (Curcuma longa), ginkgo (Ginkgo biloba), ginger (Zingiber officinale), garlic (Allium sativum), and horse chestnut (Aesculus hippocastanum).

At least half of all phenols are part of a large subgroup called flavonoids. Flavonoids contain many important antioxidant compounds that help eliminate harmful substances called free radicals from your body. The flavonoid category itself can be divided into numerous smaller subgroups of flavonoid compounds, including isoflavonoids, flavones, flavonols, flavonolignans, anthocyanins, and proanthocyanidins. When several phenol groups are attached to each other, the resulting compound is called a polyphenol.

Green tea (Camellia sinensis) is one of many phenol-rich plants with valuable health properties. It contains antioxidant phenols called catechins. Hundreds of studies conducted on these compounds suggest that they could help prevent cancer and heart disease. Many plants in the pea family (legumes) contain isoflavonoids, which also have demonstrated impressive cancer-fighting and hormone-balancing effects in modern studies. Plants rich in isoflavonoids include soy (Glycine max) and red clover (Trifolium pratense).

Many of the pigments that give plants their coloring are polyphenols. Antioxidant polyphenols called anthocyanins provide the blue and red colors of berries such as blueberries (Vaccinium angustifolium and others) and cranberries (V. macrocarpon). Red grapes (Vitis vinifera) and red wine contain anthocyanin pigments as well as resveratrol, another polyphenol. Studies have shown that berries and red wine, like green tea, could have healthful antioxidant effects.

Tannins are highly astringent polyphenols that can be used not only as tanning agents for the leather industry, but also as medicines. Astringents tone and tighten tissues throughout your body, including mucous membranes and skin; they are responsible for the mouth-puckering sensation you experience when drinking a cup of strong black tea. Oak bark (Quercus spp.), witch hazel (Hamamelis virginiana), and agrimony (Agrimonia eupatorium) are also rich in tannins.

Other important phenols include curcumin (found in turmeric, Curcuma longa), a powerful antioxidant that has anti-inflammatory and cancer-preventive properties, and silymarin, a mixture of flavonolignan compounds largely responsible for the health benefits of milk thistle (Silybum marianum). Modern studies have shown that milk thistle might protect your liver from the effects of toxins, including pharmaceutical drugs, and help it regenerate damaged cells.


While the chemical structures of these secondary metabolites (terpenes, alkaloids, and phenols) appear very different, the compounds belonging to each of them could look similar to each other yet have completely different pharmacological activities.

The alkaloids caffeine and morphine, for example, have completely different pharmacological profiles even though their chemical structures are related. Each of these compounds is a psychoactive drug, acting on your central nervous system and affecting brain function. But while caffeine is a mild stimulant that’s widely consumed in coffee and teas, morphine, the major constituent of opium, is a powerful narcotic (sleep-inducing) drug used to treat and manage moderate to severe pain.


Secondary Metabolites: Alkaloids

This group of plant chemicals can have powerful effects in the human body. Many are potent medicinal compounds that can be toxic in high doses; others are highly addictive.

Caffeine, a naturally occurring stimulant and diuretic found in coffee (Coffea arabica), green tea (Camellia sinensis), and other foods, is one familiar alkaloid. Other potent alkaloids include ephedrine, a decongestant taken from ephedra (Ephedra sinica); theophylline, a bronchial smooth-muscle relaxant present in small quantities in tea; reserpine, a tranquilizer and antihypertensive made from the Indian serpentwood plant (Rauvolfia serpentina); and vincristine and vinblastine, cancer-fighting compounds made from Madagascar periwinkle (Catharanthus roseus).

Some alkaloids have hallucinogenic effects. Examples include mescaline, which is extracted from the peyote cactus (Lophophora williamsii), and psilocybin, which is found in mushrooms of the genus Psilocybe. Other alkaloids are highly addictive. They include cocaine, a stimulant and anesthetic taken from the leaves of the South American coca plant (Erythroxylum coca); nicotine, taken from the leaves of the tobacco plant (Nicotiana tabacum); and morphine, a pain reliever extracted from the opium poppy (Papaver somniferum).

Certain pyrrolizidine alkaloids, such as those found in plants of the borage family, including comfrey (Symphytum officinale), can cause liver damage. Other poisonous alkaloids include strychnine (found in Strychnos nux-vomica), atropine (found in Atropa belladonna and Datura stramonium), and coniine (a deadly toxin made from poison hemlock, Conium maculatum).


Phytochemists use the term glycoside to describe a plant compound that has a molecule of sugar attached to a noncarbohydrate molecule, called an aglycone.

Glycosides are particularly important in the study of herbal medicine. Many have important medicinal actions; others are dangerous toxins. The cyanogenic glycosides found in apple seeds and bitter almonds, for instance, produce the deadly poison cyanide. But the cyanogenic glycoside prunasin, found in wild cherry bark, is an expectorant when taken in small quantities.

Cardiac glycosides, such as those in lily of the valley (Convallaria majalis) and foxglove (Digitalis spp.), are extremely potent chemicals that should never be taken during self-treatment. Cardiac glycosides improve your heart’s efficiency without increasing its need for oxygen. Plants containing these compounds were once the only treatments for serious heart conditions, such as congestive heart failure (a disease in which your heart loses its ability to efficiently pump blood). However, because cardiac glycosides are eliminated from your body slowly, dangerous levels can accumulate in your blood. To treat congestive heart failure today, doctors prescribe modern pharmaceuticals such as digoxin (isolated from Digitalis lanata), which they can monitor and control the dosages of more easily.

Another group of glycosides are the glucosinolates, found primarily in cruciferous vegetables (members of the mustard family, Brassicaceae) such as broccoli, cabbage, and kale (all Brassica oleracea), horseradish (Armoracia rusticana), mustard (B. nigra and others), and radish (Raphanus sativus). When one of these plants is crushed, its glucosinolates undergo a chemical reaction that creates the volatile oil compounds we know as mustard oils. When applied to the skin, mustard oils have a warming, stimulating effect — that’s why mustard poultices were traditionally used to relieve chest congestion. Research has shown that indole-3-carbinol might help prevent certain cancers, including colon and breast cancers, and indole-3-carbinol is produced in your body as you break down the glucosinolates in cruciferous vegetables.

Catharanthus roseus

Catharanthus roseus, the source of the chemotherapeutic drugs vincristine and vinblastine.