Discovery Science: Earth – Morphology and Physiology – Secondary Plant Compounds

Earth Science: Morphology and Physiology – Secondary Plant Compounds

Secondary plant compounds or secondary metabolites belong to a group of compounds with varying chemical properties. They are nonessential to a plant’s basic metabolic functions.

Secondary compounds are the metabolic products that accumulate in the plants tissues and are not directly essential to the plants survival. More than 30,000 secondary metabolites have already been discovered and, most likely, there are many more yet to be found.

Many secondary compounds are still unknown because only about 15 to 20 percent of all plant species have been tested. Secondary compounds are often only present in very small concentrations, which makes detection even more difficult. Humans are able to take
advantage of many of the properties of these secondary compounds in everyday life.

Previously, it was thought that secondary plant compounds were merely waste products that needed to be flushed out of the plant’s metabolic cycle. Today, researchers have found that these compounds play an important role in plant life. Many protect plants from herbivores by adding a bitter taste or making a plant poisonous.

They may also act as a signal for pollinators, protect the plant from aggressive UV radiation by the sun, prevent evaporation or fight pathogenic microorganisms.

Typical secondary compounds

Alkaloids are typical secondary plant compounds produced by many different plant species, e.g., the nightshade and poppy families. Many alkaloids are toxic, but may be used in small quantities for therapeutic purposes. Familiar alkaloids include caffeine, cocaine, nicotine, and strychnine. Essential oils are another type of secondary compounds.

These are aromatic, volatile, oily compounds. Their original purpose is to attract pollinating insects, but they may also be used in medicine. For example, a chamomile vapor bath can be used to treat respiratory diseases.

Some secondary compounds are used to dye cloth. Indigo, for example, is a dark blue dye extracted from Indigofera species. This dye is now usually made synthetically.


Herbal remedies often derive their properties from secondary compounds and have been used for thousands of years and, despite a drop in usage due to modern medicines, they have recently gained popularity as it is believed that they produce fewer unwanted side effects.

For example, cardiac glycosides from the fox- glove species are known active medicinal plant substances. The active substance in aspirin, acetylsahcylic acid, originally came from willow, but is now produced synthetically.


MANY secondary plant compounds occur in drugs such as cocaine or marijuana. Recreational drug abuse poses a serious and un-solved problem for society as a whole.