Plants & Human Affairs - Introduction

Plants & Human Affairs (BIOL106) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

An Introduction to Biodynamic Plants

I. What is a biodynamic plant?

exerts a physiological action in a human if ingested or otherwise taken into the body
in other words, has an effect, positive or negative, on an organism. Another way of saying this is that the plant is "biologically active".

II. Why are some plants biodynamic? Answer: because of their chemical constituents

A. Chemicals & the Plant Way of Life
Plants produce a variety of interesting chemical molecules, many of which are biologically active (biodynamic). These chemicals are a direct response to the "plant way of life". Recall that stationary organisms like plants need a defense system against predators (i.e., hungry herbivores). One solution is to make toxic, foul-tasting, or otherwise nasty chemicals - essentially the natural version of chemical warfare. As a test of this idea, we expect non-motile animals, like coral and sponges, to be good sources of biologically active chemicals - and they are! They, too, derive protection from predators from these molecules.

B. Biodynamic chemicals are secondary metabolites (chemicals)
Primary metabolites are those chemicals produced by plants that are directly involved in plant growth and development. They are absolutely essential for survival. Examples are glucose and chlorophyll. In contrast, secondary metabolites do not appear to be essential for survival. Most defensive chemicals in plants are considered secondary metabolites since a plant will grow perfectly well without them (as long as they are not attacked by a hungry herbivore). These secondary chemicals may be metabolic waste products, or may be chemicals for which we still haven't identified a particular function.

C. Biodynamic Chemicals. Some of the more important ones are:

Alkaloids - This is a very diverse group of chemical constituents that is characterized by: (a) contain a nitrogen atom in a ring; (b) alkaline pH; and (c) bitter taste. The name of an alkaloid typically ends in "-ine". Some plants are especially known for ability to produce alkaloids, such as members of the Bean family, Tomato family and Coffee family.

Steroids - These chemicals have a unique multiple-ring structure. They generally lack nitrogen. They are components of many animal hormones particularly sex hormones, (i.e., androgen, testosterone, progesterone, estrogen), adrenal and thyroid hormones, and cholesterol. They occur in plants in many families.

Terpenes - This is a diverse class of chemicals that are built from one or more five carbon-carbon units (isoprene). Many are volatile and important components of essential oils. The group gets its name from turpentine.

Assorted Others - including cyanogenic glycosides (molecules that release HCN upon breakdown) and the mustard oils.

D. These molecules can occur individually or as glycosides (attached to one or more sugars like glucose). The glycoside form is the plant way of detoxifying the chemical and make it available for transport around the plant.

III. Classification of Biodynamic Plants
Classification can be based on: (a) the effect on the body; or (b) the chemical constituents responsible for the action. One system is:

Medicinal Plants - those with biodynamic agents that are effective in treating disease and other disorders.
Poisonous Plants - those with a deleterious effect
Psychoactive Plants (including hallucinogens) - those that effect the central nervous system including stimulants (caffeine), depressants (opiates) and hallucinogens (psilocybin, mescaline).
A few points to note:

These terms are relative; that is, one person's psychoactive plant may be another person's poison.

At low dosage a plant may be a medicine; but at higher dosage, a poison. As a corollary, there is often a fine line between medicinal action and death. As dosage increases: Medicinal plant → Hallucinogenic Plant → Poisonous Plant. This is one good reason why you want to be careful when using medicinal herb. As an aside, people in some cultures actually seek out some poisonous plants for food. For example, some tribes in Kenya eat sublethal doses of black nightshade (Solanum nigrum). One advantage to this practice is that the toxic constituents of the nightshade may kill Giardia and other parasites.

Some edible plants are poisonous if not prepared correctly or if over-consumed (see Toxicants Occurring Naturally in Foods, Natural Academy of Sciences, 1971). Thus, you want to avoid eating too many apple seeds (contain cyanide-producing chemicals) or rhubarb leaves (oxalate). Cassava (manihot), a root crop that provides the major starch source for about a quarter of the world's population, produces much cyanide that must be removed during processing. In fact, people who eat cassava typically ingest more than a lethal dose of hydrogen cyanide everyday.

IV. References:

Sumner, Judith. 2000. The Natural History of Medicinal Plants. Timber Press, Portland, OR.
Fackelmann, KA. 1993. Food, Drug, or Poison? Science News 143: 312 - 314.