Trace Elements in Biology and Agriculture

Abstract

FOODS and nutrients are necessary for cell growth because of their function in generation or release of energy, building and repair of protoplasm, and regulation of metabolic processes. They are usually classified as: (a) an energy source, (b) a nitrogen source, (c) growth factors (organic compounds not synthesized by the organism and required usually in small amounts), and (d) mineral salt or inorganic nutrient. A number of inorganic elements, or minerals, are required by all living forms for normal growth and reproduction. This is not true of all the elements of the periodic table, although most have been found in living cells. The minerals essential to life are generally divided into two classes: macronutrients, or major elements, needed in relatively large amounts, and the micronutrients, or trace elements, required only in small amounts. For most organisms the macronutrient elements include sulphur, phosphorus, potassium, magnesium, calcium, and nitrogen. In addition, sodium and chloride are needed by animals and, according to recent experiments, probably by plants as well. The trace elements include iron, zinc, manganese, and copper. Molybdenum, boron, and vanadium are also needed by plants, and iodine, cobalt, and probably molybdenum by animals. Even though micronutrient elements are required in only extremely small quantities, they are no less important than the major elements. The fact that the trace metals are needed in only small quantities indicates that they are functioning in some catalytic role, usually as part of an enzyme system. In this respect the function of trace elements is similar to that of the organic micronutrients, the vitamins. A known enzymatic role has been described for most of the trace elements. Exceptions are iodine, which is a component of the thyroglobulin molecule, and cobalt, which is part of the vitamin B12 molecule. Although we can describe a specific function for most of the metals, as long as essential enzyme systems can be activated by a variety of elements we can expect nonessential metals to have a significant effect on the metabolism of plants and animals. There are many examples of multiple-metal effects on enzyme systems, some of themost striking of which are from the area of plant-animal relationship. Judging by the optimum concentration of an element for enzyme activity as well as the ratio of stimulatory and inhibitory metals present, it is evident that under physiological conditions some systems may be working at a maximum rate whereas others may be operating at less than 1 percent of maximum efficiency. Obviously there -must be some balance in the Dr. McElroy is professor of biology, chairman of the department of biology, and director of the McCollumPratt Institute, Johns Hopkins University, Baltimore, Md.