An element, usually a metal, required in minute amounts to maintain a healthy body. They are required mainly as components of enzymes and hormones, or are involved in the activation of enzymes.
Elemental components of a material that upon analysis are found to comprise less than 0.1 per cent of the substance. The analysis of trace elements sometimes allows the characterization of otherwise rather homogenous materials such as obsidian, which in turn may lead to the identification of sources and the recognition of dispersal patterns.
Physiological Roles of Trace Elements
Trace elements have several roles in living organisms. Some are essential components of enzymes where they attract substrate molecules and facilitate their conversion to specific end products. Some donate or accept electrons in reactions of reduction and oxidation, which results in the generation and utilization of metabolic energy. One trace element, iron, is involved in the binding, transporting, and releasing of oxygen in higher animals. Some trace elements impart structural stability to important biological molecules. Finally, some trace elements control important biological processes through such actions as facilitating the binding of molecules to receptor sites on cell membranes, altering the structure or ionic nature of membranes to prevent or allow specific molecules to enter or leave a cell, and inducing gene expression resulting in the formation of proteins involved in life processes.
Homeostatic Regulation of Trace Elements
The ability of the body to maintain the content of a specific substance such as a trace element within a certain range despite varying intakes is called homeostasis. Homeostasis involves the processes of absorption, storage, and excretion. The relative importance of these three processes varies among the trace elements. The homeostatic regulation of trace elements existing as positively charged cations (for example, copper, iron, zinc) occurs primarily during absorption from the gastrointestinal tract. Trace elements absorbed as negatively charged anions (for example, boron, selenium) are usually absorbed freely and completely from the gastrointestinal tract. Thus, they are homeostatically regulated primarily by excretion through the urine, bile, sweat, and breath. Storage of trace elements in inactive sites or forms is another mechanism that prevents inappropriate amounts of reactive trace elements to be present, for example, storage of iron in the form of ferritin. Release of a trace element from a storage site also can be important in preventing deficiency.