Trace elements are inorganic substances required by living things in minute quantities. There are 24 natural elements that are essential to life. The major elements, carbon, oxygen, hydrogen and nitrogen comprise 96% of the weight of living matter. In addition, the major mineral elements, macrominerals, make up another 3.7%. These include calcium (1.5), phosphorus (1.0), potassium (0.4), sulfur (0.3), sodium (0.2), chlorine (0.2) and magnesium (0.1). Trace elements, microminerals, make up the final 0.3% of the body's weight. The trace elements are iron, iodine, copper, zinc, manganese, cobalt, chromium, selenium, molybdenum, fluorine, tin, silicon, and vanadium. Although required in small amounts, organisms need these substances for their well-being. Some of the trace elements are needed by all forms of life, whereas only particular species need others. For example, all forms of life need iron because it is a component of cytochromes that function in cellular respiration. Vertebrates, on the other hand, require iodine to synthesize thyroid hormones...