02-08-2013, 10:49 PM
Organisms are composed of matter, which is anything that takes up space and has mass. Matter exists in many divers forms, each with its own characteristics. Rocks, metals, wood, glass, and you and I are just a few examples of what seems an endless assortment of matter.
Matter is made of compounds and elements.
An element is a substance that cannot be broken down to other substances by chemical reactions. Today, chemists recognize 92 elements occurring in nature, for example gold, copper, carbon and oxygen.
A compound is a substance consisting of two or more elements combined in a fixed ratio. Table salt, for example, is sodium chloride, a compound composed of the elements sodium and chlorine in 1:1 ratio. Pure sodium is a metal and pure chlorine is a poisonous gas.
Chemically combined, however, sodium and chlorine form an edible compound. This is a simple example of organized matter having emergent properties: A compound has characteristics beyond those of its combined elements.
Actuality, about 25 of the 92 natural elements are known to be essential to life. Just four of these, carbon (C), oxygen (O), hydrogen (H) and nitrogen (N) make up 96% of living matter. Phosphorus (P), sulfur (S), calcium (Ca), potassium (K), and a few other elements account for most of the remaining 4% of an organism’s weight.
Trace elements are those required by an organism in only minute quantities. Some trace elements, such as iron (Fe), are needed by all forms of life, others are required only by certain species. For example, in vertebrates (animals with back bones), the element iodine (I) is an essential ingredient of a hormone produced by the thyroid gland. A daily intake of only 0.15 milligram of iodine is adequate for normal activity of the human thyroid. An iodine deficiency in the diet causes the thyroid gland to grow to abnormal size, a condition called goiter. Where it is available, iodized salt has reduced the incidence of goiter.
requires of life about 25 chemical elements