equilibrium among the chemical species present in the oceans, soils and rocks, and the atmosphere.

Six elements are of crucial importance for all life on earth—carbon (C), hydrogen (H2), oxygen (O2), nitrogen (N2), sulfur (S), and phosphorus (P). These elements, collectively referred to as CHONSP, are essential components of the building blocks of all cells from unicellular bacteria to multicellular mammals. In different combinations and ratios, they are present in carbohydrates, lipids, proteins, and nucleic acids. They combine to form skeletal materials such as lignin and cellulose in plants, chitin in insects and crustaceans, and keratin in mammals and—with the calcium cation (Ca)—form apatite in bones and calcite in invertebrates. A wide spectrum of metabolic systems has evolved to efficiently recycle these elements. Accordingly, the products of one set of biochemical processes are used as reactants for another set, thus ensuring that the elements are not irreversibly bound in a form that is unavailable to living matter.

Biogeochemical cycling is the recycling of elements by organisms in the context of geological processes. Various microorganisms participate in every chemical transformation of CHONSP. These tiny, incredibly abundant and diverse organisms are the workhorses of biogeochemistry. They degrade previously synthesized organic material or form new organic substances by fixing carbon dioxide, both photosynthetically and in the absence of light. In carrying out their wide range of metabolic functions, they consume and then release each element, thus returning it to the biosphere, the hydrosphere, the atmosphere, and/or the solid earth. An instructive example is represented by the development of soil—the substrate on which the terrestrial food chain is based.

HUMAN PHYSIOLOGICAL PROCESSES

All of us share a bipedal upright structure and a body with distinctive anatomical parts that have discrete morphologies and functions. Acting in concert, these components maintain a state of balance (homeostasis) within the organism. The loss of body structural integrity and function can occur when homeostasis is perturbed by internal factors or by physiological response to hazardous materials in the environment. Most earth materials—solids, liquids, or gases—are essential for the body or are benign. A few can become harmful, especially if in elevated amounts, where they impinge or enter the body and disturb the normal functions of the organs. There are three usual routes of exposure to earth materials—respiration (through the nose or mouth and into the breathing apparatus), ingestion (through the mouth into the digestive system), and dermal (through the skin).



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