Answer:
Waste products may be categorized as metabolic or nonmetabolic. The difference lies in whether the substances in question are produced by the chemical processes of a living cell or are merely passed through the digestive tract of an organism without actually entering into its life processes.
Nonmetabolic wastes:
The nonmetabolic wastes are mainly materials that, by virtue of their chemical makeup, are indigestible or unusable by an organism. In addition, nonmetabolic wastes include any substances that are absorbed, ingested, or otherwise taken into a living system in excess of the needs and storage capabilities of the organism. These substances include digestible (metabolizable) as well as indigestible materials, and they may be excreted almost immediately, even though they are often usable as food.
Metabolic wastes:
Metabolic wastes may be separated into gases, liquids, solids, and heat. Heat, though usually not classified as a waste product, should be classified as such because it is a by-product of metabolic activity and must be eliminated to avoid harmful elevation of body temperatures in warm-blooded animals.
Gaseous wastes:
Oxygen produced during photosynthetic reactions in green plants and certain bacteria may be considered to be a waste product, or at least a by-product, requiring removal. Carbon dioxide is produced by all animals and by green plants in darkness. Nitrogen gas is produced by denitrifying sulfur bacteria (Thiobacillus), and ammonia is excreted by decay-causing bacteria and by most invertebrate and vertebrate animals.
Solid wastes:
Several important kinds of materials may be classified as solid wastes. Among them are nitrogenous wastes, by-products of protein and amino-acid metabolism by animals; nitrite and nitrate compounds produced by nitrifying bacteria; and sulfur and sulfates resulting from the metabolic activities of sulfur bacteria. Many other substances also enter into solid wastes to be disposed of by organisms. Iron compounds in an insoluble form are secreted by iron bacteria that have used soluble iron compounds. Various resins, fats, waxes, and complex organic chemicals are exuded from certain plants—as in the latex from rubber trees and milkweeds. Organic pigments from the breakdown of biological pigments, such as hemoglobin in vertebrates, become components of solid waste. Inorganic salts, including molecules and ions such as carbonates, bicarbonates, and phosphates resulting from life-sustaining chemical reactions, eventually may become solid waste products.
Methods of waste disposal:
Disposal of metabolic and nonmetabolic wastes involves both active and passive mechanisms. In general, gaseous wastes are eliminated through passive mechanisms without the direct expenditure of energy on the part of the living system. The solid and liquid waste-disposal mechanisms used by higher animals are active (energy consuming) systems that separate waste materials from vital substances prior to excretion. Methods of disposal may be classified into specific and nonspecific systems.
Specific elimination mechanisms
Three pathways exist in this context: (1) the alimentary canal, (2) the respiratory system, and (3) the kidneys.
Alimentary canal:
The alimentary canal is a pathway used almost exclusively for the elimination of solid wastes of an indigestible nature, and the act of elimination by this means is termed egestion. Materials disposed of in this manner have not entered the tissues of the animal but rather are the residues of enzymatic and absorptive activities occurring in the digestive tract. True metabolic wastes are excreted by means of the flow of bile from the liver into the intestine. The destruction of cells in animals produces bile pigments—residues of hemoglobin and other pigments—which may be considered to be the principal metabolic wastes eliminated via the alimentary canal. Waste disposal in this manner requires little energy expenditure other than that employed in the peristaltic contractions of muscle in the walls of the tract that act to push material along the length of the tube (see digestive system, human).