Metabolic wastes, also known as excretes, are substances produced by the metabolic activities of living organisms. These substances cannot be used by the organism and are therefore excreted. These excretes include carbon dioxide, nitrogen compounds, sulfates, phosphates and water.
Animals excrete metabolic wastes because they are not useful, but plants have the ability to transform some of these wastes into useful substances.
Metabolic wastes are excreted through excretory organs such as the kidneys and the malpighian tubules. The skin, which is the largest organ in the human body, also acts as an excretory organ. The skin has sweat glands that excrete perspiration through its pores. Perspiration is composed of salts, water and nitrogenous wastes.
Urea, ammonia and uric acid are the three nitrogenous wastes eliminated from organisms. These substances are produced from protein metabolism and are quite toxic. Plenty of water is required to excrete ammonia due to its high toxicity. Urea is less toxic than ammonia, so less water is required for its excretion. Uric acid is less toxic than urea and ammonia, and only a small amount of water is required for its excretion.
Metabolic wastes, such as carbon dioxide, are formed during the catabolism of carbohydrates and lipids in condensation reactions. Catabolism is a metabolic process in which complex molecules, such as lipids, protein and nucleic acids, are broken down into smaller units with the release of energy. Carbon dioxide is a waste product of animals and plants at night.