Metabolic waste

'Metabolic wastes or excretes are substances left over from metabolic processes (such as cellular respiration), which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, water, CO₂, phosphates, sulfates, etc. Animals treat these compounds as excretes. Plants have chemical "machinery" which transforms some of them (primarily the nitrogen compounds) into useful substances, and it has been shown by Brian J. Ford that abscissa leaves also carry wastes away from the parent plant. In this way, Ford argues that the shed leaf acts as an excretory (an organ carrying away excretory products).

All the metabolic wastes are excreted in a form of water solutes through the excretory organs (nephridia, Malpighian tubules, kidneys), with the exception of CO₂, which is excreted together with the water vapor throughout the lungs. The elimination of these compounds enables the chemical homeostasis of the organism.

The nitrogen compounds through which excess nitrogen is eliminated from organisms are called nitrogenous wastes (/naɪˈtrɒdʒᵻnəs/) or nitrogen wastes. They are ammonia, urea, uric acid, and creatinine. All of these substances are produced from protein metabolism. In many animals, the urine is the main route of excretion for such wastes; in some, the feces is.

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