Final answer:
Adequate availability of nitrogen is called nitrogen balance, achieved when dietary nitrogen intake is equal to the loss through excretion. Nitrogen balance is vital for health and is managed through the urea cycle and the nitrogen cycle in ecosystems.
Step-by-step explanation:
Adequate availability of nitrogen is expressed as nitrogen balance. This occurs when dietary intake matches its loss in urine and feces. In biological terms, the nitrogen balance indicates a state where the intake of protein nitrogen, or amino acids (AA), exactly balances the loss of nitrogen through excretion. Positive nitrogen balance is when the intake of nitrogen is greater than the excretion, typically due to retention as tissue protein, whereas in negative nitrogen balance, the intake is less than the amount lost, which can happen during periods of illness or in old age.
Of the four major macromolecules in biological systems, proteins and nucleic acids are nitrogen-containing macromolecules. When these are catabolized, nitrogen wastes are produced and must be excreted from the body in a non-toxic form such as urea or uric acid. Most animals convert toxic ammonia into urea via the urea cycle for excretion.
The nitrogen cycle is a biogeochemical process through which nitrogen is recycled in ecosystems. It includes nitrogen fixation by bacteria, which converts atmospheric nitrogen (N₂) into bioavailable forms for plants. Nitrogenous wastes from animals are then processed back into gaseous nitrogen by soil bacteria.