= Solution
\b[<Plants> generally incorporate combined <nitrogen> into organic compounds, whereas <animals> eliminate the surplus <nitrogen> of metabolized compounds chiefly as <Ammonia>, <urea> or <uric acid>.] The two processes connect <nitrogen> chemistry to the organism's <carbon> supply, <water> economy and environment.
Roots take up <nitrate> and <ammonium> from their surroundings. <Nitrate> must be reduced before its <nitrogen> can be incorporated in the usual amino form: <nitrate reductase> produces <nitrite>, and <nitrite reductase> produces <ammonium>. The balanced reduction, written without implying a single elementary reaction, is
$$
\boxed{\mathrm{NO_3^-}+10\mathrm{H^+}+8e^-\longrightarrow\mathrm{NH_4^+}+3\mathrm{H_2O}}.
$$
Reducing equivalents and <energy> must be supplied by <metabolism>; <nitrate> is not simply deposited unchanged inside every <protein>.
In the modern biochemical account, <glutamine synthetase> incorporates <ammonium> into <glutamine>, using <ATP>; <glutamate synthase> transfers that <nitrogen> to a <carbon> skeleton derived from <2-Oxoglutarate>, producing <glutamate>. Further transfer and synthesis distribute <nitrogen> into other <amino acids>, <proteins>, <nucleotides> and other <cell> constituents. <Carbon> <metabolism> therefore provides both <energy> and skeletons for <nitrogen assimilation>. These named enzymes and reactions explain the process today; they were not a completed molecular scheme in 1864.<Nitrogen uptake, assimilation and remobilization in plants>[Nitrogen Assimilation: nitrate reduction and the GS/GOGAT cycle]{ref}
Ordinary higher-plant <cells> do not themselves reduce atmospheric <molecular nitrogen> merely because the gas surrounds the leaves. Biological <nitrogen fixation> is performed by particular microorganisms; symbiotic associations can supply fixed <nitrogen> to a host plant. The later elucidation of root-nodule symbiosis should be distinguished from what the examiner could have expected. Organic <nitrogen> can also be taken up in suitable circumstances, so <nitrate> and <ammonium> are important routes rather than an exhaustive prohibition on every other source.<Biology 2e by Mary Ann Clark and others/46.3 Biogeochemical Cycles>[The Nitrogen Cycle: fixation, ammonification, nitrification and return to the atmosphere]{ref}
A useful assimilation experiment grows comparable seedlings in a defined medium containing all required <minerals> except combined <nitrogen>, with parallel <nitrate>- or <ammonium>-supplied cultures. Initial seed reserves, contaminating <nitrogen>, <water> and microbial growth must be allowed for. Growth and final <nitrogen> content are more informative than green appearance alone. A plant initially growing in a <nitrogen>-free solution may simply be consuming seed reserves; that observation does not demonstrate fixation of atmospheric <nitrogen>.
Animals obtain most of their <nitrogen> from organic <food>. <Amino acid metabolism> separates <nitrogen> handling from oxidation or reuse of the <carbon> skeleton. Free <Ammonia> is toxic at high concentrations, and mammals convert much of their waste <nitrogen> into <urea> in the <liver>; the <kidneys> then eliminate it in <urine>. <Uric acid> is also produced from <purine> breakdown. Renal <ammonium> excretion has a separate important connection with acid-base balance. \b[<Urea> formation and urinary excretion are different processes in different organs.]<Biology 2e by Mary Ann Clark and others/41.4 Nitrogenous Wastes>[Ammonia; Urea; Uric Acid: toxicity, water requirements and comparative routes]{ref}
Many aquatic animals can release <Ammonia> directly into abundant surrounding <water>. Birds, many reptiles and many insects eliminate much <nitrogen> as relatively insoluble <uric acid>, conserving <water>; mammals generally depend more on soluble <urea>. These are broad physiological strategies with exceptions, not a rule that all animals have human <urine>. Faecal <nitrogen> includes unabsorbed <food> as well as endogenous material, while smaller losses can occur through <skin> and secretions. For a stated interval the accounting identity is
$$
\boxed{N_{\rm intake}-N_{\rm total\ loss}=\Delta N_{\rm body}}.
$$
Positive retention during growth and negative balance during tissue loss need not be errors of excretion. <Nitrogen> is redistributed or lost in compounds; it is not simply breathed out as all the atmospheric <nitrogen> passing through the <lungs>.
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