systems in the form of amino acids, nitrate ions (NO3 -), nitrite ions (NO2 -), or ammonium ions (NH4 +). Most of the nitrogen in the cells and tissues of terrestrial animals can be traced back to the consumption of plants at some level of a food chain.
Plants are already known to use their leaves to absorb inorganic airborne nitrogen molecules, such as ammonia or nitrogen dioxide, and turn them into amino acids. And a relatively reactive compound called peroxyacetyl nitrate can be absorbed by leaves, although it's not clear whether plants actually use it.
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