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Why can most plants and animals not use nitrogen gas from the atmosphere directly?

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Difference in molecular structure between ammonia/amines and quaternary ammonium ions — ammonia Ammonia and amines have a slightly flattened trigonal pyramidal shape with a lone pair of electrons above the nitrogen. In quaternary ammonium ions, this area is occupied by a fourth substituent.

Most plants and animals cannot directly utilize atmospheric nitrogen gas ( $N_{2}$ ) because of its high chemical stability. The nitrogen molecule is held together by an exceptionally strong triple bond, which requires a significant amount of energy to break. Because of this high bond energy, nitrogen is relatively inert and does not readily react with other elements under ordinary biological conditions.1

Furthermore, higher life-forms lack the specific enzymes necessary to convert this stable, gaseous nitrogen into reactive, usable forms. To become biologically available, nitrogen must undergo nitrogen fixation, a process where the gas is chemically combined with other elements to form reactive compounds such as ammonia, nitrates, or nitrites. This essential task is performed primarily by specialized bacteria and archaea, which transform the atmospheric nitrogen into forms that plants can absorb. Animals subsequently obtain the nitrogen they need by consuming these plants or other organisms.234