Final answer:
Most organisms cannot use the nitrogen in the atmosphere directly due to the unreactive nature of the nitrogen-nitrogen triple bond in N₂ gas. It must be converted into ammonia through a process called nitrogen fixation, carried out by certain microorganisms. Plants and animals then utilize this fixed nitrogen for growth and maintenance.
Step-by-step explanation:
In order for most organisms to use the nitrogen in the atmosphere, dinitrogen gas must be converted into ammonia. Organisms cannot directly absorb atmospheric nitrogen (N₂) because it is unreactive due to its strong nitrogen-nitrogen triple bond. The process that facilitates the conversion of atmospheric nitrogen into a form usable by living organisms is known as nitrogen fixation. Nitrogen fixation is primarily performed by certain microorganisms, including bacteria and archaea. These microorganisms often form symbiotic relationships with plants, such as the root nodules found in legumes where bacteria fix nitrogen into ammonia, which is then utilized by the plant for growth.
This ammonia serves as the principal starting material for synthesizing essential macromolecules such as proteins and nucleic acids through the nitrogen cycle. Prokaryotes are one group of organisms that cannot use gaseous nitrogen to synthesize macromolecules directly and depend on this conversion process. The fixed nitrogen is subsequently absorbed by plants in the form of nitrate ions (NO₃⁻), ammonium ions (NH₄⁺), and urea ((NH₂)₂CO), which then enters the food chain as animals consume these plants.