Nitrogen stands as an important element for all life forms on Earth. It serves as a building block, participating in the construction of crucial molecules within living organisms. Despite its widespread presence, particularly in its atmospheric form, nitrogen is not directly usable by most life. This presents a challenge for biological systems, which must acquire this element through specific processes.
The Abundant Source: Atmospheric Nitrogen
The Earth’s atmosphere represents the largest reservoir of nitrogen, comprising about 78% of the air. It exists primarily as dinitrogen gas (N₂). The N₂ molecule has a strong triple covalent bond. This bond makes atmospheric nitrogen inert and unreactive, inaccessible to most organisms.
Nature’s Transformation: Nitrogen Fixation and Assimilation
Converting inert atmospheric nitrogen into forms usable by living organisms involves several natural and industrial processes. The most significant natural pathway is biological nitrogen fixation, performed by specialized microorganisms. These include bacteria like Rhizobium, which form symbiotic relationships in the root nodules of leguminous plants (e.g., peas and beans), converting N₂ into ammonia (NH₃). Other free-living bacteria in soil and aquatic environments also contribute to this conversion.
Atmospheric nitrogen fixation, a less significant natural process, occurs when high-energy events like lightning break the N₂ triple bond. This forms nitrogen oxides, which dissolve in rainwater and enter the soil as nitrates. Additionally, industrial nitrogen fixation, like the Haber-Bosch process, synthesizes ammonia. This energy-intensive method uses atmospheric nitrogen and hydrogen to increase nitrogen availability for agricultural fertilizers and global food production.
Ammonia, whether from biological or industrial sources, undergoes further transformations. Nitrifying bacteria in soil convert ammonia into nitrites and nitrates. Plants absorb these usable forms (nitrates and ammonium) from the soil through their roots. This process, assimilation, integrates nitrogen into organic molecules within the plant, making it available for consumption.
Our Dietary Supply: Nitrogen in Food
Humans acquire nitrogen by consuming plants or animals that have assimilated this element. Proteins are the primary dietary source of nitrogen. These molecules are constructed from amino acids, each containing nitrogen.
Foods rich in protein provide nitrogen. Animal sources include meats (beef, chicken, fish), dairy (milk, cheese), and eggs. Plant-based sources include legumes (lentils, beans), nuts, seeds, and grains. The human body cannot synthesize all amino acids; some must be obtained from dietary protein.
Nitrogen’s Vital Role in the Human Body
Once assimilated, nitrogen plays a significant role in biological functions. It is a component of amino acids, the building blocks of proteins. Proteins perform diverse functions, including building and repairing tissues, acting as enzymes for biochemical reactions, forming hormones that regulate processes, and serving as antibodies in the immune system.
Nitrogen is integral to nucleic acids (DNA and RNA). These molecules carry genetic information and direct protein synthesis. Nitrogen is also found in adenosine triphosphate (ATP), the primary energy currency for cells. Beyond these roles, nitrogen is a component of other compounds, including neurotransmitters and vitamins.