Nitrogen is fundamental to all known life forms. Despite making up about 78% of Earth’s atmosphere, this nitrogen is largely unusable by most organisms. Living systems need nitrogen to build and maintain their structures and power biological processes. Its journey from the atmosphere into living things and back again highlights its indispensable nature.
Nitrogen’s Role in Core Biological Molecules
Nitrogen serves as a building block for molecules that comprise living organisms. It is a fundamental component of amino acids, the smaller units that link together to form proteins. Proteins perform a wide array of functions within cells, acting as enzymes, providing structural support, and transporting substances. Nitrogen accounts for about 16% of protein weight, underscoring its significant presence.
Beyond proteins, nitrogen is also found in nucleic acids, DNA and RNA. These molecules carry the genetic instructions that dictate the characteristics and functions of all living things. Nitrogenous bases, such as adenine, guanine, cytosine, and thymine (in DNA) or uracil (in RNA), are nitrogen-containing structures that form the core of these nucleic acids. Their specific arrangement encodes genetic information, making nitrogen an integral part of heredity and protein synthesis.
Nitrogen’s Role in Energy and Photosynthesis
Nitrogen participates in cellular energy transfer and photosynthesis in plants. Adenosine triphosphate (ATP), often termed the “energy currency” of cells, contains nitrogen as part of its adenine component. The bonds within ATP’s triphosphate portion store and release energy for nearly all cellular activities, from muscle contraction to molecule transport.
In plants, nitrogen is a constituent of chlorophyll, the green pigment responsible for capturing light energy during photosynthesis. The chlorophyll molecule features a central magnesium atom surrounded by four nitrogen atoms within a porphyrin ring. Without adequate nitrogen, plants cannot produce sufficient chlorophyll, which directly affects their ability to convert sunlight into chemical energy and grow.
How Organisms Obtain Nitrogen
Despite its abundance as dinitrogen gas (N₂) in the atmosphere, most organisms cannot directly utilize this form due to its strong triple bond. Nitrogen must first be converted into more reactive forms through nitrogen fixation. Specialized microorganisms, primarily bacteria found in soil or in symbiotic relationships with plants (such as Rhizobium in legume root nodules), perform most of this conversion, transforming atmospheric N₂ into ammonia. Lightning also contributes a small amount of fixed nitrogen to the soil.
Once fixed, nitrogen becomes available for plants to absorb from the soil, mainly as nitrates and ammonium ions. Plants then incorporate these nitrogen compounds into their organic molecules, like proteins and nucleic acids. Animals acquire nitrogen by consuming these plants or other animals that have consumed plants. This transfer of nitrogen through the food web ensures its movement through ecosystems.
Nitrogen is continually cycled back into the environment through decomposition and microbial action. When plants and animals die or excrete waste, decomposer organisms like bacteria and fungi break down their nitrogen-containing compounds, releasing ammonia back into the soil in ammonification. Other bacteria then convert this ammonia into nitrites and subsequently into nitrates through nitrification, making it available for plant uptake. Finally, some bacteria perform denitrification, converting nitrates back into gaseous N₂, which returns to the atmosphere, completing the cycle.