why do organisms need nitrogen to survive

Nitrogen is a building-block element both in the atmosphere, where it is the most abundant gas, and in organisms. Its flow through earthвs atmospheric, geological and biological systemsвthe nitrogen cycleвis one of ecologyвs grand choreographies. Nitrogen, fundamental for cellular structure, is required by plants and animals for production of proteins and amino acids. One of the components of chlorophyll, the plant pigment that facilitates photosynthesis, is nitrogen. It plays a role in this immensely important transformation of solar energy. Although 78 percent of our atmosphere is comprised of nitrogen gas, usable nitrogen is a limited commodity. Most organisms can tap the element for growth and function only when, through a process called nitrogen fixation, it has been converted into ammonia or nitrates.

Fixation accomplished by bacteria in soilвoften in symbiotic relationship with fungi and plantsвprovides the bulk of nitrogen available to the biotic community. This gas passes through the atmosphere, rocks, lightning, plants and animals, facilitating growth and being liberated by organic waste and decay in a fundamental biogeochemical cycle.
Life depends on nitrogen, which is a basic ingredient in amino acids that make up all proteins. While a substantial percentage of the atmosphere is comprised of nitrogen gas, it must be processed into a soluble form.

This is done via a nitrogen cycle that occurs in the soil. Then plants and the animals that eat them can obtain dietary nitrogen. Seventy-eight percent of the Earth's atmosphere is nitrogen. This element, symbolized by the letter N, was discovered in 1772 by the Scot Daniel Rutherford. Nitrogen makes up 7 percent of the protein found in cereals. It causes green plants to produce healthy leaves that are strong enough to withstand a heavy wind or a frost. Plants with nitrogen deficiencies look weak. The leaves, which should be healthy and green, may look wilted and yellow. Animals and people get dietary nitrogen by eating protein-rich foods like milk, eggs, fish, beef and legumes.

Nitrogen, carbon, hydrogen and oxygen are found in amino acids, which are the main structures of every protein. Though 78 percent of the atmosphere is made of inert nitrogen, it must be transformed into a form that can be used by plants and animals. This happens via a nitrogen cycle that occurs in the soil. Earthworms, bacteria and other forces break down the proteins in organic material and animal manure in the soil to decompose them. The first byproduct is ammonia, a compound of nitrogen and hydrogen. Next, nitrates are formed. Nitrobacteria called azotobactors produce soluble nitrates that plants can get out of the soil.

Nitrogen can also enter the soil directly from the atmosphere via the rhizobium bacteria in the roots of legumes, or during a heavy rain, when it mixes with water to become nitric acid. Chemical fertilizers are another way to put nitrogen into the soil, but these can cause pollution or even nitrate poisoning in cattle. Organic sources of nitrogen fertilizer are rabbit droppings, cottonseed meal and feather meal. Digesting protein-rich food produces amino-acid nitrogen, which is a waste product the body must eliminate through reactions called transanimations. Most of this excess nitrogen is excreted in the form of urea.

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