The Nitrogen Cycle
The nitrogen cycle is one of the most difficult of the cycles to learn, simply because there are so many important forms of nitrogen, and because organisms are responsible for each of the interconversions. Nitrogen is critically important in forming the amino portions of the amino acids which in turn form the proteins of our body. Proteins make up skin and muscle, among other important structural portions of our body, and all enzymes are proteins. Since enzymes carry out almost all of the chemical reactions in our body, it's easy to see how important nitrogen is.
The chief reservoir of nitrogen is the atmosphere, which is about 78% nitrogen. Nitrogen gas in the atmosphere is composed of two nitrogen atoms bound to each other. It is a pretty non-reactive gas; it takes a lot of energy to get nitrogen gas to break up and combine with other things, such as carbon or oxygen.
Nitrogen gas can be taken from the atmosphere in two basic ways First, lightning provides enough energy to 'burn' the nitrogen and fix it in the form of nitrate, which is a nitrogen with three oxygens attached. This process is duplicated in fertilizer factories to produce nitrogen fertilizers. The second form of nitrogen fixation is by nitrogen fixing bacteria, who use special enzymes instead of the extreme amount of energy found in lighting to fix nitrogen. These nitrogen - fixing bacteria fix nitrogen either in the form of nitrate or in the form of ammonia.
Most plants can take up nitrate and convert it to amino acids. Animals acquire all of their amino acids when they eat plants or other animals. When plants or animals die or release waste the nitrogen is returned to the soil. The usual form of nitrogen returned to the soil in animal wastes or in the output of the decomposers, is ammonia. Ammonia is rather toxic, but, fortunately there are nitrite bacteria in the soil and in the water which take up ammonia and convert it to nitrite.
Nitrite is also somewhat toxic, but another type of bacteria, nitrate bacteria, take nitrite and convert it to nitrate, which can be taken up by plants to continue the cycle. There are denitrifying bacteria which take the nitrate and combine the nitrogen back into nitrogen gas.
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