Nitrogen is an essential component of proteins, nucleic acids, ATP and many other organic molecules. Nitrogen gas (N2) makes up about 80% of atmospheric air. Plants and animals unfortunately cannot use nitrogen in this form. The nitrogen that can be used by plants and animals has to be in a biologically available form. The nitrogen cycle describes how nitrogen is passed from organism to organism, from organism to environment, and from environment to organism. The diagram below shows the main stages occurring in the nitrogen cycle.
This shows the main stages and is clearly a cycle. There are other processes that can also occur transferring nitrogen between organisms and their environment. The diagram below shows all the stages and processes involved in the detail they are required for A-level.