The nitrogen cycle is important biologically because: the nitrogen cycle is not important biologically. the atmosphere is composed of 78% nitrogen; therefore, a. nitrogen is freely available to all living organisms, regardless of where it is in its cycle. b. nitrogen is a major constituent of phospholipids, atp, and other energy-storing molecules. c. nitrogen is a component of amino acids, proteins, and nucleic acids. d. nitrogen forms the framework for organic molecules that are essential to all organisms. e. the processes in the mitochondria require major amounts of nitrogen.