In which forms can nitrogen be found in the cell?

Nitrogen is a crucial element in biological systems, particularly for the synthesis of amino acids, nucleotides, and various other biomolecules. In the context of cellular processes, nitrogen can be found in various forms, but among the choices provided, ammonia and nitrate are two important and biologically relevant forms of nitrogen.

Ammonia (NH₃) is a product of the decomposition of amino acids and is a key nitrogen source for many organisms. It can be directly used by certain microorganisms and plants in the synthesis of their own organic nitrogen compounds. Nitrate (NO₃⁻), on the other hand, is one of the more oxidized forms of nitrogen commonly found in the environment, particularly in soil. It is utilized by plants and some bacteria during the process of nitrogen assimilation.

The other options present forms of nitrogen or related compounds that are either incorrect or do not contain nitrogen in a usable form for cells. For instance, while urea does contain nitrogen, it is not as readily usable by all cells as ammonia or nitrate, which serve direct roles in nitrogen metabolism. Therefore, the presence of both ammonia and nitrate highlights the significance of these forms in a biochemical context, particularly for organisms engaged in nitrogen metabolism and assimilation.