Here's why:

* Proteins: Nitrogen is a key component of amino acids, the building blocks of proteins. Each amino acid contains a nitrogen-containing amine group (-NH2).

* Nucleic acids: Nitrogen is found in the nitrogenous bases that make up DNA and RNA (adenine, guanine, cytosine, thymine, and uracil). These bases are essential for storing and transmitting genetic information.

While nitrogen is also present in other biological molecules, its high concentration in proteins and nucleic acids makes them the most significant classes of compounds containing this element.