Final answer:
Total nitrogen in the diet is crucial for the synthesis of amino acids and nucleic acids because nitrogen is a major component of these macromolecules. Lipids, although important for energy and vitamin absorption, do not contain nitrogen and are therefore not synthesized from dietary nitrogen.
Step-by-step explanation:
The question concerns which components in the diet must provide enough nitrogen for synthesis at the cellular level. Nitrogen is a critical component of both amino acids and nucleic acids, but not for lipids. Protein catabolism is a process by which dietary proteins are broken down into amino acids, which are then used for various cellular functions including the synthesis of new proteins as well as nucleotides and nucleic acids. Since the body is unable to synthesise essential amino acids, eating is the only way to get them. On the other hand, lipids are not constructed from nitrogen-containing components; rather, they are primarily composed of carbon, hydrogen, and oxygen.