Q. According to their functions how can nutrients are classified?

One possible and utile functional classification for nutrients is the one that separates them into energetic, regulatory and structural.

Energetic nutrients are those used as energy source for the metabolism mainly they are the carbohydrates but proteins and fats can also be converted into acetyl-CoA and "cycle" the Krebs cycle. Structural nutrients are those used in the support and structure of tissues and cells they are the amino acids that form structural proteins, like the membrane proteins, collagen, the cytoskeleton proteins, the contractile proteins of the muscle tissue, and so on. Regulatory nutrients are those that constitute enzymes and coenzymes of the homeostasis, metabolites of the electrolytic and osmotic equilibrium of cells and hormones some amino acids, vitamins and mineral salts are part of this group.