Organic molecules such as Fats, Proteins, and Carbohydrates all contain CARBON, OXYGEN, and HYDROGEN. Proteins also contain atoms of nitrogen. These molecules can burn as they contain high energy C-C or C-H bonds.
Cells also contain molecules of minerals and water. These molecules have atoms of nitrogen, oxygen, hydrogen, and phosphorus. As these molecules do not contain high energy C-C or C-H bonds they cannot be used for energy by the cell.