Final answer:
Lipids (fats) have the highest energy storage per gram compared to carbohydrates such as free glucose and glycogen. They provide a more dense energy source which is ideal for long-term energy storage, containing over twice the caloric content per weight of carbohydrates and proteins.
Step-by-step explanation:
The organic molecule with the highest energy storage per gram is lipids (fats). Unlike carbohydrates such as free glucose and glycogen that store energy in a form that can be quickly accessed by the body, lipids pack a much greater amount of energy per gram, making them the most efficient form of energy storage on a per gram basis. Fats are stored in adipose tissue and, when needed, are broken down into fatty acids and glycerol, which can then be used to generate ATP, the molecule that cells use for energy. Lipids can provide more than twice the caloric content per weight compared to carbohydrates and proteins, which is why they are so effective for long-term energy storage.
Carbohydrates like glucose are used for immediate energy, and glycogen serves as short-term energy storage, mainly in the liver and muscles. Glycogen can be quickly mobilized and converted back to glucose when energy is required. In contrast, the metabolism of lipids for energy happens over a longer period, and they yield a high amount of ATP per molecule, making them a denser energy source.