Final answer:
The liver is the most metabolically active organ in the human body, significantly involved in metabolism and energy consumption, with the brain coming in next regarding energy use.
Step-by-step explanation:
The most metabolically active organ in the human body is the liver. This large internal organ plays a major role in metabolism and has numerous functions including gluconeogenesis, glycogenolysis, glycogenesis, and the detoxification of blood. It is involved in carbohydrate, fat, and protein metabolism, processes vitamins and fats, synthesizes plasma proteins, and produces bile necessary for the digestion of fats.
The liver also breaks down insulin and other hormones. In terms of energy consumption, the liver, along with the spleen, utilizes the largest fraction of energy in the basal metabolic rate, with the brain being the next significant consumer of energy. The adult brain is remarkably complex and metabolically active as well, using about a quarter of the body's total energy.