The primary function of carbohydrates in living organisms is to serve as a major source of energy. Carbohydrates are composed of carbon, hydrogen, and oxygen, and are broken down by the body to produce glucose, which fuels cellular processes. In addition to energy, carbohydrates play structural roles in organisms, such as in plant cell walls (cellulose) and are involved in cell signaling. Their importance in metabolism and nutrition makes them essential for life.