Nutrition is the biochemical and physiological process by which an organism uses food to support its life. It includes ingestion, absorption, assimilation, biosynthesis, catabolism and excretion. The science that studies the physiological process of nutrition is called nutritional science. Organisms primarily provide themselves with carbon in one of two ways: autotrophy and heterotrophy. Combined with the source of energy, either light or chemical , there are four primary nutritional groups for organisms.