In any living system, there is a constant influx of both energy and matter. As we take in food material, we also take in energy in the form of the potential energy stored in chemical bonds of the food molecules. For life on earth, the ultimate source of energy is the radiant energy of the sun. Green plants, and other photosynthetic organisms, including some bacteria, are able to absorb sunlight energy and convert it, through the photolysis of water and the fixation of carbon from carbon dioxide, into the usable chemical energy of carbohydrate molecules. These carbohydrates are incorporated into the body mass of plants or used to store energy for later use by the plant.
Animals receive energy to meet their metabolic needs by directly or indirectly consuming plants.
Energy flow through living systems follows a 'one-way street'. At each transfer of energy, some energy is given off as heat. Metabolism is the overall term describing the controlled chemical reactions occurring within the body. Catabolic reactions are energy-releasing reactions that generally break large molecules down into smaller ones, allowing the potential chemical energy held in the chemical bonds to be released. Anabolic reactions are energy-storing reactions that join small molecules together to produce larger molecules.