Photosynthesis is a process that allows green plants or producers to manufacture their own food. Carbon dioxide, water and sunlight must be present to initiate this reaction. Carbon dioxide enters via stomates in the leaves, water is absorbed by roots and transported to leaves in vascular bundles and light is absorbed by chlorophyll in the leaves. Carbon fixation occurs and by combining with water, glucose is an endproduct that is produced. This glucose is a form of chemical energy that the plant can use when needed. It also forms oxygen gas and water vapor which both exit the leaves via the stomates. When a plant needs energy for growth and other life functions, the glucose, which the producer manufactured can be used. The glucose is combined with oxygen along with enzymes to catalyze the reaction, to form carbon dioxide and water vapor as waste products, as well as 36 A.T.P. molecules. These high energy molecules are then available for use by the cells. Plants are basically self-sufficient because they can re-use the endproducts of both photosynthesis and respiration to continue their life cycle. A terrarium, which is an enclosed system containing plants, soil and moisture, is self-sustaining, as, none of the gases or water vapor from photosyntheis or respiration can exit the system and the plants inside get all the materials they need to live, as long as they are near a source of light.