Animals obtain energy for their cells by feeding on photosynthetic organisms such as plants and algae or other organisms. All energy comes from the sun and some of this energy is stored in the organic molecules of food. The process which generates oxygen as well as organic molecules that are used by eukaryotes in their mitochondria for cellular respiration is photosynthesis. The chemical elements that are essential to life are recycled as the waste products of cellular respiration, carbon dioxide and water, are the key materials for photosynthesis, which takes place in the chloroplasts of plant cells.
There are two general types of metabolic pathways, catabolic pathways and anabolic pathways. Cellular respiration is an example of a catabolic pathway