Life requires a continuous input of energy. On Earth, the main source of energy is sunlight, which is transformed by photosynthesis into a form of chemical energy that can be used by photosynthetic and nonphotosynthetic organisms alike. Photosynthesis is the molecular process by which plants, algae, and certain bacteria use light energy to build molecules of sugar from carbon dioxide (CO₂) and water (H₂ O). The sugar molecules produced by photosynthetic organisms provide the energy as well as chemical building blocks needed for their growth and reproduction. In plants and algae the photosynthetic process removes CO₂ from the atmosphere while releasing molecular oxygen (O₂) as a by-product. Some photosynthetic bacteria function like plants and algae, giving off O₂; other types of photosynthetic bacteria, however, use light energy to create organic compounds without producing O₂. The type of photosynthesis that releases O₂ emerged...