Photosynthesis, computer artwork. Photosynthesis is the process by which most plants (as well as algae and some bacteria) convert sunlight into chemical energy. This energy is used to produce carbohydrate, such as starch, which is stored and used for growth. The process occurs in the green parts of the plant (especially the leaves). It requires water (absorbed by the roots) and carbon dioxide (absorbed by the leaves). A by-product is oxygen, which is released through the leaves.