Photosynthesis also is a type of chemical reaction. Plants use the energy of sunlight to synthesize glucose from carbon dioxide and water. They then use this glucose to produce such materials as starch and cellulose. In short, photosynthesis stores the energy of sunlight in the form of glucose. All animals, including humans, live by eating these plants, thereby absorbing the oxygen produced during photosynthesis and indirectly taking in the energy of sunlight. In other words, sunlight is a source of life on earth.

We do not yet know the detail of how plants conduct photosynthesis, but in green plants, chloroplasts within cells are known to play a crucial role.