Photosynthesis is the process by which green plants manufacture their own food. The process takes place in small structures within the plant’s cells called chloroplasts. Photosynthesis evolved about 3 billion years ago, yet it remains the most important method for harvesting the sun’s energy on the planet.
Most chloroplasts reside in cells located in a plant’s leaves, but they occasionally occur elsewhere as well. The chloroplasts use a substance called chlorophyll to absorb the energy carried by sunlight. Once this energy is absorbed, the chloroplasts combine it with carbon dioxide and water to produce sugars. The sugars allow the plants to store the energy for future use. Photosynthesis also produces oxygen, which is necessary for all animals to breathe.
Green plants that live in the world’s rivers, lakes and oceans engage in photosynthesis as well, but they must live relatively close to the surface. Many other simple organisms, such as algae and cyanobacteria, engage in photosynthesis. Nevertheless, scientists suspect that the process only evolved one time.
Scientists suspect that chloroplasts were initially independent organisms. Over millions of years, larger organisms engulfed them, and formed a symbiotic relationship with the small organelles. Chloroplasts have their own DNA, which most scientists interpret as evidence for the independent origin of the structures.