Photosynthesis is a process that allows plants to harvest energy in sunlight and store it chemically, by producing sugars. Sugars hold energy in their molecular bonds; when the plant breaks down these bonds, energy is released that the organism can then use. Although green plants are the most famous photosynthesizing organisms, some bacteria, algae and protists carry out the process as well.
In plants, photosynthesis takes place in small organelles found in some of the plant’s cells, called chloroplasts. Chloroplasts are most numerous in the cells that make up the plant’s leaves, although the stems and sepals may also contain small numbers of the organelles. Chloroplasts combine carbon dioxide and water with sunlight to produce the storage sugars. In the process, the plants produce oxygen and release a small amount of the water they absorbed.
Photosynthesis enables most aquatic and terrestrial ecosystems to persist. All of the planet’s ecosystems require an influx of energy. While a few species of bacteria may harvest the energy of chemical bonds and some deep-sea organisms obtain energy from thermal gradients found around geothermal vents, green plants capture sunlight and use it as an energy source. Through the synthesis of sugars, plants become food for the animals. Accordingly, scientists often refer to plants as producers, while animals are consumers.