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Photosynthesis: how plants make food

Photosynthesis is the biochemical process by which plants, algae, and certain bacteria utilize the energy of sunlight in order to synthesize carbohydrates from carbon dioxide and water, releasing oxygen as a byproduct. The process occurs within specialized organelles called chloroplasts, which contain the green pigment chlorophyll, which is responsible for absorbing energy from blue and red light waves while reflecting green light waves, making the plant appear green.

Although there are numerous steps behind the process, it can be broken down into two major stages: the light-dependent reactions and the light-independent reactions. The light-dependent reaction takes place within the thylakoid membrane and requires a steady stream of sunlight, hence the name light-dependent reaction. The light-independent stage, which is also known as the Calvin cycle, takes place in the stroma and does not require light directly.

In 1779, the Dutch physician Jan Ingenhousz demonstrated that plants release oxygen only when they are exposed to light, building on the earlier experiments of Joseph Priestley in 1771. Today scientists estimate that approximately 21% of the atmosphere consists of oxygen, and that nearly all of it originated from photosynthesis. A single large tree is capable of producing approximately 2,400 kilograms of oxygen in a year, which is sufficient for two people.

Photosynthesis requires three essential ingredients: sunlight, water, and carbon dioxide. Without photosynthesis, animals and humans would possess neither food nor oxygen, and consequently the planet would be uninhabitable.