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Chapter Four: Cell Nutrition and Respiration
Photosynthesis is a process which autotrophic organisms use to create their own food so that they do not have to obtain it from the environment. The root "photo" means light (photosynthesis requires energy from sunlight to occur), and "synthesis" means creation. A summary of the photosynthetic reaction is
CO2 + H2O + light CH2O + O2
In this reaction, the photosynthetic organism takes in three things from the environment: carbon dioxide, water, and light energy. Through a complex series of reactions, carbohydrates (the CH2O) and oxygen are formed. The cell uses the carbohydrates as food, and the oxygen is released into the environment; in fact, it often ends up in aerobic organisms which require oxygen to produce ATP. These aerobes release carbon dioxide as a result of their respiration, and the cycle between heterotrophs and autotrophs can continue.
Photosynthesis consists of two main stages. The first, called noncyclic electron flow, and the second, the Calvin cycle, are discussed in the next section.