Photosynthesis is a biochemical process for building carbohydrates using sunlight and CO2 molecules absorbed from the air. Carbohydrates have more energy stored within their chemical bonds than is contained within the bonds of the CO2 molecules from which they are synthesized during photosynthesis. Thus, to build carbohydrates using CO2 requires an input of energy. In photosynthesis, this energy comes from sunlight. Photosynthesis takes place almost everywhere there is sunlight to serve as a source of energy.
Energy is added to molecules during carbohydrate synthesis through the transfer of high-energy electrons. It is this addition of energy and electrons that allows the incoming CO2 molecules to form the higher energy bonds found in a carbohydrate molecule. Where do the electrons used to reduce CO2 come from? Ultimately these electrons come from the oxidation of other molecules. In the photosynthesis carried out by plants and many algae, the electron donor is water. The oxidation of water results in the production of electrons, protons and O2. Thus, oxygen is formed in photosynthesis as a byproduct of water’s role as a source of electrons. All of the oxygen in Earth’s atmosphere is a result of photosynthesis.
New section content