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8.5Outline of Photosynthesis

Photosynthesis consists of reactions in which light energy is absorbed and converted to chemical energy to produce ATP and reducing power (light reactions), and reactions in which carbon dioxide is fixed as organic compounds using the ATP and reducing power (dark reactions). Photosynthesis proceeds in chloroplasts (organelles unique to plant cells), light reactions occur locally in the thylakoid membrane, and dark reactions occur locally in the stroma (an aqueous space in chloroplasts). In light reactions, light energy is absorbed and transported by antenna pigment*7, and electrons are released in the photochemical reaction center*8, thus driving electron transport. The electron transport reaction of photosynthesis is different from the respiration chain in that NADPH as well as ATP is synthesized through H+ transport. The reducing power necessary in the series of reactions is obtained from the oxidation of water molecules, and oxygen that is no longer needed is disposed of. As an example, the atmosphere did not contain any oxygen when the earth was created; all the oxygen that currently exists in the air is derived from photosynthesis. In dark reactions, the carbon dioxide fixation reaction and the Calvin cycle (or saccharometabolic cycle) are driven by ATP and NADPH. A carbon dioxide fixation similar to that in dark reactions of photosynthesis also takes place in chemoautotrophic bacteria.

*7
Antenna pigment: A pigment that, following the absorption of light energy, does not perform photosynthesis and conveys energy to the reaction center.

*8
Photosynthetic reaction center: The core site containing the chlorophyll pigment that performs photosynthetic reactions on being excited by light energy.

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