ABSTRACT
Photosynthesis occurs in green plants, algae and photosynthetic bacteria. Its role is to trap solar energy and use this to drive the synthesis of carbohydrate from carbon dioxide and water. The reactions of photosynthesis occur in two distinct phases: the light reactions and the dark reactions. Green plants and algae use two types of photosystem called photosystem I (PSI) and photosystem II (PSII). The chlorophyll in the reaction center of PSI has an absorption maximum at 700 nm and so is called P700 and that in the reaction center of PSII has an absorption maximum at 680 nm and so is called P680. Chlorophyll is a porphyrin in which nitrogen atoms are coordinated to a magnesium ion. Cyanobacteria carry out photosynthesis using two photosystems as in green plants. However, other photosynthetic bacteria, such as the purple photosynthetic bacterium Rhodospirillum rubrum, have only a single photosystem reaction center.
