Describe What a Photosystem Does for Photosynthesis
Photosystems utilize light to energize an electron, which is then used in an electron transport chain to create high-energy molecules for use in the dark reactions of photosynthesis. Such reactions are known as photophosphorylation and constitute the light reaction stage of photosynthesis.
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Photosystem Structure
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Photosystems are complex arrangements of chlorophyll a with other pigments, including chlorophyll b, xanthophylls and carotenoids, which capture light energy to energize an electron removed from a water molecule. In plants, photosystems are located in the thykaloid membrane within the chloroplast. Two types of photosystems have been identified as photosystem I and photosystem II.
Photosystem I
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P680 is the form of chlorophyll a used in photosystem I, and the electron is transported from the pigments to a ferredoxin protein. Plants have photosystem I in addition to photosystem II.
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Photosystem II
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P700 is the form of chlorophyll a used in photosystem II and the electron is transported to a plastoquinone molecule. Many photosynthetic bacteria have only photosystem II. Cyanobacteria are a notable exception having both types of photosystems.
Cyclic Photophosphorylation
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In cyclic photophosphorylation, the energized electron released by the photosystem and used in the electron transport chain is returned to photosystem I. This process produces ATP.
Noncyclic Photophosphorylation
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In noncyclic photophosphorylation, the electron passes from photosystem II through a series of reactions to photosystem I, which re-energizes the electron using light for another series of reactions. The electron is not returned to the photosystems, and NADPH is created.
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