During photosynthesis, solar energy in the form of light is absorbed. This light drives charge separation in the reaction centres of photosystems I (PSI) and II (PSII), initiating electron transport which then leads to the oxidation of water and the subsequent reduction of NADP+ to NADPH in the stroma (Foyer et al., 2012). Additionally, a proton motive force is generated across the thylakoid membrane, leading to the production of ATP. Both ATP and NADPH are then in turn required for the process of carbon dioxide assimilation.
PSI and PSII are the charge separation devices that make up the photosynthetic machinery. They are connected in series by an electron transport chain and they differ in the organization of light harvesting systems and pigment compositions. The two pigments found in the photosystems of green algae are chlorophylls and carotenoids (Green and Durnford, 1996). Chlorophyll is the principal pigment that functions to trap light energy and it is present in two forms; chlorophyll a (Chl a) and chlorophyll b (Chl b), and they can be distinguished based on the...
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...nnae are associated with PSI, and the electron transfer runs in a linear fashion resulting in the reduction of NADP+ to NADPH (Delosme et al., 1996). If light intensity changes to induce preferential excitation of PSII, the plastoquinone pool is reduced by electrons from PSII. Plastoquinol binds cytochrome b6f, an electron carrier that connects PSII to PSI, leading to the activation of the LHCII kinase STT7. STT7 proceeds to phosphorylate LHCII antenna proteins causing displacement from PSII to PSI (Wollman, 2001). At this point the photosystems are in state II. Moving antenna from PSII to PSI reduces the excitation of PSII and allows balance to be reestablished between the photosystems. Essentially, excess excitation energy in PSII is quenched by PSI. This process can actually be reversed by phosphatase activity, allowing a return to state I (Pribil et al., 2010).
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