Role of Water in Photosynthesis
Photosynthesis is the series of reactions plants use to manufacture sugars from atmospheric carbon dioxide. There are two distinct phases to photosynthesis: the light reactions and the dark reactions. Water plays an important role in the light reactions.
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Process
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When a photon of light strikes a molecule of a pigment called chlorophyll and is absorbed by an electron, the electron gains energy and jumps to a higher orbital for an instant before falling back to its original orbital, transferring energy to another molecule of chlorophyll in the process. The light energy is relayed through a series of chlorophylls in this fashion until it reaches a special pair of chlorophylls called P680. The excited electron in P680 is transferred to another molecule called the primary acceptor, leaving P680 with a positive charge.
Function
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At this point, water takes center stage. An enzyme (a protein that catalyzes reactions) that can split water transfers electrons to P680 by taking them from a water molecule. By breaking up two molecules of H2O, this enzyme releases oxygen gas (O2) and four hydrogen ions. All the oxygen that you breathe was formed through this process.
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Significance
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Essentially, water's role in photosynthesis is as an electron donor to replace the electron lost by P680. The high-energy electron, meanwhile, is transferred to an electron transport chain, where it will help drive a series of events that power the synthesis of a molecule called ATP. Through this process, the plant cell will take light and store it in the form of chemical energy.
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References
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- Photo Credit spring leaf image by John Sfondilias from Fotolia.com
