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Photosynthesis Light Reactions

Light Reactions


Not all wavelengths of light are absorbed during photosynthesis. Green, the color of most plants, is actually the color that is reflected. The light that is absorbed splits water into hydrogen and oxygen:

H2O + light energy ---> ½ O2 + 2H+ + 2 electrons



  1. Excited electrons from Photosystem I can use an electron transport chain to reduce oxidized P700. This sets up a proton gradient, which can generate ATP. The end result of this looping electron flow, called cyclic phosphorylation, is the generation of ATP and P700.


  2. Excited electrons from Photosystem I could flow down a different electron transport chain to produce NADPH, which is used to synthesize carbohydratyes. This is a noncyclic pathway in which P700 is reduced by an exicted electron from Photosystem II.


  3. An excited electron from Photosystem II flows down an electron transport chain from excited P680 to the oxidized form of P700, creating a proton gradient between the stroma and thylakoids that generates ATP. The net result of this reaction is called noncyclic photophosphorylation.


  4. Water contributes the electron that is needed to regenerate the reduced P680. The reduction of each molecule of NADP+ to NADPH uses two electrons and requires four photons. Two molecules of ATP are formed.

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