Light Reactions:  Photosystem I & II



  1. When photosystem II absorbs light, an electron excited to a higher energy level in the reaction center chlorophyll (P680) is captured by the primary electron acceptor.  The oxidized chlorophyll is now a very strong oxidizing agent; its electron “hole” must be filled.
  2. An enzyme extracts electrons from water and supplies them to P680, replacing the electrons that the chlorophyll molecule lost when it absorbed light energy.  This reaction splits a water molecule into two hydrogen ions and an oxygen atom, which immediately combines with another oxygen atom to form O2.  This splitting of water is responsible for the release of O2 into the air.
  3. Each photoexcited electron (energized by light) passes from the primary electron acceptor in photosystem II to photosystem I via an electron transport chain.  This electron transport chain is very similar to the one in cellular respiration; however, the carrier proteins in the chloroplast ETC are different from those in the mitochondrial ETC.
  4. As electron move down the chain, their exergonic “fall”to a lower energy level is harnessed by the thylakoid membrane to produce ATP (by chemiosmosis).  The production of ATP in the chloroplast is called photophosphorylation because the energy harnessed in the process originally came from light.  This process of ATP production is called non-cyclic photophosphorylation.  The ATP generated in this process will provide the energy for the synthesis of glucose during the Calvin cycle (light independent reactions).
  5. When an electron reaches the “bottom” of the electron transport chain, it fills an electron “hole” in the chlorophyll a molecule in the reaction center of photosystem I (P700).  The hole was created when light energy drives an electron from P700 to the primary electron acceptor of photosystem I.
  6. The primary electron acceptor of photosystem I passes the excited electrons to a second electron transport chain which transmits them to an iron-containing protein.  An enzyme reaction transfers the electrons from the protein to NADP+ that forms NADPH (which has high chemical energy due to the energy of the electrons).  NADPH is the reducing agent needed for the synthesis of glucose in the Calvin cycle.



Cyclic vs. Non-cyclic Electron Flow


Under certain conditions, the photoexcited electrons take an alternative path called cyclic electron flow, which uses photosystem I (P700) but not photosystem II (P680).  This process produces no NADPH and no O2, but it does make ATP. This is called cyclic photophosphorylation.  The chloroplast shifts to this process when the ATP supply drops and the level of NADPH rises.  Often the amount of ATP needed to drive the Calvin cycle exceeds what is produced in non-cyclic photophosphorylation.   Without sufficient ATP, the Calvin cycle will slow or even stop.  The chloroplast will continue cyclic photophosphorylation until the ATP supply has been replenished.  ATP is produced through chemiosmosis in both cyclic and non-cyclic photophosphorylation.