Light Vs Dark Reactions of Photosynthesis

Before going into the detail of the light dependent reaction, some points have to be made clear about the light dependent phase:  

•  The Light-dependent reaction of photosynthesis takes place in  the thylakoids of the chloroplast.
• Two major processes  take place    

§  ATP and NADPH are formed.

§  Splitting of  the water molecule (photolysis) takes place to give oxygen.

·         The ATP and NADPH formed in the light reaction are utilized to form carbohydrate in the dark reaction.

•    So, what should be the  first step of the light  reaction ?

   The absorption of light (duh!)The light reaction begins with the absorption of light in the thylakoids .

• Now , you may think that why is light absorption possible only in the thylakoids ?
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• It is because the thylakoids have two very hard working friends that make it  possible , each of which is  called  a photosystem (PS680 AND PS700).
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• They are embedded in the  thylakoid membrane and absorb light.
• Close to the PS680 is the oxygen evolving complex, responsible for photolysis.

• The PS680 absorbs light ,gets excited and gives out an electron to Q with the gain of energy (electron is emitted as PS680 moves from lower to higher energy)

• But P680 must return to its initial state so that it absorbs photons again , it must gain the electron lost .This electron is provided by the splitting of water.

• In the OEC water oxidises to give  electron which is immediately accepted by the PS680 and it returns to its original state .

• By accepting the electron from PS680 Q reduces , Q gives the electron to the next member of the electron transport system and next one does the same .

• The electron reaches PS700 .PS700 transfers its electron to ferredoxin an iron-containing protein positioned at the outer surface  of the thylakoid membrane .

• The reduced ferredoxin donates its electron to NADP+.

• The NADP+ takes an electron from ferredoxin , protons from the medium (outside thylakoid )and gets reduced to NADPH  in the presence of the enzyme Ferredoxin -NADP-reductase.

• Thus the first product of the light reaction is  formed

• How is ATP formed?
•  Because of the electron transport system explained above,a proton gradient is developed across the thylakoid membrane .

       How is this gradient created?

• Proton gradient is created because ,after the splitting of  water the number of H+increase in thylakoid space.

                    

•  During the formation of NADPH , H+ is used so the number  of H+ on the stroma side of the thylakoid membrane decrease.

                     

• The number of H+  in the inner space of the thylakoid become more the number on the stroma side of the thylakoid membrane.

                    

• This transmembrane proton concentration gradient act as a driving force behind the conversion of ADP to ATP.