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the process of photosynthesis consists of two main processes .
1- light reaction or light dependent reaction
2 dark reaction of light independent reaction
1- light reaction
in the chloroplast the light capturing chlorophyll molecules , membrane bound organelles andthe electron carriers are the components which together constitute the electron transfer chain . 4 major groups of complexes are present in the membrane . these are
1 photosystem I (PS 1)
2 photosystem II(PS II)
3 cytochromo b/f complex and
4 an ATpase synthase .
PS I and PSII both contain special chlorophyll-a molecules at their centres .
the chlorophyll-a molecule has at PS I has a max absorbance of 700 nm and PS II can absorb upto 680nm.
the light reaction is further categorized in the *>electron transport chain and *>the photophosphorylation , also known as the formation of ATP.
ELECTRON TRaNSPORT CHaIN :-
when a photon of light hits the PS II the excitaion of the electron is observed.Ps II is called the plastoquinone oxidoreductase of the P680.the excited electron is carried to the primary electron that is the phaeophytin
the phaeophytin the transfers the excited elsctron ro the plastoquinone (PQ) and then from there to cytochrome b/'f complexes,( cyt b/f complex)
this point of the electron msrks the limiting step .
withing the pQ the electron is passed to cytochrome f via the FeS centre . and releasing the H+ ion from the stroma to the lumen.
the electron that arrives the PS II is obtained from the oxidation of water ,
1H2O---> O2 + 4 electrons + 4 H+
the electron from the cytochrome b/f complex is transferred to plastoyanin which in turns trasfers this electron to PSI also called a s PS I
then another photon of light hits the PS I and the electron gets exvited . thus, releasinf the energy.this energy id then usef to drive the proton and eventually ends up in NADPH+
FORMATION OF ATP or PHOTOPHOSPHORULATION :-
the energy made available by the passage of electrons down the cytochrome system is coupled to build up the ATP . some of the carrier of the cytochrome system punp H+ ions from the stroma into the thylakoid space . this thylakoid space acts as a reservoir for hydrogen ions also because for every water molecule that splits up in the beginning , 2 H+ ions stay behind in the thylakoid space . the H+ ions taken up by NADP comes from the stroma and not from the thylakoid space
b/c of the large number of H+ ions in ghe thylakoid space compared to the stroma , an extreme electrochemical graduent is present . ehen these H+ ions flow out of the thylakoid space by way of a channel protein present in the particle , called the ATP synthase complex , energy is provided for the ATP synthase enzyme to produce ATP from ADP + P . this is called chemosmotic ATP synthesis . b/c chemical and osmotic events are join to oermit ATP synthesis. the transport of the three protons from the ATPase complex are normally required for the production of onw ATP molecule .
the linear flow of the electrons from water to NADP coupled to ATP syntheses is non-cyclic photophosphorylation b/c the electrons pass on to a terminal acceptor and never back to an initial source. in cyclic photophosphorylation the electroja qre cycled from the PS I back to cytochrome complex and from there continue on to the P700 chlorophyll. the only product of this process is ATP which can be utilised to meet the ATP demand of CO2 fixation or the other processes such as protein or stardh formation
finally 4 imp events takes place at the light dependent stage
1 - photolysis of water
2 - electron transport chain i.e PSII and PSI
3- reduction kf NADP to NADPH2
4- synthesis of ATP by photophosphorylation
ATP and NADPH2 prosucts of the light dependent stage reaction play an imp role in the light independent process ....