Chemiosmosis theory of ATP synthesis by Mitchell

Chemiosmotic Hypothesis for the production of ATP

• This chemiosmotic Hypothesis is related to ATP formation. This hypothesis was given  by P. Mitchell .

• The formation of ATP  is connected  with the development of proton gradients across the membrane of thylakoid.  But mitochondria is also involved in this phenomenon.

• The photolysis of water during the light reaction of photosynthesis inside the thylakoids releases oxygen, electrons and protons or hydrogen ions.

• This process  causes the development of proton gradients across the thylakoid membrane.

• When Splitting of water molecules takes place  inside the thylakoid then  hydrogen ions or protons are produced and get collected in the lumen of the thylakoids.

What happened with the electron, proton and oxygen that are released due to photolysis of water.

• An electron passes through the photosystems and performs cyclic or noncylic photo phosphorylation.

• Protons are   transported across the membrane of thylakoid to develop the proton gradient and oxygen is diffused out from the plant  cell.

• Both photosystems are located on the membrane of thylakoids.

• The primary acceptor of the electron of both photosystems  is  also present  towards the outer side of the membrane.

• The NADP reductase enzyme is also located in the stroma side of the membrane.

• Electrons that are moved after the completion of photophosphorylation with PS I ,  are moved to acceptor of electrons  involved in PSI called ferredoxin.

• When electrons are moved from PSII to PSI  than the electrons are also gained by plastoquinone of photosystem II and plastocyanin of photosystem I . 

• Both plastocyanin and plastoquinone also diffuse protons into the lumen of thylakoids from Stroma. As a result protons are accumulated in the lumen of thylakoids.

• This creates a proton gradient across the thylakoids membrane  and puts a pressure on the thylakoid to break the proton gradient.

• And finally gradient is broken down due to movement of protons across the membrane to the stroma through the trans-membrane channel  of  ATP synthase.

• One part of  ATP synthetase  enzyme named F0 is established in such a way that this  portion of enzyme is present  in the thylakoids membrane.

• Another portion of ATP synthetase called F1  is projected out  in Stroma.

• Due to this orientation of enzymes,  a trans-membrane channel is formed for the transport of protons from thylakoids.

• As protons come out from the thylakoid membrane, ATP molecules are formed with the activity of ATP synthetase enzyme.

• The protons are  also used  for the reduction of NADP+ to NADPH2 in Stroma.

• The ATP and NADPH produced due to movement of electrons during photophosphorylation and chemiosmosis hypothesis  is used immediately for the fixation of  CO2 and form the glucose during dark reaction.

• The product of light reactions used to drive the process leading to synthesis of sugar are called biosynthetic phases of photosynthesis.