Photophosphorylation & Chemiosmosis

Photophosphorylation/ Chemiosmosis.  (Peter Mitchell 1961)

The production of ATP in the chloroplast or in other membranes during light reaction is called photophosphorylation. Photophosphorylation occurs in ATP-synthase complex or coupling factor (CF) located in stroma thylakoid membranes. The coupling factor is also responsible for transport of H+ from the thylakoid channel to the stroma.

The electron transfer during photophosphorylation takes place

  • Firstly when the quinone of PSII picks up two protons from the stromal side of the membrane and move these to Cyt b6 complex
  • Secondly when proton uptake occurs on the periphery of the PS-I complex where NADP+ is reduced to NADPH2.

Concentration of H+ in thylakoid

The concentration of H+ ions in thylakoid channels arise due to oxidation of water and PQH2. During photosynthesis the H+ concentration in the channel (pH 5) to become 1000 times as great as in the stroma (pH 8) when photosynthesis is occurring. This pH gradient across the membrane provides chemical potential energy responsible for driving photophosphorylation.

Thylakoid membranes are quite impermeable to the H+ and other ions except when transported by coupling factor (ATP synthase complex).

ATP synthesis can take place via two-processes: non-cyclic photophosphorylation and cyclic photophosphorylation

FIGURE The transfer of electrons and protons in the thylakoid membrane is carried out vectorially by four protein complexes. Water is oxidized and protons are released in the lumen by PSII. PSI reduces NADP+ to NADPH in the stroma, via the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP reductase (FNR). Protons are also transported into the lumen by the action of the cytochrome b6 f complex and contribute to the electrochemical proton gradient. These protons must then diffuse to the ATP synthase enzyme, where their diffusion down the electrochemical potential gradient is used to synthesize ATP in the stroma. Reduced plastoquinone (PQH2) and plastocyanin transfer electrons to cytochrome b6 f and to PSI, respectively. Dashed lines represent electron transfer; solid lines represent proton movement.

i. Non-Cyclic Phosphorylation

In non-cyclic photophosphorylation the electrons removed from water, do not cycle back to the water molecule. The electrons originating in water are passed by PS-II and PS-I to NADP and NADPH2 and ATP are formed together with the evolution of O2.

Cyclic Photophosphorylation

In cyclic photophosphorylation the electron takes a cyclic path from Fd and return to Cyt bf complex. During this phosphorylation ATP is formed but not NADPH2.

  • The electron removed from P700 is donated to Fe-S centers and subsequently to ferredoxin, which becomes reduced.
  • The ferredoxin instead of transferring its electrons to NADP donates its electron to cyt bf and then through electron transport cycles back to P700.
  • The evolution of oxygen does not take place during this flow of electrons

The additional ATP molecules generated during cyclic phosphorylation are used to convert CO2 into complex compounds and other processes in the cell.

FIGURE Cyclic electron transport. PSI units operating independently of PSII may return electrons from P700 through ferredoxin (fd), and PGR5 to the thylakoid plastoquinone (PQ) pool and the cytochrome b6/f complex. In cyclic electron transport, the oxidationof PQ by thecytochrome b6/f complex generates a proton gradient that can be used for ATP synthesis but no NADPH is produced.