Photosynthesis

Non-cyclic Electron Flow (yield ATP & NADPH)

 

-Light is radiated to plants' leaves and reaches chloroplast where most of the photosynthetic reactions take place. The photons strike PSII (photosystem two) consisting antenna complex and a reaction centre.

-Antenna absorbs photons and transfer the energy from one to another, until it reach to reaction centre-chlorophyll a.

-The energy is used to excite an electron at chlorophyll P680. The electron is captured by electron acceptor, pheophytin.  Through redox reactions, the electron is transferred to plastoquinone (PQ).

- When the electron is missing at PS 680,  Z protein would splits 2H₂O à O₂ + 4H⁺+ 4e^-, the electrons produced will be use to replace the missing one; the oxygen get released as a byproduct; and the hydrogen ions are kept inside in the thylakoid lumen. The process of splitting water is called, photolysis.   

-The electron carrier PQ transfer the electron to b₆-f cytochrome complex while this is happening, hydrogen ions from the chloroplast stroma are being pumped into the thylakoid lumen.

-The electron gets carried away by plastocyanin (Pc) to PSI (photosystem one) replacing the missing electron excited by the last photon energy.

-The electron in PSI excited by photon energy and transferred to FNR (ferredoxin NADP reductase) by ferredoxin the electron carrier.

-The transferred 2e^- +H⁺ floating in the stroma to convert NADP⁺ into NADPH for use of Calvin Cycle.

-The hydrogen ions accumulated through the reaction increases the electrochemical gradient. To balance the pH level between the thylakoid lumen and stroma, H⁺ is pumped out though a complex called, ATP synthase.  By doing so, ADP can be converted into ATP by adding Pi in the process of photophosphorylation.

Cyclic Electron Flow (yield only ATP)                  

-Happens when not enough light or water is presented in the plant cell.

-Photon energy excites the electron at PSI, the electron is transferred through ferredoxin to b6-f cytochrome and back to PSI.

-H+ is being pumped into the lumen adding gradient for chemiosmosis which will produce ATP same as non-cyclic electron flow.