This reaction is cyclic and yields one ATP, but not NADPH. These electrons traverse via the sequence of electron donors in the particular order of A0 (chlorophyll), A1 (phylloquinone), FeS protein, and split at ferredoxin (Fd), then traverse through cytochrome b, cytochrome b6 complex, plastocyanin (PC), and at last reaches back to P700. The PS I absorb light in the range of about 700 nm (longer wavelength) and pass it onto its reaction center P700 which gets turned on. Only PS 1 participates in this reaction and hence takes place in the stromal lamellae. These reaction clusters are often defined as the Z-scheme or non-cyclic photophosphorylation.Ĭyclic photophosphorylation- All cellular reactions happen for a reason, typically cyclic photophosphorylation occurs to satisfy the need for an additional ATP for the dark reaction. To support the source of electrons to P680, the oxygen-evolving complex (OEC) formed in the presence of Mn2+ provides it with electrons. This reaction occurs on the stromal side of the membrane that also mediates the generation of the gradient. At the same time, PS I absorbs light, gets excited, and contributes electrons to NADP+ to generate NADPH using NADP+ reductase as the catalyst. This electron flux across the photosystems generates a proton motive force that mediates ATP synthesis with the help of the enzyme, ATP synthase. From pheophytin A they are circulated through several electron donors in the particular order of plastoquinone A (PQA), plastoquinone B (PQB), cytochrome b6 complex, plastocyanin, and finally reach the P700 of the PS I.

The electrons now travel to the carrier pheophytin A (Pheo A). It then serves as an electron donor due to the creation of negative reduction potential. The photosystem II absorbs light of about 680 nm wavelengths (shorter wavelengths) and passes it to its chlorophyll- reaction center P680 which gets turned on. Non-cyclic photophosphorylation reaction takes place in the membrane of the grana present in the chloroplast which possesses both the required photosystems, they operate together and produce ATP, NADPH, and also oxygen. Two photosystems such as Photosystem I with reaction center chlorophyll PS 700 and photosystem II with reaction center chlorophyll PS 680 participate effectively in the photophosphorylation reaction. A cycle of photophosphorylation reactions takes place during which the transport of electrons occurs. The process of photosynthesis takes place in the chloroplast’s thylakoid membrane.

Proton gradient