Learn about the comparison between Non-Cyclic and Cyclic Photophosphorylation.
Comparison # Non-Cyclic Photophosphorylation:
In this a photon of light is involved to excite electron in chlorophyll b or other accessory pigments of photosystem II. Energy of two such excited electrons is accepted by an oxidized plastoquinone forming completely reduced plastoquinone and electron-deficient chlorophyll b (Chl b).
Chl b then accepts an electron from a water molecule. In this step, Mn++ and CI sup ions are needed. Consequently, water loses electron, produces oxygen (O2) and yields reduced plastoquinone.
plastoquinone + 2H ⇋ Reduced plastoquinone
The reduced plastoquinone then donates the electrons, one at a time, to cytochrome b6. From the latter, the electron may fall to cytochromeƒand then to plastocyanin and P 700 in pigment system I. P 700 can accept an electron only if it has just lost one of its own when excited by the energy in a second photon of light.
Ferredoxin is the direct acceptor of the electron lost by the excited P 700 molecule and is reduced. Two molecules of reduced ferredoxin then transfer their electrons to NADP reducing it to NADPH2 and itself is oxidized (Fig. 13-24).
Ferredoxin (Fe+2) + NADP + 2e− ⇋ 2 Ferredoxin (Fe+3) + NADPH2
In this process excess electron energy which results from the absorption of light quanta is used in the synthesis of ATP, from ADP and Pi, probably at one location between cytochrome b6 and cytochrome ƒ. In this scheme both Pigment systems participate in driving an electron from water to NADPH in an unidirectional manner.
When a herbicide dichlorophenyldimethylurea (DCMU) is added to the suspended chloroplasts, electron flow and ATP formation are blocked. DCMU inhibits the carrier chain link between PSI and PSII. If a reducing agent is added, ATP formation remains blocked but NADPH is produced. Obviously, NADPH formation is a function of PSI whereas ATP formation requires both PSI and PSII.
Comparison # Cyclic Photophosphorylation:
Here no ‘outside’ electron donor is needed. Chlorophyll absorbs photon of light of enough energy and an electron with high energy state (e–) is produced which reduces ferredoxin and the cytochrome system (cyt. b6), respectively.
The cytochrome reduction is coupled to ATP formation. The electron at a low energy level returns to chlorophyll ultimately. Clearly the electron flow was from the electron donor and back to the same compound (Fig. 13-12).
There are three types of photosynthetic bacteria (green, purple sulphur, purple non-sulphur). The former two types are autotrophic and use H2S as electron donor for CO2 reduction. However, purple non-sulphur bacteria are photoheterotrophic and use succinate or malate and not H2S for CO2 reduction.
Bacteria do not use water and therefore produce no oxygen. The reductant in bacteria is NADH + H+ and lack plastocyanin. With the production of ATP and NADPH2, the light reaction of photosynthesis is completed (Fig. 13-15, 13-16) and these two products are now utilized to reduce CO2 to the carbohydrate level in the dark phase.
Some photosynthesis bacteria have cyclic photophosphorylation as the only source of ATP. As will be seen cyclic photophosphorylation is carried out by photosystem I only. Details are clear in the following figure (Fig. 13.24). This can occur under anaerobic condition.
In brief it may be stated that biochemistry of oxygen evolution mechanisms is still a challenging problem for the biochemists and much remains to be understood.