Non-Cyclic and Cyclic Photophosphorylation | Photosynthesis

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.