Introduction to phosphorylation.
Photophosphorylation is process of trapping the solar energy by pigment molecules and its use in ATP synthesis. This is the part of light reactions of photosynthesis that occur in thylakoid of chloroplast and transfer the electron from water through a series of membrane bound carriers, producing high energy compounds vis. ATP and NADPH.
The functional arrangement of light absorbing pigments in thylakoid membranes is called Photosystem. The photosystem has many pigment molecules that are designated as photochemical reaction center or antenna molecules. The photochemical reaction centers transduce the light energy into chemical energy. The antenna molecules absorb the light energy and pass it efficiently to the reaction center molecules. Two types of photosystem operate in series for photophosphorylation:
Photosystem I (P700) absorbs far-red light of wavelengths greater than 680 nm, produces a strong reductant, capable of reducing NADP+, and a weak oxidant.
Photosystem II (P680) absorbs red light of 680 nm produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than the one produced by photosystem I.
On the basis of involvement of photosystem, the phosphorylation can be classified as Cyclic (only PS I) and non cyclic phosphorylation (both PS I and PSII).
Explanation about Cyclic Phosphorylation
Cyclic phosphorylation occurs when the conditions do not favor the non cyclic phosphorylation for example, when the chloroplasts are illuminated with light of wave length greater than 680nm. This light activate only the PS I, not PS II resulting in inhibition of electron flow from water to NADP+ and retarded CO2 fixation. This means that NADPH will no longer be oxidized, making it unavailable as electron acceptor. These conditions favor the cyclic electron transport.
Under these conditions, electrons passing from P700 to ferredoxin do not continue to NADP+, but move back through the cytochrome b6f complex to plastocyanin. Plastocyanin donates electrons to P700, which transfers them to ferredoxin when the plant is illuminated. Thus, in the light, PSI can cause electrons to cycle continuously out of and back into the reaction center of PSI, each electron propelled around the cycle by the energy yielded by the absorption of one photon. Cyclic electron flow is not accompanied by net formation of NADPH or evolution of O2. However, it is accompanied by proton pumping by the cytochrome b6f complex and by phosphorylation of ADP to ATP, referred to as cyclic photophosphorylation.
Schematic Representation of Cyclic Phosphorylation
Schematic Representation of Cyclic Photophosphorylation
Significance of Cyclic Phosphorylation
By regulating the partitioning of electrons between NADP+ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the carbon-assimilation reactions and other biosynthetic processes.
Photophosphorylation is process of trapping the solar energy by pigment molecules and its use in ATP synthesis. This is the part of light reactions of photosynthesis that occur in thylakoid of chloroplast and transfer the electron from water through a series of membrane bound carriers, producing high energy compounds vis. ATP and NADPH.
The functional arrangement of light absorbing pigments in thylakoid membranes is called Photosystem. The photosystem has many pigment molecules that are designated as photochemical reaction center or antenna molecules. The photochemical reaction centers transduce the light energy into chemical energy. The antenna molecules absorb the light energy and pass it efficiently to the reaction center molecules. Two types of photosystem operate in series for photophosphorylation:
Photosystem I (P700) absorbs far-red light of wavelengths greater than 680 nm, produces a strong reductant, capable of reducing NADP+, and a weak oxidant.
Photosystem II (P680) absorbs red light of 680 nm produces a very strong oxidant, capable of oxidizing water, and a weaker reductant than the one produced by photosystem I.
On the basis of involvement of photosystem, the phosphorylation can be classified as Cyclic (only PS I) and non cyclic phosphorylation (both PS I and PSII).
Explanation about Cyclic Phosphorylation
Cyclic phosphorylation occurs when the conditions do not favor the non cyclic phosphorylation for example, when the chloroplasts are illuminated with light of wave length greater than 680nm. This light activate only the PS I, not PS II resulting in inhibition of electron flow from water to NADP+ and retarded CO2 fixation. This means that NADPH will no longer be oxidized, making it unavailable as electron acceptor. These conditions favor the cyclic electron transport.
Under these conditions, electrons passing from P700 to ferredoxin do not continue to NADP+, but move back through the cytochrome b6f complex to plastocyanin. Plastocyanin donates electrons to P700, which transfers them to ferredoxin when the plant is illuminated. Thus, in the light, PSI can cause electrons to cycle continuously out of and back into the reaction center of PSI, each electron propelled around the cycle by the energy yielded by the absorption of one photon. Cyclic electron flow is not accompanied by net formation of NADPH or evolution of O2. However, it is accompanied by proton pumping by the cytochrome b6f complex and by phosphorylation of ADP to ATP, referred to as cyclic photophosphorylation.
Schematic Representation of Cyclic Phosphorylation
Schematic Representation of Cyclic Photophosphorylation
Significance of Cyclic Phosphorylation
By regulating the partitioning of electrons between NADP+ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the carbon-assimilation reactions and other biosynthetic processes.
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