****ANSWER ALL PARTS WITH DETAILED ANSWERS*** Analyze the Data 12-3: Balancing E
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****ANSWER ALL PARTS WITH DETAILED ANSWERS***
Analyze the Data 12-3: Balancing Energy in Photosystems
In photosynthetic organisms that evolve O2, the two photosystems, PSI and PSII, operate in series to transfer electrons from H2O to NADP+. Because PSI and PSII are excited by light of different wavelengths, the two photosystems, though acting sequentially to transport electrons, are not necessarily equally activated under different light conditions. Nonetheless, linear flow of electrons can occur if the distribution of absorbed energy between these two photosystems is balanced. The mechanism by which the balancing occurs has been subjected to analysis using Chlamydomonas reinhardtii, a single cell, green alga (see Takahashi et al (2006) Proc. Natl. Acad. Sci. USA 103: 477-482.)
Chlamydomonas reinhardtii was illuminated with light of either 700 nm to excite PSI or light of 680 nm to excite PSII. Thylakoids were then isolated from these cells, solubilized, and chlorophyll with its associated proteins separated by sucrose gradient sedimentation. Chlorophyll in the sucrose gradients can be directly observed as green bands, A-1, A-2, A-3 and A-3’ (see figure). Chlorophyll in bands A-1 and A-2 is associated with PSII and its associated light harvesting complex proteins (LHCII), whereas chlorophyll in band A-3 (and its satellite band A-3’) is associated with PSI and its associated light harvesting complex proteins (LHCI). To determine the location within the gradient of LHC proteins X, Y and Z, the gradients were fractionated and subjected to western blotting using antibodies against proteins X, Y and Z. The western blots are displayed below the gradients such that location of the bands on a western blot correspond to location in the sucrose gradient shown above the western blots. Note that the sucrose gradients are displayed horizontally with the top of each gradient at the left. The data in panel A of the figure is derived from cells illuminated with 700 nm light; the data panel B is derived from cells illuminated with 680 nm light.
a.) To which photosystem complex does each LHC protein, X, Y and Z, belong? How does the wavelength of light affect this distribution?
b.) Does your response to part (a) suggest a mechanism by which balancing of energy between the two photosystems might be achieved and thereby result in a linear flow of electrons from H2O to NADP+?
c.) When cells are illuminated with 680 nm light, LHC protein Z becomes phosphorylated. If cells are incubated with a protein kinase inhibitor and illuminated with 680 nm light, no phosphorylation of protein Z occurs and its distribution in the gradient mimics that observed in panel A of the figure. How might phosphorylation of protein Z be included in the mechanism you proposed in response to part (b)?
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PSII PSI PSII PSI A-1 A-2 A-3 A-1 A-2 A-3 A-3 700 nm PSI excited 680 nm PSII excited Identification of the mobile light-harvesting complex II polypeptides for state transitions in Chlamydomonas reinhardtii from Takahashi et al, Proc Natl Acad Sci U S A. 2006 Jan 10; 103(2): 477-482. Copyright (200) National Academy of Sciences, USAExplanation / Answer
a) From figures both A and B, the light harvesting proteins X belongs to PS-I and protein Z is clearly associated with PS-II. Protein Y is associated with both PS-I and PS-II, possibly helping in transition excitement from PS-II to PS-I. since at excitement, mobile LHC proteins II migrate from PS -II to PS-I. These LHC proteins play a key role in photoefficiency and photoprotection.LHCII proteins are phosphorylated upon PS-II excitation, and this phosphorylation triggers their distribution to PS-I. LHCII protein Z is migrated to PS-I upon excitation.
b)Yes, the mechansim depicts balancing of energy between PS-I and PS-II and translates solar energy into chemical energy.In different light environments it ensures maxium efficiency in electron flow.The core of this balance is state transitions,wherein state 1 occurs upon PS-I excitation and state 2 occurs upon PS-II excitation.A plastoquinone system plays a role via cytochromes b6 f complexes and influences thylakoid bound protein kinases.
c) At wavelength of 680nm, PS-II is excited and associated LHC complex Z is phosphorylated. This is brought about by thylakoid bound protein kinases which phophorylate LHC-II proteins, triggering their distribution to PS-I.This distribution of Z is seen in figure B. Upon incubation of cells with a protein kinase inhibitor would inhibit protein kinases of the thylakoid and thereby inhibit phosphorylation of Z. Thus protein Z of LHC-II ,PS-II would not be able to migrate to PS-I. This is as seen in Figure A.
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