****ANSWER ALL PARTS WITH DETAILED ANSWERS**** Analyze the Data 12-3: Balancing
ID: 262442 • Letter: #
Question
****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)?
****ANSWER ALL PARTS WITH DETAILED ANSWERS****
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
Photosystems are the key units involved in process of photosynthesis by absorbing light of specific wavlength and the electrons present in the reaction center upon excitation reaches higher transition states and then travel through a chain of electron transport complexes that will ultimately lead to generation of ATP.
a) From panel A we can clearly see that the protein X belongs to PSI and LHCI, Y Belongs to PSII and Z belong to both the photosystems PSI and PSII. Distribution of Y remain unchanaged upon excitation by wavelength 680 and 700nm, whereas protein Z upon excitation by wavelength 680nm shifts towards PSI indicating change in its localisation from PSII to PSI.
b)The balancing of energy between two photosystems might be achieved by state transitions.State transitions in Intersystem electron carrier(IEC) is decided by the oxidised or reduced states which is controlling the regulatory kinase activation. In the reduced state the kinase will be activated and phosphorylates LHCII, Upon phosphorylation the distribution of pLHCII protein changes they are redistributed towards PSI and transfer the energy from PSII to PSI, whereas in oxidised state the antenna will remain in non phosphorylayed form. So the shift from phsophorylated to unphosphorylated form is dependent upon the transition state.
c) Here protein Z is the kinase whose distribution shifts towards PSI and it associates with PSI. Migration of this protein towards PSI, will stimulate PSI leading to its excitation.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.