Plastocyanin shuttles electrons from cytochrome b6-f complex to PSI. Evolution of flexible non-photochemical quenching mechanisms that regulate light harvesting in oxygenic photosynthesis. Photosystem I releases electrons, which results in the formation of an NADPH molecule. Natl. Use your flowchart to answer the following questions: (a) Trace the path of electrons through PS II and PS L (b) Trace the path of energy through the light reactions. Plants absorb energy from the sun during photosynthesis. The electrons released from photosystem II enter a chain of proteins known as electron transport chain (ETC). PI occurs if the rate of D1 damage exceeds the rate of replacement, leading to loss of photosynthetic viability. FIGURE 3. Hydrogen ions flow through ATP synthase via chemiosmosis to form molecules of ATP. Importantly, P700 is oxidized (Figures 1A and 2A) despite the fact that the PQ pool is at the same time strongly reduced (Figures 1B and 2B). PSII and PSI are two major components of the photosynthetic electron transport chain, which also includes the cytochrome complex. Front. Trace the movement of electrons in noncyclic electron flow. By this mechanism, PGR5 is supposed to enhance the generation of ΔpH and thereby accelerate the induction of NPQ and slow down the Q cycle. doi: 10.1104/pp.109.150250, Tikkanen, M., Mekala, N. R., and Aro, E. M. (2014). As the name suggests, the light-dependent reaction needs light to proceed and cannot occur in darkness. (c) What is the source of energy for this process? Sci. At the reaction center of photosystem I (PSI) is a chlorophyll a molecule called P700. Indeed, only in a specific case of the stn7 mutant when energy distribution from the LHCII system to PSI is impaired, the relaxation of NPQ in low light selectively affects only PSII leading to high reduction of the PQ pool (Tikkanen et al., 2010, 2011; Grieco et al., 2012). The discovery of state transitions in photosynthesis 40 years ago. Biophys. Interestingly, the PGR5-PGRL1 complex has redox active thiol groups being able to accept electrons from ferredoxin (Hertle et al., 2013). PS II takes replacement … Required fields are marked *. Trace the movement of electrons in cyclic electron flow. As the electrons flow between PSI and PSII, they lose energy to translocate H+ ions from the stroma into the thylakoid lumen. Oxidation of P700 is, however, lost when the actinic light exceeds the intensity of growth light. PSI was the first photosystem to be discovered and absorbs maximum light of wavelength 700 nm. Ans. Therefore, it is also known as P700. Nature 403, 391–395. Photosynth. 152, 723–735. Show the path of electrons from photosystem II to NADPH in the chloroplast. Previously the interaction between proton gradient-dependent regulation of electron transfer and NPQ was studied in Chlamydomonas reinhardtii (Kukuczka et al., 2014). Importantly, pgr5 is more efficient in oxidation of P700 in low light than in high light (Figure 1). This clearly demonstrates that the control of electron flow from PSII to PSI cannot be assigned, even partially, to the down-regulation of PSII by NPQ but apparently takes place solely in Cyt b6f. trace the path of a carbon dioxide molecule from the point at which it enters a. Natl. doi: 10.1104/pp.15.00488, Munekage, Y. N., Genty, B., and Peltier, G. (2008). doi: 10.1016/j.cell.2007.12.028, Grieco, M., Suorsa, M., Jajoo, A., Tikkanen, M., and Aro, E. M. (2015). The net reaction (A) produces stoichiometric amounts of ATP, NADPH, and oxygen. FIGURE 1. The following table compares and contrasts light-dependent and independent reactions. This type of electron transport involving both PSI and PSII is called non-cyclic photophosphorylation. Regulation of cyclic and linear electron flow in higher plants. (A)(P/Pm), (B) (F′/Fm), (C) (1–Fm′/Fm) in wild type (black dots), pgr5 (blue dots), and npq4 (purple dots) during a stepwise increase in actinic light intensity. J. They are passed from PSII to PSI through an electron transport chain and are transmitted to NADP Plus....NADPH. Absence of the Proton Gradient Regulation 5 (PGR5) protein from plant chloroplasts prevents the induction of strong trans-thylakoid proton gradient (ΔpH) and consequently also the thermal dissipation of excess energy (NPQ). Sci. In addition to the slower induction, both mutants showed an impaired relaxation of NPQ during the subsequent phase of darkness (Figure 2C). Processes of the Light-Dependent Reactions –. This difference, however, decreases at extremely high light (Figure 1A), indicating that when the light intensity is strong enough, the capacity of PSBS-dependent quenching mechanism to limit excitation pressure becomes saturated. |, Creative Commons Attribution License (CC BY), Molecular Plant Biology, Department of Biochemistry, University of Turku, Turku, Finland. Non-Cyclic Electron Flow: uses PS II and PS I. Photosystem II supplies electrons to the beginning of the electron transport chain. Induction of NPQ (1–Fm′/Fm) was analyzed in order to clarify the relationship between thermal dissipation of excess excitation energy and redox state of ETC. Here, we compared the putative effects of NPQ and the reduction state of the PQ pool on P700 oxidation in WT and in the pgr5 and npq4 mutants (Schematic model, Figure 3). doi: 10.1016/j.bbabio.2012.09.011, Stiehl, H. H., and Witt, H. T. (1969). Water splits up into two hydrogen atoms, which then become ions (H, The electrons flow from PSII to PSI and lose energy to translocate the H. Hydrogen ions flow through ATP synthase via chemiosmosis to form molecules of ATP. Acta 1807, 935–944. doi: 10.1016/j.tplants.2010.11.006, Tikkanen, M., Grieco, M., Kangasjarvi, S., and Aro, E. M. (2010). Klughammer, C., and Schreiber, U. Ans. Plant Phys. The electrons flow from PSII to PSI and lose energy to translocate the H + ions from the stroma into the thylakoid lumen. Photosystem II and Photosystem I have their own minor light harvesting antennae, but the energy capture to both photosystems is largely based on the major light harvesting system that is embedded in the thylakoid membrane and composed of LHCII trimers (Wientjes et al., 2013; Grieco et al., 2015). The transport of electrons through PSII, cytochrome complex, and to PSI creates a proton gradient. The function of the light-dependent reaction is to convert light energy into chemical energy within a multi-protein complex called the photosystem, found in the thylakoid membranes. Plant Physiol. Using light energy, PSII acts first to channel an electron through a series of acceptors that drive a proton pump to generate ATP, before passing the electron on to PSI. Hydrogen ions have a positive charge. doi: 10.1016/j.bbabio.2010.10.017, Efremov, R. G., Baradaran, R., and Sazanov, L. A. Whatever the mechanism, it is clear that the pgr5 mutant cannot increase the trans-thylakoid ΔpH and consequently fails in the PSBS protein-dependent thermal dissipation of excess excitation energy (NPQ) upon increase in light intensity (Munekage et al., 2002; Suorsa et al., 2012). Cram.com makes it easy to get the grade you want! (d) How does the energy move through PS II and PS I? Acad. What is the source of the energy that moves electrons through this path? Cell 110, 361–371. 16, 126–131. 19 – Photophosphorylation at Cram.com. This challenges the idea that NPQ is required for oxidation of P700 in high light. 49, 1688–1698. Be sure to include the electron carriers plastoquinone and plastocyanin. doi: 10.1016/S0006-3495(95)80429-7, Li, X. P., Bjorkman, O., Shih, C., Grossman, A. R., Rosenquist, M., Jansson, S., and Niyogi, K. K. (2000). Plant Biol. Show where oxygen is involved in this pathway. To study the redox state PSII acceptor side, the redox state of QA was estimated by normalizing fluorescence to the maximal fluorescence (F′/Fm). Moreover, it has been reported that PSBS mutants can oxidize P700 upon increase in light intensity (Grieco et al., 2012; Roach and Krieger-Liszkay, 2012) indicating that the PSBS-dependent mechanism does not specifically downregulate PSII, but rather affects both photosystems. Trace the path of electrons during the light reactions. In growth light or light intensities below the growth light, WT, npq4, and pgr5 can keep the intersystem electron transfer chain optimally oxidized. Articles. In addition, npq4 reached a still higher level of F′/Fm than pgr5 at the high light intensities (661, 1595 μmol photons m-2 s-1), during which the QA begun to return to its oxidized state (Figure 1B). Your email address will not be published. Quantitative treatment of the function of plastoquinone in phostosynthesis. Photons absorbed by Photosystem II (PSII) are used to oxidize water in the oxygen-evolving complex (OEC) connected to PSII. Nevertheless, when the increase in light intensity is strong enough, the pgr5 mutant can induce a reasonably high NPQ that in WT occurs concomitantly with oxidation of P700 (Figure 2A). To clarify the differential roles of ΔpH-dependent control of Cyt b6f and NPQ in regulation of electron flow from PSII to PSI at high light, we investigated WT, pgr5, and npq4 with respect to the redox state of PSI, redox state of PSII electron acceptors and induction of NPQ upon changes in illumination conditions. doi: 10.1007/s11120-008-9389-8, Niyogi, K. K., and Truong, T. B. To this end, the two NPQ mutants pgr5 and npq4, the latter lacking the PSBS protein, were characterized. It was shown that the two mechanisms are complementary, both of them being needed for high light acclimation in oxygen limiting conditions. Biochim. doi: 10.1515/znb-1969-1219, Suorsa, M., Jarvi, S., Grieco, M., Nurmi, M., Pietrzykowska, M., Rantala, M., et al. Detached leaves were incubated in darkness 10 min before measurements. Photosystem I is a plastocyanin-ferredoxin oxidoreductase which uses light energy to transfer an electron from the donor P700 chlorophyll pair to the electron acceptors A0, A1, FX, FA and FB in turn. Acad. This obviously has not been considered to be a problem because the simultaneously induced energy-dependent NPQ is believed to selectively downregulate PSII, thus leading to PSII limitation of electron transfer, and consequent oxidation of PSI. The pigment is called chlorophyll and comes in two forms – chlorophyll a and chlorophyll b. Plant Sci., 08 July 2015 (1981). A saturating pulse (5000 μmol photons m-2 s-1 for 500 ms) was applied in every 1 min with increasing 635-nm actinic light (50, 127, 274, 661, and 1595 μmol photons m-2 s-1 or 127 and 1953 μmol photons m-2 s-1). ’ s trace the path of a carbon dioxide molecule from the.... Light of wavelength 680 nm and thus, called the oxygen gas exits the leaf through the.... 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