What is the primary function of cyclic electron flow quizlet?
The process of cyclic electron flow allows the cell to generate extra ATP without generating more NADPH, thereby providing enough ATP to carry out the Calvin cycle in the stroma of the chloroplasts.
What is the main purpose of cyclical electron flow within the light dependent reactions?
Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light-independent reactions. The two photosystems are protein complexes that absorb photons and are able to use this energy to create a photosynthetic electron transport chain.
What is cyclic electron flow Where does it occur?
In cyclic electron flow (CEF), electrons are recycled around photosystem I. As a result, a transthylakoid proton gradient (ΔpH) is generated, leading to the production of ATP without concomitant production of NADPH, thus increasing the ATP/NADPH ratio within the chloroplast.
What is the difference between cyclic and noncyclic electron flow?
During cyclic photophosphorylation, high energy electrons travel through electron acceptors in cyclic movements and release energy to produce ATP. During noncyclic photophosphorylation, high energy electrons flow through electron acceptors in Z-shaped noncyclic movements.
What is the main purpose of the light-dependent reactions?
The light-dependent reactions use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule ATP and the reduced electron carrier NADPH. In plants, the light reactions take place in the thylakoid membranes of organelles called chloroplasts.
What is cyclic electron flow quizlet?
Cyclic electron flow are the electrons excited from P700 in PSI are passed from Fd to the cytochrome complex and back to P700.
What is the function of the electron transport chain of Photosystem II quizlet?
The electron transport chain helps to move electrons from PS 2 to PS 1. It makes oxidation-reduction reactions within the photosystems. It also uses energy to bring in hydrogren molecules to make a concentration gradient in the thylakoid compartment, which eventually creates ATP due to ATP synthase.
What is the primary function of the Calvin cycle?
Converting Carbon Dioxide and Water Into Glucose In the most general sense, the primary function of the Calvin cycle is to make organic products that plants need using the products from the light reactions of photosynthesis (ATP and NADPH).
What is the cyclic electron flow in chloroplasts?
One of them is the cyclic electron flow around Photosystem I ( CEF), contributing to photoprotection of both Photosystem I and II (PSI , PSII) and supplying extra ATP to fix atmospheric carbon. Nonetheless, CEF remains an enigma in the field of functional photosynthesis as we lack understanding of its pathway. Here, we address the discrepancies between functional and genetic/biochemical data in the literature and formulate novel hypotheses about the pathway and regulation of CEF based on recent structural and kinetic information.
What is CEF in biology?
Therefore, CEF was defined as a rerouting of reducing equivalents from the acceptor side of PSI back to its donor side . The implication of cyt b6f in CEF, also proposed by Arnon and co-workers, was further confirmed by the sensitivity of CEF to Q o site inhibitors [ 3 ]. Later attempts to identify specific CEF transporters - in particular through genetic approaches aimed at the isolation of mutants devoid of CEF - fell short but allowed to disclose molecular bases for CEF regulation. These studies led to propose two CEF routes, which are considered by most authors to be responsible for the reduction of PQ by PSI acceptors: the NDH-1/NDH-2-, and PGR5/PGRL1-dependent pathways ( e.g. ref. [ 4 ]) ( Fig. 1 ). For the first pathway, the components allowing the transfer of electrons from PSI acceptors to the PQ pool are homologous to the respiratory NAD (P)H:PQ oxidoreductases. In land plant chloroplasts, NDH-1 shares at least 11 subunits with the mitochondrial and bacterial NADH:UQ oxidoreductase (complex I) [ 5 ]. Chloroplast NDH-1 complex however lacks an NADH binding module and has been proposed to use Fd as a substrate [ 6 ]. It probably pumps additional protons per electron transferred [ 7 ], similarly to mitochondrial and bacterial complexes I. In some microalgae, NDH-2 is present instead of NDH-1. It resembles mitochondrial Ndi1 [ 8 ], and is a monotopic membrane protein [ 9] that inserts into the stromal leaflet of the thylakoid membrane, and therefore is unable to pump protons to the lumen upon oxidation of its substrate, NAD (P)H [ 10 ]. Yet, the enzymatic pathway allowing the electron transfer from PSI acceptors back to the electron transfer chain upstream cytochrome b6f is still a matter of debate. Any legitimate candidate (enzyme or combination of enzymes) for the closing of the cycle must display a turnover rate in agreement with the highest rates of CEF measured in vivo, i.e. ~100 and ~60 electrons per second per PSI in plants [ 11, 12] and in green algae (from the companion paper, [ 1 ]).
How does Fd regulate CEF and LEF?
The regulation between CEF and LEF could be achieved by increasing the probability that Fd stays in proximity of the stromal side of the cyt. b 6f, through its anchoring by the cyt. b6f -bound FNR, as had been proposed earlier [ 12 ]. Recently, FNR was shown to be a target of the redox-sensitive STN kinase, a transmembrane enzyme which interacts with cyt. b6f [ 59, 60 ]. STN being activated upon reduction of the PQ pool, one can imagine a tentative model for an FNR phosphorylation-dependent CEF regulation - but there is a need to produce functional and biochemical data regarding the regulation of Fd-FNR- b6f binding before such a model can be critically assessed. Fd is at the very crossroads of photosynthetic electron transfer, donating electrons not only to CEF and LEF, but also to other metabolic pathways such as nitrite and sulphide reduction [ 61 ]. Furthermore, there are multiple Fd isoforms, with varying redox potentials and concentrations, which interact with multiple isoforms of FNR, some of which are soluble and some membrane-bound [ 61, 62 ]. It is obvious that a strict regulation of this hub is necessary both for photosynthesis and for poising the redox state of the entire stroma, making it easy to imagine that CEF is also governed at this level.
Does the stroma control the routing of electrons?
Therefore, we consider that a control of the routing of electrons in the stroma is sufficient for a modification of CEF/LEF parti tioning, without a need for considering the redox state of the plastoquinone pool. Competition between CEF and LEF for reduced Fd is, in our view, the hub where those two pathways require some regulatory processes. As outlined above, Fd− is readily oxidized by the CBB cycle and also by Flv proteins in microalgae and cyanobacteria [ 57 ]. We showed that such an oxidation is decreased at low oxygen concentrations when CEF increases. Furthermore an increase in the duration of CEF also was observed in an Flv mutant of moss [ 58 ], yet it was interpreted as a compensatory increase in CEF rather than a decrease in electron leak.
What is the physiological significance of cyclic flow?
We have previously shown that the PGR5-dependent pathway is essential to induce thermal dissipation under excessive light conditions and also to protect PSI from irreversible photodamage 7. However, in the double mutants, the phenotype of severe reduction in linear flow activity even at a low light intensity ( Fig. 1e) cannot be explained simply by a defect in photoprotection. The defect in the double mutants cannot be attributed to the reduced level of NPQ because electron transport is only marginally affected in the Arabidopsis mutant npq4, in which the induction of thermal dissipation is completely impaired 25. The linear flow activity was restored by adding ferricyanide to the ruptured chloroplasts ( Table 1 ). This result indicates that electron acceptors from PSI (oxidized ferredoxin and NADP +) are less available in the stroma of the double mutants, even at a low light intensity. This conclusion is supported by the lower levels of P700 oxidation in vivo ( Fig. 1f ). We have previously shown that P700 + is restored to wild-type levels in pgr5 by infiltrating leaf disks with an artificial electron acceptor from PSI (methyl viologen) 7. We conclude that cyclic flow is essential for the prevention of stroma over-reduction.
How to assess cyclic flow?
Cyclic flow activity was assayed by plastoquinone reduction upon addition of ferredoxin and NADPH to the chloroplast preparations. In this assay system, NADPH is essential for electron donation to ferredoxin through the reverse reaction of ferredoxin–NADP + reductase 21. Plastoquinone reduction was monitored as an increase in chlorophyll fluorescence emitted from PSII ( Fig. 3a ). Although NADPH did not reduce plastoquinone, it was rapidly reduced by subsequent addition of ferredoxin ( Fig. 3b ). The kinetics and the final reduced level were significantly lower in pgr5 mutants. Addition of the ferredoxin–plastoquinone reductase activity inhibitor antimycin A mimicked the pgr5 phenotype in wild-type plants but did not affect the pgr5 phenotype further, as reported previously 7.
What is the ETR of a photosynthesis mutant?
The light-intensity dependence of the electron transport rate (ETR) is indicative of the relative amount of electrons passing through PSII during steady-state photosynthesis. This was not impaired at any light intensity in crr mutants ( Fig. 1c ), indicating that NDH-dependent cyclic flow is not essential in photosynthesis, thus corroborating previous findings in tobacco 14, 16. However, the ETR was saturated at both a lower light intensity and a lower level in pgr5 than in wild type, but it was unaffected at light intensities less than 100 µmol photons m -2 s -1, which had previously been reported 7.
Does inhibition of cyclic flow affect linear flow?
Complete inhibition of cyclic flow severely affects linear flow in vivo ( Fig. 1e ). To assess the possible defect in the machinery of linear flow in the double mutants, electron transport to an artificial electron acceptor, ferricyanide, was evaluated by measuring O 2 evolution using isolated thylakoids ( Table 1 ). We did not include the crr2-2 pgr5 double mutant, which exhibits the most severe phenotype in this analysis, because PSII was photodamaged in these seedlings cultured even at 20 µmol photons m -2 s -1 ( Fv / Fm = 0.696 ± 0.02 (± s.d.), where Fm = maximum fluorescence level in the dark, and Fv = Fm - Fo, where Fo is the minimum fluorescence level in the dark). This result suggests that the restriction of electron transport is partially irreversible in crr2 pgr5 plants.
Which state do electrons go to?
The electrons go to a high energy state.
What color does chlorophyll absorb?
is the primary chlorophyll - absorbs blue-violet and red and reflects green. We say it looks "grass green".