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transport chain between photosystem II and ..

As a result of the difference in operational potential between PSII and PSI, also the donor side of the latter is significantly more reducing than that of PSII; for instance the potential of the primary acceptors, A0, is estimated at about –1.2 V, making it one of the most reducing species in nature, whereas the primary acceptors of PSII is estimated at –0.5/–0.6 V, which is about the same as the terminal electron acceptors of PSI. The differences in the operational midpoint potentials are even more obvious when comparing the PhQs (A1) in PSI (~ –0.75/–0.85 V) and the plastoquinone (QA/QB) in PSII (~ –0.03/0 V). However, whereas in the latter case the difference is also due to the different chemical species, PhQ being more reducing than PQ even in bulk organic solvents, the modulation of the redox properties of Chls a induced by the interaction with the protein subunits is rather remarkable and highlights the flexibility of these molecules as redox (as well as light harvesting) cofactors as well as the impressive influence of protein-cofactor interactions in sustaining the catalytic activity of both photosystems.

Difference Between Photosystem I and Photosystem II; Difference Between ..

A large body of investigation has been dedicated to the comprehension of photosynthesis in plants over several decades. Although many aspects of the photosynthetic process are nowadays substantially elucidated, several details, specific regulations, and even structural details about photosynthesis in plants are still little known. Hence, in the present review we will focus on the comparison between PSII and PSI of plants. We will discuss about the functioning, organisation, regulation of photosystems under different environmental conditions, by analysing common and specific aspects of each photosystem and by presenting open questions that requires further investigation in order to better understand their functioning.

Photosystems I and II - Encyclopedia Britannica

What is occurring between photosystem II and photosystem I during the light reactions of photosynthesis?

Due to the large difference in the redox potential between the electron donor (oxygen in a water molecule) and final electron acceptor during the light phase of photosynthesis (NADP+), the ancestor cyanobacteria had to evolve the capability to use two photosystems working in series in order to be able to accumulate the energy of two photons. These photosystems are called Photosystem II and Photosystem I (PSII and PSI, respectively). They are electronically connected by an intermediate membrane supercomplex called Cytochrome b6f (Cyt b6f) [,] and two electron carriers, a liposoluble quinone molecule (plastoquinone) that transports electrons between PSII and Cyt b6f, and the luminal copper-containing soluble protein plastocyanin, which links Cyt b6f to PSI.

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by convertinglight energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latterprocess has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present lowlevels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystemII are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy tocatalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems arehighly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electrontransport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activityregulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structureare different, on the other hand they share many similarities. In this review we discuss and compare various aspects ofthe organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences asobtained by structural, biochemical and spectroscopic investigations.

PHOTOSYNTHETIC REACTION CENTERS - Photobiology

Photosynthetic reaction centres are most commonly hetero-dimer pigment-protein complexes and this is the case for all known oxygenic photosystems. Structural, biochemical and biophysical analyses reveal a high degree of similarity between the two subunits composing the RC, suggesting that this common organisation originated from an ancient non-oxygenic homodimeric complex [,].

Comparison of the overall arrangement of TMH in the PsaA-PsaB dimer and the D1-D2-CP43-CP47 tetramer reveals substantial similarities in the overall structure of the photosystems, which is also supported by the partial structural homology between the D1 and D2 proteins of PSII (5 TMH each), which are homologous with each other, but which are also related to a domain of the PsaA/PsaB core proteins of PSI (11 TMH each). Similarly, internal antenna CP43 and CP47 (6 TMH) of PSII are similar to each other and are related to a different domain of PsaA and PsaB, indicating an interconnected evolution of PSI and PSII from common ancestral proteins.

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Plant Energy Transformations-Photosynthesis - …

Photosystem II contains two redox-active tyrosine residues, termed D and Z, which have different midpoint potentials and oxidation/reduction kinetics. To understand the functional properties of redox-active tyrosines, we report a difference Fourier-transform infrared (FT-IR) spectroscopic study of these species. Vibrational spectra associated with the oxidation of each tyrosine residue are acquired; electron paramagnetic resonance (EPR) and fluorescence experiments demonstrate that there is no detectable contribution of Q(A)- to these spectra. Vibrational lines are assigned to the radicals by isotopic labeling of tyrosine. Global 15N labeling, 2H exchange, and changes in pH identify differences in the reversible interactions of the two redox-active tyrosines with N-containing, titratable amino acid side chains in their environments. To identify the amino acid residue that contributes to the spectrum of D, mutations at His189 in the D2 polypeptide were examined. Mutations at this site result in substantial changes in the spectrum of tyrosine D. Previously, mutations at the analogous histidine, His190 in the D1 polypeptide, were shown to have no significant effect on the FT-IR spectrum of tyrosine Z (Bernard, M. T., et al. 1995. J. Biol. Chem. 270:1589-1594). A disparity in the number of accessible, proton-accepting groups could influence electron transfer rates and energetics and account for functional differences between the two redox-active tyrosines.

Thylakoid - definition of thylakoid by The Free Dictionary

First of all, a clear difference exists between the pigment composition of the core complexes and the external Lhc antennas: the core complexes of both photosystems bind essentially only Chl a and β-carotene molecules, while the external Lhc antenna complexes also bind Chl b and xanthophyll (oxygenated carotenoids).

photosynthesis | Importance, Process, & Reactions - …

It is also interesting to note that the large amount of Chls b in PSII as compared with PSI, as well as the particularly low energy level of certain Chls a in PSI, are at the origin of a different absorption spectrum between the two photosystems (Fig. ). This can cause an unbalanced absorption by the two photosystems under lights enriched in particular wavelengths. The “state transitions” phenomenon [-], discussed later, is an important mechanisms allowing balancing the absorption between the two photosystems with a regulation acting in few minutes.

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