Photosystem efficiency

Weis, E. Berry, J. A. (1987). Quantum efficiency of photosystem II in relation to energy -dependent quenching of chlorophyll fluorescence. Biochimica Bio-physica Acta, 894, 198-208. 

Principle Chlorophyll fluorescence is a sensitive and early indicator of damage to photosynthesis and to the physiology of the plant resulting from the effect of allelochemicals, which directly or indirectly affects the function of photosystem II (Bolhar-Nordenkemf et ah, 1989, Krause and Weiss 1991). This approach is convenient for a photosynthesis analysis in situ and in vivo and quick detection of otherwise invisible leaf damage. The photosynthetic plant efficiency was measured using the method of induced chlorophyll fluorescence kinetics of photosystem II [Fo, non-variable fluorescence Fm, maximum fluorescence Fv=Fm-Fo, variable fluorescence t /2, half the time required to reach maximum fluorescence from Fo to Fm and photosynthetic efficiency Fv/Fm]. 

The effect of zearalenone on crop development may be connected to its influence on the status and functioning of the photosynthetic apparatus (Koscielniak et al. 2008). The after-effects of zearalenone on the growth of soybean and wheat plants, net photosynthesis and transpiration rates, stomatal conductance, photochemical efficiency of photosystem 2 and on final seeds yield were determined. Modifications in leaf area were more pronounced in soybean than in wheat, and this tendency increases in successive developmental phases. The net photosynthesis was stimulated during the juvenile phase and during that of the final one by about 13.6% (average) in soybean plants. Stimulation of transpiration was also observed after... 

An alternative approach, using semiconductors as light-driven electron donors, has been demonstrated in model systems (Gratzel 1982 Nikhandrov et al. 1988). These are more stable than the photosystems, but show lower photochemical conversion efficiencies owing to short-circuiting of reducing equivalents. The presently used... 

By using either one of these photosystems, one-electron (3-activation of a,(3-unsaturated carbonyl compounds produced carbon-centered radical precursors which cyclize efficiently and stereoselectively to tethered activated olefins or carbonyl groups. The 1,2-anti-stereochemistry observed contrasts with the general trend of syn-stereochemistry expected in 5-hexenyl radical cyclizations. Application of this methodology was successfully demonstrated by the stereoselective synthesis of optically pure C-furanoside, starting from L-tartaric acid (Scheme 38) [57,58]. 

Another way in which Up may be increased is to use two separate photochemical sensitizers in two distinct photosystems, each with a different range of spectral sensitivity. Analysis of AM 1.2 radiation ( 5) shows that a device in which one sensitizer absorbs all light with X X and a second sensitizer absorbs all light in the range Xj < X < X2 could achieve an efficiency of 44% for X] = 830 nm and X2 = 1320 nm. There is a wide range of values of Xi and X2 for which Up is above 40%. 

Where ria and r c are, respectively, the anodic and cathodic overpotentials. Considering all these losses an optimum bandgap of 2.0 to 2.25 eV is required for the materials used as photoelectrodes for water photoelectrolysis. In practical cases, a reasonable value of overall solar efficiency is 10% for single bandgap devices involving two photons and 16% for dual photosystem devices involving 4 photons [102,103,110,111]. 

Rogner, M., Boekema, E.J., Barber, J. (1996) How does photosystem 2 split water The structural basis of efficient energy conversion. Trends Biochem. Sci. 21, 44 49. 

These observations could be explained by the Z scheme if the absorption spectra of the antennas associated with photosystems I and II are different. Because the two photosystems must operate in series, light is used most efficiently when the flux of electrons through photosystem II is equal to that through photosystem I. If light of a particular wavelength excites one photosystem more frequently than the other, some of the light is wasted. 

Chl-coated semiconductor (n-type) electrodes have thus far been studied using ZnO, CdS, and Sn02, all of which act as efficient photoanodes for converting visible light. Such Chl-sensi-tized photoanodes could be regarded as in vitro models for the photosystem II (oxygen evolution) function in photosynthesis, p-type semiconductor electrodes have not been utilized successfully to produce cathodic Chl-sensitized photocurrents with satisfactory efficiencies. On the other hand, Chl-coated metal electrode systems seem to overcome this problem. 

Photosynthesis, another potential target of bioconjugation, is one of the most important processes in nature. In photosynthesis, photoelectric conversion with nearly 100% efficiency is involved in the primary process.47-48 Such a high performance of photosystem I (PSI) is due to its well-designed spatial configuration. A large number of trials have applied such biological systems to electronic devices. For example, chloroplasts coated on an SnC>2 electrode have been examined as photoelectrochemical cells.49-53... 

The combination of surface-associated reactants with surface-bound H-atoms, occasionally leads to poor photoinduced hydrogenation of the reactant and parallelly to inhibition of H2-evolution. For such systems, tailored bifunctional heterogeneous catalysts have been developed [141], where cooperative catalytic effects are observed in the photohydrogenation reactions. Substitution of ethylene by acetylene, C2H2, in the photosystem composed of Ru(bpy) +/MV2+/Na2EDTA and the Pt colloid results in inefficient hydrogenation of acetylene to ethylene,

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