Photosynthesis quanta

Dutton HJ(1997) Carotenoid-sensitized photosynthesis Quantum efficiency, fluorescence and energy transfer. Photosynth Res 52 175-185... 

The enhancement effect" (Emerson s 2nd effect), i. e. the increase of photosynthesis quantum yield in the far-red spectral region at simultaneous illumination of chlorophyll-containing cells with shorter-wave light, has been drawing attention of researchers for almost 30 years. 

Weber S, Ohmes E, Thurnauer M C, Norris J R and Kothe G 1995 Light-generated nuclear quantum beats a signature of photosynthesis Proc. Natl Acad. Sc/. USA 92 7789-93... 

The quantum yield of photosynthesis, the amount of product formed per equivalent of light input, has traditionally been expressed as the ratio of COg fixed or Og evolved per quantum absorbed. At each reaction center, one photon or quantum yields one electron. Interestingly, an overall stoichiometry of one translocated into the thylakoid vesicle for each photon has also been observed. Two photons per center would allow a pair of electrons to flow from HgO to NADP (Figure 22.12), resulting in the formation of 1 NADPH and Og. If one ATP were formed for every 3 H translocated during photosynthetic electron transport, 1 ATP would be synthesized. More appropriately, 4 hv per center (8 quanta total) would drive the evolution of 1 Og, the reduction of 2 NADP, and the phosphorylation of 2 ATP. 

Krause, G.H. Laasch, H. (1987). Energy-dependent chlorophyll fluorescence quenching in chloroplasts correlated with quantum yield of photosynthesis. Zeitschrift fiir Naturforschung, 42, 581. ... 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

A potentially more serious problem than that of low efficiency is the requirement that the photochemical absorber operate without any significant side reactions. For example, if the quantum yield for side reactions were 1%, then after only 100 cycles the concentration of the absorber would have decreased to 37% of its original concentration. It is interesting that in photosynthesis each chlorophyll molecule processes at least 10 photons in its lifetime in a leaf. This means that the quantum yield for reactions leading to the degradation of chlorophyll must be less than 10-5 ... 

When plants receive more light energy than can be used for photochemistry, they show a characteristic decline in the quantum efficiency of photosynthesis, termed photoinhibition (reviewed in Ref. 110). Most plants experience photoinhibition at some point over the course of their lives, often on a regular or even a daily basis. The excess quanta from photoinhibitory... 

Measurement of Quantum Yield, Quantum Requirement, and Energetic Efficiency of the 02-Evolving System of Photosynthesis by a Simple Dye Reaction 127... 

Thus, electron tunneling appears to be capable of providing simultaneously conditions for a high quantum yield of charge separation and a high efficiency of the multistage process of photosynthesis as a whole. 

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