Photosynthesis quantum requirements

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

If one assumes that the ratio H /e is 2, then an ATP/2 e of 1.66 can be calculated. Izawa and Good [91] (see also Ref. 92) have established that a ratio ATP/2 e of 2 can be calculated for phosphorylating electron transport, if one subtracts the independently occuring electron transport rate observed in the absence of ADP-Pji obviously, such a correction cannot be made if the purpose is to evaluate the quantum requirement of photosynthesis, but it is important in the investigation of the efficiency of energy coupling. 

Under conditions of low photorespiration, about 90% of photochemical product is allocated to COj fixation. The progression from light limitation to light saturation in photosynthesis is illustrated in Pig. 3 A which shows a typical light saturation curve for photosynthesis illustrated in the shmb Juanulloa aurantiaca. The quantum requirement per mole COj fixed (on an incident irradiance basis) increases as the irradiance and rate of CO2 assimilation increase. The higher the flux the lower the efficiency of COj assimilation (Fig. 3B). As the ATP and NADPH requirements for COj fixation are independent of... 

Ley AC, Mauzerall DC Absolute absorption cross-sections for photosystem II and the minimum quantum requirement for photosynthesis in Chtoretta vulgaris, Biochim Biophys Acta Bioenerg 680(1) 95-106, 1982. 

Fig.8 Diurnal variation of quantum requirement of photosynthesis in bamboo leaf...

Table 4. The apparent quantum requirement of photosynthesis measured at places of different altitude...

Table 5 The effect of atmospheric pressure and gas composition on the apparent quantum requirement of photosynthesis In wheat leaves...

Quantum requirement the number of light quanta required for the formation of one molecule of O2 in Photosynthesis (see). Two quanta are required per electron. The theoretical value of Q.r. is eight, since the production of one molecule of Oj proceeds according to the following equation, with transfer of four electrons from water (see Photolysis of water) to NADP+ 2H2O ->02 + 4H+ + 4e. The experimentally determined value of Q. r. lies between 8 and 10 for leaves, and between 10 and 14 for isolated chloro-plasts. It is influenced by the physiological state of the experimental system. 

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 ... 

In his rejection of the Emerson model, Rabinowitch went on to say The difficulty of the phosphate storage theory appears most clearly when one considers the fact that, in weak light, eight or ten quanta of light are sufficient to reduce one molecule of carbon dioxide. If each quantum should produce one molecule of high-energy phosphate, the accumulated energy would be only 80-100 kcal per Einstein—while photosynthesis requires at least 112 kcal per mole, and probably more, because of losses in irreversible partial reactions (Vol. 1, p. 228). 

Consideration of the energy involved in the process and of the energy of the quantum absorbed by chlorophyll showed that interaction with at least three excited molecules was required for each molecule of CO2 formed [186]. The investigations by Emil Warburg, fostered also by the formulation of the Einstein equivalence law (1905 in the first formulation see Chap. 2), and by his son Otto gave a quantitative framework to photosynthesis substituting well-defined chemical measurements to qualitative observations [188]. 

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