Photosynthetic cycle

One of the following molecules (a)-(d) is D-erythrose 4-phosphale, an intermediate in the Calvin photosynthetic cycle by which plants incorporate C02 into carbo- hydrates. If D-erythrose 4-phosphate has R stereochemistry at both chirality centers, which of the structures is it Which of the remaining three structures is the enantiomer of D-erythrose 4-phosphate, and which are diastereomers ... 

Figure 9.4 Membrane model for electron transfer reaction in photosynthetic cycle with acceptor A and donor D on either side of the membrane (a) P, P. P+ are respectively normal state electronically excited state and oxidised form of pigment molecule, (b) Illustrating energy levels of ground and excited states of pigment molecule in the membrane and acceptor and donor molecules in solution. tk.G=riFE is theoretically available electrochemical free energy e=electron, +=positive hole.

The implementation of an artificial photosynthetic cycle which could recycle C02 [4, 5], thus complementing the natural cycle, could also make an effective contribution to reducing C02 emissions. 

Due to the fact that lasers can be focused into a very small volume, small slits can be used together with a fast rotating disk to make the time resolution in the one-slit experiment in the tens of nanoseconds when using very sensitive detection techniques and samples with good Raman enhancements. This technique will probably be most useful in the microsecond time regime. Fig. 1 shows the results of this technique when used in the measurement of the time development of the bands characteristic of the intermediates produced in the bacteriorhodopsin photosynthetic cycle (8). Using optical flash photolysis (17) techniques, the rise time of the intermediate having a Raman band at 1570 cm l is known to be in the microsecond time scale. 

Various possible time resolved techniques are discussed which enable one to measure the vibrational spectra (and what they entail of structural information) of the distinct transient intermediates formed in different photochemical decomposition schemes and at different times (in the sec-picosec range). The techniques make use of 1) the difference in the time development behavior of the different intermediates, 2) the difference in the absorption maxima and thus the difference in the resonance Raman enhancements for the different intermediates, and 3) the laser power. The techniques use one or two lasers for the photolytic and probe sources as well as an optical multichannel analyzer as a detector. Some of the results are shown for the intermediates in the photosynthetic cycle of bacteriorhodopsin. 

In a sense, the chemistry of O2 itself represents something of a nonproblem the photosynthetic cycle keeps in balance the production and removal (through respiration) of molecular oxygen. Net removal of O2 accompanies the combustion of fossil fuel but even were the Earth s entire reserve of carbon burned to yield CO2, the concomitant decrease in partial pressure of O2 would be small and probably of little environmental consequence when compared to the enormous increase in atmospheric carbon dioxide. Our research efforts are primarily concerned with the establishment of a reliable base of kinetic and photochemical data that may be applied to the development of advanced models of atmospheric chemistry. Field measurements alone provide only part of the answer in that one cannot measure everything simultaneously and there are certain minor constituents (such as HO2) which even now cannot be adequately monitored at ambient concentrations. All models are in a sense underdefined in that were one to vary freely the available parameters, perfectly good fits to the limited field data would be possible. The goal of laboratory studies is to constrain the number of free parameters that are available to a large extent I think that we have been successful. 

As stated earlier, photosynthesis generates electronic "holes" in the photosynthetic apparatus on the outside of the membrane. These holes are powerful oxidizing agents that is, they have very high affinities for electrons and can pull electrons from many types of molecules. They can even oxidize water. Thus, for many photosynthetic organisms, the electron donor that completes the photosynthetic cycle is water. The product of water oxidation is oxygen gas—that is, molecular oxygen (O2). 

In bacterial chromatophores the RC and the b/c, complex are arranged to form a cyclic electron transfer system possibly mediated by the diffusion of ubiquinone and cyt. Cj these carriers are, however, also coupled to other multienzyme complexes forming the respiratory chain and perform the aerobic metabolism of these facultative photosynthetic organisms [254]. The electrogenic steps of the photosynthetic cycle take place both within the RC and the 6/cj complexes and can be monitored by the electrochromic spectral shift of endogenous carotenoids and on the basis of their response to specific inhibitors and kinetics. When induced by a short laser flash the carotenoid signal displays three distinct kinetic phases (r,/2 10 h/i 5 jas... 

Figure 7.51. Schematic representation of charge separation in the photosynthetic cycle a) in green plants involving photosystems PS II and PS I and b) in photosynthetic active bacteria (by permission from Rettig, 1986).

Fig. 6.1.2. Comparison of elemental sulfur-linked and oxygen-linked respiratory-photosynthetic cycles.

Respiration (oxidation) in plants and animals and oxidation in soils complete the photosynthetic cycle by utilizing the energy stored in the carbohydrates and organic compounds derived from the carbohydrates, by disposing of organic wastes, and by producing the C02 needed for more photosynthesis by the reaction ... 

Investigations into the effects of ultraviolet and visible light on carbohydrates have derived much of their impetus from the desire to understand the photodegradation of cellulosic materials, for such reactions are of commercial significance. An understanding of the photosensitized degradation of carbohydrates may also be of value in the study of processes which operate in the photosynthetic cycle. However, many of the investigations... 

Figure 1. Sequence of photochemical reactions in the photosynthetic cycle (Z-scheme).

The most significant is the photos5mthetic mechanism. There are two classes of the photosynthetic cycle, the Calvin-Benson photosynthetic cycle and the Hatch-Slack photos5mthetic cycle. 

On the other hand, the Hatch-Slack photosynthetic cycle is important for tropical grasses, com and sugarcane. Phosphenol-pyr-uvate carboxylase is responsible for the primary carboxylation reaction. The first stable carbon compound is a C-4 acid, which is subsequently decarboxylated (101,102). The fundamentals and sys-tematics of the non-statistical distributions of isotopes in natural compounds have been reviewed (103). 

In the photosynthetic center II exactly the opposite process happens water gets reduced to give dioxygen. So the potential expected for reaction, at pH = 5, will be (H20/02) = —0.933 V. The full reaction for each photosynthetic cycle involves two water molecules, so we write... 

Other terpenoids in addition to the carotenoids are involved in the photosynthetic cycle of electron transfer and energy conversion in green plants. Plastoquinones A and C [1312], Vitamin Kj and a-tocopherolquinone occur in sufficient quantity to be considered candidates for a role in electron transport [1313]. Much of this evidence is considered in a recent publication [1314]. 

According to the ideas of Calvin, the priming reaction for photosynthesis, that is the transformation of water under the influence of light, furnishes much TPNH and ATP from the electromagnetic energy of the light. Also pyruvic acid is diverted to the photosynthetic cycle as a result of the presence of thioctic acid in its dithiol form. 

Calvin and his school believe that ribulose diphosphate is the immediate precursor of 3-phosphoglyceric add. Thus the photosynthetic cycle would begin by a carboxylation of Ru-PP with formation Cj + C = 2Ca) of two molecules of phosphoglyceric acid (PGA). Starting with PGA, all... 

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