Overall Process of Photosynthesis

The best-understood reaction for the synthesis of glucose, and probably the most important quantitatively, is photosynthesis. Photosynthesis converts carbon from carbon dioxide to glucose with reducing equivalents supplied from water and energy supplied from light. 

The energy in light is dependent on its wavelength, and is given by the following relationship. 

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To begin our discussion of a mechanistic model for photosynthesis, let us return to Eq. (12.1b) for the overall process of photosynthesis in green plants ... 

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

With respect to mechanistic consequences of the H /ATP ratio of four we refer to a recent coupling model described in [11]. With respect to the overall process of photosynthesis we have to accept that on the basis of the H /ATP ratio of four, taken together with the probable H/e ratio of 2.5 (see [1]), linear electron transport alone does not produce enough ATP to drive CO2 reduction. 

A series of experiments using tracers was carried out in the late 1950s by Melvin Calvin at the University of California, Berkeley, to discover the mechanism of photosynthesis in plants. The overall process of photosynthesis involves the reaction of CO2 and H2O to give glucose, C6H12O6, and O2. Energy for photosynthesis comes from the sun. 

Chi a fluorescence is a property exhibited by all photosynthetic organisms due to the essential role of chlorophyll in the structure and function of the photosynthetic apparatus. Typically less than 3% of the absorbed light is ever re-emitted as chlorophyll fluorescence and, at room temperature, the latter primarily emanates from PSII (Krause and Jahns, 2003). Quantification of chlorophyll a fluorescence induction has proven to be an extremely useful tool to assess the structure and ftmction of PSII and the overall process of photosynthesis (Krause and Weis, 1991 Krause and Jahns, 2003). Since reduced Qb is in equilibrium with the reduced PQ pool, the relative reduction state of the PQ pool, also called excitation pressure, can be estimated in vivo as the relative reduction state of Qa, that is [Qa ] / ([Qa] + [Qa ]), which can be conveniently measured in vivo as 1-qP using pulse amplitude modulated... 

Photosynthesis in green plants is by far the most crucial redox reaction on Earth, for life depends upon the transduction of solar energy into biochemical energy. In simplest terms, the overall process of photosynthesis can be considered as light-driven, coupled CO2 reduction and H2O oxidation. The important landmarks of photosynthesis research are summarized in Table 2. In spite of more than 200 years of efforts, however, we are still far from solving all the major problems of photo-synthesis 42. Among the outstanding problems that remain to be solved at the molecular level are (i) the primary events of quantum conversion. 

In the process of photosynthesis, plants use C02 and water to produce sugars according to the overall reaction... 

An alternative way is to divide the overall process of water splitting into two stages, each being conducted in a separate photoelectrochemical cell. Both cells are coupled with the aid of a certain intermediate substance, which acts as a charge transfer agent and is not consumed in the course of the overall process, but only provides the connection in series of the chemical potentials developed in both cells. Apparently, such a scheme imitates the combination of two photosystems in photosynthesis occurring in green plants. 

All the free energy consumed in biological systans arises from solar energy that is trapped by the process of photosynthesis. The overall process can be represented by Equation 11.54, but it actually proceeds by a series of reactions some of which are quite complicated and not fully understood. The end product, the carbohydrate glucose, C6H,20g, is actually stored in cells in the highly polymerised forms known as polysaccharides, (CgH,206) which are obtained from glucose by a process known as glycogenesis. 

The fixation of carbon dioxide to form hexose, the dark reactions of photosynthesis, requires considerable energy. The overall stoichiometry of this process (Eq. 22.3) involves 12 NADPH and 18 ATP. To generate 12 equivalents of NADPH necessitates the consumption of 48 Einsteins of light, minimally 170 kj each. However, if the preceding ratio of l ATP per NADPH were correct, insufficient ATP for COg fixation would be produced. Six additional Einsteins would provide the necessary two additional ATP. Prom 54 Einsteins, or 9180 kJ, one mole of hexose would be synthesized. The standard free energy change, AG°, for hexose formation from carbon dioxide and water (the exact reverse of cellular respiration) is +2870 kj/mol. 

In the overall scheme of the photosynthesis of green plants the electron transport cycle starts with the excitation of chlorophyll a in photosystem 2. The excited electron then follows a downward electron acceptor chain which eventually reaches the chlorophyll a of photosystem 1 (P700) in which it can fill the positive hole left by electronic excitation. The energy released in the electron transport chain which links photosystems 2 and 1 is used for other biochemical processes which are thereby related to photosynthesis. One of these is the process of photophosphorylation which is the production of molecules with phosphate chains used as energy transfer agents in many biochemical reactions. 

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