Reactions photosynthesis

Suppose that the photosynthesis reaction (20b) in Chapter 24 (p. 430) could be based upon light of wavelength 2400 A (this light is absorbed heavily by ozone). How many moles of these photons would provide the 673 kcal of energy needed to produce one mole of glucose (Remember, E = hv, and h = 9.5 X 10 14 kcal sec/mole). Compare your answer with that of Exercise 24-10. 

This would ignore the conditions of the two reactions, and in particular the special circumstances under which the endothemtic photosynthesis reaction takes place. This also ignores how multi-stage reactions are being summarised in a single equation. It is not unusual to see energy included as a term, especially in the respiration equation ... 

The individual steps of the multistep chemical reduction of COj with the aid of NADPHj require an energy supply. This supply is secured by participation of ATP molecules in these steps. The chloroplasts of plants contain few mitochondria. Hence, the ATP molecules are formed in plants not by oxidative phosphorylation of ADP but by a phosphorylation reaction coupled with the individual steps of the photosynthesis reaction, particularly with the steps in the transition from PSII to PSI. The mechanism of ATP synthesis evidently is similar to the electrochemical mechanism involved in their formation by oxidative phosphorylation owing to concentration gradients of the hydrogen ions between the two sides of internal chloroplast membranes, a certain membrane potential develops on account of which the ATP can be synthesized from ADP. Three molecules of ATP are involved in the reaction per molecule of COj. 

Figure 12.1. Schematic diagram of a photosynthesis reaction center. Light is absorbed by pigments in the light-gathering antenna and absorbed energy is transferred to a photochemically active site P, where it is utilized to initiate photosynthetic reactions.

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

It is helpful to think of the photosynthesis reaction as the sum of an oxidation half reaction and a reduction half reaction as shown in Figure 1. In fact, nature does separate these half reactions, in that the reduction of C02 to carbohydrates occurs in the stroma of the chloroplast, the organelle in the leaf where the photosynthesis reaction occurs, - whereas, the light-driven oxidation half reaction takes place on the thylakoid membranes which make up the grana stacks within the chloroplast. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) carries the reducing power and most of the energy to the stroma to drive the fixation of C02 with the help of some additional energy provided... 

Much has been written about artifical solar-energy converters - the reader is referred to references 10, 12, 14-17 for detailed treatments. Here I shall deal exclusively with those artificial systems designed to mimic various aspects of the photosynthesis reaction. 

Taking into account the requirements listed in Section A, it would be desirable if the reactants A and B be compounds which are very cheap and readily available. Naturally, the constituents of the atmosphere and liquid water fill this requirement admirably. Table 1 lists most of the endergonic fuel generation reactions which involve N2, CO2 and H2O as reactants including the reaction of photosynthesis. It is significant that the potential difference AE , which is the potential stored per electron transferred, is between 1.02 V and 1.48 V for all of the reactions in Table 1. Thus, the energy requirements for the photochemistry are about the same for each of these reactions. We immediately see that the reaction of photosynthesis (reaction 9 of Table 1) is in troiable for one photosystem because is known to be 700 nm. The imp-... 

The most famous example of nature manipulating thermodynamics is probably the photosynthesis reaction, without which most life forms would not be present on the Earth. Thermodynamically, CO2 + H2O are much more stable than carbohydrate + O2. However, nature can convert CO2 + H2O to carbohydrate + O2 through the photosynthesis reaction ... 

The respiration reaction is, basically, the opposite of the photosynthesis reaction. During respiration, glucose within the cells reacts with oxygen to produce carbon dioxide, water, and energy. 

In the second set of reactions of photosynthesis, carbohydrates are synthesized from carbon dioxide and hydrogen (from NADPH). The ATP and NADPH produced by the first set of photosynthesis reactions provide the energy for these reactions. 

In the marine photosynthesis reaction, for convenience, the proportion of sulfur was raised from 1.7 to 2, and for land photosynthesis the proportion of S was rounded up to 1. The proportions of hydrogen and oxygen in organic matter are nearly H 0 = 2 1 this is the reason for the abbreviated chemical notation of organic matter usually written as CH2O. 

In subsequent discussions, other examples will be given which support the view that allelopathic interference with crops may operate through effects on water balance and mineral nutrition. A number of allelopathic chemicals other than phenolic acids may also have their initial effects on cellular and organelle membranes. A disruption of membranes not only affects nutrient transport, but respiratory coupling, photosynthesis reactions, and other membrane associated processes. 

The oft quoted example of the isothermal photosynthesis reaction is fructose production. Carbon dioxide and water are forced together, by solar power, catalysed by chlorophyll. Oxygen is released. That is. 

Porphyrin-quinone compounds as models of photosynthesis reaction center 89UK1032. 

Although such a 2-MHz ESR apparatus was very sophisticated, its time resolution was not enough for measurement of CIDEP. In 1973, Fessenden [5] found that the direct ESR measurement without field modulation improved the time resolution, observing CIDEP signals in solution with pulse radiolysis. This method was applied for laser-photolysis measurements in solids [6] and in solution [7]. A spin-echo ESR technique was also found to be useful for CIDEP [8]. Since then, CIDEP experiments with cw-ESR and pulsed-ESR spectrometers without field modulation have become much more popular than before. Through such transient ESR measurements, CIDEP due to not only the radical pair mechanism but also several other mechanisms have been observed in many chemical reactions including biologically important ones such as photosynthesis reactions. In this chapter, we will show several mechanisms for CIDEP with several typical examples. 

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