Photosynthesizing systems

The present chapter discusses briefly modern ideas on the mechanisms of electron transfer during photosynthesis and the experimental data pointing to the important part played by electron tunneling reactions in the operation of the reaction centres of photosynthesizing systems. 

Electron tunneling in the reaction centres of photosynthesizing systems... 

Photosystem 1 is basically similar to the photosynthesizing system of bacteria just discussed. The difference between PSl and the photosystem of bacteria lies mainly in the fact that, instead of bacteriochlorophyll P890, the photochemical active centre of PSl contains chlorophyll a as a primary electron donor having the peak in the differential absorption spectrum at 700 nm and thus denoted as P700. In PS2 the primary donor of electrons is a chlorophyll molecule P680 with the peak in the differential optical spectrum at 680 nm. Photosystems 1 and 2 are located close to each other. Between them there is an electron transport chain containing molecules of plasto-quinones and cytochromes. 

P Sogo, M dost and M Calvin (1959) Evidence forfree-radicalproduction in photosynthesizing systems. Radiation Res, Supplement 1 511-518... 

Our results indicate a protective function of carotenoids, especially those in the xanthophyll cycle (V and Z) against photoinhibition as also observed by (7,8) and upto lesser extent to salt stress. Such damage appears to occur when the photosynthesizing system can not increase the content of protective compound, Z thus we observed more damage at 5 C than at 20 C. This suggest that changes in the xanthophyll cycle are temperature dependent. Under chilling conditions dissipation of excessive radiant... 

This process has been studied intensively from many points of view, but the fundamental mechanisms by which light affects a chemical reaction are still unknown. Many properties of photosynthesizing systems are known, but these properties cannot be ascribed to defined chemical reactions. The enzyme chemistry of photosynthesis is an almost completely unknown area. 

But nature has chosen the unlikely candidate, water, as the primary reducing agent for most photosynthesizing systems, probably because of its abundance when the process of photosynthesis was evolving. 

Fig. 8. Steady-state model for the earth s surface geochemical system. The kiteraction of water with rocks ki the presence of photosynthesized organic matter contkiuously produces reactive material of high surface area. This process provides nutrient supply to the biosphere and, along with biota, forms the array of small particles (sods). Weatheriag imparts solutes to the water, and erosion brings particles kito surface waters and oceans.

In the artificial system Figure 4b, a polymerized surfactant vesicle is substituted for the thylakoid membrane. Energy is harvested by semiconductors, rather than by PSI and PSII. Electron transfer is rather simple. Water (rather than C02) is reduced in the reduction half cycle to hydrogen, at the expense of benzyl alcohol. In spite of these differences, the basic principles in plant and mimetic photosyntheses are similar. Components of both are compartmentalized. The sequence of events is identical in both systems energy harvesting, vectorial charge separation, and reduction. 

Experimental research on the role of electron tunneling in biological systems is typically carried out on subchromatophore and subchloroplast fragments at low temperatures. The operation of photosynthesizing objects at low temperatures was first discovered by Chance and Nishimura [28] who reported the oxidation of cytochrome c under the action of light in photosyn-... 

PA Loach, GM Androes, AF Maksim and M Caivin (1963) Variation in electron paramagnetic resonance signals of photosynthetic systems with the redox level of their environment. Photochem Photobiol 2 443-454 B Rumberg (1964) Analyse der Photosynthese mit Blitzlicht. II. Die Eigenschaften des Reaktionscyclus von Chlorophyll-a,-430-703. Z Naturforschung 19B 707-716... 

Phosphorus is one of the most important elements on the Earth. It participates in or controls many of the biogeochemical processes occurring in the biosphere. The important role of phosphorus in the biosphere is owed to its vital role in protein synthesis. The exothermal reaction of adenosine triphosphate with photosynthesized hydrocarbons provides the subsequent biochemical reactions with energy (see structure and functions of DNA and RNA in Chapter 2, Box 5). The N P ratio in plant issues is within 8-15 (Bazilevich, 1974 Romankevich, 1982, Vitoushek and Howarth, 1991). Almost in all biogeochemical systems P was found as a deficient element limiting the productivity of ecosystems. 

---