Photosynthesis in plants

Photocatalysis is a fundamental feature of life processes on our planet [1] (it provides photosynthesis in plants and bacteria) and of the chemistry of its atmosphere [2]. Work is under way to develop photocatalytic technologies for abatement of environmental problems [3,4]. Photocatalysis is anticipated to become in the coming years important also for selective organic synthesis [4]. In a more distant future thermal catalytic processes induced by heating with solcir radiation, together with photocatalytic processes may become important for environmentally friendly technologies of solar energy utilization [5-9]. 

Long-term aim of our project is the construction of a biomolecular device for hydrogen production in combination with light-driven water-splitting as it occurs in the natural process of photosynthesis in plants. Such a semiartificial device should combine the best suited components found in various native systems which - up to now - cannot be found in an individual native system due to incompatibilities and/or different origin (pro- and eukaryotic, meso- and thermophilic, aerobic - anaerobic environment). Advantage of such a system is... 

Water stress reduces the rate of photosynthesis in plants. In soybean crops, water stress leads to a loss of root integrity. The failure of the lower leaves to intercept light in a mature crop is compounded by the water-related limitations. The result is that lower leaves abscise and, because seed pods (yield) are supplied with assimilate mainly from leaves at the same nodal position on the stem, they in turn are lost and overall yield is reduced. The control of water stress in crops is a major PGR target in yield maintenance. The alleviation of water stress in developing crops, particularly in arid areas, is also desirable. 

Excited state redox potentials may be different from those of the ground state values. Redox reactions can be initiated on electronic excitation against the electrochemical gradients. Electron transfer in the excited state may be reversed in the ground state. An important example is photosynthesis in plants. 

Though most of the oxygen in the atmosphere has been formed by photosynthesis in plants, some is produced by photolysis of water vapour in the vacuum ultraviolet region A <200 nm. Photolysis of N2, NO, N02, NHa, CO, 002 and small aliphatic hydrocarbons (alkanes) set up complex reactions in the upper atmosphere. 

ATP synthesis is not the only energy-conserving reaction of photosynthesis in plants the NADPH formed in the final electron transfer is (like its close analog NADH) also energetically rich. The overall equation for noncyclic photophosphorylation (a term explained below) is... 

The fundamental similarity of photosynthesis in plants and bacteria, despite the differences in the electron donors they employ, becomes more obvious when the equation of photosynthesis is written in the more general form... 

The control of glycogen phosphorylase by the phosphorylation-dephosphorylation cycle was discovered in 1955 by Edmond Fischer and Edwin Krebs50 and was at first regarded as peculiar to glycogen breakdown. However, it is now abundantly clear that similar reactions control most aspects of metabolism.51 Phosphorylation of proteins is involved in control of carbohydrate, lipid, and amino acid metabolism in control of muscular contraction, regulation of photosynthesis in plants,52 transcription of genes,51 protein syntheses,53 and cell division and in mediating most effects of hormones. 

Cellulose is the principal structural component of vegetable matter. Wood is 30-40% cellulose, cotton over 90%. Photosynthesis in plants is responsible for the formation of 109 tons per year of cellulose. Structurally, cellulose is a polysaccharide composed of several thousand D-glucose units joined by (3(l,4)-glycosidic linkages (Figure 25.8). Complete hydrolysis of all the glycosidic bonds of cellulose yields D-glucose. The disaccharide fraction that results from partial hydrolysis is cellobiose. 

Oxygen in the earth s atmosphere is produced by photosynthesis in plants, such as this underwater elodea. 

In a broad sense photosynthesis in plants is a photoinduced electron transport reaction. Chlorophyll molecules in the green plants are the main light harvesting molecules. They are assisted by carotenoids and phycocyanins in this act. These molecules have absorption in the visible region covering the whole spectrum from blue to red. The energy absorbed by all these molecules is transferred to chlorophyll a (Chi a), which is the main light sensitive molecule, by mechanisms discussed in Section 6.6.4. 

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