Carbohydrates, photosynthetic cycle

One of the following molecules (a)-(d) is n-erytlirose 4-phosphale, an intermediate in the Calvin photosynthetic cycle by which plants incorporate CO2 into carbohydrates. If o-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 o-erythrose 4-phosphate, and which are diastereomers ... 

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... 

Recent work by Calvin and co-workers and by Horecker et al. have demonstrated the importance of sedoheptulose in the photosynthetic cycle of the carbohydrates as being a transitory intermediate in the regeneration of D-erythropentulose, the monosaccharide involved in the fixation of carbon dioxide . This sugar also plays a part in animal carbohydrate metabolism. Its most characteristic chemical property is its ready conversion into sedoheptulosan.(2,7-anhydro-j8-D-altroheptulopyranose (LVII). 

This cycle represents the quantitatively most important C02 fixation pathway in Nature. It is found in most aerobic autotrophic organisms, ranging from diverse photosynthetic and chemolithoautotrophic bacteria to chloroplasts of eukaryotic algae and higher plants [5]. It is centered around carbohydrates, with ribulose 1,5-bisphosphate being the C02 acceptor (Figure 3.1). 

Despite the fact that many heptoses are by far less prominent in Nature than hexoses these monosaccharides are found both as metabolic intermediates, and as structural carbohydrates of bacterial cell walls.D-Sedoheptulose 7-phosphate is an important intermediate of the pentose cycle, and D-sedoheptulose 1,7-bisphosphate is present in plants as an intermediate of the dark phase of photosynthetic reactions. L-Glycero-D-manno-heptose was isolated from the oligosaccharides obtained by partial acid hydrolysis of the lipopolysaccharide from Escherichia coli K-12 strain W3100 [153] and Haemophilus influenzae [154]. Both L-glycero-D-wtanno-heptose and D-glycero-D-ma o-heptose were isolated from the lipopolysaccharide of Vibrio parahaemolyticus [155]. 

Some aspects of the carbon cycle are detailed in Figure 2 and more details are shown in Section 2. One can see that the reduction of 1 mole of CO2 in oxygenic photosynthetic formation of carbohydrates, and the concomitant splitting of 1 mole of H2O, releases 1 mole of O2. This oxygen, in turn, will take part in oxidation processes like respiration. It is known that the content of atmospheric oxygen is about 21%, and to maintain this value the rapid cycling of carbon in tbe atmosphere, hydrosphere, and biosphere is highly necessary. 

Overall, the hydrogen stored in NADPH is used to reduce C02 to carbohydrate units (0H2O). This is not a direct reaction because the C02 is first combined with a C5 compound, ribulose diphosphate (RDP), which then spontaneously splits into two identical C3 molecules, phosphoglyceric acid (PGA). Most of the PGA is used to synthesize further RDP but some is reduced by NADPH, using energy supplied by the ATP/ADP system, to give triose phosphate, which in turn is converted into the glucose phosphate from which various carbohydrates are synthesized. This assimilatory path is known as the Calvin cycle and is involved in all autotrophic carbon fixation, whether photosynthetic or chemosynthetic. 

The (CH20) is the general formula for a carbohydrate. It was then assumed that the energy stored in the carbohydrate was used in other chemical reactions to synthesize all the other plant materials (proteins, lipids, fats, and so on). It is now clear that amino acids, for example, are immediate products of the photosynthetic reduction of carbon dioxide, and that carbohydrate need not be synthesized first. This is not to minimize the importance of the photosynthesis of carbohydrate, but only to note that many other types of compounds are produced photosynthetically. The overall mechanism and many of the details of the carbon reduction cycle, CO2 to carbohydrate, were worked out by Melvin Calvin and his colleagues, for which he received the Nobel Prize. 

Respiration is the reduction of O2 to H2O during the oxidation of carbohydrate to CO2. There are two types of respiration in photosynthetic organisms a dark respiration and a photorespiration [3]. Dark respiration includes O2 reduction and the oxidation of NADH and FADH2 in mitochondrial membranes, glycolysis, the Krebs cycle, and the oxidative pentose phosphate pathway. Respiration is commonly subdivided into two functional components growth respiration, supplying energy for new biomass production, and... 

If items made of PHAs are composted, they are completely degraded to water and carbon dioxide as the final products of their oxidative breakdown. Here it has to be emphasized that these final oxidation products are the basic materials for the photosynthetic regeneration of carbohydrates by green plants. This demonstrates that, in contrast to petrol-based plastics, PHAs are embedded into the natural closed cycle of carbon. The range of applications for PHAs is not limited to simple packaging materials, but encompasses commodity items, materials for agro-industrial purposes and pharmaceutical and medical applications. The major advantageous characteristics of PHAs can be summarized as follows ... 

Maintenance of a constant photosynthetic rate over 4 hours (it increased by 1%) in warm-grown sunflower at 30 C contrasts with the 9.5% decline in photosynthetic rate at 30 C in warm-grown rape over the same period. At higher temperatures the movement of triose phosphate towards sucrose synthesis may be too rapid in rape depleting Calvin cycle pools and depressing photosynthetic rate. In cold-grown plants of sunflower and rape photosynthetic rate declined over 4 hours by 7.4% and 3.6% respectively. The decline in photosynthetic rate in the cold may be due to a progressive diminuition of Pi caused by slow consumption of triose phosphate. The accumulation of carbohydrates in the cold (Table 1), may also depress photosynthesis by feedback inhibition. 

The synthetic reactions requiring electrons and ATP are not limited to the initial reduction of the inorganic oxides. Many secondary photosynthetic pathways in the chloroplast convert the products of the primary carbon reduction cycle plus ammonium and sulfhydryl to a host of secondary products. Among these are carbohydrates, fats, proteins, nucleic acids, various coenzymes, and many other substances needed both for the growth and activity of the chloroplasts and for export to other parts of the cell or organism. 

---