Photosynthesis starch synthesis

Potassium is required for enzyme activity in a few special cases, the most widely studied example of which is the enzyme pymvate kinase. In plants it is required for protein and starch synthesis. Potassium is also involved in water and nutrient transport within and into the plant, and has a role in photosynthesis. Although sodium and potassium are similar in their inorganic chemical behavior, these ions are different in their physiological activities. In fact, their functions are often mutually antagonistic. For example, increases both the respiration rate in muscle tissue and the rate of protein synthesis, whereas inhibits both processes (42). 

Sucrose synthesis in the cytosol and starch synthesis in the chloroplast are the major pathways by which the excess triose phosphate from photosynthesis is harvested. Sucrose synthesis (described below) releases four Pi molecules from the four triose phosphates required to make sucrose. For every molecule of triose phosphate removed from the chloroplast, one Pj is transported into the chloroplast, providing the ninth Pj mentioned above, to be used in regenerating ATP. If this exchange were blocked, triose phosphate synthesis would quickly deplete the available Pj in the chloroplast, slowing ATP synthesis and suppressing assimilation of C02 into starch. 

FIGURE 20-28 Regulation of ADP-glucose phosphorylase by 3-phosphoglycerate and Pj. This enzyme, which produces the precursor for starch synthesis, is rate-limiting in starch production. The enzyme is stimulated allosterically by 3-phosphoglycerate (3-PGA) and inhibited by P, in effect, the ratio [3-PGA]/[Pi], which rises with increasing rates of photosynthesis, controls starch synthesis at this step. 

The partitioning of triose phosphates between sucrose synthesis and starch synthesis is regulated by fructose 2,6-bisphosphate (F2,6BP), an allosteric effector of the enzymes that determine the level of fructose 6-phosphate. F2,6BP concentration varies inversely with the rate of photosynthesis, and F2,6BP inhibits the synthesis of fructose 6-phosphate, the precursor to sucrose. 

During the day, the rates of starch and sucrose synthesis and the rate of photosynthetic carbon assimilation must be coordinated. There is a clear need to determine how much assimilated carbon can be diverted into sucrose and starch synthesis without decreasing too much the amount that returns to the RPPP. Conversely, when sucrose accumulates in the cytosol because the rate of export diminishes (and/or photosynthesis increases), starch begins to accumulate inside the chloroplast. During the night, the... 

Libessart, N., Maddelein, M. L., Van den Koomhuise, N., Decq., A., Deirue, B., and Ball, S. 1995. Storage, photosynthesis and growth The conditional nature of mutations affecting starch synthesis and structure in Chalamydomonas reinhardtii. Plant Cell 7, 1117-1127. 

Both potato tuber and potato leaf ADP-Glc PPases are plas-tidic the leaf enzyme is in the chloroplast, and the tuber enzyme is in the amyloplast (74). The ferredoxin-thioredoxin system is located in the chloroplast and thus, with photosynthesis, reduced thioredoxin is formed and activated within the leaf ADP-Glc PPase. At night, oxidized thioredoxin is formed it oxidizes and inactivates the ADP-Glc PPase. This activation/inactivation process during the light/dark cycle allows a fine tuning and dynamic regulation of starch synthesis in the chloroplasts. Thioredoxin isoforms are present in many different subcellular locations of plant tissues cytosol, mitochondria, chloroplasts, and even nuclei (75) and are also present in amyloplasts (76). 

So the CO2 and O2 g insensitivity observed in water stressed plants indicates an effect on the biochemistry of photosynthesis, and from the analytical gas exchange analysis presented above, it is predicted that the effect is on sucrose or starch synthesis, or both. Vassey and Sharkey (11) found that the extractable activity of sucrose-phosphate synthase (SPS) was reduced by nearly 70% by mild water stress (Figure 4). We have made a number of additional tests and have been able to demonstrate the following 1. SPS in water stressed plants recovers when the plant is placed in high CO2 (1 mbar) even though the stress is not relieved. 2. Unstressed plants held in a low CO2 (75 /ibar) environment for one hour exhibit reduced SPS activity. 3. The recovery of SPS activity in water stressed plants requires one to five hours and can be prevented by feeding cycloheximide. 

Starch synthesis is closely related to sucrose synthesis (see Chapters IX and XII). Experiments with isotopically labeled sugars have shown that leaves can form starch from externally supplied glucose, fructose, and sucrose, as well as from several other compounds. Starch is also formed from carbon dioxide during photosynthesis. It was found (78) that when labeled carbon dioxide was fed to leaves, the starch became labeled before the free sugars. Labeled glucose and glucose 1-phosphate did not contribute an... 

Mature dry seeds of Lupinus luteus contain no starch, but starch grains appear during mobilization of the protein and lipid reserves [102]. The source of material for starch synthesis is not photosynthesis (for starch grains also form in dark-germinated seeds), nor the proteins, nor the small quantities of... 

The main question is whether synthesis of PHA in plants can succeed in bringing the cost of the polymer down to the range of 0.5 -1 US /kg. Bacterial production of PHA typically relies on a carbon source, such as sucrose or glucose, which is produced from photosynthesis and extracted from plants. Synthesis of PHA directly in plants would, therefore, represent a saving in terms of the number of intermediary steps linking C02 fixation to PHA production. Furthermore, starch is one of the cheapest plant commodity product on the market, at about 0.25 US /kg [86]. It is, thus, likely that the production cost of PHA in plants will be substantially cheaper than bacterial fermentation. The final cost of producing PHA in plants will depend on a number of factors. 

Chloroplasts (29-36) are the sites of photosynthesis and their ribosomes can carry out protein synthesis. Chloroplasts that contain chlorophylls and carotenoids, are disc shaped and 4-6 pm in diameter. These plastids are comprised of a ground substance (stroma) and are traversed by thylakoids (flattened membranous sacs). The thylakoids are stacked as grana. In addition, the chloroplasts of green algae and plants contain starch grains, small lipid oil droplets, and DNA. 

Research had confirmed that no parent simazine residues were found in treated com plants, and additional data on the dissipation pathway of simazine needed to be developed. Research also indicated that triazines interfered with the photosynthetic process on susceptible growing weeds, as evidenced by the appearance of chlorotic leaves. Steps were undertaken to elucidate simazine s dissipation pathway and herbicidal mode of action. In Basel, Dr. Gast (1958) showed that the accumulation of starch by common coleus (Coleus blumei Benth.) plants was inhibited from treatment with 2-chloro-4,6-bis-(alkyl-amino)-triazines due to the inhibition of sugar synthesis. At the same time, Moreland et al. (1958) found weed control activity could be reduced by supplying carbohydrates to the plants through their leaves and that simazine was a strong inhibitor of the Hill reaction in photosynthesis. Exer (1958) found that triazines inhibited the Hill reaction as strongly as urea of the CMU (monuron) type. 

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