Sucrose assimilation

As shown in Fig. 6B, a two-phase pattern occurred for the substrate uptake. It can be observed that during the exponential growth phase, sucrose assimilation by the bacteria was small, corresponding to about 20% of the initial amount introduced into the medium. However, after a 40-h process corresponding to the end of the growth phase, there was a rise in the substrate uptake, suggesting that the carbon source was directed to biosurfactant production, for the conditions tested. It should be emphasized that the fermentative process, when the medium was supplemented with microsalts and EDTA (Fig. 6A), generated a different substrate kinetics in comparison with that obtained for the nonsupplemented medium (Fig. 6B). 

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. 

Once C02 is fixed into 3-phosphoglycerate in the bundle-sheath cells, the other reactions of the Calvin cycle take place exactly as described earlier. Thus in C4 plants, mesophyll cells carry out C02 assimilation by the C4 pathway and bundle-sheath cells synthesize starch and sucrose by the C3 pathway. 

The accumulation and redistribution of assimilates can be viewed as changes in dry matter, carbon, individual nutrient elements, or specific compounds (e.g., sucrose) or classes of compounds (e.g., inulins). In the following subsections, the quantitative and temporal accumulation and subsequent redistribution of dry matter, carbon, and nutrient elements in the various locations within the plant are described. 

Riesmeier, J.W., Willmitzer, L., and Frommer, W.B., Evidence for an essential role of the sucrose transporter in phloem loading and assimilate partitioning, EMBO J., 13, 1-7, 1994. 

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... 

The C4 cycle can be viewed as an ATP-dependent C02 pump that delivers C02 from the mesophyll cells to the bundle-sheath cells, thereby suppressing photorespiration (Hatch and Osmond, 1976). The development of the C4 syndrome has resulted in considerable modifications of inter- and intracellular transport processes. Perhaps the most striking development with regard to the formation of assimilates is that sucrose and starch formation are not only compartmented within cells, but in C4 plants also may be largely compartmented between mesophyll and bundle-sheath cells. This has been achieved together with a profound alteration of the Benson-Calvin cycle function, in that 3PGA reduction is shared between the bundle-sheath and mesophyll chloroplasts in all the C4 subtypes. Moreover, since C4 plants are polyphyletic in origin, several different metabolic and structural answers have arisen in response to the same problem of how to concentrate C02. C4 plants have three distinct mechanisms based on decarboxylation by NADP+-malic enzyme, by NAD+-malic enzyme, or by phosphoenolpy-ruvate (PEP) carboxykinase in the bundle-sheath (Hatch and Osmond, 1976). 

The characteristics of the phloem loading system can be summarized as follows. Sucrose loading is (1) dependent on metabolism (2) carrier-mediated (3) selective for sucrose (4) maintains a high concentration inside the phloem which is the basis for the osmotically-driven mass flow of solutions and (5) dependent on the factors which control assimilate supply to the loading sites (e.g., photosynthesis, sucrose synthesis, and sucrose movement between leaf cells, and within subcellular compartments such as the cytoplasm and vacuole) ((>, 7 ). 

Komor, E. (2000). Source physiology and assimilate transport the interaction of sucrose... 

During photosynthesis, chloroplasts convert CO2, water and Pj to triose phosphates that are exported to the cytosol. Phosphate is therefore a substrate of this process and the continued operation of the RPP cycle is dependent on the utilization of triose phosphate for the synthesis of starch (in the chloroplast) and sucrose (in the cytosol). These synthetic processes release Pj, preventing the level of free Pj in the cell from falling to a concentration where photosynthesis may be limited by its availability. Such a limitation of photosynthesis is observed during O2-insensitive CO2 assimilation [56] and is suggested by the increase in CO2 fixation detected on feeding P via the transpiration stream to a cut leaf [57]. It has long been known that isolated chloroplasts require a continuous supply of P in order to sustain photosynthesis. 

However, for economy of production, maximum yields of alkaloids, and ease of recovery of the products, certain culture media containing relatively simple nutrient sources are preferred. For example, the media which are useful in the production of the alkaloids include an assimilable source of carbon such as glucose, sucrose, starch, molasses, dex-trins, corn steep solids, corn syrup liquor, sorbitol, mannitol, lactose, and the like. A preferred source of carbon is mannitol. Additionally, the media employed contain a source of assimilable nitrogen such as oatmeal meat extracts, peptones, amino acids and their mixtures, proteins and their hydrolysates, com steep liquor, soybean meal, peanut meal and ammonium salts of organic acids such as the citrate, acetate, malate, oxalate, succinate, tartrate and like salts. 

The data show that SSHB increased the root dry matter in the same way it did in the other experiments. Simultaneously, we observed an increase in leaf length and leaf area which corresponded with an increase in invertase activity. The higher enzyme activity appeared to be the key to the whole problem. It is well known that acid invertase splits the disaccharide sucrose so that the two monosaccharides formed from it can become building stones for additional leaf substances. That is, glucose is recycled through the Embden-Meyerhof-Parnas pathway. Likewise, fructose is readily available for conversion into fructose 1,6-diphosphate. Subsequently, more leaf area arises which is able to produce more assimilates. The consequence is an increased yield. Thus, photosynthetic efficiency increases considerably. 

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