Model based design of a biochemical cultivation process

In this paper a simple design procedure is used to enhance the performance of a biotechnical cultivation process. A model supported approach is proposed. It starts with a simple classical process model obtained from literature data, which is used to design the first experiment. Then, the main procedure is an iteration of (i) improving the model making use of the deviations between the experimental data and the data predicted by the model, (ii) of designing the next experiment by determining the optimal control profiles from the current model, and (iii) of executing that designed experiment. Important for the success of the procedure is that the model development is oriented at the process performance. The procedure is demonstrated at the simple practical example of a laboratory-scale fed-batch cultivation of Escherichia coli.     

On spatial periodicity in the formation of cell adhesions to a substrate

Interference reflection microscopy reveals a fairly regular array of marginal focal contacts in early states of cell spreading. A theoretical explanation of the onset of patchiness is given, based on the positively cooperative binding to the substrate of laterally diffusing receptor molecules.     

Inward and outward transport of ammonium in roots of maize and sorghum: contrasting effects of methionine sulphoximine

To examine the adaptive responses of ammonium efflux of nitrate-grownplants during first exposure to ammonium nutrition, isotopicflows of ammonium in and out of root tissue of maize (Zea maysL.) and sorghum (Sorghum bicolor L.) were delineated. Appreciableefflux of ¹⁴N-ammonium occurred during the first 2 min of exposure,suggesting that ammonium efflux is characteristic of plantsgrown with nitrate as the sole source of nitrogen. Treatmentwith methionine sulphoximine (MSX) enhanced the initial ¹⁴N-ammoniumefflux 5-fold in maize but less than 2-fold in sorghum. Increasingambient acidity from pH 6 to pH 4 had relatively little effecton 14N-ammonium efflux in either species. Initial influx ofexogenous ¹⁵N-ammonium was stimulated by MSX in maize (slightlyless at pH 4 than at pH 6), but either was not affected (atpH 6) or was inhibited (at pH 4) in sorghum. Assuming that therate of ammonium influx remained constant during the 2 h adaptationperiod, and that efflux of ¹⁵N-ammonium was negligible duringthe 0-2 min period, permitted estimates to be made of the unidirectionalinward and outward flow of the two isotopic ammonium species.The decline in efflux of endogenous ¹⁴N-ammonium after the 2min exposure was more than compensated for by an increase inefflux of the exo-genously-supplied 15N-ammonium. As a consequence,total ammonium efflux increased significantly during the adaptationperiod. By this analysis about 60% of the total efflux in maizewas as 1BN-ammonium within the 2-5 min period whereas in sorghumit was 20% to 40%. The data indicate that appreciable ammoniumefflux occurs in nitrate-grown seedlings in spite of low rootammonium concentrations, and that substantial futile cyclingof ammonium occurs in both maize and sorghum during net transportof ammonium into their root tissue. Key words: Ammonium, cycling, transport, maize, sorghum