3,880 research outputs found
Temperature modelling and model predictive control of a pilot-scale batch reaction system
The temperature control equipment on a pilot scale batch reaction system located at EAFIT University in Medelln, Colombia, is modeled and a new controller is designed aiming at using it in the reactor current PLC-based control system. Some mathematical models are developed from experimental data to describe the system behavior and using them several model based predictive controllers are designed. The simplest, yet reliable, model obtained is an ARX polynomial model of order (1,1,1) that yields a four states ane model for which an explicit MPC was calculated.
This controller has a reduced mathematical complexity and can probably be used directly on the existing control system.Preprin
Existence of multiple solutions for a quasilinear elliptic problem
In this paper we prove the existence of multiple solutions for a quasilinear
elliptic boundary value problem, when the p-derivative at zero and the
p-derivative at infinity of the nonlinearity are greater than the first
eigenvalue of the p-Laplace operator. Our proof uses bifurcation from infinity
and bifurcation from zero to prove the existence of unbounded branches of
positive solutions (resp. of negative solutions). We show the existence of
multiple solutions and we provide qualitative properties of these solutions
The Impacts of Cash and In-Kind Transfers on Consumption and Labor Supply: Experimental Evidence from Rural Mexico
The authors use the unique experimental design of the Food Support Program (Programa Apoyo Alimentario) to analyze in-kind and cash transfers in the poor rural areas of southern states of Mexico. They compare the impacts of monthly in-kind and cash transfers of equivalent value (mean share 11.5 percent of pre-program consumption) on household welfare as measured by food and total consumption, adult labor supply, and poverty. The results show that approximately two years later the transfer has a large and positive impact on total and food consumption. There are no differences in the size of the effect of transfer in cash versus transfers in-kind on consumption. The transfer, irrespective of type, does not affect overall participation in labor market activities but induces beneficiary households to switch their labor allocation from agricultural to nonagricultural activities. The analysis finds that the program leads to a significant reduction in poverty. Overall, the findings suggest that the Food Support Program intervention is able to relax the binding liquidity constraints faced by poor agricultural households, and thus increases both equity and efficiency.Adult Work Incentives; Cash Transfers; Consumption; Difference-in-Differences; In-Kind Transfers; Mexico; Poverty Measures; PAL; Randomized design.
Limits of aerobic metabolism in cancer cells
Cancer cells exhibit high rates of glycolysis and glutaminolysis. Glycolysis can provide energy and glutaminolysis can provide carbon for anaplerosis and reductive carboxylation to citrate. However, all these metabolic requirements could be in principle satisfied from glucose. Here we investigate why cancer cells do not satisfy their metabolic demands using aerobic biosynthesis from glucose. Based on the typical composition of a mammalian cell we quantify the energy demand and the OxPhos burden of cell biosynthesis from glucose. Our calculation demonstrates that aerobic growth from glucose is feasible up to a minimum doubling time that is proportional to the OxPhos burden and inversely proportional to the mitochondria OxPhos capacity. To grow faster cancer cells must activate aerobic glycolysis for energy generation and uncouple NADH generation from biosynthesis. To uncouple biosynthesis from NADH generation cancer cells can synthesize lipids from carbon sources that do not produce NADH in their catabolism, including acetate and the amino acids glutamate, glutamine, phenylalanine and tyrosine. Finally, we show that cancer cell lines have an OxPhos capacity that is insufficient to support aerobic biosynthesis from glucose. We conclude that selection for high rate of biosynthesis implies a selection for aerobic glycolysis and uncoupling biosynthesis from NADH generation
A physical model of cell metabolism
Cell metabolism is characterized by three fundamental energy demands: to sustain cell maintenance, to trigger aerobic fermentation and to achieve maximum metabolic rate. The transition to aerobic fermentation and the maximum metabolic rate are currently understood based on enzymatic cost constraints. Yet, we are lacking a theory explaining the maintenance energy demand. Here we report a physical model of cell metabolism that explains the origin of these three energy scales. Our key hypothesis is that the maintenance energy demand is rooted on the energy expended by molecular motors to fluidize the cytoplasm and counteract molecular crowding. Using this model and independent parameter estimates we make predictions for the three energy scales that are in quantitative agreement with experimental values. The model also recapitulates the dependencies of cell growth with extracellular osmolarity and temperature. This theory brings together biophysics and cell biology in a tractable model that can be applied to understand key principles of cell metabolism
Vargar
Squinting my eyes and resuming the frenzied spurring, I headed unflinchingly towards the setting sun. The
gap between my steed and the couple of pursuers was widening, we were almost outside their shooting
range
Characterizing steady states of genome-scale metabolic networks in continuous cell cultures
We present a model for continuous cell culture coupling intra-cellular
metabolism to extracellular variables describing the state of the bioreactor,
taking into account the growth capacity of the cell and the impact of toxic
byproduct accumulation. We provide a method to determine the steady states of
this system that is tractable for metabolic networks of arbitrary complexity.
We demonstrate our approach in a toy model first, and then in a genome-scale
metabolic network of the Chinese hamster ovary cell line, obtaining results
that are in qualitative agreement with experimental observations. More
importantly, we derive a number of consequences from the model that are
independent of parameter values. First, that the ratio between cell density and
dilution rate is an ideal control parameter to fix a steady state with desired
metabolic properties invariant across perfusion systems. This conclusion is
robust even in the presence of multi-stability, which is explained in our model
by the negative feedback loop on cell growth due to toxic byproduct
accumulation. Moreover, a complex landscape of steady states in continuous cell
culture emerges from our simulations, including multiple metabolic switches,
which also explain why cell-line and media benchmarks carried out in batch
culture cannot be extrapolated to perfusion. On the other hand, we predict
invariance laws between continuous cell cultures with different parameters. A
practical consequence is that the chemostat is an ideal experimental model for
large-scale high-density perfusion cultures, where the complex landscape of
metabolic transitions is faithfully reproduced. Thus, in order to actually
reflect the expected behavior in perfusion, performance benchmarks of
cell-lines and culture media should be carried out in a chemostat
- …