No abstract

Changes in slope of Arrhenius plots for transport can, in some instances, be detected at two different temperatures for cells that have a relatively simple fatty-acid composition in the membrane lipids. These characteristic temperatures correlate with the characteristic temperatures that define changes of state in membrane phospholipids as revealed by the paramagnetic resonance of the spin label TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl). The higher of these characteristic temperatures is that at which the formation of solid patches of membrane lipids is first detected. The lower is the end point of the course of lateral phase separations, at which all the membrane lipids are in a solid phase. For cells enriched for elaidic acid, the rate of transport increases by as much as 2-fold as the temperature is decreased by less than 10, at the higher characteristic temperature. At this characteristic temperature, lateral phase separations begin in the membrane phospholipids. This is also the temperature where one predicts a striking increase in the lateral compressibility of the membrane lipids. These data are thus interpreted to indicate that a component of the transport system vertically penetrates one or both monolayer faces of the membrane during transport, or that some other event involving the lateral compression of the phospholipids is important for transport.Essential fatty-acid auxotrophs of Escherichia coli are powerful tools that have been exploited to study the influence of lipid physical properties on the function and assembly of cellular membranes (1-7). Arrhenius plots for transport have been shown to have a biphasic shape, and the characteristic temperature that defines the change in slope in these plots is exquisitely responsive to the physical characteristics of the essential fatty-acid supplement present during cellular growth. The characteristic temperatures reflect the melting characteristics of the essential fatty-acid supplement, e.g., the characteristic temperatures are in decreasing order when determined in cells grown in media supplemented with fatty acids that are trans-monoenoic, cis-monoenoic, and cis, cisdienoic (or cis, cis, cis-trienoic), (2-4, 6). The first strong indications that the characteristic temperatures correspond to a change of state in the membrane lipids came from two independent studies: (1) The characteristic temperatures for two unrelated transport systems were identical for cells grown with a single essential fatty acid (2, 4). (2) A change in state detected in a monolayer of phosphatidylethaAbbreviations: T is the absolute temperature (degrees Kelvin), th and tI refer to "higher" and "lower" characteristic temperatures (0C) as revealed by spin labeling (or other physical techniques), and th* and tt* refer to the higher and lower temperatures (0C) as revealed by transport rate. The spin label TEMPO is 2,2,6,6-tetramethylpiperidine-1-oxyl. MATERIALS AND METHODSGrowth and Properties of Bacterial Strains. Strain 30E,3ox-was used exclusively in the studies reporte...

[1] The aim of this research is to estimate the parameters of a large-scale numerical model of a geothermal reservoir using Markov chain Monte Carlo (MCMC) sampling, within the framework of Bayesian inference. All feasible parameters that are consistent with the measured data are summarized by the posterior distribution, and hence parameter estimation and uncertainty quantification are both given by calculating expected values of statistics of interest over the posterior distribution. It appears to be computationally infeasible to use the standard Metropolis-Hastings algorithm (MH) to sample the high dimensional computationally expensive posterior distribution. To improve the sampling efficiency, a new adaptive delayed-acceptance MH algorithm (ADAMH) is implemented to adaptively build a stochastic model of the error introduced by the use of a reduced-order model. This use of adaptivity differs from existing adaptive MCMC algorithms that tune proposal distributions of the Metropolis-Hastings algorithm (MH), though ADAMH also implements that technique. For the 3-D geothermal reservoir model we present here, ADAMH shows a great improvement in the computational efficiency of the MCMC sampling, and promising results for parameter estimation and uncertainty quantification are obtained. This algorithm could offer significant improvement in computational efficiency when implementing sample-based inference in other large-scale inverse problems.

The reflection and transmission of ocean waves at a sea ice boundary is reconsidered. The sea ice is modelled as a continuous, thin elastic plate of uniform thickness, floating on water of arbitrary constant depth. Unlike earlier solutions, matching of potentials between the free surface domain and the ice‐covered domain is done at all depths; previous solutions were incompletely matched, as the potentials in each domain were deficient. In the present solution a number of the infinity of evanescent modes, which were hitherto ignored, are included in the solution and allow matching to be carried out by minimization of an integrated error term from surface to seafloor. Reflection and transmission are found to be markedly influenced by the inclusion of these modes, suggesting that conclusions based on the incomplete potentials may be substantially in error.

The inositol (1,4,5)-trisphosphate receptor (IPR) plays a crucial role in calcium dynamics in a wide range of cell types, and is often a central feature in quantitative models of calcium oscillations and waves. We compare three mathematical models of the IPR, fitting each of them to the same data set to determine ranges for the parameter values. Each of the fits indicates that fast activation of the receptor, followed by slow inactivation, is an important feature of the model, and also that the speed of inositol trisphosphate IP3 binding cannot necessarily be assumed to be faster than Ca2+ activation. In addition, the model which assumed saturating binding rates of Ca2+ to the IPR demonstrated the best fit. However, lack of convergence in the fitting procedure indicates that responses to step increases of Ca2+ and IP3 provide insufficient data to determine the parameters unambiguously in any of the models.