3,562 research outputs found
Diffusion and bulk flow in phloem loading - a theoretical analysis of the polymer trap mechanism in plants
Plants create sugar in the mesophyll cells of their leaves by photosynthesis.
This sugar, mostly sucrose, has to be loaded via the bundle sheath into the
phloem vascular system (the sieve elements), where it is distributed to growing
parts of the plant. We analyze the feasibility of a particular loading
mechanism, active symplasmic loading, also called the polymer trap mechanism,
where sucrose is transformed into heavier sugars, such as raffinose and
stachyose, in the intermediary-type companion cells bordering the sieve
elements in the minor veins of the phloem. Keeping the heavier sugars from
diffusing back requires that the plasmodesmata connecting the bundle sheath
with the intermediary cell act as extremely precise filters, which are able to
distinguish between molecules that differ by less than 20% in size. In our
modeling, we take into account the coupled water and sugar movement across the
relevant interfaces, without explicitly considering the chemical reactions
transforming the sucrose into the heavier sugars. Based on the available data
for plasmodesmata geometry, sugar concentrations and flux rates, we conclude
that this mechanism can in principle function. We find that the water flow
through the plasmodesmata, which has not been quantified before, contributes
only 10-20% to the sucrose flux into the intermediary cells, while the main
part is transported by diffusion. On the other hand, the subsequent sugar
translocation into the sieve elements would very likely be carried
predominantly by bulk water flow through the plasmodesmata. Thus, in contrast
to apoplasmic loaders, all the necessary water for phloem translocation would
be supplied in this way with no need for additional water uptake across the
plasma membranes of the phloem.Comment: 29 pages with 5 figure
Stochastic Parameter Estimation of Poroelastic Processes Using Geomechanical Measurements
Understanding the structure and material properties of hydrologic systems is important for a number of applications, including carbon dioxide injection for geological carbon storage or enhanced oil recovery, monitoring of hydraulic fracturing projects, mine dewatering, environmental remediation and managing geothermal reservoirs. These applications require a detailed knowledge of the geologic systems being impacted, in order to optimize their operation and safety. In order to evaluate, monitor and manage such hydrologic systems, a stochastic estimation framework was developed which is capable of characterizing the structure and physical parameters of the subsurface. This software framework uses a set of stochastic optimization algorithms to calibrate a heterogeneous subsurface flow model to available field data, and to construct an ensemble of models which represent the range of system states that would explain this data.
Many of these systems, such as oil reservoirs, are deep and hydraulically isolted from the shallow subsurface making near-surface fluid pressure measurements uninformative. Near-surface strainmeter, tiltmeter and extensometer signals were therefore evaluated in terms of their potential information content for calibrating poroelastic flow models. Such geomechanical signals are caused by mechanical deformation, and therefore travel through hydraulically impermeable rock much more quickly. A numerical geomechanics model was therefore developed using Geocentric, which couples subsurface flow and elastic deformation equations to simulate geomechanical signals (e.g. pressure, strain, tilt and displacement) given a set of model parameters. A high-performance cluster computer performs this computationally expensive simulation for each set of parameters, and compares the simulation results to measured data in order to evaluate the likelihood of each model. The set of data-model comparisons are then used to estimate each unknown parameter, as well as the uncertainty of each parameter estimate. This uncertainty can be inuenced by limitations in the measured dataset such as random noise, instrument drift, and the number and location of sensors, as well as by conceptual model errors and false underlying assumptions.
In this study we find that strain measurements taken from the shallow subsurface can be used to estimate the structure and material parameters of geologic layers much deeper in the subsurface. This can signicantly mitigate drilling and installation costs of monitoring wells, as well as reduce the risk of puncturing or fracturing a target reservoir. These parameter estimates were also used to develop an ensemble of calibrated hydromechanical models which can predict the range of system behavior and inform decision-making on the management of an aquifer or reservoir
Combining Sentiment Lexica with a Multi-View Variational Autoencoder
When assigning quantitative labels to a dataset, different methodologies may
rely on different scales. In particular, when assigning polarities to words in
a sentiment lexicon, annotators may use binary, categorical, or continuous
labels. Naturally, it is of interest to unify these labels from disparate
scales to both achieve maximal coverage over words and to create a single, more
robust sentiment lexicon while retaining scale coherence. We introduce a
generative model of sentiment lexica to combine disparate scales into a common
latent representation. We realize this model with a novel multi-view
variational autoencoder (VAE), called SentiVAE. We evaluate our approach via a
downstream text classification task involving nine English-Language sentiment
analysis datasets; our representation outperforms six individual sentiment
lexica, as well as a straightforward combination thereof.Comment: To appear in NAACL-HLT 201
On a Generalization of the Frobenius Number
We consider a generalization of the Frobenius Problem where the object of
interest is the greatest integer which has exactly representations by a
collection of positive relatively prime integers. We prove an analogue of a
theorem of Brauer and Shockley and show how it can be used for computation.Comment: 5 page
Synthetic X-ray and radio maps for two different models of Stephan's Quintet
We present simulations of the compact galaxy group Stephan's Quintet (SQ)
including magnetic fields, performed with the N-body/smoothed particle
hydrodynamics (SPH) code \textsc{Gadget}. The simulations include radiative
cooling, star formation and supernova feedback. Magnetohydrodynamics (MHD) is
implemented using the standard smoothed particle magnetohydrodynamics (SPMHD)
method. We adapt two different initial models for SQ based on Renaud et al. and
Hwang et al., both including four galaxies (NGC 7319, NGC 7320c, NGC 7318a and
NGC 7318b). Additionally, the galaxies are embedded in a magnetized, low
density intergalactic medium (IGM). The ambient IGM has an initial magnetic
field of G and the four progenitor discs have initial magnetic fields
of G. We investigate the morphology, regions of star
formation, temperature, X-ray emission, magnetic field structure and radio
emission within the two different SQ models. In general, the enhancement and
propagation of the studied gaseous properties (temperature, X-ray emission,
magnetic field strength and synchrotron intensity) is more efficient for the SQ
model based on Renaud et al., whose galaxies are more massive, whereas the less
massive SQ model based on Hwang et al. shows generally similar effects but with
smaller efficiency. We show that the large shock found in observations of SQ is
most likely the result of a collision of the galaxy NGC 7318b with the IGM.
This large group-wide shock is clearly visible in the X-ray emission and
synchrotron intensity within the simulations of both SQ models. The order of
magnitude of the observed synchrotron emission within the shock front is
slightly better reproduced by the SQ model based on Renaud et al., whereas the
distribution and structure of the synchrotron emission is better reproduced by
the SQ model based on Hwang et al..Comment: 20 pages, 15 figures, accepted to MNRA
Gel phantom data for dynamic X-ray tomography
This is the documentation for dynamic X-ray tomography measurements of a gel
phantom diffused with potassium iodide contrast agent. The measured data and
additional materials are available at http://www.fips.fi/dataset.php and Zenodo
(https://doi.org/10.5281/zenodo.3696817) for open use by the scientific
community, as long as the data and this documentation at arXiv are
appropriately referenced.
The files contain: (1) 17 consecutive measurements of the gel phantom
organized into sinograms at two different resolutions (binning levels) and some
additional metadata which can be used to create matrix-free forward operators
and filtered back-projection reconstructions; (2) the first measurement using a
greater number of projections, and an additional measurement of an 18th time
step using very dense angular sampling, for example to be used for reference
reconstructions; (3) short example codes to showcase how the data could be used
to test and validate reconstruction algorithms.Comment: Actual data can be found at https://doi.org/10.5281/zenodo.369681
Giellariššu: Indigenous language revitalisation in the city
Source at https://multiethnica.org/About 10% of all pupils in grades 1–10 who learn Sámi in Norwegian schools live in the city of Tromsø in Northern Norway. This group totalled 232 pupils in the school year 2020/2021, and the pupils went to over 20 different schools. All but one were pupils of North Sámi, and a handful also received instruction in South Sámi. In Umeå in Sweden, 42 pupils attended mother tongue classes in Sámi in the winter of 2021, divided among four different varieties of Sámi, reflecting the diverse composition of the Sámi population in the area. In neither city, the Sámi pupils exceed 3 percent of their municipal peer group and they are embedded in local communities fully dominated by the Norwegian and Swed- ish majority language, respectively.We discuss the challenges and opportunities that Sámi children who grow up in two urban environments face when reclaiming, maintaining, and developing their indigenous heritage language, and we report from piloted language (re)vitalisation activities. Giellariššu gathers pupils from different schools regularly for activities in Sámi, led by adult proficient speakers with the goal to strengthen the pupils’ language skills and the social bonds between children who otherwise do not meet on a regular basis
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