7,428 research outputs found
Polyamine uptake in the malaria parasite, Plasmodium falciparum, is dependent on the parasite's membrane potential
Polyamines are present at high levels in proliferating cells, including cancerous cells and protozoan parasites and the inhibition of their synthesis has been exploited in antiproliferative strategies. Inhibition of the malaria parasite’s polyamine biosynthetic pathway causes cytostatic arrest in the trophozoite stage but does not cure in vivo infections in the murine model of malaria. This is possibly due to exogenous polyamine salvage from the host, which replenishes the intracellular polyamine pool. This implies that disruption of polyamine metabolism as an antimalarial chemotherapy strategy may require targeting both polyamine biosynthesis and transport simultaneously
Spontaneous emergence of spatial patterns ina a predator-prey model
We present studies for an individual based model of three interacting
populations whose individuals are mobile in a 2D-lattice. We focus on the
pattern formation in the spatial distributions of the populations. Also
relevant is the relationship between pattern formation and features of the
populations' time series. Our model displays travelling waves solutions,
clustering and uniform distributions, all related to the parameters values. We
also observed that the regeneration rate, the parameter associated to the
primary level of trophic chain, the plants, regulated the presence of
predators, as well as the type of spatial configuration.Comment: 17 pages and 15 figure
A variational problem on Stiefel manifolds
In their paper on discrete analogues of some classical systems such as the
rigid body and the geodesic flow on an ellipsoid, Moser and Veselov introduced
their analysis in the general context of flows on Stiefel manifolds. We
consider here a general class of continuous time, quadratic cost, optimal
control problems on Stiefel manifolds, which in the extreme dimensions again
yield these classical physical geodesic flows. We have already shown that this
optimal control setting gives a new symmetric representation of the rigid body
flow and in this paper we extend this representation to the geodesic flow on
the ellipsoid and the more general Stiefel manifold case. The metric we choose
on the Stiefel manifolds is the same as that used in the symmetric
representation of the rigid body flow and that used by Moser and Veselov. In
the extreme cases of the ellipsoid and the rigid body, the geodesic flows are
known to be integrable. We obtain the extremal flows using both variational and
optimal control approaches and elucidate the structure of the flows on general
Stiefel manifolds.Comment: 30 page
Multivalued SK-contractions with respect to b-generalized pseudodistances
A new class of multivalued non-self-mappings, called SK-contractions with respect to
b-generalized pseudodistances, is introduced and used to investigate the existence of
best proximity points by using an appropriate geometric property. Some new fixed
point results in b-metric spaces are also obtained. Examples are given to support the
usability of our main result
Lepton Acceleration in Pulsar Wind Nebulae
Pulsar Wind Nebulae (PWNe) act as calorimeters for the relativistic pair
winds emanating from within the pulsar light cylinder. Their radiative
dissipation in various wavebands is significantly different from that of their
pulsar central engines: the broadband spectra of PWNe possess characteristics
distinct from those of pulsars, thereby demanding a site of lepton acceleration
remote from the pulsar magnetosphere. A principal candidate for this locale is
the pulsar wind termination shock, a putatively highly-oblique,
ultra-relativistic MHD discontinuity. This paper summarizes key characteristics
of relativistic shock acceleration germane to PWNe, using predominantly Monte
Carlo simulation techniques that compare well with semi-analytic solutions of
the diffusion-convection equation. The array of potential spectral indices for
the pair distribution function is explored, defining how these depend
critically on the parameters of the turbulent plasma in the shock environs.
Injection efficiencies into the acceleration process are also addressed.
Informative constraints on the frequency of particle scattering and the level
of field turbulence are identified using the multiwavelength observations of
selected PWNe. These suggest that the termination shock can be comfortably
invoked as a principal injector of energetic leptons into PWNe without
resorting to unrealistic properties for the shock layer turbulence or MHD
structure.Comment: 19 pages, 5 figures, invited review to appear in Proc. of the
inaugural ICREA Workshop on "The High-Energy Emission from Pulsars and their
Systems" (2010), eds. N. Rea and D. Torres, (Springer Astrophysics and Space
Science series
Current Star Formation in the Perseus Molecular Cloud: Constraints from Unbiased Submillimeter and Mid-Infrared Surveys
We present a census of the population of deeply embedded young stellar
objects (YSOs) in the Perseus molecular cloud complex based on a combination of
Spitzer Space Telescope mid-IR data from the c2d legacy team and JCMT/SCUBA
submillimeter maps from the COMPLETE team. The mid-IR sources detected at 24
micron and having [3.6]-[4.5] > 1 are located close to the center of the SCUBA
cores, typically within 15" of their peaks. The narrowness of the spatial
distribution of mid-IR sources around the peaks of the SCUBA cores suggests
that no significant dispersal of the newly formed YSOs has occurred. This
argues against the suggestion that motions of protostars regulate the time
scales over which significant (Bondi-Hoyle) accretion can occur. The most
deeply embedded YSOs are found in regions with high extinction, AV > 5, similar
to the extinction threshold observed for the SCUBA cores. All the SCUBA cores
with high concentrations have embedded YSOs, but not all cores with low
concentrations are starless. An unbiased sample of 49 deeply embedded YSOs is
constructed. Embedded YSOs are found in 40 of the 72 SCUBA cores with only
three cores harboring multiple embedded YSOs within 15". The equal number of
SCUBA cores with and without embedded YSOs suggests that the time scale for the
evolution through the dense prestellar stages, where the cores are recognized
in the submillimeter maps and have central densities of 5e4-1e5 cm^{-3}, is
similar to the time scale for the embedded protostellar stages. The current
star formation efficiency of cores is estimated to be approximately 10-15%. In
contrast, the star formation efficiency averaged over the cloud life time and
compared to the total cloud mass is only a few percent, reflecting also the
efficiency in assembling cloud material into the dense cores forming stars.Comment: Accepted for publication in ApJ (36 pages, 13 figures
- …