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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Biological cybernetics 63 (1990), S. 51-60 
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract We present models based on transmitter gating and competition for SUSTAINED and ON-OFF units located in the lamina of the fly visual system. Predictions of the models are compared with data in several experimental paradigms. The overshoot and the plateau of the response are explained by the adaptation process of transmitter dynamics. Furthermore, it is demonstrated that this transmitter adaptation when coupled with fast competition can explain response features arising from different combinations of ON and OFF field inputs.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Biological cybernetics 56 (1987), S. 27-35 
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract A mathematical model predicting many facets of the PDA phenomenon is presented. The phototransduction process is divided into three interacting sublevels: pigment kinetics, transmitter dynamics and coupling of the signal to the membrane potential. The dynamics of each is studied, while showing their characteristics and limitations. Control mechanisms are proposed to overcome these limitations and to achieve good sensitivity in their dynamic range. For example, it is claimed that enzymatic gain modulation underlies the post-stimulus behaviour of the invertebrate photoreceptor cells. This hypothesis is further explored through a quantitative and qualitative analysis of the model. The results agree with the general PDA characteristics inferred from the large amount of data available from the literature in the last decade. This work provides a theoretical framework which unifies the phototransduction data into an overall system organization.
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  • 3
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Journal of Insect Physiology 21 (1975), S. 237-248 
    ISSN: 0022-1910
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Journal of comparative physiology 165 (1989), S. 75-81 
    ISSN: 1432-1351
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary The photochemical cycle of the visual pigment molecules in the blowflyCalliphora erythrocephala was investigated by transmission measurements, making use of the fact that intermediate states of the visual pigment molecules each have a characteristic absorption spectrum. It is shown that the conversion of metaxanthopsin (M 580) to the native xanthopsin state (P 490) induced by an orange-red light pulse proceeds through a newly discovered intermediate (N), which thermally decays with a time constant of about 13 ms at room temperature. The absorption spectrum of N peaks in the blue-green at about 490 nm. In the green and orange N absorbs more strongly than the native xanthopsin, but in the blue N and xanthopsin absorb almost equally. The latter finding explains why N has remained undetected in earlier studies.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Applied Polymer Science 59 (1996), S. 1151-1158 
    ISSN: 0021-8995
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Surface morphology of asymmetric and homogeneous membranes prepared from poly(phenylene oxide) (PPO) was studied by tapping mode atomic force microscopy (TMAFM). As expected, a significant difference in the morphology between the top and the bottom surfaces of the asymmetric membrane was observed. The images of the top surface revealed a small variation in the vertical direction (6.7 nm), compared to the mean diameter of nodules (62 nm), while the images of the bottom surface were very porous (microfiltration structure). On the other hand, the observed difference in morphology between the top and the bottom surfaces of the membrane prepared by the complete evaporation of the solvent (homogeneous membrane) was rather unexpected. The nodules on the bottom surface were twice as large as those on the top surface. These studies also revealed some differences in the morphology of the top surface of asymmetric and homogeneous membranes. Both surface were made up of nodules having a similar size (62-64 nm); however, roughness parameters calculated for the top surface of the asymmetric membrane were approximately two times greater than those for the top surface of the homogeneous membrane. © 1996 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
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  • 6
    Publication Date: 2012-07-16
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 7
    Publication Date: 2014-07-19
    Description: Nuclear magnetic resonance (NMR) is a powerful tool for observing the motion of biomolecules at the atomic level. One technique, the analysis of relaxation dispersion phenomenon, is highly suited for studying the kinetics and thermodynamics of biological processes. Built on top of the relax computational environment for NMR dynamics is a new dispersion analysis designed to be comprehensive, accurate and easy-to-use. The software supports more models, both numeric and analytic, than current solutions. An automated protocol, available for scripting and driving the graphical user interface (GUI), is designed to simplify the analysis of dispersion data for NMR spectroscopists. Decreases in optimization time are granted by parallelization for running on computer clusters and by skipping an initial grid search by using parameters from one solution as the starting point for another —using analytic model results for the numeric models, taking advantage of model nesting, and using averaged non-clustered results for the clustered analysis. Availability and implementation: The software relax is written in Python with C modules and is released under the GPLv3+ license. Source code and precompiled binaries for all major operating systems are available from http://www.nmr-relax.com . Contact: edward@nmr-relax.com
    Print ISSN: 1367-4803
    Electronic ISSN: 1460-2059
    Topics: Biology , Computer Science , Medicine
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  • 8
    Publication Date: 2013-07-16
    Description: Interactions between headwater aquifers and peatlands have received limited scientific attention. Hydrological stresses, including those related to climate change, may adversely impact these interactions. In this study, the dynamics of a southern Quebec headwater system where a peatland is present is simulated under current conditions and with climate change. The model is calibrated in steady-state on field-measured data and provides satisfactory results for transient state conditions. Under current conditions, simulations confirm that the peatland is fed by the fractured bedrock aquifer year round and provides continuous baseflow to its outlets. Climate change is simulated through its impact on groundwater recharge. Predicted precipitation and temperature data from a suite of Regional Climate Model scenarios provide a net precipitation variation range from +10% to -30% for the 2041-2070 horizon. Calibrated recharge is modified within this range to perform a sensitivity analysis of the headwater model to recharge variations (+10%, -15% and -30%). Total contribution from the aquifer to rivers and streams varies from +14% to -44% of the baseline for +10% to -30% recharge changes from spring 2010 data, for example. With higher recharge, the peatland receives more groundwater, which could significantly change its vegetation pattern and eventually ecosystem functions. For -30% recharge, the peatland becomes perched above the aquifer during the summer, fall and winter. Recharge reductions also induce sharp declines in groundwater levels and drying streams. This article is protected by copyright. All rights reserved.
    Print ISSN: 0885-6087
    Electronic ISSN: 1099-1085
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley
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  • 9
    Publication Date: 2012-08-01
    Description: Δ9-Tetrahydrocannabinol (THC) and other cannabinoids are responsible for the psychoactive and medicinal properties of Cannabis sativa L. (marijuana). The first intermediate in the cannabinoid biosynthetic pathway is proposed to be olivetolic acid (OA), an alkylresorcinolic acid that forms the polyketide nucleus of the cannabinoids. OA has been postulated to be synthesized by a type III polyketide synthase (PKS) enzyme, but so far type III PKSs from cannabis have been shown to produce catalytic byproducts instead of OA. We analyzed the transcriptome of glandular trichomes from female cannabis flowers, which are the primary site of cannabinoid biosynthesis, and searched for polyketide cyclase-like enzymes that could assist in OA cyclization. Here, we show that a type III PKS (tetraketide synthase) from cannabis trichomes requires the presence of a polyketide cyclase enzyme, olivetolic acid cyclase (OAC), which catalyzes a C2–C7 intramolecular aldol condensation with carboxylate retention to form OA. OAC is a dimeric α+β barrel (DABB) protein that is structurally similar to polyketide cyclases from Streptomyces species. OAC transcript is present at high levels in glandular trichomes, an expression profile that parallels other cannabinoid pathway enzymes. Our identification of OAC both clarifies the cannabinoid pathway and demonstrates unexpected evolutionary parallels between polyketide biosynthesis in plants and bacteria. In addition, the widespread occurrence of DABB proteins in plants suggests that polyketide cyclases may play an overlooked role in generating plant chemical diversity.
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 10
    Publication Date: 1987-04-01
    Print ISSN: 0340-1200
    Electronic ISSN: 1432-0770
    Topics: Biology , Computer Science , Physics
    Published by Springer
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