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691.
J. Caspar Rüegg 《Die Naturwissenschaften》1998,85(12):575-582
2+ concentrations tropomyosin is located at the edge of the thin filament, thereby interfering with the formation of strong
actin-myosin linkages (blocked state). An increase in Ca2+ activity causes an azimuthal shift of tropomyosin around the filament (by about 30°), thereby increasing the probability
of low-force crossbridge interaction, a process which by cooperative effects induces further tropomyosin movement (by an additional
10°) which results in the open state of the filament characterized by forceful crossbridge interaction. (This mechanism may
be analogous to that in ligand-gated ion channels, where ligand binding increases the open probability of the pore.) The extent
of activation then depends on the free Ca2+ concentration and on the calcium sensitivity of the thin filament that may be affected by protein phosphorylation, crossbridge
attachment, the troponin isoform composition of the filament, and novel calcium-sensitizing drugs that act on the contractile
or regulatory proteins and thus increase the force of the heart. 相似文献
692.
Kurz Werner A. Beukema Sarah J. Apps Michael J. 《Mitigation and Adaptation Strategies for Global Change》1997,2(4):405-421
Land-use change from an unmanaged to a managed forested landscape in northern forests is associated with a reduction of the
area annually affected by natural disturbances (wildfires and forest insects) and the introduction of harvesting as a new
disturbance. This study examined the impacts of changes in the disturbance regime-the frequency and type of disturbance-on
landscape-level carbon (C) content and fluxes. The Carbon Budget Model of the Canadian Forest Sector was used to assess these
impacts in six representative landscapes (100,000 ha each) with a range of disturbance regimes that are characteristic of
conditions in coastal British Columbia, the interior of British Columbia, and the eastern boreal forest in Canada. The model
was used to simulate ecosystem C fluxes during a period of natural disturbances, a 50-year transition period during which
harvesting replaced natural disturbances, followed by 150 years of harvesting. The initial landscape-level biomass C content
under natural disturbance regimes in the six example landscapes was 22 to 75% of their potential maximum content which is
often used as the reference or baseline case. After 200 years of forest management, the C stored in the landscape plus the
C retained in forest products manufactured from harvested biomass was between 58 and 101% of the landscape C content prior
to the onset of harvesting. Landscape-level ecosystem C content was found to be affected by changes in the disturbance frequency,
the age-dependence of the disturbance probabilities, and the disturbance-specific impacts on ecosystem C content. The results
indicate that using the potential maximum C content of a landscape as the baseline always overestimates the actual C release
due to land use change. A more meaningful procedure would be to assess the actual differences in landscape-level C content
between the natural and the managed disturbance regime. 相似文献
693.
694.
重金属在牡蛎(Crassostrea virginica)中的生物积累及其影响因素的研究 总被引:11,自引:0,他引:11
牡蛎软体,贝壳和沉积物中Cd、Cr、Cu、Fe、Mn、Pb和Zn等重金属元素被同步分析研究,研究结果表明,重金属在牡蛎中的生物积累是牡自身的新陈代谢、重金属元素的地球化学性质和环境诸因素综合影响的结果,其中牡蛎斩生理作用对重金属在软体中的积累产生的影响尤为重要,而贝壳中重金属的积累对来自环境的影响更为敏感。 相似文献
695.
Mitigation and Adaptation Strategies for Global Change - Guidelines of the Intergovernmental Panel on Climate Change (IPCC) were used to assess a greenhouse gas inventory for land use change and... 相似文献
696.
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699.
In several branches of science and technology a gaseous phase is dispersed into a liquid in the form of bubbles, a gaseous
component then dissolves into the liquid and subsequently undergoes chemical reaction. The overall process performance can
be improved substantially when the area of gas–liquid contact is increased. By subjecting the liquid phase to low frequency
vibrations, the bubbles are shown to suffer significant breakage, induced by resonance. When the vibration is properly tuned,
the interfacial area is found to increase by a factor of 1.8–2.4, depending on the properties of the liquid. Resonance-induced
bubble breakage phenomena have a great potential for improving the rates of chemical processes involving fast reactions, with
minimal energy input.
Received: 7 July 2000 / Accepted in revised form: 28 August 2000 相似文献
700.
The groundwater regime in Upper Palar basin, Tamilnadu has been highly contaminated in several locations due to discharge of effluents from a large number of tanneries. At some places total dissolved solids (TDS) concentration in groundwater was found as high as 8000 mg/l. Transmissivity and storativity of the regional aquifer were estimated at a few locations. The porosity and dispersivity values were not determined in the field. These parameters were assumed based on data available for similar geological formations elsewhere. The aquifer conceptualization thus arrived at formed the basis of a numerical groundwater flow model which was constructed using the finite difference method. The flow model was calibrated for steady state and then for transient condition for the period of 1984-92. The computed heads and calibrated parameters of the flow model were used to compute groundwater velocities. The migration of contaminants for a 20 year period was computed using the hydraulic heads and effective porosity value in a pathline model using FLOWPATH software. Mass transport model was constructed using Method of Characteristics (MOC) computer code in a separate model. The seepage rate of effluent is assumed at a rate of 30% of that discharged on the surface. The mass concentration of solute in the effluent reaching the water table was assumed as 40%, the same as in the surface effluent. The mass transport model was calibrated for a 20 year period. Prediction of contaminant migration from different clusters in the basin was analyzed. The prediction results indicated elevated TDS concentration of more than 4000 mg/l from most clusters. Also the area of the contaminated zone is likely to double in 20 years from contaminated zone of 1992. 相似文献