首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   609篇
  免费   56篇
  2023年   3篇
  2021年   6篇
  2020年   4篇
  2019年   5篇
  2018年   6篇
  2017年   9篇
  2016年   7篇
  2015年   15篇
  2014年   16篇
  2013年   22篇
  2012年   38篇
  2011年   30篇
  2010年   28篇
  2009年   22篇
  2008年   23篇
  2007年   22篇
  2006年   23篇
  2005年   16篇
  2004年   19篇
  2003年   23篇
  2002年   22篇
  2001年   16篇
  2000年   21篇
  1999年   17篇
  1998年   9篇
  1997年   13篇
  1996年   6篇
  1995年   7篇
  1994年   8篇
  1993年   12篇
  1992年   10篇
  1991年   6篇
  1990年   10篇
  1989年   11篇
  1988年   14篇
  1987年   13篇
  1986年   14篇
  1985年   13篇
  1984年   11篇
  1983年   22篇
  1982年   12篇
  1981年   3篇
  1980年   8篇
  1979年   5篇
  1978年   8篇
  1977年   3篇
  1976年   3篇
  1974年   3篇
  1936年   3篇
  1934年   3篇
排序方式: 共有665条查询结果,搜索用时 531 毫秒
81.
1. Studies assessing human impacts on freshwater ecosystems are typically based on a single taxonomic group, often macroinvertebrates or fish. Unfortunately, the degree to which such macroinvertebrate or fish‐based surveys can be generalised across other taxonomic groups remains largely unknown. A prerequisite for useful generalisations is that different taxonomic groups exhibit concordant patterns of community structure across sites. 2. We examined the concordance among fish, benthic macroinvertebrates and bryophytes in 32 streams in a boreal catchment in Finland. Our goal was to test how consistently different taxonomic groups classify stream sites; for example, can site groupings based on macroinvertebrates be used as a surrogate for bryophyte or fish assemblage classification? 3. Our results show that community classifications in headwater streams are not concordant across taxonomic groups, at least not at the within‐river system scale. The lack of concordance reflected the fact that all three groups responded to different environmental factors. Macroinvertebrate community structure was mainly correlated with stream size and pH, whereas bryophytes were related to water colour, nutrient content and in‐stream habitat variability. Fish community structure was best described by stream depth, substrate size and water oxygen concentration. 4. Our results suggest that great care should be taken when typologies based on benthic macroinvertebrates, or any other taxonomic group, are extrapolated to other groups in creating typologies of lotic environments.  相似文献   
82.
Localization of laminin alpha4-chain in developing and adult human tissues.   总被引:3,自引:0,他引:3  
Recent studies suggest important functions for laminin-8 (Ln-8; alpha4beta1gamma1) in vascular and blood cell biology, but its distribution in human tissues has remained elusive. We have raised a monoclonal antibody (MAb) FC10, and by enzyme-linked immunoassay (EIA) and Western blotting techniques we show that it recognizes the human Ln alpha4-chain. Immunoreactivity for the Ln alpha4-chain was localized in tissues of mesodermal origin, such as basement membranes (BMs) of endothelia, adipocytes, and skeletal, smooth, and cardiac muscle cells. In addition, the Ln alpha4-chain was found in regions of some epithelial BMs, including epidermis, salivary glands, pancreas, esophageal and gastric glands, intestinal crypts, and some renal medullary tubules. Developmental differences in the distribution of Ln alpha4-chain were detected in skeletal muscle, walls of vessels, and intestinal crypts. Ln alpha4- and Ln alpha2-chains co-localized in BMs of fetal skeletal muscle cells and in some epithelial BMs, e.g., in gastric glands and acini of pancreas. Cultured human pulmonary artery endothelial (HPAE) cells produced Ln alpha4-chain as M(r) 180,000 and 200,000 doublet and rapidly deposited it to the growth substratum. In cell-free extracellular matrices of human kidney and lung, Ln alpha4-chain was found as M(r) 180,000 protein.  相似文献   
83.
Allergens from various sources have been shown to comprise several isoforms. In the present study, a series of chromatographic steps was carried out to separate the lipocalin allergen Bos d 2 isoforms present in cow dander. Subsequent HPLC-MS–MS analyses revealed two new Bos d 2 variants. In one of the proteins, tyrosine (Y83) was substituted by aspartic acid, and in the other protein valine (V102) was replaced by alanine. We propose the three Bos d 2 variants be named as Bos d 2.0101 (previously sequenced Bos d 2), Bos d 2.0102 and Bos d 2.0103. Our results suggest that molecular polymorphism is a common property among lipocalin allergens. Since allergen isoforms may show variation in their IgE binding and/or T-cell reactivity, all of the many allergen forms should be taken into account when planning preparations for immunotherapy.  相似文献   
84.
We have devised a simple chromatographic procedure which isolates five polyadenylation factors that are required for polyadenylation of eukaryotic mRNA. The factors were separated from each other by fractionation of HeLa cell nuclear extract in two consecutive chromatographic steps. RNA cleavage at the L3 polyadenylation site of human adenovirus 2 required at least four factors. Addition of adenosine residues required only two of these factors. The fractionation procedure separates two components that are both likely to be poly(A) polymerases. The candidate poly(A) polymerases were interchangeable and participated during both RNA cleavage and adenosine addition. They were discriminated from each other by chromatographic properties, heat sensitivity and divalent cation requirement. We have compared our data with published information and have been able to correlate the activities that we have isolated to previously identified polyadenylation factors. However, we have not been able to assign one of the candidate poly(A) polymerases to a previously identified poly(A) polymerase. This simple fractionation procedure can be used for generating an in vitro reconstituted system for polyadenylation within a short period of time.  相似文献   
85.
Triggering of the activity of phospholipase A2 by an electric field   总被引:3,自引:0,他引:3  
In this paper we show that the action of phospholipase A2 can be triggered by applying an electric field across a 1,2-didodecanoyl-sn-3-phosphoglycerol monolayer residing between an alkylated silicon surface and water. When the silicon wafer served as a cathode, rapid activation of porcine pancreatic phospholipase was observed and did depend on the magnitude of the applied potential. The degree of activation was different for the pancreatic phospholipase A2 and snake and bee venom enzymes. Maximally, a 7-fold activation of pancreatic phospholipase A2 was observed when the applied potential was 75 V. The effective field over the lipid film could be estimated to be approximately 25-175 mV, i.e., in the range of membrane potentials found in cells. On the basis of these results, we suggest that changes in membrane potential might be an important factor in the regulation of the action of intracellular phospholipases A2 in vivo.  相似文献   
86.
This paper puts forward a framework for probabilistic and holistic cost-effectiveness analysis to provide support in selecting the least-cost set of measures to reach a multidimensional environmental objective. Following the principles of ecosystem-based management, the framework includes a flexible methodology for deriving and populating criteria for effectiveness and costs and analyzing complex ecological-economic trade-offs under uncertainty. The framework is applied in the development of the Finnish Programme of Measures (PoM) for reaching the targets of the EU Marine Strategy Framework Directive (MSFD). The numerical results demonstrate that substantial cost savings can be realized from careful consideration of the costs and multiple effects of management measures. If adopted, the proposed PoM would yield improvements in the state of the Baltic Sea, but the overall objective of the MSFD would not be reached by the target year of 2020; for various environmental and administrative reasons, it would take longer for most measures to take full effect.  相似文献   
87.
88.
P Virtanen  V Lassila 《Acta anatomica》1986,127(4):285-289
Experimental liver injury with different stages was induced to adult female test rats with daily injection of thioacetamide (ThAA). The doses administered intraperitoneally were 50 mg/kg body weight. In the liver sections progressive changes of damage, regeneration and fat substitution were noticed. Kidney sections revealed enhanced glomerular atrophy, particularly in the cortical tubules, provoked in the 3-week traumatization period. The influence of ThAA on female rat blood was assayed using standard biochemical methods. The analyses done were: the percentage of blood obtainable and the serum/blood ratio; the serum alanine transferase; serum alkaline phosphatase; serum creatinine; serum hydroxyproline and serum beta-glucuronidase activity in the acute, subacute, chronic and highly chronic stage of liver injury. The biochemical findings show continuously progressing damages when traumatization proceeds. In the 3-week test period the histological findings processed showed an increase in osteoclastic resorption in the alveolar bone around the occlusally stressed tooth simultaneously with a horizontal bone loss. Some indications of recovering incidents were seen, too. Only in the histological findings was no difference seen in the deterioration between both sexes, contrarily to the biochemical results also discussed in this study.  相似文献   
89.
Phase separation in mixed monolayers of phosphatidylcholine (PC) and pyrene-labeled phosphatidic acid (PA) was observed by fluorescence microscopy on an air/water interface as a function of subphase Ca2+ concentration and lateral packing pressure of the film. Below 45 mN m-1 and in the absence of Ca2+ no indications of phase immiscibility were observed. Addition of 1 mM Ca2+ caused extensive phase separation, which was evident immediately after spreading of the film. Further increase in Ca2+ concentration up to 30 mM increased the pyrene excimer intensity of the separated phosphatidic acid enriched domains. In the presence of Ca2+ (1-30 mM) and at surface pressures below 10 mN m-1 phase separation was always evident. However, as surface pressure exceeded 10 mN m-1, mixing of PC and PA occurred. Upon decompression of the film, phase separation reappeared at surface pressures close to 10 mN m-1. The surface textures of the film before and after the compression and subsequent relaxation were different. Inclusion of 30 mol% cholesterol increased the number and decreased the size of the PA domains. In films containing 50 mol% cholesterol no phase separation could be detected at the resolution available.  相似文献   
90.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号