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   1117篇
  免费   60篇
  2021年   14篇
  2020年   9篇
  2019年   13篇
  2018年   19篇
  2017年   9篇
  2016年   16篇
  2015年   41篇
  2014年   49篇
  2013年   49篇
  2012年   71篇
  2011年   48篇
  2010年   28篇
  2009年   48篇
  2008年   53篇
  2007年   40篇
  2006年   53篇
  2005年   61篇
  2004年   46篇
  2003年   36篇
  2002年   31篇
  2001年   34篇
  2000年   42篇
  1999年   22篇
  1998年   19篇
  1997年   18篇
  1996年   17篇
  1994年   9篇
  1993年   9篇
  1992年   21篇
  1991年   11篇
  1990年   14篇
  1989年   14篇
  1988年   12篇
  1987年   7篇
  1986年   7篇
  1984年   8篇
  1982年   7篇
  1980年   11篇
  1979年   10篇
  1976年   6篇
  1975年   9篇
  1974年   7篇
  1973年   12篇
  1972年   9篇
  1971年   10篇
  1970年   6篇
  1969年   11篇
  1968年   8篇
  1965年   7篇
  1958年   6篇
排序方式: 共有1177条查询结果,搜索用时 15 毫秒
71.
Species of Ascocotyle Looss, 1899 (subgenus Phagicola Faust, 1920) (Digenea: Heterophyidae) related to A. (P.) longa Ransom, 1920, i.e. those with a bipartite gonotyl and a single crown of 16 circumoral spines, were revised on the basis of examination of type and voucher specimens. The species A. (P.) arnaldoi Travassos, 1929, A. (P.) byrdi (Robinson, 1956) and A. (P.) longicollis (Kuntz & Chandler, 1956), all possessing 16 circumoral spines, are synonymised with A. (P.) longa. A. (P.) ascolonga Witenberg, 1929, the types of which were not available, is provisionally retained as a valid species despite its close similarity to A. (P.) longa (identical number of circumoral spines and morphology of the ventrogenital sac). It differs from A. (P.) longa in the presence of uterine loops posterior to the testes and intestinal caeca ending anterior to the ovary (versus the pretesticular uterus and intestinal caeca reaching to the posterior margin of the ovary in A. (P.) longa). A. (P.) inglei (Hutton & Sogandares-Bernal, 1959), found only once in a single specimen, is also tentatively considered a valid species because it differs in number of circumoral spines (19), slightly larger eggs (23–26×13–16 m) and the distribution of refractile bodies of the gonotyl arranged in four separate groups from A. (P.) longa and A. (P.) ascolonga (16 circumoral spines; eggs 15–20×–12 m; refractile bodies in two groups).  相似文献   
72.
The caryophyllidean tapeworm Atractolytocestus huronensis Anthony, 1958, originally described from the common carp Cyprinus carpio carpio L. from North America, has recently been introduced into Europe (UK, Hungary). In the present study, the dispersion of the tapeworm in other countries of Central Europe (Slovakia and the Czech Republic) is described, including its first observation in the common carp of the Tisa River, southeastern Slovakia. Because of the transboundary location of the river, there are possible consequences of the rapid distribution of the parasite to other regions throughout the Danube River basin. The parasite has also been found in cultured carp from fishponds in South Bohemia, from where the carp are imported to many European countries. Rapid dissemination of this cestode in Europe indicates its ability to colonise new regions, and represents another example of man-made introduction of potential pathogens of carp and other farmed fish. The morphology of the A. huronensis specimens found is compared with that of specimens from Hungary and North America as well as that of Atractolytocestus sagittatus (Kulakovskaya and Akhmerov, 1965) from the same fish host in eastern Asia.  相似文献   
73.
The purpose of this case study was to optimise design, operation and maintenance guidelines, and to assess the water treatment potential of a storm water pond system after 15 months of operation. The system was based on a combined silt trap, attenuation pond and vegetated infiltration basin. This combination was used as the basis for construction of a roof water runoff system from a single domestic property. United Kingdom Building Research Establishment and Construction Industry Research and Information Association, and German Association for Water, Wastewater and Waste design guidelines were tested. These design guidelines failed because they did not consider local conditions. The infiltration function for the infiltration basin was logarithmic. Algal control techniques were successfully applied, and treatment of rainwater runoff from roofs was found to be largely unnecessary for recycling (e.g., watering plants). However, seasonal and diurnal variations of biochemical oxygen demand, dissolved oxygen and pH were recorded.  相似文献   
74.
The 17-alpha-substituted triterpene 1 [(17alpha)-23-(E)-dammara-20,23-diene-3beta,25-diol] showed promising activity in animal models of immunosuppression and inflammation. Using a mouse model for inflammatory skin diseases (oxazolone-induced allergic contact dermatitis, ACD) as the directing in vivo test system, Structure-activity-relationship studies with the aim to understand the necessary structural requirements for the biological activity of 1 were conducted. Furthermore, we anticipated to identify biologically active compounds with the 17beta configuration, which are thermodynamically more stable and much easier to synthesize. This was achieved by identifying the 17-beta substituted dammarane 5B and its analogues.  相似文献   
75.
A new natural-synthetic hybrid biomaterial has been isolated from the growth of Pseudomonas oleovorans in the presence of diethylene glycol (DEG). DEG was consumed by P. oleovorans with 20 mM sodium octanoate in modified E* medium, but its presence in the fermentation medium retarded cell growth and viability, influencing production and composition of polyhydroxyalkanoates with medium chain length substituents (mclPHAs) and consequently attenuating PHA yield. DEG affected the composition of the mclPHA with an increase in the C8 component: polyhydroxyoctanoate (PHO). Gas chromatography-mass spectrometry (GC-MS) was used to quantitatively monitor DEG in the system and reveal its cellular adsorption and penetration. Intracellularly, the DEG significantly reduced the molar mass of the mclPHA; PHO with a bimodal distribution of high and low molecular weight fractions was observed. 1H NMR, 2-D COSY, and heteronuclear single quantum coherence spectra confirmed that the high molecular weight fraction consisted of PHO chains terminated by DEG. Thus, the synthesis of this natural-synthetic hybrid copolymer, PHO-DEG, opens the way for microbial synthesis of a wide variety of PHA-DEG copolymers with a range of bioactive properties.  相似文献   
76.
We studied the role of the adventitia in adaptive arteriogenesis during the phase of active growth of coronary collateral vessels (CV) induced by chronic occlusion of the left circumflex coronary artery in canine hearts. We used electron microscopy and immunoconfocal (IF) labeling for bFGF, matrix metalloproteinase (MMP)-2, MMP-9, tissue-type plasminogen activator (tPA), its inhibitor (PAI-1), fibronectin (FN), and Ki-67. Proliferation of smooth muscle cells and adventitial fibroblasts was evident. Quantitative IF showed that adventitial MMP-2, MMP-9, and FN were 9.2-, 7.5-, and 8.6-fold, bFGF was 5.1-fold, and PAI-1 was 3.4-fold higher in CV than in normal vessels (NV). The number of fibroblasts was 5-fold elevated in CV, but the elastic fiber content was 25-fold greater in NV than in CV. Perivascular myocyte damage and induction of endothelial nitric oxide synthase in peri-CV capillaries indicate expansion of CV. It was concluded that adventitial activation is associated with the development of CV through cell proliferation, production of growth factors, and induction of extracellular proteolysis thereby contributing to remodeling during adaptive arteriogenesis.  相似文献   
77.
The caspase-activated DNase (CAD) is an important nuclease involved in apoptotic DNA degradation. Results of a sequence comparison of CAD proteins with beta beta alpha-Me-finger nucleases in conjunction with a mutational and chemical modification analysis suggest that CAD proteins constitute a new family of beta beta alpha-Me-finger nucleases. Nucleases of this family have widely different functions but are characterized by a common active-site fold and similar catalytic mechanisms. According to our results and comparisons with related nucleases, the active site of CAD displays features that partly resemble those of the colicin E9 and partly those of the T4 endonuclease VII active sites. We suggest that the catalytic mechanism of CAD involves a conserved histidine residue, acting as a general base, and another histidine as well as an aspartic acid residue required for cofactor binding. Our findings provide a first insight into the likely active-site structure and catalytic mechanism of a nuclease involved in the degradation of chromosomal DNA during programmed cell death.  相似文献   
78.
Beta-carbolines have been suggested to be involved in the pathogenesis of Parkinson's disease as a result of their structural similarity to the neurotoxin N -methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The chloral-derived beta-carboline derivative 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) causes cell loss in neuronal and glial cell cultures and induces a slowly developing neurodegenerative process in rats. In our experiments, effects of TaClo and its derivatives 2-methyl-TaClo (2-Me-TaClo), and 1-dichloromethylene-1,2,3,4-tetrahydro-beta-carboline (1-CCl(2) -THbetaC) on tyrosine hydroxylase (TH) activity were investigated in TH assays using homogenate preparations of the rat nucleus accumbens and recombinant human TH (hTH1). TH activity was determined in vitro by measuring l-DOPA production with HPLC-ECD. Using homogenate preparations, TaClo, 2-Me-TaClo, and 1-CCl(2) -THbetaC inhibited TH in concentrations of 0.1 mm, while 1-CCl(2) -THbetaC in low concentrations enhanced TH activity. When TH was activated by PACAP-27, TaClo, 2-Me-TaClo, or 1-CCl(2) -THbetaC also inhibited activated enzyme activity in high concentrations. However, in the case of 2-Me-TaClo and 1-CCl(2) -THbetaC a biphasic effect was observed with a marked increase of TH activity in the nanomolar range. In our experiments using recombinant hTH1, TaClo, 2-Me-TaClo, or 1-CCl(2) -THbetaC did not modify enzyme activity. After activation of hTH1 by PKA all the tetrahydro-beta-carbolines investigated in this study decreased l-DOPA formation. We suggest that these beta-carbolines modulate dopamine synthesis by interacting with a protein kinase TH-activating system.  相似文献   
79.
80.
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