全文获取类型
收费全文 | 4516篇 |
免费 | 359篇 |
国内免费 | 2篇 |
专业分类
4877篇 |
出版年
2023年 | 16篇 |
2022年 | 40篇 |
2021年 | 71篇 |
2020年 | 40篇 |
2019年 | 56篇 |
2018年 | 66篇 |
2017年 | 68篇 |
2016年 | 119篇 |
2015年 | 165篇 |
2014年 | 213篇 |
2013年 | 276篇 |
2012年 | 308篇 |
2011年 | 298篇 |
2010年 | 237篇 |
2009年 | 185篇 |
2008年 | 265篇 |
2007年 | 265篇 |
2006年 | 219篇 |
2005年 | 227篇 |
2004年 | 243篇 |
2003年 | 247篇 |
2002年 | 219篇 |
2001年 | 72篇 |
2000年 | 53篇 |
1999年 | 60篇 |
1998年 | 80篇 |
1997年 | 41篇 |
1996年 | 34篇 |
1995年 | 37篇 |
1994年 | 30篇 |
1993年 | 35篇 |
1992年 | 52篇 |
1991年 | 50篇 |
1990年 | 35篇 |
1989年 | 37篇 |
1988年 | 33篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 19篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 22篇 |
1981年 | 18篇 |
1980年 | 11篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1974年 | 16篇 |
1973年 | 11篇 |
1972年 | 13篇 |
1970年 | 11篇 |
排序方式: 共有4877条查询结果,搜索用时 15 毫秒
181.
Properties of the P-Type ATPases Encoded by the copAP Operons of Helicobacter pylori and Helicobacter felis 下载免费PDF全文
Denis Bayle Sabine Wngler Thomas Weitzenegger Wolfram Steinhilber Jürgen Volz Michael Przybylski Klaus P. Schfer George Sachs Klaus Melchers 《Journal of bacteriology》1998,180(2):317-329
The cop operons of Helicobacter pylori and Helicobacter felis were cloned by gene library screening. Both operons contain open reading frames for a P-type ion pump (CopA) with homology to Cd2+ and Cu2+ ATPases and a putative ion binding protein (CopP), the latter representing a CopZ homolog of the copYZAB operon of Enterococcus hirae. The predicted CopA ATPases contained an N-terminal GMXCXXC ion binding motif and a membrane-associated CPC sequence. A synthetic N-terminal peptide of the H. pylori CopA ATPase bound to Cu2+ specifically, and gene disruption mutagenesis of CopA resulted in an enhanced growth sensitivity of H. pylori to Cu2+ but not to other divalent cations. As determined experimentally, H. pylori CopA contains four pairs of transmembrane segments (H1 to H8), with the ATP binding and phosphorylation domains lying between H6 and H7, as found for another putative transition metal pump of H. pylori (K. Melchers, T. Weitzenegger, A. Buhmann, W. Steinhilber, G. Sachs, and K. P. Schäfer, J. Biol. Chem. 271:446–457, 1996). The corresponding transmembrane segments of the H. felis CopA pump were identified by hydrophobicity analysis and via sequence similarity. To define functional domains, similarly oriented regions of the two enzymes were examined for sequence identity. Regions with high degrees of identity included the N-terminal Cu2+ binding domain, the regions of ATP binding and phosphorylation in the energy transduction domain, and a transport domain consisting of the last six transmembrane segments with conserved cysteines in H4, H6, and H7. The data suggest that H. pylori and H. felis employ conserved mechanisms of ATPase-dependent copper resistance. 相似文献
182.
Genetic imbalances in preleukemic thymuses 总被引:4,自引:0,他引:4
Verlaet M Deregowski V Denis G Humblet C Stalmans MT Bours V Castronovo V Boniver J Defresne MP 《Biochemical and biophysical research communications》2001,283(1):12-18
To understand the molecular mechanisms involved in preleukemia, the suppression subtractive hybridization method was used in a murine radiation-induced thymic lymphoma model. Seventeen mRNAs overexpressed in preleukemic thymuses were identified: mouse laminin binding protein (p40/37LBP), E25 protein, Rattus norvegicus clone BB.1.4.1, profilin, poly(A) binding protein (PABP), mouse high mobility group protein 1, topoisomerase I, clusterin, proteasome RC1 subunit, rat prostatein C3 and C1 subunits; two ESTs and four unknown genes. The overexpression of PABP, clusterin, profilin, and the p40/37LBP mRNAs was confirmed in preleukemic thymuses and can be related to some cellular events observed during the preleukemic period, i.e., alterations of cell cycle and apoptosis properties. The p40/37LBP and 67-kDa laminin receptor proteins were upregulated during the preleukemic period. The data suggest that additional studies on p40/37LBP and 67-kDa laminin receptor regulation are required to evaluate their potential role in the lymphoma prevention by TNF-alpha and IFN-gamma. 相似文献
183.
Gallant M Carrière MC Chateauneuf A Denis D Gareau Y Godbout C Greig G Juteau H Lachance N Lacombe P Lamontagne S Metters KM Rochette C Ruel R Slipetz D Sawyer N Tremblay N Labelle M 《Bioorganic & medicinal chemistry letters》2002,12(18):2583-2586
Potent and selective ligands for the human EP3 prostanoid receptor are described. Biaryl compounds bearing a tethered ortho substituted acidic moiety were identified as potent EP3 antagonists based on the SAR described herein. The binding affinity of key compounds on all eight human prostanoid receptors is reported. 相似文献
184.
185.
The principal sulfatide of virulentMycobacterium tuberculosis, sulfolipid-I (SL-I), both directly stimulates neutrophil superoxide (O
2
–
) release and, at substimulatory concentrations, primes these cells for markedly enhanced oxidative responsiveness to other stimuli. The present study was undertaken to clarify the priming mechanisms by comparing cellular events following priming doses of SL-I with those following priming with N-formyl-methionyl-leucyl-phenylalanine (FMLP). We compared the involvement of the calcium cation (Ca2+), as well as membrane protein kinase C (PKC) activity and the translocation of NADPH oxidase-cytosolic cofactor effected by priming levels of the two agonists. The investigation led to two important conclusions. First, we clearly demonstrate that priming by both SL-I and FMLP results from activation of cellular processes that are not involved in direct oxidative activation. For example, whereas direct induction of O
2
–
generation by FMLP and SL-I required increases in intracellular Ca2+, an increase in intracellular calcium concentration ([Ca2+]i) above basal levels was not required for priming. Second, we identified key differences in the cellular responses to priming doses of SL-I and FMLP. Whereas increased membrane PKC activity caused by priming doses of FMLP was only partially blocked by chelation of intracellular Ca2+, Ca2+ chelation completely inhibited the increase in membrane PKC activity caused by SL-I. NADPH oxidase-cytosolic factor translocation to plasma membranes was completely blocked by pertussis toxin when priming doses of SL-I were used. This guanine-nucleotide-binding protein inhibitor had no effect on FMLP-dependent translocation of the oxidase cofactors. The comparative approach introduced in this report provides a valuable and novel method to discern the complex interactions of various cellular processes that regulate the state of activation of stimulated cells. 相似文献
186.
187.
Peiretti F Bernot D Lopez S Bonardo B Deprez-Beauclair P Juhan-Vague I Nalbone G 《Journal of cellular physiology》2003,196(2):346-353
During phorbol ester-induced differentiation of HL-60 monocytic cells, tumor necrosis factoralpha (TNFalpha) synthesis and secretion are increased, which contributes to the autocrine regulation of TNFalpha-responsive genes. We investigated how, during phorbol ester-induced differentiation of HL-60 cells, the secreted TNFalpha modulated plasminogen activator inhibitor type I (PAI-1) and gelatinase B (MMP-9) syntheses, two proteins involved in pericellular proteolysis. The differentiation-induced release of TNFalpha, was abolished by the hydroxamate-based matrix metalloproteinase (MMP) inhibitor, RU36156. RU36156 or a neutralizing anti-TNFalpha significantly down-regulated PAI-1 synthesis exclusively during the early phases of differentiation (from promyelocyte to monocytic-like cells), which underlined the activating role of autocrine TNFalpha during this time range. As cells progressed to monocyte/macrophage phenotype, they still released TNFalpha, but RU36156 or anti-TNFalpha no longer had an effect on PAI-1 synthesis. This lack of effect was not due to a default of TNFalpha signaling since PAI-1 synthesis was still stimulated in response to exogenous TNFalpha. TNFalpha receptor RI was also actively released and was shown to reduce TNFalpha activity which may account for the inability of soluble TNFalpha to up-regulate PAI-1 synthesis. In later mature stage, cells became susceptible to exogenous TNFalpha-induced apoptosis and rapidly lost their ability to respond to TNFalpha. The MMP-9 synthesis followed similar regulation as PAI-1. Isolated human blood monocytes-derived macrophages behave like HL-60-derived macrophages. In conclusion, these results show that during leukocyte differentiation, time windows exist during which the autocrine TNFalpha is active and then down-regulated by RI, which may temper a continuous up-regulation of the synthesis of proteins involved in pericellular proteolysis. 相似文献
188.
Dr. Denis G. Baskin 《Cell and tissue research》1976,174(1):55-67
Summary Epidermal septate junctions of Nereis sp. and Cirriformia sp. fixed with OsO4 or glutaraldehyde/OsO4 display variable structure in electron micrographs. In transverse section the septa are often indistinct and obscured by opaque material that fills the junctional cleft. Septa (spaced at 180–280 Å) are more clearly defined in slightly oblique transverse section; they exhibit an electron lucent center and appear to be linked by arms. En face views of the junction show a honeycomb pattern. Cytoplasmic faces of junctional membranes are backed with plaques opposite the septa. Lanthanum used as a tracer delineates junctional structure in negative contrast. In transverse section a chain-like lattice is present in the junctional cleft. En face views show parallel rows of pleated elements often linked by arms into honeycomb arrays. Oblique sections demonstrate that these pleated elements are continuous with the chain-like lattice seen in transverse sections. Lanthanum does not pass entirely through the junction. Lanthanum reveals that the septa have a very intricate substructure, but it is difficult to visualize the architecture that could generate the various images presented by these junctions when seen in different orientations. However, it is clear that these junctions possess some features that are diagnostic of several supposedly different types of septate junctions in invertebrates.Supported by USPHS grants NIH 5 P01 NS-07512, NIH 2701 GM-00102, and NB-00840, and by a grant from the Pomona College Research CommitteeI thank Sarah Wurzelmann, Stanley Brown, Nancy Kelly, and Gerhard Ott for excellent technical assistance. Portions of this study were carried out while I was a Postdoctoral Fellow in the Department of Anatomy, Albert Einstein College of Medicine. I dedicate this article to Berta Scharrer as a token of appreciation and affection for her guidance, encouragement, inspiration, and example of excellence 相似文献
189.
Litter decay controlled by temperature,not soil properties,affecting future soil carbon 总被引:3,自引:0,他引:3 下载免费PDF全文
Edward G. Gregorich Henry Janzen Benjamin H. Ellert Bobbi L. Helgason Budong Qian Bernie J. Zebarth Denis A. Angers Ronald P. Beyaert Craig F. Drury Scott D. Duguid William E. May Brian G. McConkey Miles F. Dyck 《Global Change Biology》2017,23(4):1725-1734
Widespread global changes, including rising atmospheric CO2 concentrations, climate warming and loss of biodiversity, are predicted for this century; all of these will affect terrestrial ecosystem processes like plant litter decomposition. Conversely, increased plant litter decomposition can have potential carbon‐cycle feedbacks on atmospheric CO2 levels, climate warming and biodiversity. But predicting litter decomposition is difficult because of many interacting factors related to the chemical, physical and biological properties of soil, as well as to climate and agricultural management practices. We applied 13C‐labelled plant litter to soil at ten sites spanning a 3500‐km transect across the agricultural regions of Canada and measured its decomposition over five years. Despite large differences in soil type and climatic conditions, we found that the kinetics of litter decomposition were similar once the effect of temperature had been removed, indicating no measurable effect of soil properties. A two‐pool exponential decay model expressing undecomposed carbon simply as a function of thermal time accurately described kinetics of decomposition. (R2 = 0.94; RMSE = 0.0508). Soil properties such as texture, cation exchange capacity, pH and moisture, although very different among sites, had minimal discernible influence on decomposition kinetics. Using this kinetic model under different climate change scenarios, we projected that the time required to decompose 50% of the litter (i.e. the labile fractions) would be reduced by 1–4 months, whereas time required to decompose 90% of the litter (including recalcitrant fractions) would be reduced by 1 year in cooler sites to as much as 2 years in warmer sites. These findings confirm quantitatively the sensitivity of litter decomposition to temperature increases and demonstrate how climate change may constrain future soil carbon storage, an effect apparently not influenced by soil properties. 相似文献
190.
Olga A. Snytnikova Anastasiya A. Khlichkina Lyudmila V. Yanshole Vadim V. Yanshole Igor A. Iskakov Elena V. Egorova Denis A. Stepakov Vladimir P. Novoselov Yuri P. Tsentalovich 《Metabolomics : Official journal of the Metabolomic Society》2017,13(1):5