全文获取类型
收费全文 | 3328篇 |
免费 | 268篇 |
国内免费 | 3篇 |
专业分类
3599篇 |
出版年
2021年 | 19篇 |
2020年 | 22篇 |
2019年 | 23篇 |
2018年 | 22篇 |
2017年 | 24篇 |
2016年 | 49篇 |
2015年 | 84篇 |
2014年 | 91篇 |
2013年 | 139篇 |
2012年 | 154篇 |
2011年 | 162篇 |
2010年 | 96篇 |
2009年 | 113篇 |
2008年 | 138篇 |
2007年 | 156篇 |
2006年 | 162篇 |
2005年 | 148篇 |
2004年 | 128篇 |
2003年 | 133篇 |
2002年 | 128篇 |
2001年 | 58篇 |
2000年 | 57篇 |
1999年 | 51篇 |
1998年 | 53篇 |
1997年 | 53篇 |
1996年 | 44篇 |
1995年 | 34篇 |
1994年 | 44篇 |
1993年 | 35篇 |
1992年 | 41篇 |
1991年 | 47篇 |
1990年 | 30篇 |
1989年 | 40篇 |
1988年 | 35篇 |
1987年 | 29篇 |
1986年 | 29篇 |
1985年 | 35篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 28篇 |
1981年 | 28篇 |
1980年 | 39篇 |
1977年 | 29篇 |
1975年 | 22篇 |
1974年 | 25篇 |
1973年 | 20篇 |
1970年 | 17篇 |
1968年 | 22篇 |
1936年 | 17篇 |
1932年 | 18篇 |
排序方式: 共有3599条查询结果,搜索用时 15 毫秒
1.
Ernst Wm. Spannhake Terry L. Adams Steven R. Kleeberger 《Prostaglandins & other lipid mediators》1985,30(6):1041-1055
The effects of repeated antigen exposure on the synthesis of mediators by lung tissues are not well understood. To investigate the influence of antigen challenge on the synthesis of prostaglandins by central airway and peripheral lung tissues, fourteen sensitive sheep underwent biweekly exposure to aerosolized antigen (7) or saline (7). Following the fifth exposure, microsomal and high speed supernatant fractions were prepared from trachealis muscle and lung parenchyma. Synthesis of thromboxane (TX) A2, prostaglandin (PG) D2 and PGI2 from the PG endoperoxide intermediate, PGH2, was assayed over a range of substrate concentrations from 3–200 uM. Synthesis of PGI2 by trachealis microsomes was approximately 5-fold greater than that of TXA2. PGI2 and TXA2 production was identical in tracheal preparations from Ascaris- and saline-exposed animals. In parenchymal tissues, where TXA2 production predominated over PGI2 by 9-fold, preparations from Ascaris- exposed animals synthesized 50% more TXA2 than controls at PGH2 concentrations of 25 uM and above, whereas synthesis of PGI2 and PGD2 were similar in preparations from both groups of animals. The density of pulmonary mast cells was decreased by 21% in the Ascaris group, whereas polymorphonuclear leukocyte density was unchanged. These results demonstrate the differential synthesis of TXA2 and PGI2 in central airways and peripheral lung regions of the sheep. They further indicate that repeated exposure of the airways to antigen selectively enhances TXA2 synthesis in the lung periphery of sensitized animals. The site of this increased enzymatic activity, whether in resident cells or newly-infiltrated cells, has not been determined. 相似文献
2.
Liane Wehder Günther Ernst Anna C. Crecelius Orlando Guntinas-Lichius Christian Melle Ulrich S. Schubert Ferdinand von Eggeling 《The journal of histochemistry and cytochemistry》2010,58(10):929-937
Carcinoma tissue consists of not only tumor cells but also fibroblasts, endothelial cells or vascular structures, and inflammatory cells forming the supportive tumor stroma. Therefore, the spatial distribution of proteins that promote growth and proliferation in these complex functional units is of high interest. Matrix-assisted laser desorption/ionization imaging mass spectrometry is a newly developed technique that generates spatially resolved profiles of protein signals directly from thin tissue sections. Surface-enhanced laser desorption/ionization mass spectrometry (MS)combined with tissue microdissection allows analysis of defined parts of the tissue with a higher sensitivity and a broader mass range. Nevertheless, both MS-based techniques have a limited spatial resolution. IHC is a technique that allows a resolution down to the subcellular level. However, the detection and measurement of a specific protein expression level is possible only by semiquantitative methods. Moreover, prior knowledge about the identity of the proteins of interest is necessary. In this study, we combined all three techniques to gain highest spatial resolution, sensitivity, and quantitative information. We used frozen tissue from head and neck tumors and chose two exemplary proteins (HNP1–3 and S100A8) to highlight the advantages and disadvantages of each technique. It could be shown that the combination of these three techniques results in congruent but also synergetic data. (J Histochem Cytochem 58:929–937, 2010) 相似文献
3.
4.
N. A. Volkova N. A. Zinovieva L. A. Volkova L. K. Ernst 《Russian Journal of Genetics》2006,42(1):72-75
The data on the in vitro and in vivo (into embryonic disk) retroviral-mediated transfer of genetic information into chicken embryonic cells are presented. The estimated transformation frequency of the cultured target cells constituted 8 × 10?4 to 5 × 10?3. A transgenic rooster, carrying recombinant DNA in blood, heart, liver, and intestine cells, was obtained. 相似文献
5.
Structural analysis of human profilin has revealed two tryptophan residues, W3 and W31, which interact with polyproline. The codons for these residues were mutated to encode phenylalanine and the mutant proteins overexpressed in Eschericia coli. The isolated proteins were diminished in their ability to bind polyproline, whereas phosphatidylinositol 4,5-bisphosphate (PIP2) binding remained unchanged. In many strains of Saccharomyces cerevisiae, disruption of the gene encoding profilin, PFY1, is lethal. It was found that expression of the gene for human profilin is capable of suppressing this lethality. The polyproline-binding mutant alleles of the human gene were cloned into various yeast expression vectors. Each of the mutant genes resulted in suppression of the lethality of pfy1Delta. It was observed that the mutant protein expression levels paralleled the growth rates of the strains. The severity of various morphological abnormalities of the strains was also attenuated with increased protein levels, suggesting that profilin polyproline-binding mutations are deleterious to cell growth unless overexpressed. Both tryptophan mutations were combined to give a third mutant allele that was found both unable to bind polyproline and to suppress the lethality of a pfy1 deletion. Immunoprecipitation experiments suggested that the mutants were unaltered in their affinity for actin and PIP2. These data strongly suggest that polyproline binding is an essential function of profilin. 相似文献
6.
The three-dimensional structure of ricin at 2.8 A 总被引:22,自引:0,他引:22
W Montfort J E Villafranca A F Monzingo S R Ernst B Katzin E Rutenber N H Xuong R Hamlin J D Robertus 《The Journal of biological chemistry》1987,262(11):5398-5403
The x-ray crystallographic structure of the heterodimeric plant toxin ricin has been determined at 2.8-A resolution. The A chain enzyme is a globular protein with extensive secondary structure and a reasonably prominent cleft assumed to be the active site. The B chain lectin folds into two topologically similar domains, each binding lactose in a shallow cleft. In each site a glutamine residue forms a hydrogen bond to the OH-4 of galactose, accounting for the epimerimic specificity of binding. The interface between the A and B chains shows some hydrophobic contacts in which proline and phenylalanine side chains play a prominent role. 相似文献
7.
Use of a novel strategy for the preparation and characterization of an antipeptide antibody capable of recognizing members of the annexin family 总被引:2,自引:0,他引:2
J D Ernst E Hoye R A Blackwood 《Biochemical and biophysical research communications》1989,161(3):959-964
Annexins are structurally-related proteins which bind phospholipids in a Ca2+-dependent manner. We have used a novel coupling strategy to prepare an antiserum directed against a 17-amino acid synthetic peptide that resembles the sequence of a highly-conserved portion of these proteins. This antipeptide serum specifically recognizes 5 of 6 human annexins on Western blots, despite differences between the protein and peptide sequences of 3 or 4 amino acids. The antiserum does not recognize endonexin II, whose sequence differs from that of the peptide by 6 amino acids. The availability of multiple proteins with known amino acid sequence has allowed analysis of structural requirements for recognition by this antibody. In some situations, use of such an antibody may allow the identification of a protein as a member of a family. 相似文献
8.
A novel system useful for aeration and cell retention in continuous perfused microcarrier cultures is described. The system is based on a vibrating cage that separates cells and microcarriers from the oxygenation chamber and allows gas bubble free oxygen transfer. In the cultivation of monkey kidney cells (VERO) on gelatin coated microcarriers, using different concentrations (5, 10 and 15 g Cytodex 3/liter) cell densities up to 107 cells per ml were obtained. The described system is scaleable. 相似文献
9.
The mutation of Scenedesmus obliquus strain C-6D is expressed in the dark only. Under these conditions, this strain synthesizes acyclic poly-cis carotenes. Cyclic carotenoids like -carotene or xanthophylls are absent. In the light the normal cyclic carotenes and xanthophylls are synthesized in the all-trans configuration. The poly-cis carotencs present in dark cultures have been analysed and quantitated. It could be shown that these poly-cis carotenes are identical with the poly-cis carotenes synthesized by the tomato mutant Lycopersicon esculentum var. Tangella. The poly-cis pathways, however, are different regulated in the two organisms. The tomato mutant accumulates prolycopene as the major carotene, whereas the mutant C-6D accumulates mainly pro--carotene. Furthermore, the mutation in Tangella is constitutive in light in contrast to Scenedesmus C-6D. Besides that, Scenedesmus C-6D synthesizes a further cis-carotene isomer of phytofluene as well as of -carotene. The configuration of these carotenes still have to be elucidated. The occurrence of this poly-cis carotene biosynthetic pathway by a mutation of only one enzyme, the phytoene desaturase which, however, is only expressed in darkness under heterotrophic conditions, is discussed. 相似文献
10.
Ernst Mayr 《Biology & philosophy》1988,3(4):431-441
The biological species concept deals both with the meaning of the sexual species as a harmonious gene pool and with its protection against deleterious outbreeding (effected by isolating mechanisms). According to the Darwin-Muller-Mayr theory isolating mechanisms are acquired by incipient species during alloparty. Isolating mechanisms are not the result of ad hoc selection, but of a change of function of properties acquired during the preceding isolation of the incipient species. The role of behavioral properties (recognition) among the isolating mechanisms has long been recognized and described by naturalists but was rejected as basis of a species definition for a number of valid reasons. 相似文献