全文获取类型
收费全文 | 1236篇 |
免费 | 96篇 |
国内免费 | 1篇 |
出版年
2021年 | 15篇 |
2020年 | 12篇 |
2019年 | 11篇 |
2018年 | 16篇 |
2017年 | 17篇 |
2016年 | 16篇 |
2015年 | 30篇 |
2014年 | 46篇 |
2013年 | 74篇 |
2012年 | 65篇 |
2011年 | 64篇 |
2010年 | 39篇 |
2009年 | 40篇 |
2008年 | 66篇 |
2007年 | 62篇 |
2006年 | 68篇 |
2005年 | 66篇 |
2004年 | 64篇 |
2003年 | 55篇 |
2002年 | 62篇 |
2001年 | 11篇 |
2000年 | 13篇 |
1999年 | 23篇 |
1998年 | 20篇 |
1997年 | 8篇 |
1996年 | 9篇 |
1995年 | 12篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1988年 | 7篇 |
1987年 | 13篇 |
1986年 | 6篇 |
1985年 | 15篇 |
1984年 | 25篇 |
1983年 | 18篇 |
1982年 | 26篇 |
1981年 | 11篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 8篇 |
1968年 | 6篇 |
排序方式: 共有1333条查询结果,搜索用时 15 毫秒
991.
Plant Ecology - Selection against inbred progeny is a well-documented phenomenon in natural and experimental populations of trees with mixed mating systems. This inbreeding depression can be... 相似文献
992.
This study is dedicated to the environmental monitoring of radionuclides released in the course of the Fukushima nuclear accident. The activity concentrations of β− -emitting 90Sr and β−/γ-emitting 134Cs and 137Cs from several hot spots in Japan were determined in soil and vegetation samples. The 90Sr contamination levels of the samples were relatively low and did not exceed the Bq⋅g−1 range. They were up four orders of magnitude lower than the respective 137Cs levels. This study, therefore, experimentally confirms previous predictions indicating a low release of 90Sr from the Fukushima reactors, due to its low volatility. The radiocesium contamination could be clearly attributed to the Fukushima nuclear accident via its activity ratio fingerprint (134Cs/137Cs). Although the correlation between 90Sr and 137Cs is relatively weak, the data set suggests an intrinsic coexistence of both radionuclides in the contaminations caused by the Fukushima nuclear accident. This observation is of great importance not only for remediation campaigns but also for the current food monitoring campaigns, which currently rely on the assumption that the activity concentrations of β−-emitting 90Sr (which is relatively laborious to determine) is not higher than 10% of the level of γ-emitting 137Cs (which can be measured quickly). This assumption could be confirmed for the samples investigated herein. 相似文献
993.
Siegfried W. De Laat Paul T. Van Der Saag S. Ad Nelemans Meir Shinitzky 《生物化学与生物物理学报:生物膜》1978,509(1):188-193
Microviscosity of the plasma-membrane lipid matrix was measured in exponentially growing and differentiating C1300 mouse neuroblastoma cells, attached to a glass substratum, by fluorescence polarisation of 1,6-diphenyl-1,3,5-hexatriene. Upon differentiation decreases progressively, reaching values below those observed in the growth phase. Treatment of the cells with dipalmitoyl phosphatidylcholine vesicles reversibly inhibits morphological differentiation. The results show that a high membrane fluidity is a prerequisite for differentiation. 相似文献
994.
Abdel-Majid Khatib Rachid Lahlil Nathalie Scamuffa Marie-Andrée Akimenko Sylvain Ernest Abdderahim Lomri Claude Lalou Nabil G. Seidah Bruno O. Villoutreix Fabien Calvo Geraldine Siegfried 《PloS one》2010,5(7)
Background
In zebrafish, vascular endothelial growth factor-C precursor (proVEGF-C) processing occurs within the dibasic motif HSIIRR214 suggesting the involvement of one or more basic amino acid-specific proprotein convertases (PCs) in this process. In the present study, we examined zebrafish proVEGF-C expression and processing and the effect of unprocessed proVEGF-C on caudal fin regeneration.Methodology/Principal Findings
Cell transfection assays revealed that the cleavage of proVEGF-C, mainly mediated by the proprotein convertases Furin and PC5 and to a less degree by PACE4 and PC7, is abolished by PCs inhibitors or by mutation of its cleavage site (HSIIRR214 into HSIISS214). In vitro, unprocessed proVEGF-C failed to activate its signaling proteins Akt and ERK and to induce cell proliferation. In vivo, following caudal fin amputation, the induction of VEGF-C, Furin and PC5 expression occurs as early as 2 days post-amputation (dpa) with a maximum levels at 4–7 dpa. Using immunofluorescence staining we localized high expression of VEGF-C and the convertases Furin and PC5 surrounding the apical growth zone of the regenerating fin. While expression of wild-type proVEGF-C in this area had no effect, unprocessed proVEGF-C inhibited fin regeneration.Conclusions/Significances
Taken together, these data indicate that zebrafish fin regeneration is associated with up-regulation of VEGF-C and the convertases Furin and PC5 and highlight the inhibitory effect of unprocessed proVEGF-C on fin regeneration. 相似文献995.
Background
DNA extraction is a routine step in many insect molecular studies. A variety of methods have been used to isolate DNA molecules from insects, and many commercial kits are available. Extraction methods need to be evaluated for their efficiency, cost, and side effects such as DNA degradation during extraction.Methodology/Principal Findings
From individual western corn rootworm beetles, Diabrotica virgifera virgifera, DNA extractions by the SDS method, CTAB method, DNAzol® reagent, Puregene® solutions and DNeasy® column were compared in terms of DNA quantity and quality, cost of materials, and time consumed. Although all five methods resulted in acceptable DNA concentrations and absorbance ratios, the SDS and CTAB methods resulted in higher DNA yield (ng DNA vs. mg tissue) at much lower cost and less degradation as revealed on agarose gels. The DNeasy® kit was most time-efficient but was the costliest among the methods tested. The effects of ethanol volume, temperature and incubation time on precipitation of DNA were also investigated. The DNA samples obtained by the five methods were tested in PCR for six microsatellites located in various positions of the beetle''s genome, and all samples showed successful amplifications.Conclusion/Significance
These evaluations provide a guide for choosing methods of DNA extraction from western corn rootworm beetles based on expected DNA yield and quality, extraction time, cost, and waste control. The extraction conditions for this mid-size insect were optimized. The DNA extracted by the five methods was suitable for further molecular applications such as PCR and sequencing by synthesis. 相似文献996.
Sara Leschner Kathrin Westphal Nicole Dietrich Nuno Viegas Jadwiga Jablonska Marcin Lyszkiewicz Stefan Lienenklaus Werner Falk Nelson Gekara Holger Loessner Siegfried Weiss 《PloS one》2009,4(8)
Background
Several facultative anaerobic bacteria with potential therapeutic abilities are known to preferentially colonize solid tumors after systemic administration. How they efficiently find and invade the tumors is still unclear. However, this is an important issue to be clarified when bacteria should be tailored for application in cancer therapy.Methodology/Principal Findings
We describe the initial events of colonization of an ectopic transplantable tumor by Salmonella enterica serovar Typhimurium. Initially, after intravenous administration, bacteria were found in blood, spleen, and liver. Low numbers were also detected in tumors associated with blood vessels as could be observed by immunohistochemistry. A rapid increase of TNF-α in blood was observed at that time, in addition to other pro-inflammatory cytokines. This induced a tremendous influx of blood into the tumors by vascular disruption that could be visualized in H&E stainings and quantified by hemoglobin measurements of tumor homogenate. Most likely, together with the blood, bacteria were flushed into the tumor. In addition, blood influx was followed by necrosis formation, bacterial growth, and infiltration of neutrophilic granulocytes. Depletion of TNF-α retarded blood influx and delayed bacterial tumor-colonization.Conclusion
Our findings emphasize similarities between Gram-negative tumor-colonizing bacteria and tumor vascular disrupting agents and show the involvement of TNF-α in the initial phase of tumor-colonization by bacteria. 相似文献997.
Effect of average phospholipid curvature on supported bilayer formation on glass by vesicle fusion 下载免费PDF全文
The adsorption of large unilamellar vesicles composed of various combinations of phosphatidylcholine, phosphatidylethanolamine (PE), monomethyl PE, and dimethyl PE (PE-Me2) onto a glass surface was studied using fluorescence microscopy. The average lipid geometry within the vesicles, described mathematically by the average intrinsic curvature, C(0,ave), was methodically altered by changing the lipid ratios to determine the effect of intrinsic curvature on the ability of vesicles to rupture and form a supported lipid bilayer. We show that the ability of vesicles to create fluid planar bilayers is dependent on C(0,ave) and independent of the identity of the component lipids. When the C(0,ave) was approximately -0.1 nm(-1), the vesicles readily formed supported lipid bilayers with almost full mobility. In contrast, when the C(0,ave) ranged from approximately -0.2 to approximately -0.3 nm(-1), the adsorbed vesicles remained intact upon the surface. The results indicate that the average shape of lipid molecules within a vesicle (C(0,ave)) is essential for determining kinetically viable reactions that are responsible for global geometric changes. 相似文献
998.
Oleic acid is one of the major components of a variety of oils and its spectra are very complex due to the presence of a large number of conformational states (alpha, beta, gamma). In the present communication, an attempt has been made to unravel the complex spectra by calculating the normal modes and examining the spectral features such as line position, line width, band intensity, group frequency concept and spectral relationship between the finite and infinite systems. 相似文献
999.
1000.
Triantafilou M Gamper FG Haston RM Mouratis MA Morath S Hartung T Triantafilou K 《The Journal of biological chemistry》2006,281(41):31002-31011
Toll-like receptors (TLRs) are receptors of the innate immune system responsible for recognizing pathogen-associated molecular patterns. TLR2 seems to be the most promiscuous TLR receptor able to recognize the most diverse set of pathogen-associated patterns. Its promiscuity has been attributed to its unique ability to heterodimerize with TLRs 1 and 6 and, most recently, to its association with CD36 in response to diacylated lipoproteins. Thus, it seems that TLR2 forms receptor clusters in response to different microbial ligands. In this study we investigated TLR2 cell surface heterotypic interactions in response to different ligands as well as internalization and intracellular trafficking. Our data show that TLR2 forms heterodimers with TLR1 and TLR6 and that these heterodimer pre-exist and are not induced by the ligand. Upon stimulation by the specific ligand, these heterodimers are recruited within lipid rafts. In contrast, heterotypic associations of TLR2/6 with CD36 are not preformed and are ligand-induced. All TLR2 receptor clusters accumulate in lipid rafts and are targeted to the Golgi apparatus. This localization and targeting is ligand-specific. Activation occurs at the cell surface, and the observed trafficking is independent of signaling. 相似文献