全文获取类型
收费全文 | 5742篇 |
免费 | 466篇 |
出版年
2023年 | 17篇 |
2022年 | 39篇 |
2021年 | 102篇 |
2020年 | 59篇 |
2019年 | 84篇 |
2018年 | 93篇 |
2017年 | 103篇 |
2016年 | 124篇 |
2015年 | 243篇 |
2014年 | 269篇 |
2013年 | 395篇 |
2012年 | 463篇 |
2011年 | 454篇 |
2010年 | 323篇 |
2009年 | 272篇 |
2008年 | 388篇 |
2007年 | 396篇 |
2006年 | 371篇 |
2005年 | 326篇 |
2004年 | 319篇 |
2003年 | 279篇 |
2002年 | 279篇 |
2001年 | 68篇 |
2000年 | 62篇 |
1999年 | 62篇 |
1998年 | 69篇 |
1997年 | 48篇 |
1996年 | 37篇 |
1995年 | 34篇 |
1994年 | 26篇 |
1993年 | 22篇 |
1992年 | 37篇 |
1991年 | 22篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 30篇 |
1987年 | 17篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 16篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1979年 | 10篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1975年 | 10篇 |
1974年 | 10篇 |
1973年 | 12篇 |
1966年 | 8篇 |
排序方式: 共有6208条查询结果,搜索用时 31 毫秒
951.
Yeasts are often exposed to variations in osmotic pressure in their natural environments or in their substrates when used in fermentation industries. Such changes may lead to cell death or activity loss. Although the involvement of the plasma membrane is strongly suspected, the mechanism remains unclear. Here, the integrity and functionality of the yeast plasma membrane at different levels of dehydration and rehydration during an osmotic treatment were assessed using various fluorescent dyes. Flow cytometry and confocal microscopy of cells stained with oxonol, propidium iodide, and lucifer yellow were used to study changes in membrane polarization, permeabilization, and endocytosis, respectively. Cell volume contraction, reversible depolarization, permeabilization, and endovesicle formation were successively observed with increasing levels of osmotic pressure during dehydration. The maximum survival rate was also detected at a specific rehydration level, of 20 MPa, above which cells were strongly permeabilized. Thus, we show that the two steps of an osmotic treatment, dehydration and rehydration, are both involved in the induction of cell death. Permeabilization of the plasma membranes is the critical event related to cell death. It may result from lipidic phase transitions in the membrane and from variations in the area-to-volume ratio during the osmotic treatment. 相似文献
952.
953.
A Technique to Identify Changes in Hemlock Forest Health over Space and Time Using Satellite Image Data 总被引:1,自引:0,他引:1
The objective of this study was to develop a technique to classify health of eastern hemlock stands using historical satellite images. While remote sensing and geographic information systems have been used successfully to classify forest health using recent images, applying this process to older images is problematic because contemporaneous field data are not available to measure the accuracy of the classification of historical images. Data ranges were established for each hemlock health class using a contemporary image and field data. These ranges were used to level-slice archived images to create a series of health-class maps that show changes in forest health over time. By applying cross-tabulation procedures to pairs of classified images, it is possible to construct a transition map that indicates how the hemlock health class of each pixel in the images of the study area has changed over time. The resulting maps provide a look back at forest conditions of the past and can be used to identify areas of special interest. 相似文献
954.
Mechanical Load Enhances Procollagen Processing in Dermal Fibroblasts by Regulating Levels of Procollagen C-Proteinase 总被引:2,自引:0,他引:2
M. Parsons E. Kessler G. J. Laurent R. A. Brown J. E. Bishop 《Experimental cell research》1999,252(2):319-331
Mechanical forces are emerging as key regulators of cell function. We hypothesize that mechanical load may influence dermal fibroblast activity. We assessed the direct effects of mechanical load on human dermal fibroblast procollagen synthesis and processing in vitro. Cells were loaded in a biaxial loading system (Flexercell 3000). Hydroxyproline levels were measured in the medium and cell layer as an estimate of procollagen synthesis and processing to insoluble collagen. Mechanical load (in the presence of serum or TGF-beta) enhanced procollagen synthesis by 45 +/- 3% (P < 0.001), and 38 +/- 4% (P < 0.001), respectively, over unloaded growth factor controls after 48 h. Insoluble collagen deposition was enhanced in the same cultures by 115 +/- 8% (P < 0.01) and 72% +/- 9% (P < 0.01), respectively. This effect was inhibited using l-arginine suggesting that procollagen C-proteinase, the enzyme which directly cleaves the C-terminal propeptide of procollagen to form insoluble collagen, is required for the fiber formation observed. Procollagen mRNA levels in loaded samples increased by more than two-fold in both serum and TGF-beta-treated cultures at 48 h. Procollagen C-proteinase mRNA levels were also enhanced by a similar magnitude, although the increase was observed at 24 h. Procollagen C-proteinase protein levels were also increased at this time. Protein and mRNA levels of the procollagen C-proteinase enhancer protein, which binds the C-terminal propeptide of procollagen to enhance the rate of peptide cleavage, were unaffected by mechanical load. This study demonstrates that mechanical load promotes procollagen synthesis in dermal fibroblasts by enhancing gene expression and posttranslational processing of procollagen. 相似文献
955.
Physical and Functional Interactions between Cellular Retinoic Acid Binding Protein II and the Retinoic Acid-Dependent Nuclear Complex 总被引:8,自引:0,他引:8 下载免费PDF全文
956.
Regulation of Peroxisome Proliferator-Activated Receptor γ Expression by Adipocyte Differentiation and Determination Factor 1/Sterol Regulatory Element Binding Protein 1: Implications for Adipocyte Differentiation and Metabolism 总被引:1,自引:0,他引:1 下载免费PDF全文
957.
958.
C Egile T P Loisel V Laurent R Li D Pantaloni P J Sansonetti M F Carlier 《The Journal of cell biology》1999,146(6):1319-1332
To propel itself in infected cells, the pathogen Shigella flexneri subverts the Cdc42-controlled machinery responsible for actin assembly during filopodia formation. Using a combination of bacterial motility assays in platelet extracts with Escherichia coli expressing the Shigella IcsA protein and in vitro analysis of reconstituted systems from purified proteins, we show here that the bacterial protein IcsA binds N-WASP and activates it in a Cdc42-like fashion. Dramatic stimulation of actin assembly is linked to the formation of a ternary IcsA-N-WASP-Arp2/3 complex, which nucleates actin polymerization. The Arp2/3 complex is essential in initiation of actin assembly and Shigella movement, as previously observed for Listeria monocytogenes. Activation of N-WASP by IcsA unmasks two domains acting together in insertional actin polymerization. The isolated COOH-terminal domain of N-WASP containing a verprolin-homology region, a cofilin-homology sequence, and an acidic terminal segment (VCA) interacts with G-actin in a unique profilin-like functional fashion. Hence, when N-WASP is activated, its COOH-terminal domain feeds barbed end growth of filaments and lowers the critical concentration at the bacterial surface. On the other hand, the NH(2)-terminal domain of N-WASP interacts with F-actin, mediating the attachment of the actin tail to the bacterium surface. VASP is not involved in Shigella movement, and the function of profilin does not require its binding to proline-rich regions. 相似文献
959.
The carbon footprint of flying overseas to conferences, meetings, and workshops to share and build knowledge has been increasingly questioned over the last two decades, especially in environmental and climate sciences, due to the related colossal carbon emissions. Here, we infer the value of scientific meetings through the number of publications produced either directly or indirectly after attending a scientific conference, symposium, or workshop (i.e., the conference‐related production) and the number of publications produced per meeting (i.e., the conference‐related productivity) as proxies for the academic value of these meetings, and relate them to both the number of meetings attended and the related carbon emissions. We show that conference‐related production and productivity, respectively, increase and decay with the number of meetings attended, and noticeably that the less productive people exhibit the largest carbon footprint. Taken together, our results imply that a twofold decrease in the carbon footprint of a given scientist would result in a twofold increase in productivity through a fivefold decrease in the number of meeting attended. In light of these figures, we call for both the implementation of objective and quantitative criteria related to the optimum number of conferences to attend in an effort to maximize scientific productivity while minimizing the related carbon footprint, and the development of a rationale to minimize the carbon emission related to scientific activities. 相似文献
960.