全文获取类型
收费全文 | 19775篇 |
免费 | 1814篇 |
国内免费 | 19篇 |
专业分类
21608篇 |
出版年
2023年 | 127篇 |
2022年 | 306篇 |
2021年 | 592篇 |
2020年 | 324篇 |
2019年 | 410篇 |
2018年 | 462篇 |
2017年 | 381篇 |
2016年 | 659篇 |
2015年 | 1135篇 |
2014年 | 1163篇 |
2013年 | 1309篇 |
2012年 | 1773篇 |
2011年 | 1605篇 |
2010年 | 949篇 |
2009年 | 837篇 |
2008年 | 1132篇 |
2007年 | 1107篇 |
2006年 | 982篇 |
2005年 | 874篇 |
2004年 | 869篇 |
2003年 | 764篇 |
2002年 | 699篇 |
2001年 | 186篇 |
2000年 | 152篇 |
1999年 | 157篇 |
1998年 | 171篇 |
1997年 | 96篇 |
1996年 | 98篇 |
1995年 | 84篇 |
1994年 | 94篇 |
1993年 | 92篇 |
1992年 | 123篇 |
1991年 | 100篇 |
1990年 | 99篇 |
1989年 | 96篇 |
1988年 | 86篇 |
1987年 | 73篇 |
1986年 | 72篇 |
1985年 | 79篇 |
1984年 | 85篇 |
1983年 | 88篇 |
1982年 | 66篇 |
1981年 | 76篇 |
1980年 | 72篇 |
1979年 | 81篇 |
1978年 | 61篇 |
1977年 | 57篇 |
1976年 | 70篇 |
1975年 | 56篇 |
1974年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
Istem Fer Anthony K. Gardella Alexey N. Shiklomanov Eleanor E. Campbell Elizabeth M. Cowdery Martin G. De Kauwe Ankur Desai Matthew J. Duveneck Joshua B. Fisher Katherine D. Haynes Forrest M. Hoffman Miriam R. Johnston Rob Kooper David S. LeBauer Joshua Mantooth William J. Parton Benjamin Poulter Tristan Quaife Ann Raiho Kevin Schaefer Shawn P. Serbin James Simkins Kevin R. Wilcox Toni Viskari Michael C. Dietze 《Global Change Biology》2021,27(1):13-26
In an era of rapid global change, our ability to understand and predict Earth's natural systems is lagging behind our ability to monitor and measure changes in the biosphere. Bottlenecks to informing models with observations have reduced our capacity to fully exploit the growing volume and variety of available data. Here, we take a critical look at the information infrastructure that connects ecosystem modeling and measurement efforts, and propose a roadmap to community cyberinfrastructure development that can reduce the divisions between empirical research and modeling and accelerate the pace of discovery. A new era of data‐model integration requires investment in accessible, scalable, and transparent tools that integrate the expertise of the whole community, including both modelers and empiricists. This roadmap focuses on five key opportunities for community tools: the underlying foundations of community cyberinfrastructure; data ingest; calibration of models to data; model‐data benchmarking; and data assimilation and ecological forecasting. This community‐driven approach is a key to meeting the pressing needs of science and society in the 21st century. 相似文献
82.
Cyanidin-3-glucoside, a natural product derived from blackberry, exhibits chemopreventive and chemotherapeutic activity 总被引:2,自引:0,他引:2
Ding M Feng R Wang SY Bowman L Lu Y Qian Y Castranova V Jiang BH Shi X 《The Journal of biological chemistry》2006,281(25):17359-17368
Epidemiological data suggest that consumption of fruits and vegetables has been associated with a lower incidence of cancer. Cyanidin-3-glucoside (C3G), a compound found in blackberry and other food products, was shown to possess chemopreventive and chemotherapeutic activity in the present study. In cultured JB6 cells, C3G was able to scavenge ultraviolet B-induced *OH and O2-* radicals. In vivo studies indicated that C3G treatment decreased the number of non-malignant and malignant skin tumors per mouse induced by 12-O-tetradecanolyphorbol-13-acetate (TPA) in 7,12-dimethylbenz[a]anthracene-initiated mouse skin. Pretreatment of JB6 cells with C3G inhibited UVB- and TPA-induced transactivation of NF-kappaB and AP-1 and expression of cyclooxygenase-2 and tumor necrosis factor-alpha. These inhibitory effects appear to be mediated through the inhibition of MAPK activity. C3G also blocked TPA-induced neoplastic transformation in JB6 cells. In addition, C3G inhibited proliferation of a human lung carcinoma cell line, A549. Animal studies showed that C3G reduced the size of A549 tumor xenograft growth and significantly inhibited metastasis in nude mice. Mechanistic studies indicated that C3G inhibited migration and invasion of A549 tumor cells. These finding demonstrate for the first time that a purified compound of anthocyanin inhibits tumor promoter-induced carcinogenesis and tumor metastasis in vivo. 相似文献
83.
84.
The study is a survey of the shape and carbohydrate histochemistry of the endothelial cells lining the conus arteriosus of 10 species of elasmobranchs and the bulbus arteriosus of 80 species of teleosts. Intensely PAS-positive cells that were often tall, were found in many teleosts and were typical of phylogenetically advanced species. They were not seen in any elasmobranch. Most of the teleosts with strongly PAS-positive cells were marine fish or euryhaline animals that were caught in freshwater. Beyond this, it is difficult to generalize on their life-style or habitat, for they included small fish, large fish, fast swimmers, bottom-living forms, deep-water fish, littoral species and Antarctic "bloodless" forms. It is likely that the PAS-positivity of the endothelial cells can be attributed to the moderately-dense granules revealed by E.M., but as yet the significance of these is unclear. 相似文献
85.
Garcia BA Hake SB Diaz RL Kauer M Morris SA Recht J Shabanowitz J Mishra N Strahl BD Allis CD Hunt DF 《The Journal of biological chemistry》2007,282(10):7641-7655
Post-translational modifications (PTMs) of histones play an important role in many cellular processes, notably gene regulation. Using a combination of mass spectrometric and immunobiochemical approaches, we show that the PTM profile of histone H3 differs significantly among the various model organisms examined. Unicellular eukaryotes, such as Saccharomyces cerevisiae (yeast) and Tetrahymena thermophila (Tet), for example, contain more activation than silencing marks as compared with mammalian cells (mouse and human), which are generally enriched in PTMs more often associated with gene silencing. Close examination reveals that many of the better-known modified lysines (Lys) can be either methylated or acetylated and that the overall modification patterns become more complex from unicellular eukaryotes to mammals. Additionally, novel species-specific H3 PTMs from wild-type asynchronously grown cells are also detected by mass spectrometry. Our results suggest that some PTMs are more conserved than previously thought, including H3K9me1 and H4K20me2 in yeast and H3K27me1, -me2, and -me3 in Tet. On histone H4, methylation at Lys-20 showed a similar pattern as H3 methylation at Lys-9, with mammals containing more methylation than the unicellular organisms. Additionally, modification profiles of H4 acetylation were very similar among the organisms examined. 相似文献
86.
Linda S. M. Ooi Wing-Shan Ho Karry L. K. Ngai Li Tian Paul K. S. Chan Samuel S. M. Sun Vincent E. C. Ooi 《Journal of biosciences》2010,35(1):95-103
A mannose-binding lectin (Narcissus tazetta lectin [NTL]) with potent antiviral activity was isolated and purified from the bulbs of the Chinese daffodil Narcissus tazetta var. chinensis, using ion exchange chromatography on diethylaminoethyl (DEAE)-cellulose, affinity chromatography on mannose-agarose and
fast protein liquid chromatography (FPLC)-gel filtration on Superose 12. The purified lectin was shown to have an apparent
molecular mass of 26 kDa by gel filtration and 13 kDa by SDS-PAGE, indicating that it is probably a dimer with two identical
subunits. The cDNA-derived amino acid sequence of NTL as determined by molecular cloning also reveals that NTL protein contains
a mature polypeptide consisting of 105 amino acids and a C-terminal peptide extension. Three-dimensional modelling study demonstrated
that the NTL primary polypeptide contains three subdomains, each with a conserved mannose-binding site. It shows a high homology
of about 60%–80% similarity with the existing monocot mannose-binding lectins. NTL could significantly inhibit plaque formation
by the human respiratory syncytial virus (RSV) with an IC50 of 2.30 μg/ml and exhibit strong antiviral properties against influenza A (H1N1, H3N2, H5N1) and influenza B viruses with
IC50 values ranging from 0.20 μg/ml to 1.33 μg/ml in a dose-dependent manner. It is worth noting that the modes of antiviral action
of NTL against RSV and influenza A virus are significantly different. NTL is effective in the inhibition of RSV during the
whole viral infection cycle, but the antiviral activity of NTL is mainly expressed at the early stage of the viral cycle of
influenza A (H1N1) virus. NTL with a high selective index (SI=CC50/IC50≥141) resulting from its potent antiviral activity and low cytotoxicity demonstrates a potential for biotechnological development
as an antiviral agent. 相似文献
87.
We present evidence that ethanol alters intracellular poly(adenosine diphosphoribose) metabolism and we further describe the mechanism by which ethanol exerts its effect on polymer synthesis. One percent ethanol stimulates polymer accumulation as much as 2.5-fold but does not alter polymer degradation in intact cells following DNA damage. Ethanol directly stimulates polymer synthesis following low doses of DNA damage induce by deoxyribonuclease I in a nucleotide-permeable cell system that does not possess a functional polymer turnover system. Ethanol has no measurable effect on polymer synthesis in undamaged nucleotide-permeable cells or in permeable cells treated with high doses of deoxyribonuclease I. Ethanol concentrations that stimulate poly(adenosine diphosphoribose) polymerase activity in vitro specifically lower KDNA without affecting KNAD or Vmax. The results clearly show that ethanol alters the binding of this enzyme to the DNA component of chromatin and that this altered binding is responsible for the activation of the enzyme. Altered affinity of poly(adenosine diphosphoribose) polymerase and perhaps other regulatory proteins for chromatin may play an important role in the pathology of alcohol. 相似文献
88.
Benjamin Zwirzitz Sarah Thalguter Stefanie U. Wetzels Beatrix Stessl Martin Wagner Evelyne Selberherr 《Microbial biotechnology》2022,15(5):1392-1403
Raw meat sausage represents a unique ecological niche rich in nutrients for microbial consumption, making it particularly vulnerable to microbial spoilage. Starter cultures are applied to improve product stability and safety as well as flavour characteristics. However, the influence of starter cultures on microbial community assembly and succession throughout the fermentation process is largely unknown. In particular the effect on the fungal community has not yet been explored. We evaluate the microbiological status of four different raw meat sausages using high-throughput 16S rRNA gene and ITS2 gene sequencing. The objective was to study temporal changes of microbial composition during the fermentation process and to identify potential keystone species that play an important role within the microbial community. Our results suggest that fungi assigned to the species Debaryomyces hansenii and Alternaria alternata play a key role in microbial community dynamics during fermentation. In addition, bacteria related to the starter culture Lactobacillus sakei and the spoilage-associated genera Acinetobacter, Pseudomonas and Psychrobacter are central components of the microbial ecosystem in raw fermented sausages. Elucidating the exact role and interactions of these microorganisms has the potential to have direct impacts on the quality and safety of fermented foods. 相似文献
89.
Evangelia Stergiakouli Romy Gaillard Jeremy M. Tavaré Nina Balthasar Ruth J. Loos Hendrik R. Taal David M. Evans Fernando Rivadeneira Beate St Pourcain André G. Uitterlinden John P. Kemp Albert Hofman Susan M. Ring Tim J. Cole Vincent W.V. Jaddoe George Davey Smith Nicholas J. Timpson 《Obesity (Silver Spring, Md.)》2014,22(10):2252-2259
90.
Nitrate transport in plants: which gene and which control? 总被引:7,自引:0,他引:7