全文获取类型
收费全文 | 6641篇 |
免费 | 389篇 |
国内免费 | 60篇 |
专业分类
7090篇 |
出版年
2023年 | 37篇 |
2022年 | 65篇 |
2021年 | 99篇 |
2020年 | 78篇 |
2019年 | 106篇 |
2018年 | 181篇 |
2017年 | 139篇 |
2016年 | 215篇 |
2015年 | 277篇 |
2014年 | 290篇 |
2013年 | 386篇 |
2012年 | 557篇 |
2011年 | 593篇 |
2010年 | 278篇 |
2009年 | 215篇 |
2008年 | 419篇 |
2007年 | 374篇 |
2006年 | 428篇 |
2005年 | 349篇 |
2004年 | 369篇 |
2003年 | 308篇 |
2002年 | 316篇 |
2001年 | 155篇 |
2000年 | 175篇 |
1999年 | 102篇 |
1998年 | 49篇 |
1997年 | 35篇 |
1996年 | 30篇 |
1995年 | 25篇 |
1994年 | 24篇 |
1993年 | 26篇 |
1992年 | 24篇 |
1991年 | 36篇 |
1990年 | 22篇 |
1989年 | 27篇 |
1988年 | 15篇 |
1987年 | 18篇 |
1986年 | 17篇 |
1985年 | 26篇 |
1984年 | 17篇 |
1983年 | 19篇 |
1982年 | 14篇 |
1981年 | 21篇 |
1980年 | 16篇 |
1979年 | 25篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1972年 | 10篇 |
排序方式: 共有7090条查询结果,搜索用时 15 毫秒
41.
42.
Marzec-Wróblewska U Kamiński P Lakota P Szymański M Wasilow K Ludwikowski G Kuligowska-Prusińska M Odrowąż-Sypniewska G Stuczyński T Michałkiewicz J 《Biological trace element research》2011,143(1):167-177
The aim of the present study was to measure zinc (Zn) and iron (Fe) concentration in human semen and superoxide dismutase
(SOD) activity in seminal plasma and correlate the results with sperm quality. Semen samples were obtained from men (N = 168) undergoing routine infertility evaluation. The study design included two groups based on the ejaculate parameters.
Group I (n = 39) consisted of males with normal ejaculate (normozoospermia), and group II (n = 129) consisted of males with pathological spermiogram. Seminal Zn and Fe were measured in 162 samples (group I, n = 38; group II, n = 124) and SOD activity in 149 samples (group I, n = 37; group II, n = 112). Correlations were found between SOD activity and Fe and Zn concentration, and between Fe and Zn concentration. SOD
activity was negatively associated with volume of semen and positively associated with rapid progressive motility, nonprogressive
motility, and concentration. Negative correlation was stated between Fe concentration and normal morphology. Mean SOD activity
in seminal plasma of semen from men of group I was higher than in seminal plasma of semen from men of group II. Fe concentration
was higher in teratozoospermic males than in males with normal morphology of spermatozoa in group II. Our results suggest
that Fe may influence spermatozoa morphology. 相似文献
43.
菱纹叶蝉属中国种类记要(同翅目,叶蝉科,殃叶蝉亚科) 总被引:1,自引:0,他引:1
记述中国菱纹叶蝉属Hishimonus Ishihara 17种,其中有2新种,即长突菱纹叶蝉H.prolongatus sp.nov.和褐斑菱纹叶蝉H.fuscomaculatus sp.nov..新种模式标本保存在贵州大学昆虫研究所. 相似文献
44.
Chris Kypridemos Piotr Bandosz Graeme L. Hickey Maria Guzman-Castillo Kirk Allen Iain Buchan Simon Capewell Martin O’Flaherty 《PloS one》2015,10(4)
Background
Serum total cholesterol is one of the major targets for cardiovascular disease prevention. Statins are effective for cholesterol control in individual patients. At the population level, however, their contribution to total cholesterol decline remains unclear. The aim of this study was to quantify the contribution of statins to the observed fall in population mean cholesterol levels in England over the past two decades, and explore any differences between socioeconomic groups.Methods and Findings
This is a modelling study based on data from the Health Survey for England. We analysed changes in observed mean total cholesterol levels in the adult England population between 1991-92 (baseline) and 2011-12. We then compared the observed changes with a counterfactual ‘no statins’ scenario, where the impact of statins on population total cholesterol was estimated and removed. We estimated uncertainty intervals (UI) using Monte Carlo simulation, where confidence intervals (CI) were impractical. In 2011-12, 13.2% (95% CI: 12.5-14.0%) of the English adult population used statins at least once per week, compared with 1991-92 when the proportion was just 0.5% (95% CI: 0.3-1.0%). Between 1991-92 and 2011-12, mean total cholesterol declined from 5.86 mmol/L (95% CI: 5.82-5.90) to 5.17 mmol/L (95% CI: 5.14-5.20). For 2011-12, mean total cholesterol was lower in more deprived groups. In our ‘no statins’ scenario we predicted a mean total cholesterol of 5.36 mmol/L (95% CI: 5.33-5.40) for 2011-12. Statins were responsible for approximately 33.7% (95% UI: 28.9-38.8%) of the total cholesterol reduction since 1991-92. The statin contribution to cholesterol reduction was greater among the more deprived groups of women, while showing little socio-economic gradient among men.Conclusions
Our model suggests that statins explained around a third of the substantial falls in total cholesterol observed in England since 1991. Approximately two thirds of the cholesterol decrease can reasonably be attributed non-pharmacological determinants. 相似文献45.
Górska-Frączek S Sandström C Kenne L Rybka J Strus M Heczko P Gamian A 《Carbohydrate research》2011,346(18):2926-2932
A novel structure of exopolysaccharide from the lactic acid bacteria (LAB) Lactobacillus rhamnosus KL37B, from the human intestinal flora, is described. During the structural investigation of the exopolysaccharide it was found that the repeating unit is a nonasaccharide, which is the largest repeating unit found in LAB exopolysaccharides to date. The polysaccharide material was prepared by TCA extraction of a bacterial cell mass, purified by anion-exchange and gel permeation chromatography and characterized using chemical and enzymatic methods. On the basis of monosaccharide and methylation analysis and also 1D and 2D 1H and 13C NMR spectroscopy the exopolysaccharide was shown to be composed of the following nonasaccharide repeating unit:The physicochemical cell surface study and adhesive properties indicated distinct surface properties of Lactobacillus rhamnosus strain KL37B with high adhesive abilities to Caco-2 cells, hydrophobicity and slime production, in comparison to other Lactobacillus strains used as controls. 相似文献
46.
47.
Repression of microRNA‐382 inhibits glomerular mesangial cell proliferation and extracellular matrix accumulation via FoxO1 in mice with diabetic nephropathy 下载免费PDF全文
48.
49.
The physiological responses to TGF-β stimulation are diverse and vary amongst different cell types and environmental conditions. Even though the principal molecular components of the canonical and the non-canonical TGF-β signaling pathways have been largely identified, the mechanism that underlies the well-established context dependent physiological responses remains a mystery. Understanding how the components of TGF-β signaling function as a system and how this system functions in the context of the global cellular regulatory network requires a more quantitative and systematic approach. Here, we review the recent progress in understanding TGF-β biology using integration of mathematical modeling and quantitative experimental analysis. These studies reveal many interesting dynamics of TGF-β signaling and how cells quantitatively decode variable doses of TGF-β stimulation. 相似文献
50.
Severe growth abnormalities, including leaf yellowing, sprout proliferation and flower virescence and phyllody, were found on Brassica rapa subsp. pekinensis plants in Poland. The presence of phytoplasma in naturally infected plants was demonstrated by polymerase chain reaction assay employing phytoplasma universal P1/P7 followed by R16F2n/R16R2 primer pairs. The detected phytoplasma was identified using restriction fragment length polymorphism analysis (RFLP) of the 16S rRNA gene fragment with AluI, HhaI, MseI and RsaI endonucleases. After enzymatic digestion, all tested samples showed restriction pattern similar to that of ‘Candidatus phytoplasma asteris’. Nested PCR‐amplified products, obtained with primers R16F2n/R16R2, were sequenced. Sequences of the 16S rDNA gene fragment of analysed phytoplasma isolates were nearly identical. They revealed high nucleotide sequence identity (>98%) with corresponding sequences of other phytoplasma isolates from subgroup 16SrI‐B, and they were classified as members of ‘Candidatus phytoplasma asteris’. This is the first report of the natural occurrence of phytoplasma‐associated disease in plants of Chinese cabbage. 相似文献