全文获取类型
收费全文 | 99174篇 |
免费 | 8421篇 |
国内免费 | 10609篇 |
出版年
2024年 | 220篇 |
2023年 | 1235篇 |
2022年 | 2852篇 |
2021年 | 5110篇 |
2020年 | 3627篇 |
2019年 | 4471篇 |
2018年 | 4153篇 |
2017年 | 3189篇 |
2016年 | 4413篇 |
2015年 | 6334篇 |
2014年 | 7614篇 |
2013年 | 7834篇 |
2012年 | 9341篇 |
2011年 | 8581篇 |
2010年 | 5341篇 |
2009年 | 4837篇 |
2008年 | 5447篇 |
2007年 | 4911篇 |
2006年 | 4319篇 |
2005年 | 3469篇 |
2004年 | 2868篇 |
2003年 | 2717篇 |
2002年 | 2186篇 |
2001年 | 1663篇 |
2000年 | 1520篇 |
1999年 | 1411篇 |
1998年 | 908篇 |
1997年 | 803篇 |
1996年 | 788篇 |
1995年 | 680篇 |
1994年 | 646篇 |
1993年 | 494篇 |
1992年 | 652篇 |
1991年 | 479篇 |
1990年 | 409篇 |
1989年 | 383篇 |
1988年 | 310篇 |
1987年 | 258篇 |
1986年 | 246篇 |
1985年 | 233篇 |
1984年 | 154篇 |
1983年 | 159篇 |
1982年 | 127篇 |
1981年 | 78篇 |
1980年 | 69篇 |
1979年 | 94篇 |
1976年 | 55篇 |
1974年 | 65篇 |
1973年 | 63篇 |
1972年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 398 毫秒
41.
42.
<正>Geminiviruses are a group of circular single-stranded DNA viruses that constitute the largest family of plant viruses.Many diseases resulting from geminivirus infections, such as maize streak disease, cassava mosaic disease, tomato yellow leaf curl disease and cotton leaf curl diseases, cause significant problems in terms of economic losses, posing a serious threat to global crop productivity (Hanley-Bowdoin et al., 2013; Yang et al., 2019). As obligate intracellular 相似文献
43.
Wavefront shaping can compensate the wavefront distortions in deep tissue focusing, leading to an improved penetration depth. However, when using the backscattered signals as the feedback, unexpected compensation bias may be introduced, resulting in focusing position deviations or even no focus in the illumination focal plane. Here we investigated the reliability of wavefront shaping based on coherent optical adaptive technique in deep tissue focusing by measuring the position deviations between the foci in the illumination focal plane and the epi‐detection plane. The experimental results show that when the penetration depth reaches 150 μm in mouse brain tissue (with scattering coefficient ~22.42 mm?1) using a 488 nm laser and an objective lens with 0.75 numerical aperture, the center of the real focus will deviate out of one radius range of the Airy disk while the optimized focus in the epi‐detection plane maintained basically at the center. With the penetration depth increases, the peak to background ratio of the focus in the illumination focal plane decreases faster than that in the epi‐detection plane. The results indicate that when the penetration depth reaches 150 μm, feedback based on backscattered signals will make wavefront shaping lose its reliability, which may provide a guidance for applications of non‐invasive precise optogenetics or deep tissue optical stimulation using wavefront shaping methods. A, Intensity distribution in the epi‐detection plane and the illumination focal plane before and after correction, corresponding to brain sections with 250 and 300 μm thickness, respectively. Scale bar is 2 μm. B, Averaged focusing deviations in the epi‐detection plane (optimized) and the illumination focal plane (monitored) after compensation. The unit of the ordinate is one Airy disk diameter. Black dashed line represents one Airy disk radius. Bars represent the SE of each measurement set. 相似文献
44.
Two distinct types of cell growth of suspension-cultured Mentha were formed when the cells maintained in the medium containing 1000 micrograms l-1 2,4-D were subcultured into different 2,4-D concentrations. Few cell elongation of Mentha (average cell length: 34-40 microns) was observed after division in the medium containing 1-200 micrograms l-1 2,4-D; and significant cell elongation (average cell length: 95-130 microns) was observed after cell division in the medium containing 500-2000 micrograms l-1 2,4-D. A close correlation between culture medium and water content in the cells indicated that 2,4-D promoted cell elongation by water uptake. Amounts of phenolic acid in cell walls were much higher in unelongated cell walls than in elongated ones during the cultivation, and there was a close correlation between the amounts and the level of PAL activity in elongated and unelongated cells. However, there was no significant difference in cell wall components and its neutral sugar composition between elongated and unelongated cells. 相似文献
45.
46.
Jun‐juan Wang Qi‐min Zhang Wei‐wei Ni Xin Zhang Ying Li Ai‐li Li Peng Du Chun Li Su‐su Yu 《Microbiology and immunology》2019,63(8):303-315
We investigated the correlation between the beneficial effect of Lactobacillus acidophilus on gut microbiota composition, metabolic activities, and reducing cow's milk protein allergy. Mice sensitized with β‐lactoglobulin (β‐Lg) were treated with different doses of L. acidophilus KLDS 1.0738 for 4 weeks, starting 1 week before allergen induction. The results showed that intake of L. acidophilus significantly suppressed the hypersensitivity responses, together with increased fecal microbiota diversity and short‐chain fatty acids (SCFAs) concentration (including propionate, butyrate, isobutyrate, and isovalerate) when compared with the allergic group. Moreover, treatment with L. acidophilus induced the expression of SCFAs receptors, G‐protein–coupled receptors 41 (GPR41) and 43 (GPR43), in the spleen and colon of the allergic mice. Further analysis revealed that the GPR41 and GPR43 messenger RNA expression both positively correlated with the serum concentrations of transforming growth factor‐β and IFN‐γ (p < .05), but negatively with the serum concentrations of IL‐17, IL‐4, and IL‐6 in the L. acidophilus–treated group compared with the allergic group (p < .05). These results suggested that L. acidophilus protected against the development of allergic inflammation by improving the intestinal flora, as well as upregulating SCFAs and their receptors GPR41/43. 相似文献
47.
S C Lee J Y Shew F H Chang M J Chou C S Yang 《Proceedings of the National Science Council, Republic of China. Part B, Life sciences》1984,8(2):143-147
Protein kinase activity has been found in hepatitis B virions (Dane particles) purified from the plasma of hepatitis B surface antigen carriers [Albin, C., and Robinson, W.S. (1980) J. Virol. 34, 297-302]. Dane particles were purified from the pooled, HBeAg-positive plasma. When this preparation was incubated with [gamma 32P]ATP in the presence of 10mM MnCl2 and 0.5% NP-40 for 15 seconds at 30 degrees C, several phosphorylated polypeptides of 20,000, 42,000, 48,000, 50,000 and 56,000 daltons were detected in sodium dodecyl sulfate-polyacrylamide gels. When the Dane particles were incubated with [gamma 32P]ATP, 10 mM MnCl2, and 0.5% NP-40 in the presence of human hepatoma cell (J-5) particulate fraction at 30 degrees C, 15 seconds, the 42,000, 48,000 and 50,000 daltons phosphorylated polypeptides were not found. When human peripheral blood lymphocytes particulate fraction was incubated with Dane particles under the same conditions, no change of Dane particle phosphorylated polypeptides was detected. Previous publications [Albin, C., and Robinson, W.S. (1980) J. Virol. 34, 297-302; Gerlich, W.H. et al. (1982) J. Virol. 42, 761-766] showed that when hepatitis B core particles purified from hepatoma tissues contained protein kinase activity, only phosphorylated polypeptide was 20,000 daltons. Our data suggested that when Dane particles were put in an environment of hepatoma cells (or tissues), the protein kinase could only phosphorylate selected polypeptides in these particles. 相似文献
48.
Hongchi Yu Yang Shen Jingsi Jin Yingying Zhang Tang Feng 《Cell Adhesion & Migration》2018,12(1):56-68
Hepatocellular carcinoma (HCC) is a subtype of malignant liver cancer with poor prognosis and limited treatment options. It is noteworthy that mechanical forces in tumor microenvironment play a pivotal role in mediating the behaviors and functions of tumor cells. As an instrumental type of mechanical forces in vivo, fluid shear stress (FSS) has been reported having potent physiologic and pathologic effects on cancer progression. However, the time-dependent mechanochemical transduction in HCC induced by FSS remains unclear. In this study, hepatocellular carcinoma HepG2 cells were exposed to 1.4 dyn/cm2 FSS for transient duration (15s and 30s), short duration (5 min, 15 min and 30 min) and long duration (1h, 2h and 4h), respectively. The expression and translocation of Integrins induced FAK-Rho GTPases signaling events were examined. Our results showed that FSS endowed HepG2 cells with higher migration ability via reorganizing cellular F-actin and disrupting intercellular tight junctions. We further demonstrated that FSS regulated the expression and translocation of Integrins and their downstream signaling cascade in time-dependent patterns. The FSS downregulated focal adhesion components (Paxillin, Vinculin and Talin) while upregulated the expression of Rho GTPases (Cdc42, Rac1 and RhoA) in long durations. These results indicated that FSS enhanced tumor cell migration through Integrins-FAK-Rho GTPases signaling pathway in time-dependent manners. Our in vitro findings shed new light on the role of FSS acting in physiologic and pathological processes during tumor progression, which has emerged as a promising clinical strategy for liver carcinoma. 相似文献
49.
Jianjun Wang Janne Soininen Yong Zhang Beixin Wang Xiangdong Yang Ji Shen 《Journal of Biogeography》2011,38(3):595-603
Aim Data and analyses of elevational gradients in diversity have been central to the development and evaluation of a range of general theories of biodiversity. Elevational diversity patterns have, however, been severely understudied for microbes, which often represent decomposer subsystems. Consequently, generalities in the patterns of elevational diversity across different trophic levels remain poorly understood. Our aim was to examine elevational gradients in the diversity of macroinvertebrates, diatoms and bacteria along a stony stream that covered a large elevational gradient. Location Laojun Mountain, Yunnan province, China. Methods The sampling scheme included 26 sites spaced at elevational intervals of 89 m from 1820 to 4050 m elevation along a stony stream. Macroinvertebrate and diatom richness were determined based on the morphology of the specimens. Taxonomic richness for bacteria was quantified using a molecular fingerprinting method. Over 50 environmental variables were measured at each site to quantify environmental variables that could correlate with the patterns of diversity. We used eigenvector‐based spatial filters with multiple regressions to account for spatial autocorrelation. Results The bacterial richness followed an unexpected monotonic increase with elevation. Diatoms decreased monotonically, and macroinvertebrate richness showed a clear unimodal pattern with elevation. The unimodal richness pattern for macroinvertebrates was best explained by the mid‐domain effect (r2 = 0.72). The diatom richness was best explained by the variation in nutrient supply, and the increase in bacterial richness with elevation may be related to an increased carbon supply. Main conclusions We found contrasting patterns in elevational diversity among the three studied multi‐trophic groups comprising unicellular and multicellular aquatic taxa. We also found that there may be fundamental differences in the mechanisms underlying these species diversity patterns. 相似文献
50.
<正>Structural variations (SVs) are mutations with large-scale changes (generally50 bp) in the genome. SVs are major sources of the genetic diversity of organisms and thus are of high relevance to phenotype variations, gene dosage and evolutionary genetics. Except detecting SVs through comparative genetic analyses, dozens of software had been developed based on the alignment of short-reads to a single linear genome in the past decades (Guan and Sung, 2016). 相似文献