全文获取类型
收费全文 | 39174篇 |
免费 | 3288篇 |
国内免费 | 4349篇 |
专业分类
46811篇 |
出版年
2024年 | 124篇 |
2023年 | 677篇 |
2022年 | 1434篇 |
2021年 | 2220篇 |
2020年 | 1566篇 |
2019年 | 1945篇 |
2018年 | 1798篇 |
2017年 | 1303篇 |
2016年 | 1809篇 |
2015年 | 2585篇 |
2014年 | 3118篇 |
2013年 | 3260篇 |
2012年 | 3904篇 |
2011年 | 3486篇 |
2010年 | 2162篇 |
2009年 | 1821篇 |
2008年 | 2054篇 |
2007年 | 1769篇 |
2006年 | 1458篇 |
2005年 | 1270篇 |
2004年 | 1002篇 |
2003年 | 839篇 |
2002年 | 761篇 |
2001年 | 539篇 |
2000年 | 527篇 |
1999年 | 527篇 |
1998年 | 341篇 |
1997年 | 334篇 |
1996年 | 314篇 |
1995年 | 257篇 |
1994年 | 250篇 |
1993年 | 177篇 |
1992年 | 237篇 |
1991年 | 178篇 |
1990年 | 162篇 |
1989年 | 131篇 |
1988年 | 93篇 |
1987年 | 63篇 |
1986年 | 57篇 |
1985年 | 71篇 |
1984年 | 37篇 |
1983年 | 40篇 |
1982年 | 28篇 |
1981年 | 13篇 |
1980年 | 8篇 |
1979年 | 14篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1974年 | 4篇 |
1950年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Li W Tian H Li L Li S Yue W Chen Z Qi L Hu W Zhu Y Hao B Gao C Si L Gao F 《生物化学与生物物理学报(英文版)》2012,44(7):577-583
Lung cancer is the leading cause of cancer-related mortality all over the world. In recent years, pulmonary adenocarcinoma has surpassed squamous cell carcinoma in frequency and is the predominant form of lung cancer in many countries. Epidemiological investigations have shown an inverse relationship between garlic (Allium sativum) consumption and death rate from many cancers. Diallyl trisulfide (DATS) is one of the garlic-derived compounds (also known as: organosulfer compounds, OSC). DATS can induce apoptosis and inhibit the growth of many cancer cell lines. Our study demonstrated that the apoptotic incidents induced by DATS were a mitochondria-dependent caspase cascade through a significant decrease of the anti-apoptotic Bcl-2 that resulted in up-regulation of the ratio of Bax/Bcl-2 and the activity of caspase-3, -8, and -9. Eventually, DATS induced the apoptosis and inhibited the proliferation in a concentration- and time-dependent manner. Furthermore, by establishing an animal model of female BALB/c nude mice with A549 xenografts, we found that oral gavage of DATS significantly retarded growth of A549 xenografts in nude mice without causing weight loss or any other side effects compared with the control group. All the evidence both in vitro and in vivo suggested that DATS could be an ideal anti-cancer drug. 相似文献
52.
To identify salt stress-responsive genes, we constructed a cDNA library with the salttolerant rice cultivar, Lansheng. About 15000 plasmids were extracted and dotted on filters with Biomeck 2000 HDRT system or by hand. Thirty genes were identified to display altered expression levels responding to 150 mmol/L NaCl. Among them eighteen genes were up-regulated and the remainders downregulated. Twenty-seven genes have their homologous genes in GenBank Databases. The expression of twelve genes was studied by Northern analysis. Based on the functions, these genes can be classified into five categories, including photosynthesis-related gene, transportrelated gene, metabolismrelated gene, stress-or resistancerelated gene and the others with various functions. The results showed that salt stress influenced many aspects of rice growth. Some of these genes may play important roles in plant salt tolerance. 相似文献
53.
Recent work has identified the topology of almost all the inner membrane proteins in Escherichia coli, and advances in nuclear magnetic resonance spectroscopy now allow the determination of α-helical membrane protein structures at high resolution. Together these developments will help overcome the current limitations of high-throughput determination of membrane protein structures. 相似文献
54.
Xiong XH Han S Wang JH Jiang ZH Chen W Jia N Wei HL Cheng H Yang YX Zhu B You S He JY Hou W Chen MX Yu CJ Jiao YH Zhang WC 《Journal of bacteriology》2011,193(1):315-316
Ketogulonicigenium vulgare is characterized by the efficient production of 2KGA from L-sorbose. Ketogulonicigenium vulgare Y25 is known as a 2-keto-L-gulonic acid-producing strain in the vitamin C industry. Here we report the finished, annotated genome sequence of Ketogulonicigenium vulgare Y25. 相似文献
55.
采用随机区组试验设计,对引进的8个冷季型牧草品种进行产草量和营养成分的比较分析.结果表明:4次刈割的总鲜重和总干重在品种间差异极显著,且以蓝天堂黑麦草(7号)最高.品种间的粗蛋白含量差异显著,其中第1次刈割以奇可利(3号)最高,蓝天堂黑麦草(7号)居第2位;第2次仍以奇可利(3号)最高,特高黑麦草(1号)和蓝天堂黑麦草(7号)居其次;第3次以紫花苜蓿(8号)最高,其次是多年生黑麦草(4号);第4次以奇可利(3号)最高,多年生黑麦草(4号)居第2位;粗纤维、粗脂肪和粗灰分在品种间的差别较小.综合产草量和品质两方面考虑,蓝天堂黑麦草(7号)可作为本地首选推广的冷季型牧草品种,奇可利(3号)和多年生黑麦草(4号)可作为备选推广的牧草品种. 相似文献
56.
The Tibetan Plateau is an essential area to study the potential feedback effects of soils to climate change due to the rapid rise in its air temperature in the past several decades and the large amounts of soil organic carbon (SOC) stocks, particularly in the permafrost. Yet it is one of the most under-investigated regions in soil respiration (Rs) studies. Here, Rs rates were measured at 42 sites in alpine grasslands (including alpine steppes and meadows) along a transect across the Tibetan Plateau during the peak growing season of 2006 and 2007 in order to test whether: (1) belowground biomass (BGB) is most closely related to spatial variation in Rs due to high root biomass density, and (2) soil temperature significantly influences spatial pattern of Rs owing to metabolic limitation from the low temperature in cold, high-altitude ecosystems. The average daily mean Rs of the alpine grasslands at peak growing season was 3.92 μmol CO(2) m(-2) s(-1), ranging from 0.39 to 12.88 μmol CO(2) m(-2) s(-1), with average daily mean Rs of 2.01 and 5.49 μmol CO(2) m(-2) s(-1) for steppes and meadows, respectively. By regression tree analysis, BGB, aboveground biomass (AGB), SOC, soil moisture (SM), and vegetation type were selected out of 15 variables examined, as the factors influencing large-scale variation in Rs. With a structural equation modelling approach, we found only BGB and SM had direct effects on Rs, while other factors indirectly affecting Rs through BGB or SM. Most (80%) of the variation in Rs could be attributed to the difference in BGB among sites. BGB and SM together accounted for the majority (82%) of spatial patterns of Rs. Our results only support the first hypothesis, suggesting that models incorporating BGB and SM can improve Rs estimation at regional scale. 相似文献
57.
58.
为了优化天麻素的提取工艺,本研究采用超声法提取天麻素,以天麻素含量为指标,在单因素筛选基础上,采用正交试验优化提取工艺,选择料液比、提取次数和提取时间3个影响因素,每个因素3个水平进行正交试验。结果表明,最优提取工艺组合为:提取时间20min、料液比1∶15和提取次数2次;按照此工艺平行制备3批样品,天麻素的提取率为(0.339±0.13)%。 相似文献
59.
60.
Fei Yang Ying Wang Shanshan Liu Chen He Xiaoqiu Tao Huimin Deng Gangling Tang Zhaoyang Bian Ziyan Fan 《Chirality》2020,32(5):505-514
We reported a new methodology for the stereoselective determination of metalaxyl enantiomers in tobacco and soil. The QuEChERS (quick, easy, cheap, effective, rugged, and safe) method was used for the extraction and clean-up of the tobacco and soil samples. Separation of the metalaxyl enantiomers was performed on an ACQUITY UPC2 Trefoil CEL1 chiral column coupled with supercritical fluid chromatography with tandem mass spectrometry (SFC-MS/MS), and the run time was only 5 minutes. Under the optimized conditions, the recoveries for the enantiomers were between 78.2% and 93.3% with intraday relative standard deviations (RSDs) ranging from 1.1% to 5.4%. The limit of detection (LOD) for the enantiomers in tobacco and soil varied from 0.005 to 0.007 mg/kg, and the limit of quantitation (LOQ) ranged from 0.017 to 0.020 mg/kg. In this method, only a small amount of methanol was consumed to obtain a rapid stereoselective separation. This proposed method showed good accuracy and precision and might be suitable for fast enantioselective determination of metalaxyl in food and environmental samples. The developed method was further validated by application to the analysis of authentic samples. 相似文献