全文获取类型
收费全文 | 21883篇 |
免费 | 2289篇 |
国内免费 | 17篇 |
出版年
2021年 | 268篇 |
2020年 | 149篇 |
2019年 | 214篇 |
2018年 | 266篇 |
2017年 | 204篇 |
2016年 | 402篇 |
2015年 | 643篇 |
2014年 | 699篇 |
2013年 | 964篇 |
2012年 | 1226篇 |
2011年 | 1219篇 |
2010年 | 784篇 |
2009年 | 789篇 |
2008年 | 1079篇 |
2007年 | 1107篇 |
2006年 | 1068篇 |
2005年 | 1070篇 |
2004年 | 1094篇 |
2003年 | 1086篇 |
2002年 | 1086篇 |
2001年 | 401篇 |
2000年 | 341篇 |
1999年 | 367篇 |
1998年 | 324篇 |
1997年 | 282篇 |
1996年 | 237篇 |
1995年 | 226篇 |
1994年 | 233篇 |
1993年 | 248篇 |
1992年 | 311篇 |
1991年 | 323篇 |
1990年 | 277篇 |
1989年 | 247篇 |
1988年 | 264篇 |
1987年 | 253篇 |
1986年 | 245篇 |
1985年 | 291篇 |
1984年 | 244篇 |
1983年 | 219篇 |
1982年 | 249篇 |
1981年 | 207篇 |
1980年 | 194篇 |
1979年 | 193篇 |
1978年 | 195篇 |
1977年 | 176篇 |
1976年 | 172篇 |
1975年 | 139篇 |
1974年 | 183篇 |
1973年 | 172篇 |
1972年 | 104篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
41.
The membrane environments in normal and tumor rat tissue and the effect of hyperthermia thereon are studied with 19F-NMR spectroscopy of the general anesthetic halothane. Normal and tumor cell types are clearly differentiated by the halothane resonance. A hydrophobic environment prominent in tumor tissue is more sensitive to heat treatment than the corresponding environments of normal cells. Studies of extracted lipids suggest that this may be due in part to the considerable difference in lipid temperature response which exists between normal and kidney tumor cells. 相似文献
42.
Mark Andrews Alan Brown Jean-Yves Chiva David Fradet David Gordon Mark Lansdell Malcolm MacKenny 《Bioorganic & medicinal chemistry letters》2009,19(8):2329-2332
The reported selective serotonin Re-uptake Inhibitor Litoxetine was used as the starting point in the design of a range of potential SSRIs with high ease of synthetic accessibility. Preparation and subsequent optimization yielded a range of potent and highly selective SSRIs. 相似文献
43.
44.
Alan Rumsey 《The Australian journal of anthropology》2006,17(1):47-69
One of the leading challenges for contemporary anthropology is to try to contribute to an understanding of the interaction between indigenous and exogenous socio‐cultural orders, especially at the frontiers of globalisation. Here I review three recent attempts to do so: (1) a model of structural transformation as developed by Marshall Sahlins; (2) a model of articulation as developed by James Clifford; (3) a model of ‘adoption’ proposed by Joel Robbins. As a test case for these models, I consider them in relation to some recent developments in local segmentary politics and verbal art in the Ku Waru region of Highland New Guinea. I show that all three models are in certain respects inadequate for understanding those developments, and offer some proposals as to what kinds of theory might be more adequate to the task. 相似文献
45.
When expressed in Escherichia coli, a truncated form of phytochrome (oat PHYA AP3 residues 464-1129) self associates to form a series of products ranging in size from monomers to aggregates of greater than 20 subunits. When these same phytochrome sequences are coexpressed with the chaperonins GroEL and GroES, the truncated phytochrome migrates as a native-like dimer in size exclusion chromatography and no higher-order aggregates were detected. GroEL and GroES inhibition of phytochrome aggregation in E. coli presumably occurs via the suppression of folding pathways leading to incorrectly folded phytochrome. 相似文献
46.
47.
Alan B Oelkers Erick J Schutte David R Tyler 《Photochemical & photobiological sciences》2008,7(2):228-234
The photochemistry of the [(CpR)Mo(CO)(3)](2) molecules, where CpR = eta(5)-C(5)H(4)(CH(2))(2)C(O)NCH(3)(CH(2))(n)CH(3) (n = 3, 8, 13, and 18), was examined using femtosecond pump-probe transient absorption spectroscopy. The goal of this study was to investigate the importance of radical size and mass on the dynamics and efficiency of geminate radical-radical recombination. The femtosecond results demonstrated the lack of any size/mass dependence of the recombination efficiency. This finding contrasts with results from a prior study that did find a size/mass dependence using a steady-state photochemical technique. To explain these conflicting results, it is proposed that the femtosecond pump-probe results are a measurement of the efficiency of primary geminate recombination whereas the steady-state method results are a measurement of the sum of primary and secondary geminate recombination efficiencies. The size/mass dependence is evident in the latter because secondary geminate recombination is a slower diffusive recombination process and therefore depends on the steric properties of the radicals. Although the existence of primary and secondary recombination channels is often taken for granted, experimental differentiation of primary and secondary caging has proven to be difficult because it is not possible for a single experimental technique to span the entire timescale for recombination of a radical cage pair and adequately resolve these recombination pathways. 相似文献
48.
49.
Metabolism of Caffeine and Related Purine Alkaloids in Leaves of Tea (Camellia sinensis L.) 总被引:3,自引:0,他引:3
Purine alkaloid catabolism pathways in young, mature and agedleaves of tea (Camellia sinensis L.) were investigated by incubatingleaf sections with 14C-labelled theobromine, caffeine, theophyllineand xanthine. Incorporation of label into CO2 was determinedand methanol-soluble metabolites were analysed by high-performanceliquid chromatography-radiocounting and thin layer chro-matography.The data obtained demonstrate that theobromine is the immediateprecursor of caffeine, which accumulates in tea leaves becauseits conversion to theophylline is the rate limiting step inthe purine alkaloid catabolism pathway. The main fate of [8-14C]theophyllineincubated with mature and aged leaves, and to a lesser extentyoung leaves, is conversion to 3-methylxanthine and onto xanthinewhich is degraded to 14CO2 via the purine catabolism pathway.However, with young leaves, sizable amounts of [8-14C]-theophyllinewere salvaged for the synthesis of caffeine via a 3-methylxanthine 相似文献
50.
K Yamazaki R A Roberts E Spooncer T M Dexter T D Allen 《Journal of cellular physiology》1989,139(2):301-312
With the aid of a multipotent stem cell line (FDCP-mix cells) co-cultured with either normal or irradiated Swiss 3T3, cellular interactions between stromal cells and haemopoietic stem cells were studied by electron microscopy and time-lapse video microscopy. When cultured in the presence of interleukin 3 (IL-3) but in the absence of stromal cells, the FDCP-mix cells have a characteristic blast morphology. In the absence of IL-3, the cells die unless they are co-cultured with marrow stromal cells or 3T3 cells. In the latter case, they attach, proliferate, and differentiate on both normal and irradiated Swiss 3T3 cell layers without the addition of extrinsic growth factor (IL-3). At the initial attachment sites of these two cell lines, cellular recognition seemed to be mediated by the formation of microvillus cytoplasmic projections and extracellular matrix. These areas may well be the sites of plasma-membrane-bound signalling/adhesional molecules between the interacting cells. 相似文献