全文获取类型
收费全文 | 9910篇 |
免费 | 797篇 |
国内免费 | 8篇 |
专业分类
10715篇 |
出版年
2023年 | 52篇 |
2022年 | 110篇 |
2021年 | 220篇 |
2020年 | 138篇 |
2019年 | 150篇 |
2018年 | 243篇 |
2017年 | 211篇 |
2016年 | 290篇 |
2015年 | 524篇 |
2014年 | 482篇 |
2013年 | 619篇 |
2012年 | 810篇 |
2011年 | 743篇 |
2010年 | 479篇 |
2009年 | 390篇 |
2008年 | 553篇 |
2007年 | 499篇 |
2006年 | 482篇 |
2005年 | 469篇 |
2004年 | 476篇 |
2003年 | 447篇 |
2002年 | 353篇 |
2001年 | 125篇 |
2000年 | 110篇 |
1999年 | 120篇 |
1998年 | 96篇 |
1997年 | 66篇 |
1996年 | 72篇 |
1995年 | 64篇 |
1994年 | 67篇 |
1993年 | 52篇 |
1992年 | 85篇 |
1991年 | 75篇 |
1990年 | 60篇 |
1989年 | 63篇 |
1988年 | 54篇 |
1987年 | 55篇 |
1986年 | 51篇 |
1985年 | 61篇 |
1984年 | 50篇 |
1983年 | 56篇 |
1982年 | 45篇 |
1981年 | 42篇 |
1980年 | 46篇 |
1979年 | 43篇 |
1978年 | 39篇 |
1977年 | 39篇 |
1976年 | 39篇 |
1975年 | 34篇 |
1973年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Salivary glands incubated in various concentrations of pyridoxine (Vitamin B6) show increasing tyrosine aminotransferase (TAT) activity at concentrations up to 10–5 M and then decreasing activity up to 10–2 M but in all cases the activity is greater than that of the controls. This increase in activity is demonstrable for up to 6 h, the longest period tested, and is dependent on the synthesis of new mRNA. A similar increase in TAT activity is observed in salivary glands subjected to heat shock. Antibodies prepared against purified tyrosine aminotransferase precipitate a peptide of the same molecular weight (40 KD) as that induced by pyridoxine. 相似文献
192.
I-Chian Li Diane A. Blake Irwin J. Goldstein Ernest H. Y. Chu 《Experimental cell research》1980,129(2)
Stable and heritable variants of Chinese hamster ovary (CHO) cells which are resistant to different levels (0.1, 1.0 and 10 μg/ml) of the toxin abrin have been isolated and characterized. The frequency of resistant colonies to abrin was increased with the concentration of a chemical mutagen. There was no effect of cell density or cross-feeding on the recovery of variants. In experiments using fluorescein-labeled abrin and ricin which bind to terminal (non-sialylated) galactose residues of cell-surface oligosaccharides, parental cells exhibited strong binding toward both toxins, whereas no fluorescence was observed in the resistant clones. A fluorescein-conjugated lectin, BS II, which is specific for terminal N-acetyl-
-glucosaminyl residues, did not interact with the parental cells, but did with the resistant clones. This suggests that on the surface of resistant cells the number of terminal galactosyl residues of oligosaccharide chains in glycoproteins was reduced, exposing the penultimate N-acetyl-
-glucosaminyl residues. The number of available endogenous acceptor sites for galactosyl transferase in the abrin-resistant clones was directly proportional to the degree of resistance. In the presence of great excess of exogenous acceptor, the rates of galactosyl transfer were similar in all the abrin-resistant cell types tested, with levels ranging from 1.4 to 1.7 times parental cell values. Studies with tetraploid cell hybrids reveal that resistance was a recessive trait. Fluctuation analysis showed that abrin resistance occurred in CHO cell populations at a rate of 4−7 × 10−8/cell/generation. The system may serve as a new marker for quantitative mutagenesis studies. 相似文献
193.
Tomáš Trnka Miloslav Černý Anna Ya. Shmyrina Alexander S. Shashkov Alexander F. Sviridov Oleg S. Chizhov 《Carbohydrate research》1979,76(1):39-44
13C-N.m.r. spectra of all possible 1,6:2,3- and 1,6:3,4-dianhydro-β-D-hexo-pyranoses and their O-acetyl and deoxy derivatives are presented. Relations between chemical shifts of certain carbon atoms and the structure of the dianhydrides are outlined, and their application in structural analysis is discussed. Inversion of configuration of the oxirane ring from the endo to the exo position is associated with typical upfield-shifts for oxirane-ring carbon atoms C-2 or C-4, respectively. Possible inter-relationships between 13C-chemical shifts and steric and polar interactions in the dianhydro derivatives are discussed. 相似文献
194.
195.
Derivative melting profiles of DNA have been obtained directly by recording the difference in absorbance between two identical solutions maintained at a small constant temperature differential. This deltaA is monitored continuously with increasing temperature in a ratio recording spectrophotometer. Resolution of complex hyperfine structure in the profiles of small homogeneous viral DNAs appears to be significantly better than has been produced by various numerical methods of differentiation. In addition, a spectral method has been modified that permits easy analysis for DNA base composition from the ratio of derivative melting curves obtained at 282 and 260 nm. Eight bacterial and three vertebrate DNAs have been analyzed for total base composition from the product of the instantaneous base composition at small temperature intervals (0.05 degrees C) throughout the entire melting region and the integrated area of the 282 nm profile. The results are in excellent agreement with values determined by traditional methods. 相似文献
196.
Severe freezing injury was produced in the hind foot of 26 mongrel dogs. All dogs were given daily whirlpool treatment and protective bandaging for 14 days following injury. In addition, certain dogs received a vasodilator, fasciotomy, or both vasodilator and fasciotomy following injury. Deep foot temperatures, foot volumes, tissue pressures, and 14 day tissue loss-salvage scores were compared. Significant differences between fasciotomy and nonfasciotomy dogs were seen in foot temperature, volume, and tissue pressure immediately following fasciotomy. Though there was no significant difference in 14 day tissue loss, there was clinically apparent prolongation of integrity of the local vascular system for 2 to 5 days following fasciotomy, and total foot salvage in several dogs receiving fasciotomy. 相似文献
197.
Tom M. Harris 《Review of Palaeobotany and Palynology》1976,21(1):119-134
Features needing elucidation in Mesozoic Cycadales and Bennettitales are given and the Ginkgo—Czekanowskia assemblage is surveyed. Florin's interpretation of the conifer cone scale is discussed in relation to Schweizer's work on Pseudovoltzia. Taxus jurassica Florin is revised and shown to differ generically from Taxus but to share features seen among other Taxaceae: it thus helps to unify the family, but it does not relate the Taxaceae to ordinary conifers. A possible way is offered by which early conifers like Walchia could shift their ovules from a lateral to a terminal position and if this were acceptable, then the class Taxopsida would become unnecessary. 相似文献
198.
199.
200.