全文获取类型
收费全文 | 60338篇 |
免费 | 4933篇 |
国内免费 | 31篇 |
专业分类
65302篇 |
出版年
2022年 | 413篇 |
2021年 | 855篇 |
2020年 | 528篇 |
2019年 | 638篇 |
2018年 | 831篇 |
2017年 | 772篇 |
2016年 | 1357篇 |
2015年 | 2392篇 |
2014年 | 2513篇 |
2013年 | 3334篇 |
2012年 | 4236篇 |
2011年 | 4243篇 |
2010年 | 2755篇 |
2009年 | 2440篇 |
2008年 | 3531篇 |
2007年 | 3590篇 |
2006年 | 3392篇 |
2005年 | 3418篇 |
2004年 | 3355篇 |
2003年 | 3150篇 |
2002年 | 3116篇 |
2001年 | 711篇 |
2000年 | 532篇 |
1999年 | 731篇 |
1998年 | 865篇 |
1997年 | 597篇 |
1996年 | 602篇 |
1995年 | 592篇 |
1994年 | 573篇 |
1993年 | 596篇 |
1992年 | 535篇 |
1991年 | 464篇 |
1990年 | 390篇 |
1989年 | 405篇 |
1988年 | 402篇 |
1987年 | 338篇 |
1986年 | 340篇 |
1985年 | 395篇 |
1984年 | 444篇 |
1983年 | 374篇 |
1982年 | 459篇 |
1981年 | 409篇 |
1980年 | 363篇 |
1979年 | 240篇 |
1978年 | 293篇 |
1977年 | 284篇 |
1976年 | 231篇 |
1975年 | 216篇 |
1974年 | 236篇 |
1973年 | 219篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
972.
The high affinity uptake of choline into homogenates from hippocampi and striata of two groups of rats was investigated. One group of rats was habituated to an experimental situation (E I), the other consisted of naive animals (E II). The choline transport into hippocampal homogenates of rats of E I exceeded that of E II whereas the values of striatal tissue were not affected. The results support the notion that the high affinity uptake of choline may vary depending on the behavioral situation. 相似文献
973.
974.
975.
Peter F. T. Vaughan David F. Kaye Helen L. Reeve Stephen G. Ball Chris Peers 《Journal of neurochemistry》1993,60(6):2159-2166
Abstract: Dimethylphenylpiperazinium iodide (a nicotinic agonist) evokes noradrenaline release from human neuroblastoma SH-SY5Y cells that have been pretreated with 12- O -tetradecanoylphorbol 13-acetate for 8 min. This effect of dimethylphenylpiperazinium iodide was inhibited by 1 μ M mecamylamine but not by 1 μ M atropine, which suggests that SH-SY5Y cells express nicotinic receptors coupled to the release of noradrenaline. Dimethylphenylpiperazinium iodide-evoked release was enhanced by 5 μ M Bay K 8644 (an L-type calcium agonist) and inhibited by 1 μ M nifedipine. Dimethylphenylpiperazinium iodide depolarised SH-SY5Y cells and enhanced the level of intracellular calcium in cells loaded with fura 2. The effects of dimethylphenylpiperazinium iodide on noradrenaline release, depolarisation, and intracellular calcium levels were all inhibited by 1 μ M desmethylimipramine. The results of this study show that nicotinic receptors in SH-SY5Y cells stimulate noradrenaline release by activation of L-type calcium channels. 相似文献
976.
In Peninsular Malaysia ten species of lycaenid butterflies use leaf flushes or inflorescences of the legume tree Saraca thaipingensis as larval hostplant. Resource partitioning among these species is regulated by a complex mixture of patterns of interaction with ants. Females of obligately myrmecophilous species lay their eggs exclusively on trees colonized by their specific host ants. On trees colonized by weaver ants, only specialist mutualists adapted to these territorial ants are able to survive, while larvae of other species are killed. The formicine ant Cladomyrma petalae, which inhabits hollow twigs of the myrmecophytic hostplant, likewise precludes oviposition by female butterflies. Lycaenid larvae confronted with this ant species never survive, but one concealed feeding species (Jamides caeruleus) escapes removal due to the cryptic life-habits of the larvae. Two facultative myrmecophiles associate in a mutualistic way with a wide and largely overlapping range of ant genera which forage at the extrafloral nectaries of leaf flushes. One species (Cheritra freja) is not myrmecophilous, but is tolerated by all but the most territorial ants. Ant-dependent hostplant selection and egg-clustering characterize the obligate mutualists, whereas facultative myrmecophiles and the non-myrmecophile distribute their eggs singly over appropriate hostplants. Signals mediating caterpillar-ant communication are highly specialized in one obligate myrmecophile (Drupadia theda), but rather unspecific in four other species tested. Altogether our observations indicate that colonization and establishment of lycaenid butterflies on S. thaipingensis trees are governed by specializations as well as opportunistic use of resources (ants and hostplant parts). Therefore, the diversity of this species assemblage is maintained by deterministic as well as stochastic factors. 相似文献
977.
Stephen J. Assinder Lisa V.J. Eynstone R. Peter Shellis George H. Dibdin 《FEMS microbiology letters》1995,134(2-3):287-292
Abstract The pac gene encoding the penicillin G acylase (PGA) of Bacillus megaterium ATCC 14945 has been cloned in Escherichia coli HB101 ( proA, leuB ) using a selective minimal medium containing phenylacetyl-L-leucine instead of L-leucine. The nucleotide sequence of this gene has been determined and contains an open reading frame of 2406 nucleotides. The deduced amino acid sequence shows significant similarity with other β-lactam acylases. Although the PGA of B. megaterium is extracellular, the enzyme produced in E. coli appears to have a cytoplasmic localization. 相似文献
978.
979.
980.