全文获取类型
收费全文 | 17980篇 |
免费 | 100篇 |
国内免费 | 227篇 |
专业分类
系统科学 | 383篇 |
丛书文集 | 381篇 |
教育与普及 | 72篇 |
理论与方法论 | 60篇 |
现状及发展 | 6248篇 |
研究方法 | 776篇 |
综合类 | 9866篇 |
自然研究 | 521篇 |
出版年
2014年 | 140篇 |
2013年 | 278篇 |
2012年 | 480篇 |
2011年 | 1013篇 |
2010年 | 360篇 |
2009年 | 343篇 |
2008年 | 528篇 |
2007年 | 612篇 |
2006年 | 607篇 |
2005年 | 544篇 |
2004年 | 399篇 |
2003年 | 308篇 |
2002年 | 317篇 |
2001年 | 507篇 |
2000年 | 559篇 |
1999年 | 411篇 |
1992年 | 288篇 |
1991年 | 252篇 |
1990年 | 250篇 |
1989年 | 229篇 |
1988年 | 209篇 |
1987年 | 226篇 |
1986年 | 203篇 |
1985年 | 245篇 |
1984年 | 235篇 |
1983年 | 159篇 |
1982年 | 172篇 |
1981年 | 157篇 |
1980年 | 199篇 |
1979年 | 445篇 |
1978年 | 340篇 |
1977年 | 367篇 |
1976年 | 264篇 |
1975年 | 322篇 |
1974年 | 446篇 |
1973年 | 379篇 |
1972年 | 360篇 |
1971年 | 421篇 |
1970年 | 484篇 |
1969年 | 425篇 |
1968年 | 407篇 |
1967年 | 442篇 |
1966年 | 384篇 |
1965年 | 301篇 |
1959年 | 155篇 |
1958年 | 231篇 |
1957年 | 179篇 |
1956年 | 138篇 |
1955年 | 154篇 |
1954年 | 132篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
21.
Low concentrations of inositol 1,4,5-trisphosphate (InsP3) evoke a very rapid mobilization of intracellular Ca2+ stores in many cell types, which can be followed by a further, much slower efflux. Two explanations have been suggested for this biphasic release. The first proposes that the Ca2+ stores vary in their sensitivity to InsP3, and each store releases either its entire contents or nothing (all-or-none release); the second proposes instead that the stores are uniformly sensitive to the effects of InsP3, but that they can release only a fraction of their Ca2+ before their sensitivity is somehow attenuated (steady-state release). Experiments using purified InsP3 receptor molecules reconstituted into lipid vesicles have shown heterogeneity of the receptors in their response to InsP3 under conditions in which the total Ca2+ level at both sides of the receptor is held constant. We now report that in permeabilized A7r5 smooth-muscle cells incubated in Ca(2+)-free medium, the amount of 45Ca2+ remaining in the stores after the rapid transient phase of release is independent of their initial Ca2+ levels, indicating that partially depleted stores are less sensitive to InsP3. Moreover, if the stores are reloaded with 40Ca2+ after the first stimulus, reapplication of the same low concentration of InsP3 will release further 45Ca2+. This recovery of InsP3 sensitivity is almost complete. Under these conditions, Ca2+ release must thus occur by a steady-state mechanism, in which the decreasing Ca2+ content of the stores slows down further release. 相似文献
22.
23.
研究了Al3+离子的固溶对C-S-H表面吸附Na+离子量的影响.实验结果表明,Al3+离子固溶于C-S-H结构中将使C-S-H表面所带的负电量增大,因而也就增大了C-S-H表面对Na+离子的吸附能力. 相似文献
24.
25.
26.
Rogaeva E Meng Y Lee JH Gu Y Kawarai T Zou F Katayama T Baldwin CT Cheng R Hasegawa H Chen F Shibata N Lunetta KL Pardossi-Piquard R Bohm C Wakutani Y Cupples LA Cuenco KT Green RC Pinessi L Rainero I Sorbi S Bruni A Duara R Friedland RP Inzelberg R Hampe W Bujo H Song YQ Andersen OM Willnow TE Graff-Radford N Petersen RC Dickson D Der SD Fraser PE Schmitt-Ulms G Younkin S Mayeux R Farrer LA St George-Hyslop P 《Nature genetics》2007,39(2):168-177
The recycling of the amyloid precursor protein (APP) from the cell surface via the endocytic pathways plays a key role in the generation of amyloid beta peptide (Abeta) in Alzheimer disease. We report here that inherited variants in the SORL1 neuronal sorting receptor are associated with late-onset Alzheimer disease. These variants, which occur in at least two different clusters of intronic sequences within the SORL1 gene (also known as LR11 or SORLA) may regulate tissue-specific expression of SORL1. We also show that SORL1 directs trafficking of APP into recycling pathways and that when SORL1 is underexpressed, APP is sorted into Abeta-generating compartments. These data suggest that inherited or acquired changes in SORL1 expression or function are mechanistically involved in causing Alzheimer disease. 相似文献
27.
1 Results Pyrimidinones (PMs) are a class of important heterocycles which have been well documented throughout the literature due to their biological importance. They exhibit a wide range of pharmaceutical and therapeutic properties[1].A rapid and efficient one-pot method for the synthesis of 4,6-diarylpyrimidin-2(1H)- ones and related heterocycles is described.The condensation of acetophenone derivatives,aldehydes and urea in the presence of sulfamic acid was employed to synthesize a variety of pyrimid... 相似文献
28.
29.
吕玉芳 《济源职业技术学院学报》2002,1(1):48-51
中国共产党始终代表中国先进生产力的发展要求的论断,是站在历史唯物主义的角度,揭示出我们党兴旺发达的根本动力. 相似文献
30.
Nuclear localization and signalling activity of phosphoinositidase C beta in Swiss 3T3 cells. 总被引:14,自引:0,他引:14
The hydrolysis of phosphatidylinositol 4,5-bisphosphate (PtdInsP2) is a widespread receptor-coupled signalling system at the plasma membrane of most eukaryotic cells. The existence of an entirely separate nuclear phosphoinositide signalling system is suggested from evidence that purified nuclei synthesize PtdInsP2 and phosphatidylinositol 4-phosphate (PtdInsP) in vitro and that a transient decrease in the mass of these lipids occurs when Swiss 3T3 cells are cultured in the presence of insulin-like growth factor-1 (IGF-1). These IGF-1-dependent changes in inositol lipids coincide with an increase in nuclear diacyglycerol and precede translocation to the nucleus and activation of protein kinase C (refs 5, 6). Circumstantial evidence that links these changes with mitosis comes from the isolation of a 3T3 clone that expresses the type-1 IGF receptor and binds IGF-1 peptide but does not respond mitogenically or show transient mass changes in nuclear inositol lipids. A key question is how IGF-1 initiates the rapid breakdown of PtdInsP and PtdInsP2 in the nucleus. Here we present evidence that nuclei of 3T3 cells contain the beta-isozyme of phosphoinositidase C, whereas the gamma-isozyme is confined to the cytoplasm and that IGF-1 treatment stimulates exclusively the activity of nuclear phosphoinositidase C. 相似文献