全文获取类型
收费全文 | 59695篇 |
免费 | 271篇 |
国内免费 | 543篇 |
专业分类
系统科学 | 1240篇 |
丛书文集 | 566篇 |
教育与普及 | 285篇 |
理论与方法论 | 530篇 |
现状及发展 | 32866篇 |
研究方法 | 879篇 |
综合类 | 21954篇 |
自然研究 | 2189篇 |
出版年
2013年 | 803篇 |
2012年 | 621篇 |
2011年 | 2666篇 |
2009年 | 597篇 |
2008年 | 822篇 |
2007年 | 935篇 |
2006年 | 1047篇 |
2005年 | 1198篇 |
2004年 | 2294篇 |
2003年 | 1925篇 |
2002年 | 1574篇 |
2001年 | 1539篇 |
2000年 | 1098篇 |
1999年 | 1030篇 |
1998年 | 645篇 |
1997年 | 774篇 |
1996年 | 532篇 |
1994年 | 684篇 |
1993年 | 695篇 |
1992年 | 967篇 |
1991年 | 858篇 |
1990年 | 941篇 |
1989年 | 730篇 |
1988年 | 727篇 |
1987年 | 726篇 |
1986年 | 772篇 |
1985年 | 988篇 |
1984年 | 835篇 |
1983年 | 720篇 |
1982年 | 811篇 |
1981年 | 848篇 |
1980年 | 961篇 |
1979年 | 1511篇 |
1978年 | 1355篇 |
1977年 | 1323篇 |
1976年 | 1176篇 |
1975年 | 1187篇 |
1974年 | 1120篇 |
1973年 | 1323篇 |
1972年 | 1400篇 |
1971年 | 1445篇 |
1970年 | 1670篇 |
1969年 | 1441篇 |
1968年 | 1335篇 |
1967年 | 1253篇 |
1966年 | 1076篇 |
1965年 | 850篇 |
1959年 | 519篇 |
1958年 | 679篇 |
1957年 | 574篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
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
42.
43.
44.
45.
R. Poggioli A. V. Vergoni A. Bertolini 《Cellular and molecular life sciences : CMLS》1985,41(2):265-266
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma. 相似文献
46.
H R Brenner 《Nature》1985,317(6038):572-573
47.
1 Results This paper presents the development of platinum nanocomposites structures based on organically modified c.a.2 nm core platinum nanoparticles.The chemical modification of the 4-mercaptoaniline functionalized particles by various in coming molecules is evidenced and precisely quantified.The particles can be dissolved like molecules in various solvents depending on the features of the new crown and X-rays shows that the interparticle distance is affected by the crown modification.These platinum n... 相似文献
48.
49.
1 Results The development of the concepts of nanotechnology has given an important impact on the design of new polymer based materials which are in most cases characterized by a multiphase morphology. When at least one phase has nanometric dimension(s) the system can be considered as a nanocomposite where the interface is not only determining for the adhesion but also may play a role in some bulk properties. Indeed in nanostructured multiphase solids the interface is significant as a bulk component. The... 相似文献
50.
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