全文获取类型
收费全文 | 29880篇 |
免费 | 74篇 |
国内免费 | 75篇 |
专业分类
系统科学 | 145篇 |
丛书文集 | 363篇 |
教育与普及 | 57篇 |
理论与方法论 | 121篇 |
现状及发展 | 12817篇 |
研究方法 | 1186篇 |
综合类 | 14885篇 |
自然研究 | 455篇 |
出版年
2013年 | 250篇 |
2012年 | 369篇 |
2011年 | 839篇 |
2010年 | 158篇 |
2008年 | 430篇 |
2007年 | 513篇 |
2006年 | 540篇 |
2005年 | 513篇 |
2004年 | 598篇 |
2003年 | 482篇 |
2002年 | 481篇 |
2001年 | 996篇 |
2000年 | 957篇 |
1999年 | 584篇 |
1992年 | 538篇 |
1991年 | 433篇 |
1990年 | 460篇 |
1989年 | 487篇 |
1988年 | 434篇 |
1987年 | 434篇 |
1986年 | 490篇 |
1985年 | 572篇 |
1984年 | 440篇 |
1983年 | 423篇 |
1982年 | 363篇 |
1981年 | 363篇 |
1980年 | 397篇 |
1979年 | 967篇 |
1978年 | 782篇 |
1977年 | 739篇 |
1976年 | 573篇 |
1975年 | 695篇 |
1974年 | 968篇 |
1973年 | 766篇 |
1972年 | 802篇 |
1971年 | 960篇 |
1970年 | 1204篇 |
1969年 | 883篇 |
1968年 | 910篇 |
1967年 | 848篇 |
1966年 | 764篇 |
1965年 | 589篇 |
1964年 | 156篇 |
1959年 | 313篇 |
1958年 | 523篇 |
1957年 | 380篇 |
1956年 | 293篇 |
1955年 | 295篇 |
1954年 | 282篇 |
1948年 | 207篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
22.
M. Lazzari V. Franceschini G. Minelli F. Ciani 《Cellular and molecular life sciences : CMLS》1993,49(4):277-281
The corrosion cast technique provided for the first time an excellent three-dimensional visualization of the vascular pattern of the choroid and iris in the newt eye. The results show the presence of a single arterial afference to the choroidal and iris capillaries: the ophthalmic artery is the origin fo both ciliary arteries and the long posterior ciliary artery. Slightly behind the equatorial circumference of the eyeball the venous drainage consists of a single vessel on the dorsal side and two distinct vessels on the ventral one. It receives blood from both iris and choroid. The surface of the plastic endocasts shows some details of fine luminal structures of the endothelial cells. Shallow depressions may be regarded as imprints of endothelial cell nuclei, and they are distinctly different for arteries and capillaries. The angioarchitecture of the newt eye differs from that of brain in that hairpin-shaped capillary loops are not observed at all. 相似文献
23.
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
24.
25.
26.
27.
煤矸石空心砖是一种新型环保墙体材料,符合国家及山西省出台的限制生产和使用实心黏土砖,鼓励发展新型建筑材料的政策。利用煤矸石制造的空心砖可以作为保温材料,既可节约建筑能耗,又可节约土地资源,减少环境污染。煤矸石空心砖同黏土实心砖相比,具有高强、轻质、隔音、隔热、保温、防震等优点,市场前景良好。 相似文献
28.
29.
Human cytomegalovirus UL97 open reading frame encodes a protein that phosphorylates the antiviral nucleoside analogue ganciclovir. 总被引:32,自引:0,他引:32
Human cytomegalovirus (HCMV, a betaherpes virus) is the cause of serious disease in immunologically compromised individuals, including those with acquired immunodeficiency syndrome. One of the compounds used in the chemotherapy of HCMV infections is the nucleoside analogue 9-(1,3-dihydroxy-2-propoxymethyl)-guanine (ganciclovir). The mechanism of action of this drug is dependent on the formation of the nucleoside triphosphate, which is a strong inhibitor of the viral DNA polymerase. Thymidine kinase, which is encoded by many of the herpesviruses, catalyses the initial phosphorylation of ganciclovir. But there is no evidence for the coding of this enzyme by HCMV, and DNA sequence analysis of the HCMV genome has shown that there is no open reading frame characteristic of a herpesvirus thymidine kinase. Here we present biochemical and immunological evidence that the HCMV UL97 open reading frame codes for a protein capable of phosphorylating ganciclovir. This protein seems to be responsible for the selectivity of ganciclovir and will be useful tool in the understanding and refinement of the antiviral activity of new selective anti-HCMV compounds. 相似文献
30.
A. F. McDonagh Y. -M. Pu D. A. Lightner 《Cellular and molecular life sciences : CMLS》1992,48(3):246-248
The characteristic circular dichroism of bilirubin bound to human serum albumin undergoes a remarkable sign inversion on addition of halothane, chloroform and other volatile anesthetics. This sign inversion, which is completely reversed by removal of the anesthetic, reflects a pronounced conformational change of the bound ligand; probably a complete inversion of chirality. The observation suggests that association of volatile anesthetics with proteins can markedly alter the internal topography of receptor sites and potentially influence the stereoselectivity of ligand binding. 相似文献