全文获取类型
收费全文 | 27830篇 |
免费 | 123篇 |
国内免费 | 259篇 |
专业分类
系统科学 | 435篇 |
丛书文集 | 557篇 |
教育与普及 | 106篇 |
理论与方法论 | 112篇 |
现状及发展 | 11112篇 |
研究方法 | 1129篇 |
综合类 | 14137篇 |
自然研究 | 624篇 |
出版年
2014年 | 184篇 |
2013年 | 379篇 |
2012年 | 619篇 |
2011年 | 1266篇 |
2010年 | 440篇 |
2009年 | 394篇 |
2008年 | 709篇 |
2007年 | 814篇 |
2006年 | 808篇 |
2005年 | 747篇 |
2004年 | 564篇 |
2003年 | 495篇 |
2002年 | 517篇 |
2001年 | 720篇 |
2000年 | 790篇 |
1999年 | 545篇 |
1992年 | 428篇 |
1991年 | 355篇 |
1990年 | 378篇 |
1989年 | 359篇 |
1988年 | 344篇 |
1987年 | 354篇 |
1986年 | 333篇 |
1985年 | 440篇 |
1984年 | 345篇 |
1983年 | 274篇 |
1982年 | 257篇 |
1981年 | 259篇 |
1980年 | 329篇 |
1979年 | 764篇 |
1978年 | 582篇 |
1977年 | 620篇 |
1976年 | 457篇 |
1975年 | 521篇 |
1974年 | 739篇 |
1973年 | 611篇 |
1972年 | 623篇 |
1971年 | 700篇 |
1970年 | 873篇 |
1969年 | 752篇 |
1968年 | 723篇 |
1967年 | 741篇 |
1966年 | 648篇 |
1965年 | 483篇 |
1959年 | 268篇 |
1958年 | 413篇 |
1957年 | 330篇 |
1956年 | 288篇 |
1955年 | 278篇 |
1954年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 515 毫秒
51.
A. Gutnisky P. Glikman M. F. Gimeno A. L. Gimeno 《Cellular and molecular life sciences : CMLS》1982,38(1):116-117
Summary The administration of 15(R)-15-methyl prostaglandin E2 (15(R)-15-M-PGE2) in vivo significantly diminished the uptake of59Fe into blood, spleen, liver, femur and dried intestine of rats, whereas acetylsalicylic acid (ASA) increased the counts significantly. This effect of ASA was counteracted by 15(R)-15-M-PGE2. It is suggested that prostaglandins (PGs) might play an important role in inhibiting iron absorption at the intestinal level.This work was supported by grant No.6638 from CONICET (Argentina). The technical assistance of Mrs María E. Castro and Norma Rizzo is gratefully acknowleged. 相似文献
52.
During the evolution of proteins the pressure to optimize biological activity is moderated by a need for efficient folding. For most proteins, this is accomplished through spontaneous folding to a thermodynamically stable and active native state. However, in the extracellular bacterial alpha-lytic protease (alphaLP) these two processes have become decoupled. The native state of alphaLP is thermodynamically unstable, and when denatured, requires millennia (t1/2 approximately 1,800 years) to refold. Folding is made possible by an attached folding catalyst, the pro-region, which is degraded on completion of folding, leaving alphaLP trapped in its native state by a large kinetic unfolding barrier (t1/2 approximately 1.2 years). alphaLP faces two very different folding landscapes: one in the presence of the pro-region controlling folding, and one in its absence restricting unfolding. Here we demonstrate that this separation of folding and unfolding pathways has removed constraints placed on the folding of thermodynamically stable proteins, and allowed the evolution of a native state having markedly reduced dynamic fluctuations. This, in turn, has led to a significant extension of the functional lifetime of alphaLP by the optimal suppression of proteolytic sensitivity. 相似文献
53.
A. E. Yassen H. A. Hassan L. S. Kawashti 《Cellular and molecular life sciences : CMLS》1994,50(11-12):1111-1114
A detailed karyotypic analysis of two Egyptian species of bats,Taphozous perforatus andTaphozous nudiventris, was made on the basis of conventional data and G-band patterns. No detectable karyotypic difference was found between the two species (2n=42, F.N. 64). By comparison of G-band patterns, some differences between the two species were seen in the corresponding autosomal pairs. These results are reported for the first time in Egypt. 相似文献
54.
The gas-liquid-liquid reactive extraction system for preparing hydrogen peroxide via anthraquinone was investigated. The oxidation reaction of hydrogenated working solution was combined with the extraction of hydrogen peroxide from working solution in a sieve plate column. The reaction of 2-ethylanthrahydroquionone with oxygen and the liquid-liquid extraction of hydrogen peroxide take place simultaneously. The oxygen was introduced with hydrogenated working solution through a nozzle in the bottom of the column, which worked as agitated air as well as oxidation reagent. The results showed the oxidation and extraction do not hamper each other, on the contrary, the presence of oxidation gas in the column can promote the transfer of hydrogen peroxide from organic phase to aqueous phase, thus the reaction efficiency and extraction efficiency increased with increasing gas superficial velocity. Furthermore, the oxidation efficiency is almost 100% and the extraction efficiency is higher than 90% in this process. 相似文献
55.
F. Acerbi 《Archive for History of Exact Sciences》2003,57(6):465-502
56.
Contemporary developments in economicmethodology have produced a vibrant agenda ofcompeting positions. These include, amongothers, constructivism, critical realism andrhetoric, with each contributing to the Realistvs. Pragmatism debate in the philosophies of thesocial sciences. A major development in theneo-pragmatist contribution to economicmethodology has been Quine's pragmatic assaulton the dogmas of empiricism, which are nowclearly acknowledged within contemporaryeconomic methodology. This assault isencapsulated in the celebrated Duhem-Quinethesis, which according to a number ofcontemporary leading philosophers of economics,poses a particularly serious methodologicalproblem for economics. This problem, asreflected in Hausman's analysis, consists ofthe inability of economics to learn fromexperience, thereby subverting the capacity totest economic theories. In this paper wedispute this position. Our argument is basedon a combination of Quine's holism with VanFraassen's constructive empiricism, especiallythe latter's analysis of empirical adequacy andhis pragmatic approach to explanation. Theresulting reorientation of economic methodologyrestores the capacity of economics to learnfrom experience and reinstates the imperativeof developing alternatives to orthodoxtheorizing in economics. 相似文献
57.
Coalignment of vimentin intermediate filaments with microtubules depends on kinesin. 总被引:19,自引:0,他引:19
Intermediate filaments in most types of cultured cells coalign with microtubules. Depolymerization of microtubules results in collapse of vimentin and desmin intermediate filaments to the nucleus where they form a perinuclear cap. Collapse can also be induced by microinjection of antibodies against intermediate filament or microtubule proteins. Thus, two filament systems interact with each other. But the molecules mediating this interaction are unknown. One of the candidates for this role is a microtubule motor kinesin. Recent data showed that kinesin is involved in the plus end-directed movement of the membranous organelles along microtubules such as radial extension of lysosomes in macrophages and centrifugal movement of pigment in melanophores. Here we report that injection of the anti-kinesin antibody into human fibroblasts results in the redistribution of intermediate filaments to a tight perinuclear aggregate but had no effect on the distribution of microtubules. Thus, kinesin is involved not only in organelle movement but also in interaction of the two major cytoskeletal systems, intermediate filaments and microtubules. 相似文献
58.
卢武星 《北京师范大学学报(自然科学版)》1994,(4)
1.0MeV208Pb离子在非晶Si中的投影射程RP和射程偏差ΔRP作为注量和温度二者的函数用背散射法进行测定.注量的变化范围为5×1013~7×1014cm-2.注入是在室温和t=-120℃下完成的.由由实验所确定的投影射程,射程偏差与注量或温度无关,并且分别等于295和72.2nm.与TRIM86的计算结果相比较,发现RP的偏离为18%,而ΔRP的偏离为36%.RP和ΔRP二者与注量及温度的无关性,排除了所观察到的与TRIM的矛盾是由于注入期间辐射增强扩散或离子束混合效应而引起的解释。 相似文献
59.
60.