全文获取类型
收费全文 | 94622篇 |
免费 | 410篇 |
国内免费 | 885篇 |
专业分类
95917篇 |
出版年
2023年 | 10篇 |
2022年 | 33篇 |
2021年 | 62篇 |
2020年 | 28篇 |
2019年 | 34篇 |
2018年 | 11885篇 |
2017年 | 10697篇 |
2016年 | 7495篇 |
2015年 | 688篇 |
2014年 | 397篇 |
2013年 | 417篇 |
2012年 | 4351篇 |
2011年 | 12916篇 |
2010年 | 12073篇 |
2009年 | 8275篇 |
2008年 | 9860篇 |
2007年 | 11420篇 |
2006年 | 334篇 |
2005年 | 580篇 |
2004年 | 1037篇 |
2003年 | 1092篇 |
2002年 | 857篇 |
2001年 | 261篇 |
2000年 | 163篇 |
1999年 | 34篇 |
1998年 | 18篇 |
1997年 | 27篇 |
1996年 | 19篇 |
1995年 | 7篇 |
1994年 | 11篇 |
1993年 | 36篇 |
1992年 | 21篇 |
1991年 | 43篇 |
1990年 | 14篇 |
1989年 | 12篇 |
1988年 | 20篇 |
1987年 | 16篇 |
1984年 | 9篇 |
1983年 | 20篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1975年 | 9篇 |
1972年 | 250篇 |
1971年 | 276篇 |
1970年 | 6篇 |
1965年 | 13篇 |
1962年 | 24篇 |
1944年 | 12篇 |
1940年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
62.
63.
64.
65.
Summary Non-symbiotic heterotrophic N2 fixation in coniferous bark litter was investigated with the acetylene reduction assay under aerobic and anaerobic conditions. The litter studied was composed essentially of bark, of pH 5 and a C/N ratio of 101; the ratio of available C to available N, which governs N2 fixation, was considerably higher. The rate of N2 fixation was estimated as 2.5–4.4 g N. g–1 dry wt. day–1. Nitrogenase activity was still evident after seven months of incubation under aerobic conditions. The N2-ase activity was O2 dependent: under anaerobic conditions no N2-ase activity was found unless a fermentable C source was added. The importance of N2 fixation in N-poor litter for the maintenance of soil fertility is emphasized. 相似文献
66.
The kinetics of haemolysis of rabbit erythrocytes byCroton tiglium lectin was studied as a function of concentration of the lectin and erythrocytes. The length of the prelytic period decreased with increasing lectin concentrations, indicating that the secondary events at the membrane which follow the binding of the lectin to cell surface carbohydrate receptors are accelerated at higher surface concentrations of the lectin. The rate or extent of haemolysis was not affected by the inclusion of ions like K+, Ca2+ and Mg2+ in the medium or by the substitution of ionic medium by a non-ionic medium. The inhibition of haemagglutination and haemolysis of rabbit red cells byCroton tiglium lectin by antilectin rabbit serum was observed. A possible mechanism of haemolysis by the lectin is discussed. 相似文献
67.
N-Acetyl-D-galactosamine in β-linkage being ubiquitous in cell surface glycoproteins, their interaction with lectins specific for this sugar moiety may be a significant event in cell adhesion phenomena. This article discusses the common β-N-acetyl galactosamine-specific lectins, with particular stress on the lectin from winged beans (Psophocarpus tetragonolobus). 相似文献
68.
Demonstration of an “Excitation-Contraction Recoupling” Mechanism in Mammalian Ventricular Myocardium 总被引:1,自引:0,他引:1
A PROCESS called “excitation-contraction coupling” has been generally accepted to take place only in the direction of excitation to contraction. Through this mechanism a propagated action potential initiates an active state in skeletal or cardiac muscle and the muscle contracts. We propose that, in the mammalian ventricular myocardium at least, the process is not unidirectional and an important reverse coupling between the contractile system and the excitable plasma membrane has been overlooked. Through this feedback interaction the mode of contraction (that is, isotonic or isometric) not only determines the instantaneous electrical state of the plasma membrane, but also influences the mechanical events of the subsequent beats. Thus when Kaufmann et al.1 recorded intracellular action potentials from cat papillary muscle, the time course of the repolarization was altered depending on the mode of contraction. Some kind of contraction-excitation feedback has also been suggested by Stauch2 and Lab3,4. They showed a difference in the shape of the monophasic action potential, as recorded by a suction electrode, when comparing isotonic and isovolumic contraction of the intact ventricle. But their experimental conditions did not allow satisfactory analysis of the phenomenon. 相似文献
69.
70.
Function of the <Emphasis Type="Italic">rel</Emphasis> Gene in <Emphasis Type="Italic">Escherichia coli</Emphasis> 总被引:6,自引:0,他引:6
Sokawa et al. suggest that rel- strains of Escherichia coli possess abnormal protein synthesizing machinery, which cannot carry out normal protein synthesis when the supply of amino-acids is limited. 相似文献