全文获取类型
收费全文 | 857025篇 |
免费 | 87594篇 |
国内免费 | 276篇 |
专业分类
944895篇 |
出版年
2018年 | 8533篇 |
2017年 | 8116篇 |
2016年 | 11444篇 |
2015年 | 14506篇 |
2014年 | 17302篇 |
2013年 | 24960篇 |
2012年 | 27890篇 |
2011年 | 28632篇 |
2010年 | 19651篇 |
2009年 | 18094篇 |
2008年 | 25636篇 |
2007年 | 26637篇 |
2006年 | 24839篇 |
2005年 | 23885篇 |
2004年 | 23665篇 |
2003年 | 22789篇 |
2002年 | 22208篇 |
2001年 | 37128篇 |
2000年 | 36495篇 |
1999年 | 29371篇 |
1998年 | 10858篇 |
1997年 | 10945篇 |
1996年 | 10427篇 |
1995年 | 9599篇 |
1994年 | 9260篇 |
1993年 | 9304篇 |
1992年 | 23826篇 |
1991年 | 23376篇 |
1990年 | 22846篇 |
1989年 | 22228篇 |
1988年 | 20373篇 |
1987年 | 19554篇 |
1986年 | 18283篇 |
1985年 | 18122篇 |
1984年 | 14951篇 |
1983年 | 13054篇 |
1982年 | 9876篇 |
1981年 | 8971篇 |
1980年 | 8332篇 |
1979年 | 13949篇 |
1978年 | 11021篇 |
1977年 | 9930篇 |
1976年 | 9512篇 |
1975年 | 10671篇 |
1974年 | 11398篇 |
1973年 | 11343篇 |
1972年 | 10358篇 |
1971年 | 9234篇 |
1970年 | 8108篇 |
1969年 | 7988篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Cristian A. Acevedo Elizabeth Y. Sanchez Juan G. Reyes Manuel E. Young 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2010,878(3-4):449-455
It is known that skin releases volatile organic compounds to the environment, and also that its emission pattern changes with aging of the skin. It could be considered, that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, a simple and non-destructive method consisting of SPME sampling and GC/MS analysis was developed to identify volatile organic emanations from cell cultures. This technique, applied to skin cells culture, indicates that the cells or cell metabolism produce several skin emissions. Chemometric analysis was performed in order to explore the relationship between a volatile profile and the senescence of cell cultures. Volatile profiles were different for cell cultures in different degrees of senescence, indicating that volatile compound patterns could be used to provide information about the age of skin cells. 相似文献
52.
Recent experimental evidence suggests that coordinated expression of ion channels plays a role in constraining neuronal electrical activity. In particular, each neuronal cell type of the crustacean stomatogastric ganglion exhibits a unique set of positive linear correlations between ionic membrane conductances. These data suggest a causal relationship between expressed conductance correlations and features of cellular identity, namely electrical activity type. To test this idea, we used an existing database of conductance-based model neurons. We partitioned this database based on various measures of intrinsic activity, to approximate distinctions between biological cell types. We then tested individual conductance pairs for linear dependence to identify correlations. Contrary to experimental evidence, in which all conductance correlations are positive, 32% of correlations seen in this database were negative relationships. In addition, 80% of correlations seen here involved at least one calcium conductance, which have been difficult to measure experimentally. Similar to experimental results, each activity type investigated had a unique combination of correlated conductances. Finally, we found that populations of models that conform to a specific conductance correlation have a higher likelihood of exhibiting a particular feature of electrical activity. We conclude that regulating conductance ratios can support proper electrical activity of a wide range of cell types, particularly when the identity of the cell is well-defined by one or two features of its activity. Furthermore, we predict that previously unseen negative correlations and correlations involving calcium conductances are biologically plausible. 相似文献
53.
Formation of rings from Drosophila DNA fragments 总被引:1,自引:0,他引:1
54.
Four myeloid cell lines (M1, WEHI-3B D+, FDC-P1, and 32D) were screened for the presence of J11d antigen. One of these cell lines, the myeloid leukemia M1, was found to express a high level of J11d antigen on the cell surface. Recombinant mouse leukemic inhibitory factor (rm-LIF), recombinant human LIF (rh-LIF), and steroids (hydrocortisone and dexamethasone) could induce M1 cells to undergo monocytic differentiation. The level of J11d antigen was greatly reduced after treatment of the cells with LIF or steroids. Western blotting revealed that the apparent molecular weight of the J11d antigen on M1 cells was 45-48 kDa. Furthermore, the level of J11d mRNA was also reduced during LIF-induced differentiation of M1 cells. 相似文献
55.
56.
A. R. E. SINCLAIR 《African Journal of Ecology》1973,11(1):93-107
Censuses of buffalo and wildebeest in the Serengeti-Mara region have been carried out by various workers at intervals since 1958. The methods of these censuses, which normally employ a total count, are described and the errors inherent in such techniques are analysed in detail. After the appropriate corrections had been made the results showed that both populations have increased considerably over the years 1961–1971. 相似文献
57.
58.
T. B. Lapirova V. R. Mikryakova A. S. Mavrin G. A. Yinogradova 《Journal of Evolutionary Biochemistry and Physiology》2000,36(1):47-51
The paper presents data on a change in lysozyme content in tissues of spleen, liver and heart in fry of the Lena River sturgeon
exposed to the presence of sublethal concentrations of Hg2+, Cd2+, and Cu2+ under conditions of chronic experiment. It has been shown that the lysozyme content in fish tissues varies and has a phasic
character. The amplitude of fluctuations of this parameter depends on the moment of sampling, nature of the toxicant, and
structural-functional organization of the studied organs. 相似文献
59.
To maximize fitness, many animals must trade off their need to forage efficiently against their need to avoid predators. We studied such a trade-off in four species of tits (Paridae) in a forest near Oxford, UK. During winter, tits form flocks which increase feeding efficiency and reduce predation risk. These flocks feed extensively on beech (Fagus sylvatica) seeds, the abundance of which may be critical for winter survival. Because these seeds drop to the ground, where birds are exposed to sparrowhawk (Accipiter nisus) attack, tits need to trade off their need to find seeds against the proximity to protective cover, provided by dense clusters of hawthorn (Crataegus spp.). The quality of the beech crop differs markedly between trees and years. During a year of abundant beechmast, most tits searched for seeds close to protective cover. This 'safety-first' strategy precluded visits to superabundant food patches if they were too far from protective cover. Among beech trees near to cover, tits tended to prefer those with high seed density. Tits benefited from foraging under trees with high seed density because this correlated significantly with seed mass per square metre and because mean search times decreased with increasing seed density. Finally, we show experimentally that great tits, Parus major, can discriminate between edible (viable) and inedible (empty) seeds. 相似文献
60.