全文获取类型
收费全文 | 8697篇 |
免费 | 680篇 |
专业分类
9377篇 |
出版年
2023年 | 89篇 |
2022年 | 128篇 |
2021年 | 253篇 |
2020年 | 135篇 |
2019年 | 179篇 |
2018年 | 215篇 |
2017年 | 199篇 |
2016年 | 305篇 |
2015年 | 471篇 |
2014年 | 517篇 |
2013年 | 662篇 |
2012年 | 742篇 |
2011年 | 691篇 |
2010年 | 408篇 |
2009年 | 441篇 |
2008年 | 526篇 |
2007年 | 447篇 |
2006年 | 422篇 |
2005年 | 408篇 |
2004年 | 374篇 |
2003年 | 331篇 |
2002年 | 316篇 |
2001年 | 100篇 |
2000年 | 63篇 |
1999年 | 85篇 |
1998年 | 94篇 |
1997年 | 80篇 |
1996年 | 63篇 |
1995年 | 50篇 |
1994年 | 44篇 |
1993年 | 49篇 |
1992年 | 36篇 |
1991年 | 28篇 |
1990年 | 29篇 |
1989年 | 27篇 |
1988年 | 22篇 |
1987年 | 29篇 |
1986年 | 13篇 |
1985年 | 28篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 22篇 |
1980年 | 14篇 |
1978年 | 15篇 |
1977年 | 23篇 |
1975年 | 12篇 |
1974年 | 17篇 |
1973年 | 12篇 |
1972年 | 11篇 |
排序方式: 共有9377条查询结果,搜索用时 8 毫秒
1.
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
2.
Tanner Miest Dyana Saenz Anne Meehan Manuel Llano Eric M. Poeschla 《Methods (San Diego, Calif.)》2009,47(4):298-303
RNAi is a powerful technology for analyzing gene function in human cells. However, its utility can be compromised by inadequate knockdown of the target mRNA or by interpretation of effects without rigorous controls. We review lentiviral vector-based methods that enable transient or stable knockdowns to trace mRNA levels in human CD4+ T cell lines and other targets. Critical controls are reviewed, including rescue of the pre-knockdown phenotype by re-expression of the targeted gene. The time from thinking about a potential knockdown target to analysis of phenotypes can be as short as a few weeks. 相似文献
3.
W. Kelley Thomas J. T. Vida Linda M. Frisse Manuel Mundo James G. Baldwin 《Journal of nematology》1997,29(3):250-254
To effectively integrate DNA sequence analysis and classical nematode taxonomy, we must be able to obtain DNA sequences from formalin-fixed specimens. Microdissected sections of nematodes were removed from specimens fixed in formalin, using standard protocols and without destroying morphological features. The fixed sections provided sufficient template for multiple polymerase chain reaction-based DNA sequence analyses. 相似文献
4.
Maria João Feio Trefor B. Reynoldson Verónica Ferreira Manuel Augusto S. Graça 《Hydrobiologia》2007,579(1):55-68
We sampled macroinvertebrates at 75 locations in the Mondego river catchment, Central Portugal, and developed a predictive
model for water quality assessment of this basin, based on the Reference Condition Approach. Sampling was done from June to
September 2001. Fifty-five sites were identified as “Reference sites” and 20 sites were used as “Test sites” to test the model.
At each site we also measured 40 habitat variables to characterize water physics and chemistry, habitat type, land use, stream
hydrology and geographic location. Macroinvertebrates were generally identified to species or genus level; a total of 207
taxa were found. By Unweighted Pair Group Method with Arithmetic mean (UPGMA) clustering and analysis of species contribution
to similarities percentage (SIMPER), two groups of reference sites were established. Using Discriminant Analysis (stepwise
forward), four variables correctly predicted 78% of the reference sites to the appropriate group: stream order, pool quality,
substrate quality and current velocity. Test sites’ environmental quality was established from their relative distance to
reference sites, in MDS ordination space, using a series of bands (BEAST methodology). The model performed well at upstream
sites, but at downstream sites it was compromised by the lack of reference sites. As with the English RIVPACS predictive model,
the Mondego model should be continually improved with the addition of new reference sites. The adaptation of the Mondego model
methodology to the Water Framework Directive is possible and would consist mainly of the integration of the WFD typology and
increasing the number of ellipses that define quality bands.
Handling editor: K. Martens 相似文献
5.
Luís M. S. Loura Fábio Fernandes Manuel Prieto 《European biophysics journal : EBJ》2010,39(4):589-607
Lateral membrane heterogeneity, in the form of lipid rafts and microdomains, is currently implicated in cell processes including signal transduction, endocytosis, and cholesterol trafficking. Various biophysical techniques have been used to detect and characterize lateral membrane domains. Among these, Förster resonance energy transfer (FRET) has the crucial advantage of being sensitive to domain sizes smaller than 50-100 nm, below the resolution of optical microscopy but, apparently, similar to those of rafts in cell membranes. In the last decade, several formalisms for the analysis of FRET in heterogeneous membrane systems have been derived and applied to the study of microdomains. They are critically described and illustrated here. 相似文献
6.
Paleoecology allows construction of paleoenvironmental models, faunal changes and evolutionary trends of paleontological taxa using modern analogs. However, when linking modern analogs to paleontological taxa in paleoecological reconstruction, differential taxonomic preservation in the fossil record has to be taken into account. Paleontologists have known the biased nature of the fossil record since Efremov’s publication on taphonomy in 1940, yet many ecological models of habitats associated with hominins in paleontological and archaeological sites in Africa and elsewhere barely address the complexity of the fossil record. We use randomly sampled ungulates from modern biomes in a comparative taxonomic abundance to demonstrate how the combination of modern thanatocoenoses and taphocoenoses, when used in reference to habitat-specific biocoenosis, produce better inferences of past habitats in paleontological and archaeological sites than approaches currently used. 相似文献
7.
Quince (Cydonia oblonga Mill.), pear (Pyrus communis L.) and olive (Olea europaea L.) genotypes were evaluated for their tolerance to iron deficiency stress by growing young plants in three types of aerated nutrient solutions: (1) with iron, (2) without iron or (3) low in iron and with 10 mM bicarbonate. Plants were obtained either from rooted softwood cuttings or from germination of seeds. The degree of tolerance was evaluated with several indices: (1) the chlorophyll content, (2) the root Fe3+ reducing capacity and (3) the whole plant relative growth. Fifteen hours before Fe3+ reducing capacity determination, iron was applied to the roots of plants with iron-stress, since this method resulted in increasing the reductase activity. All quince and pear genotypes increased the root Fe3+ reducing capacity when grown in the treatments for iron-stress, in relation to control plants of the same genotypes. In olive cultivars, the Fe3+ reducing capacity was lower in the iron-stress treatments than in the control one. Studying the relationship between relative growth and chlorophyll content for each genotype under iron-stress, in relation to both indices in control plants, a classification of species and genotypes was established. According to that, most olive cultivars and some pear rootstocks and cultivars appear more iron-efficient than quince rootstocks. Our study shows that in some woody species, determining root Fe3+ reducing capacity is not the best method to establish tolerance to iron deficiency stress. 相似文献
8.
9.
Esteve Padrs Mireia Duach Antoni Morros Manuel Sabs Joan Maosa 《Trends in biochemical sciences》1984,9(12):508-510
Fourth-derivative spectrophotometry offers several advantages over classical absorption or difference spectrophotometry in examining the characteristics of aromatic amino acids in proteins. The basic principles of the technique and its applications are outlined. 相似文献
10.
The effects produced on bacteriorhodopsin by low concentrations of several detergents have been studied by absorption and fourth-derivative spectrophotometry. Sodium dodecyl sulfate induces the appearance of the blue form of bacteriorhodopsin (λmax = 600 nm) at pH values up to 7.0 in a reversible manner. The apparent pK of the purple-to-blue transition raised with increasing concentration of SDS. Of the other detergents tested, only sodium dodecyl-N-sarcosinate showed a slight red-shift of the absorption band to 580 nm, whereas sodium taurocholate, Triton X-100 and cetyltrimethylammonium bromide did not favour the appearance of the blue form. The effect of SDS was found to be consistent with a localized conformational change that moves away the counter-ion of the protonated Schiff base. 相似文献