全文获取类型
收费全文 | 12022篇 |
免费 | 1401篇 |
国内免费 | 8篇 |
出版年
2023年 | 55篇 |
2022年 | 84篇 |
2021年 | 322篇 |
2020年 | 173篇 |
2019年 | 215篇 |
2018年 | 249篇 |
2017年 | 244篇 |
2016年 | 403篇 |
2015年 | 662篇 |
2014年 | 665篇 |
2013年 | 753篇 |
2012年 | 909篇 |
2011年 | 972篇 |
2010年 | 618篇 |
2009年 | 493篇 |
2008年 | 733篇 |
2007年 | 744篇 |
2006年 | 589篇 |
2005年 | 587篇 |
2004年 | 587篇 |
2003年 | 568篇 |
2002年 | 505篇 |
2001年 | 163篇 |
2000年 | 151篇 |
1999年 | 167篇 |
1998年 | 145篇 |
1997年 | 92篇 |
1996年 | 101篇 |
1995年 | 79篇 |
1994年 | 72篇 |
1993年 | 60篇 |
1992年 | 78篇 |
1991年 | 74篇 |
1990年 | 77篇 |
1989年 | 67篇 |
1988年 | 53篇 |
1987年 | 58篇 |
1986年 | 52篇 |
1985年 | 72篇 |
1984年 | 73篇 |
1983年 | 52篇 |
1982年 | 52篇 |
1981年 | 59篇 |
1980年 | 61篇 |
1979年 | 40篇 |
1978年 | 39篇 |
1977年 | 39篇 |
1976年 | 25篇 |
1974年 | 38篇 |
1971年 | 26篇 |
排序方式: 共有10000条查询结果,搜索用时 214 毫秒
1.
Donald E. Rivett Dean Hewish Alan Kirkpatrick Jerome Werkmeister 《Journal of Protein Chemistry》1999,18(3):291-295
Nine fatty acid–peptide hybrid molecules were constructed using the general formula CH3(CH2)
n
CO-Phe Asp Cys-amide and tested for their ability to inhibit cell lysis induced by the membrane-active peptide melittin. All of these molecules, where n = 4–14, inhibited the action of melittin to some extent, but the longer carbon chains were most effective. Several potential inhibitors were also constructed with conservative substitutions in the peptide portion of the molecule. All were effective to varying degrees. We concluded that in the hexapeptide inhibitor published by Blondelle et al. (1993), the role of the first three residues is only to provide hydrophobic interaction with the melittin and has no particular amino acid sequence specificity. Some of these inhibitors were found to inhibit the lytic activity of a melittin analogue which had only superficial sequence similarity to melittin and also a truncated form of melittin, indicating the generality of the action of the inhibitors.Deceased 5/4/98 相似文献
2.
3.
Rebecca R. Sharitz Susan A. Wineriter Michael H. Smith Edwin H. Liu 《American journal of botany》1980,67(9):1297-1303
Biochemical phenotsypes of four taxa of Typha from the eastern United States were determined by starch gel electrophoresis. The isozyme banding patterns of T. latifolia, T. angustifolia and T. domingensis are distinct and allow unambiguous species identification when morphological characters are inadequate or unsuitable. The fourth form, T. glauca, is not an F1 hybrid, but it does appear to be intermediate between T. latifolia and T. angustifolia. The status of T. glauca and evolutionary relationships among the four forms may now be clarified by additional sampling because of the distinct and relatively invariant isozyme banding patterns which are described. 相似文献
4.
John G. Neuhoff Katherine L. Long Rebecca C. Worthington 《Evolution and human behavior》2012,33(4):318-322
Listeners consistently perceive approaching sounds to be closer than they actually are and perceptually underestimate the time to arrival of looming sound sources. In a natural environment, this underestimation results in more time than expected to evade or engage the source and affords a “margin of safety” that may provide a selective advantage. However, a key component in the proposed evolutionary origins of the perceptual bias is the appropriate timing of anticipatory motor behaviors. Here we show that listeners with poorer physical fitness respond sooner to looming sounds and with a larger margin of safety than listeners with better physical fitness. The anticipatory perceptual bias for looming sounds is negatively correlated with physical strength and positively correlated with recovery heart rate (a measure of aerobic fitness). The results suggest that the auditory perception of looming sounds may be modulated by the response capacity of the motor system. 相似文献
5.
The cost of mutualism in a fly-fungus interaction 总被引:2,自引:0,他引:2
The movement ability of individuals has become increasingly important to a variety of ecological questions. In this study,
I investigate how plant structure and changes in body size through development affect the movement ability of a predaceous
stinkbug, Podisus maculiventris, on three species of goldenrod (Solidago spp.) representing a wide range of surface complexities. I adapt existing techniques for quantifying movement in two dimensions
to the study of movement on natural plant structures in three dimensions. These experiments indicate that plant structure
and insect size are significant factors affecting the movement ability of P. maculiventris. Changes in movement ability due to factors of ontogeny and different habitat structures suggest that the scale of an individual’s
ambit or ecological sphere of influence may vary within its lifespan. Considering the influence of ontogeny and habitat structure
on movement ability may be useful to investigations of population dynamics, foraging behavior, and pest management.
Received: 14 July 1999 / Accepted: 23 March 2000 相似文献
6.
Big, beautiful organisms are useful for biological education, increasing evolution literacy, and biodiversity conservation. But if educators gloss over the ubiquity of streamlined and miniaturized organisms, they unwittingly leave students and the public vulnerable to the idea that the primary evolutionary plot of every metazoan lineage is “progressive” and "favors" complexity. We show that simple, small, and intriguingly repulsive invertebrate animals provide a counterpoint to misconceptions about evolution. Our examples can be immediately deployed in biology courses and outreach. This context emphasizes that chordates are not the pinnacle of evolution. Rather, in the evolution of animals, miniaturization, trait loss, and lack of perfection are at least as frequent as their opposites. Teaching about invertebrate animals in a “tree thinking” context uproots evolution misconceptions (for students and the public alike), provides a mental scaffold for understanding all animals, and helps to cultivate future ambassadors and experts on these little‐known, weird, and fascinating taxa. 相似文献
7.
During the progression of malignant peritoneal mesothelioma (MPeM), tumor nodules propagate diffusely within the abdomen and tumors are characterized by distinct phenotypic sub-types. Recent studies in solid organ cancers have shown that cancer stem cells (CSCs) play a pivotal role in the initiation and progression of tumors. However, it is not known whether tumorigenic stem cells exist and whether they promote tumor growth in MPeM. In this study, we developed and characterized a CSC model for MPeM using stably expandable tumorigenic stem cells derived from patient tumors. We found morphologically distinct populations of CSCs that divide asymmetrically or symmetrically in MPeM in vitro cell culture. The MPeM stem cells (MPeMSCs) express stem cell markers c-MYC, NES and VEGFR2 and in the presence of matrix components cells form colony spheres. MPeMSCs are multipotent, differentiate into neuronal, vascular and adipose progeny upon defined induction and the differentiating cells express lineage-specific markers such as TUBB3, an early neuronal marker; vWF, VEGFA, VEGFC and IL-8, endothelial markers; and PPARγ and FABP4, adipose markers. Xenotransplantation experiments using MPeMSCs demonstrated early tumor growth compared with parental cells. Limiting dilution experiments using MPeMSCs and endothelial lineage-induced cells derived from a single MPeMSC resulted in early tumor growth in the latter group indicating that endothelial differentiation of MPeMSCs is important for MPeM tumor initiation. Our observation that the MPeM tumors contain stem cells with tumorigenic potential has important implications for understanding the cells of origin and tumor progression in MPeM and hence targeting CSCs may be a useful strategy to inhibit malignant progression. 相似文献
8.
We have constructed four deletion derivatives of the cloned dnaK gene. Plasmid pDD1, in which the last 10 amino acids of the DnaK protein have been replaced by three different amino acids derived from the pBR322 vector, was as effective as plasmid pKP31, from which it was derived, in restoring the ability of a dnaK null mutant, Escherichia coli BB1553, to plate lambda phage and to grow at high temperatures. The other three mutations, involving much larger deletions of the dnaK gene, did not restore the ability to plate lambda phage or the ability to grow at high temperatures. Plasmid pKUC2, which contains the whole dnaK gene and its promoters, was capable of restoring the ability of E. coli BB1553 to plate lambda phage but, surprisingly, it did not restore the ability to grow at high temperatures, even though it was shown that the DnaK protein was efficiently expressed in these cultures. By transposon mutagenesis and sub-cloning, we have shown the presence of a second gene in plasmid pKP31 which is required for high-temperature growth of E. coli BB1553. This gene, which we call htg A, is presumably also defective in the dnaK null mutant E. coli BB1553. We have also demonstrated that the inability of E. coli K756 to grow above 43.5 degrees C is complemented by sub-clones which contain the htg A gene, but not by plasmid pKUC2. 相似文献
9.
10.
Household‐level correlates of children's physical activity levels in and across 12 countries 下载免费PDF全文
Deirdre M. Harrington Fiona Gillison Stephanie T. Broyles Jean‐Philippe Chaput Mikael Fogelholm Gang Hu Rebecca Kuriyan Anura Kurpad Allana G. LeBlanc Carol Maher Jose Maia Victor Matsudo Timothy Olds Vincent Onywera Olga L. Sarmiento Martyn Standage Mark S. Tremblay Catrine Tudor‐Locke Pei Zhao Peter T. Katzmarzyk for the ISCOLE Research Group 《Obesity (Silver Spring, Md.)》2016,24(10):2150-2157