全文获取类型
收费全文 | 19654篇 |
免费 | 2688篇 |
国内免费 | 8篇 |
出版年
2021年 | 213篇 |
2018年 | 210篇 |
2017年 | 190篇 |
2016年 | 332篇 |
2015年 | 481篇 |
2014年 | 574篇 |
2013年 | 785篇 |
2012年 | 874篇 |
2011年 | 866篇 |
2010年 | 542篇 |
2009年 | 512篇 |
2008年 | 729篇 |
2007年 | 730篇 |
2006年 | 672篇 |
2005年 | 615篇 |
2004年 | 669篇 |
2003年 | 653篇 |
2002年 | 612篇 |
2001年 | 627篇 |
2000年 | 622篇 |
1999年 | 542篇 |
1998年 | 312篇 |
1997年 | 300篇 |
1996年 | 278篇 |
1995年 | 274篇 |
1994年 | 261篇 |
1993年 | 230篇 |
1992年 | 457篇 |
1991年 | 409篇 |
1990年 | 453篇 |
1989年 | 391篇 |
1988年 | 376篇 |
1987年 | 381篇 |
1986年 | 346篇 |
1985年 | 379篇 |
1984年 | 325篇 |
1983年 | 270篇 |
1982年 | 242篇 |
1981年 | 232篇 |
1980年 | 189篇 |
1979年 | 310篇 |
1978年 | 255篇 |
1977年 | 225篇 |
1976年 | 195篇 |
1975年 | 204篇 |
1974年 | 214篇 |
1973年 | 203篇 |
1972年 | 203篇 |
1970年 | 185篇 |
1969年 | 186篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
82.
Leonid S Brown 《Photochemical & photobiological sciences》2004,3(6):555-565
In the last decade, several genome sequencing projects revealed the existence of previously unknown photoreceptors. Among those are eukaryotic rhodopsins of haloarchaeal type, mostly represented by fungal sequences. We have classified and analyzed seventy-seven of these fungal proteins, which show a high similarity of their putative transmembrane regions to those of bacteriorhodopsin. Those sequences can be divided into the two subgroups, fungal rhodopsins (RDs) and opsin-related proteins (ORPs), the latter lacking the lysine residue necessary for retinal binding. We have analyzed the conservation pattern of the residues known to have functional or structural importance in bacteriorhodopsin and discussed dramatic differences in the conservation between RDs and ORPs. We found many cases of multiple forms of RDs and/or ORPs and examined possible reasons for such multiplicity. For some species the reason may lie in functional photobiological diversification, while for the others it follows the pattern of evolutionary recent genome duplication and possible functional redundancy. 相似文献
83.
Jerry F. Downhower Lawrence S. Blumer Luther Brown 《Evolution; international journal of organic evolution》1987,41(6):1386-1394
Seasonal variation in sexual and natural selection in male mottled sculpins (Cottus bairdi) can be evaluated by calculating selection differentials, which measure the magnitude of phenotypic change resulting from selection, and by calculating indices of the opportunity for selection, which indicate the potential for phenotypic selection in a given interval. Selection differentials are high at the beginning of the breeding season and decline throughout the breeding season. The magnitude and direction of selection differentials depend on when spawning occurs and are independent of the size or age of the females that spawn. Annual selection differentials due to differences in mating success (female choice) are nearly constant between years. Annual selection differentials associated with hatching success are variable. Opportunities for selection (I = fitness variance/[mean fitness]2) show clear seasonal patterns. They are highest at the beginning and at the end of the spawning season. However, this variation is dependent on the mean used to calculate I, and hence variation in I values does not indicate a significant change in the variance of male fitness. 相似文献
84.
85.
86.
87.
Copper is an essential trace element that is utilized in a number of oxygenases and electron transport proteins, but it is also a highly toxic heavy metal, against which all organisms must protect themselves. Known bacterial determinants of copper resistance are plasmid-encoded. The mechanisms which confer resistance must be integrated with the normal metabolism of copper. Different bacteria have adopted diverse strategies for copper resistance, and this review outlines what is known about bacterial copper resistance mechanisms and their genetic regulation. 相似文献
88.
Stressful life events, social support networks, and the physical and mental health of urban black adults 总被引:2,自引:0,他引:2
The direct and the buffering effects of social support networks have been documented, but few studies have examined their health outcomes for blacks and, specifically, the differences in physical and mental health. This study uses six measures of social support to examine the relationships of social support to health outcomes for black males and females. The data are from a community sample of 451 urban black adults. Results show significant differences by sex according to source of social support and particular health outcomes. Number of nearby relatives and perceived social support have direct and buffering effects, respectively, on mental health for black females, but no significant influences emerge for black males. On the other hand, the number of confidants for black males and the degree of religiosity among black females were inversely related to physical health, suggesting differences in conceptual models of social support for mental and physical health. 相似文献
89.
M. Mes-Hartree E. K. C. Yu I. D. Reid J. N. Saddler 《Applied microbiology and biotechnology》1987,26(2):120-125
Summary Enzymatic conversion of lignocellulosic material to fuels and chemicals depends on a initial pretreatment to render the cellulose more susceptible to enzymatic attack. Biological delignification of aspenwood with the fungus Phlebia tremellosus was compared to steaming as a pretreatment method.The biologically delignified aspenwood (BDA) had a high pentosan content and did not contain inhibitors of enzymatic hydrolysis or subsequent fermentation. In contrast, the steamed aspenwood required a water extraction step to remove the inhibitory material and this step also removed most of the pentosan. The yield of treated material was 90% from biological delignification and 70% from steaming.The cellulose in the BDA was less accessible to the cellulase enzymes than the steamed aspenwood. Combined hydrolysis and fermentation with Saccharomyces cerevisiae gave a lower yield of ethanol from BDA than from the steamed aspenwood, but the yields based on the weight of substrate before pretreatment were comparable. Combined hydrolysis and fermentation with Klebsiella pneumoniae gave higher yields of butanediol from BDA than from steamed aspenwood, because Klebsiella can ferment the xylose which was present in the biologically treated aspenwood. Trichoderma harzianum produced lower levels of cellulase enzymes when grown on BDA than when grown on steamed aspenwood and this was related to the xylan found in the biologically treated material.Abbreviations BDA
biologically delignified aspenwood
- SEA-WI
steam-exploded, water-extracted aspenwood
- AI-SEA-WI
acid-impregnated, steam-exploded, water-extracted aspenwood
- CHF
combined hydrolysis and fermentation
- FP
filter paper 相似文献
90.
Flow cytometric analysis and sorting of plant chromosomes from Petunia hybrida protoplasts 总被引:2,自引:0,他引:2
A method to obtain a high metaphase index and thereafter a plant chromosome suspension is described for Petunia hybrida (2n = 14). Mesophyll protoplast cultures have been used, giving easily disrupted cell walls and a high percentage of dividing cells after 42 h. On 2.5 mM colchicine-treated cells, metaphase indexes reaching 10% were routinely obtained. The lysis medium in which the protoplast-derived cells were disrupted was a simplified culture medium. After chromosome release, samples were stained with Hoechst 33342 dye and analysed by flow cytofluorometry. The histogram of fluorescence intensities included three peaks of metaphase chromosomes and a duplication of this flow karyotype provoked by "monochromatid chromosome." This interpretation was established after flow sorting; micronuclei could also be observed and sorted. Of the 7 chromosomes, only the largest formed a distinct peak while the others were incompletely resolved, due to the similar DNA content of various chromosomes. Model distributions of Petunia hybrida chromosomes have been computed according to the relative chromosome length. The theoretical histograms indicated that low variability is indispensable for resolving distinctive chromosome peaks. The experimental flow karyotype was consistent with one of the models having CV of 2.5%. 相似文献