全文获取类型
收费全文 | 625422篇 |
免费 | 67949篇 |
国内免费 | 1058篇 |
专业分类
694429篇 |
出版年
2018年 | 15514篇 |
2017年 | 13902篇 |
2016年 | 12978篇 |
2015年 | 9101篇 |
2014年 | 10416篇 |
2013年 | 14476篇 |
2012年 | 19874篇 |
2011年 | 28029篇 |
2010年 | 21665篇 |
2009年 | 17479篇 |
2008年 | 23164篇 |
2007年 | 25195篇 |
2006年 | 14005篇 |
2005年 | 13823篇 |
2004年 | 14059篇 |
2003年 | 13595篇 |
2002年 | 12917篇 |
2001年 | 22991篇 |
2000年 | 23400篇 |
1999年 | 18701篇 |
1998年 | 6771篇 |
1997年 | 7133篇 |
1996年 | 6905篇 |
1995年 | 6617篇 |
1994年 | 6596篇 |
1993年 | 6544篇 |
1992年 | 16300篇 |
1991年 | 16190篇 |
1990年 | 15847篇 |
1989年 | 15412篇 |
1988年 | 14392篇 |
1987年 | 13836篇 |
1986年 | 12931篇 |
1985年 | 13072篇 |
1984年 | 10982篇 |
1983年 | 9708篇 |
1982年 | 7478篇 |
1981年 | 7017篇 |
1980年 | 6595篇 |
1979年 | 10964篇 |
1978年 | 8520篇 |
1977年 | 7850篇 |
1976年 | 7457篇 |
1975年 | 8272篇 |
1974年 | 8564篇 |
1973年 | 8403篇 |
1972年 | 8177篇 |
1971年 | 7167篇 |
1970年 | 6055篇 |
1969年 | 5714篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Accumulation of malto-oligosaccharides in the syncytiotrophoblastic cells of first-trimester human placentas. 总被引:1,自引:1,他引:0
A cell-surface microvillar fraction that was isolated from the syncytiotrophoblastic cells of first-trimester human placentas was found to contain very high concentrations (890 +/- 32 microgram of hexose/mg of protein) of a class of low-molecular-weight oligosaccharides that were comprised entirely of glucose. T.l.c. and gel filtration showed that the saccharides contained from one to six glucose residues. The structures of the most prominent members of the series, a tetra- and a tri-saccharide, were determined. The anomeric configuration of the glucose residues was alpha, and methylation linkage analysis gave terminal and 4-linked hexose residues. These malto-oligosaccharides contained one reducing terminus per molecule, indicating that they were free and not bound to other structural elements of the cells. Within the placenta they appeared to be concentrated in the first-trimester trophoblastic cells, since crude membrane and particulate fractions isolated from either term trophoblastic cells or cultured placental fibroblasts did not contain detectable amounts of glucose oligomers. This series of oligosaccharides was similar to the products that are formed when glycogen is degraded by alpha-amylase in liver homogenates and may be indicative of a similar, highly active enzymic reaction closely associated with the brush border of the syncytiotrophoblastic cells of the first-trimester human placenta. Although the role of these oligosaccharides remains obscure they are probably involved in foetal metabolism. 相似文献
993.
The number of orientation, feeding and social calls emitted by pipistrelle batsPipistrellus pipistrellus Schreber, 1774 andP. pygmaeus Leach, 1825 was recorded throughout the night at eight different sites. Social calls were unaffected by weather variables,
whereas orientation calls and feeding buzzes were both significantly affected by cloud and temperature conditions. The number
of emissions of each call type was significantly different between sites, indicating that the bats used different sites for
different activities. Significant positive correlations between all three combinations of call types occurred only during
the middle of the night, corresponding with the nadir of flying insects. This suggests that bats were engaged in activities
other than feeding at this time, such as territory protection or mate attraction. 相似文献
994.
995.
The flap endonuclease, FEN1, plays a critical role in DNA replication and repair. Human FEN1 exhibits both a 5' to 3' exonucleolytic and a structure-specific endonucleolytic activity. On primer-template substrates containing an unannealed 5'-tail, or flap structure, FEN1 employs a unique mechanism to cleave at the point of annealing, releasing the 5'-tail intact. FEN1 appears to track along the full length of the flap from the 5'-end to the point of cleavage. Substrates containing structural modifications to the flap have been used to explore the mechanism of tracking. To determine whether the nuclease must recognize a succession of nucleotides on the flap, chemical linkers were used to replace an interior nucleotide. The nuclease could readily traverse this site. The footprint of the nuclease at the time of cleavage does not extend beyond 25 nucleotides on the flap. Eleven-nucleotide branches attached to the flap beyond the footprinted region do not prevent cleavage. Single- or double-thymine dimers also allow cleavage. cis-Platinum adducts outside the protected region are moderately inhibitory. Platinum-modified branch structures are completely inert to cleavage. These results show that some flap modifications can prevent or inhibit tracking, but the tracking mechanism tolerates a variety of flap modifications. FEN1 has a flexible loop structure through which the flap has been proposed to thread. However, efficient cleavage of branched structures is inconsistent with threading the flap through a hole in the protein. 相似文献
996.
997.
998.
Are all plant populations metapopulations? 总被引:2,自引:1,他引:1
999.
1000.
P Sinibaldi R Muraro M Centanni M G Giganti 《Bollettino della Società italiana di biologia sperimentale》1983,59(1):112-116
The behaviour of Na+/K+ ATPase during cell growth has been studied. Human cultured fibroblasts were used in the presence or absence of EGF. Sample and control cultures were stopped by gathering and washing the cells with tris buffer. Homogenates were tested for Na+/K+ ATPase activity by the method of incubating and for the -SH groups content (Ellman). Na+/K+ ATPase activity that slightly increases in the controls is strongly reduced by the addition of EGF. The behaviour shows evidence for a double mechanism of action: I) involvement of the cAMP system 2) decrease of the -SH group availability. 相似文献