全文获取类型
收费全文 | 8578篇 |
免费 | 792篇 |
国内免费 | 4篇 |
专业分类
9374篇 |
出版年
2023年 | 37篇 |
2022年 | 62篇 |
2021年 | 128篇 |
2020年 | 87篇 |
2019年 | 137篇 |
2018年 | 162篇 |
2017年 | 145篇 |
2016年 | 241篇 |
2015年 | 380篇 |
2014年 | 399篇 |
2013年 | 498篇 |
2012年 | 681篇 |
2011年 | 704篇 |
2010年 | 426篇 |
2009年 | 369篇 |
2008年 | 463篇 |
2007年 | 471篇 |
2006年 | 431篇 |
2005年 | 444篇 |
2004年 | 403篇 |
2003年 | 416篇 |
2002年 | 357篇 |
2001年 | 142篇 |
2000年 | 155篇 |
1999年 | 143篇 |
1998年 | 93篇 |
1997年 | 76篇 |
1996年 | 72篇 |
1995年 | 77篇 |
1994年 | 75篇 |
1993年 | 65篇 |
1992年 | 95篇 |
1991年 | 68篇 |
1990年 | 64篇 |
1989年 | 74篇 |
1988年 | 37篇 |
1987年 | 53篇 |
1986年 | 69篇 |
1985年 | 41篇 |
1984年 | 42篇 |
1983年 | 49篇 |
1982年 | 41篇 |
1981年 | 42篇 |
1980年 | 27篇 |
1979年 | 35篇 |
1978年 | 37篇 |
1977年 | 28篇 |
1975年 | 25篇 |
1974年 | 21篇 |
1973年 | 23篇 |
排序方式: 共有9374条查询结果,搜索用时 15 毫秒
101.
Ulla Vogt Andersen 《Nordic Journal of Botany》1995,15(1):91-104
By the creation of Køge Bay Seaside Park in 1978 the opportunity was given to study the establishment of vegetation and primary succession in a man-made coastal area. As the soil consists of marine material from the Baltic Sea, no organic matter or seedbank was present. The first steps of primary succession were followed in 1980, when the initial inventories of flora immigration and soil development were carried out in coastal grasslands and plantings. The surveys were repeated in 1992 and 1993. Except for the planted woody species and a few sown grasses, all other species of plants have reached the area through natural dispersal of diaspores. The total number of species in the permanent plots has increased from 26 in 1980 to 91 in 1993. The results indicate that this number will continue to increase in the coming years until a certain level. Then it will probably decrease as a result of competition from woody species, unless the vegetation is kept in a steady state by disturbances or management. Today the area is very far from the initial situation, and the off-shore barrier has changed towards a landscape dominated by small groves and grasslands of an urban common type. 相似文献
102.
Uracil uptake in Escherichia coli K-12: isolation of uraA mutants and cloning of the gene. 总被引:3,自引:2,他引:1 下载免费PDF全文
Mutants defective in utilization of uracil at low concentrations have been isolated and characterized. The mutations in question (uraA) map close to the upp gene encoding uracil phosphoribosyltransferase. By complementation analysis, a plasmid that complements the uraA mutation has been isolated. The uraA gene was shown to be the second gene in a bicistronic operon with upp as the promoter proximal gene. The nucleotide sequence of the gene was determined, and the gene encodes a hydrophobic membrane protein with a calculated Mr of 45,030. The UraA protein has been identified in sodium dodecyl sulfate-polyacrylamide gels in the membrane fraction of minicells harboring the uraA plasmids. 相似文献
103.
Mikkel Nissum Jens-Jakob Karlsson Jens Ulstrup Palle Waage Jensen G. Smulevich 《Journal of biological inorganic chemistry》1997,2(3):302-307
Di-heme Pseudomonas stutzeri cytochrome c
4 has been characterized by electronic absorption and resonance Raman spectroscopies in the ferric and ferrous forms at pH 7.5
and at room temperature. The data indicate that the two hemes are inequivalent. It is proposed that the N-terminal contains
a more relaxed heme as a consequence of the relative orientation of the methionine and histidine ligands with respect to the
N-Fe-N directions of the heme plane. This causes a weakening of the Fe-S bond with concomitant partial dissociation of the
methionine and the formation of an Fe-aquo bond. Heme group relaxation is further accompanied by less distortion of the heme
group than that associated with cytochrome c, expansion of the "core" and a negative shift of the redox potential.
Received: 17 December 1996 / Accepted: 6 March 1997 相似文献
104.
Small-subunit ribosomal RNA nucleotide sequences were inferred for Giraudyopsis stellifera Dangeard (Chrysomeridales), as well as for Pulvinaria sp. and Sarcinochrysis marina Geitler (Sarcinochrysidales,). Phylogenetic analyses of the molecular data indicate that the former is weakly related to the Phaeophyceae/Xanthophyceae clade, whereas the latter two have affinities to the Pelagophyceae, and the Sarcinochrysidales sensu stricto is transferred to this class. A recent study proposed that the Pelagophyceae belongs to a larger assemblage of chromophytic species characterized by reduced flagellar apparatuses. Although the flagellar apparatus characterizing the Sarcinochrysidales is reduced relative to the Chysomeridaels and some other chromophytes, it is the most complicated to be associated with “the reduced flagellar apparatus” lineage. Cladistic analyses of a traditional data set (largely ultrastructural features of the flagellar apparatus) and a combined traditional/molecular data set were used to assess the evolutionary trends of reduction in the flagellar apparatus within the heterokont chromophytes. 相似文献
105.
Susanne J. Andersen 《Antonie van Leeuwenhoek》1995,68(2):165-171
A large number ofPenicillium nalgiovense isolates from mould fermented sausages and the ex type culture were examined for characters of morphology, physiology and production of secondary metabolites. To separate biotypes within theP. nalgiovense species, the data obtained were evaluated using multivariate statistical methods. The macromorphological characters of the ex type culture and isolates from meat products appeared to be distinctive. The ex type culture is characterized by a brown reverse on both Czapek yeast extract and malt extract agar while the isolates from meat products have a yellow to orange reverse. Proteolytic and/or lipolytic activity was demonstrated by 75% of the examined cultures and all of them demonstrated ability to utilize lactate as sole carbon source. Growth on creatine sucrose agar was very inhibited and acid production was absent or very weak. TLC analysis showed production of three unknown secondary metabolites that constituted the characteristic profile. HPLC analysis showed production of only three known secondary metabolites; chrysogine (96%), nalgiolaxin and nalgiovensin (9%). The ex type culture produced nalgiolaxin and nalgiovensin but not chrysogine. The chemometric evaluation showed thatP. nalgiovense isolates from meat products from a homogenous species, which can not be divided into biotypes. The only indication of grouping, beside a separation of the ex type culture, was related to the conidium colour (white, turquoise or grey green). The examinedP. nalgiovense isolates showed some resemblance (morphologically and chemically) toP. chrysogenum. 相似文献
106.
Wolf-Dietrich Hard Jens M. Warnecke Roland K. Hartmann 《Molecular biology reports》1995,22(2-3):161-169
Modification interference is a powerful method to identify important functional groups in RNA molecules. We review here recent developments of techniques to screen for chemical modifications that interfere with (i) binding of(pre-)tRNA to bacterial RNase P RNA or (ii) pre-tRNA cleavage by this ribozyme. For example, two studies have analyzed positions at which a substitution of sulfur for thepro-Rp oxygen affects tRNA binding [1] or catalysis [2]. The results emphasize the functional key role of a central core element present in all known RNase P RNA subunits. The four sulfur substitutions identified in one study [2] to inhibit the catalytic step also interfered with binding of tRNA toE. coli RNase P RNA [1]. This suggests that losses in binding energy due to the modification at these positions affect the enzyme-substrate and the enzyme-transition state complex. In addition, the two studies have revealed, for the first time, sites of direct metal ion coordination in RNase P RNA. The potentials, limitations and interpretational ambiguities of modification interference experiments as well as factors influencing their outcome are discussed.Abbreviations nt
nucleotide(s)
- PAGE
polyacrylamide gel electrophoresis 相似文献
107.
Jens Meyer Norbert Elsner 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1995,176(4):563-573
1. | Up to 9 kHz, the tympanal membrane of the grasshopper Chorthippus biguttulus responds with equal sensitivity at the attachment sites of the low and the high-frequency receptors; at the latter site it is also particularly sensitive between 10 and 20 kHz. |
2. | The frequency spectra of the songs of both sexes exhibit maxima at 7–8 kHz, to which the membrane is well matched. In the high-frequency region, where the male songs have a peak at 30 kHz, there is no corresponding maximum in the membrane oscillation. |
3. | Because the tympanal membrane is immediately adjacent to air sacs in the tracheal system, it is deflected inward and outward by as much as 80 m during the respiratory cycle. |
4. | Measurements by laser vibrometry show that acoustically induced membrane oscillations are attenuated severely due to the respiratory displacement of the membrane for frequencies up to 10–12 kHz. By contrast, at higher frequencies the membrane sensitivity is doubled or tripled. |
5. | As a result of these membrane effects, the discharge in the tympanal nerve was profoundly reduced in the low-frequency range, whereas above 11 kHz there was a marked increase. This modulation of auditory sensitivity affects the animals' ability to detect conspecific songs. |
108.
109.
Sara S. Tynan Niels H. Andersen Max T. Wills Laurence A. Harker Stephen R. Hanson 《Prostaglandins & other lipid mediators》1984,27(5):683-696
The ω-chain variant analogs of prostacyclin (PGI2) and PGD2 in which the n-amyl side-chain has been replaced by a cyclohexyl group have been prepared and their cardiovascular activities have been compared to those of BW-245C(Fig. 1)(1) a potent anti-aggregatory vasodilator bearing a cyclohexyl-terminated side-chain on a hydantoin skeleton. The cyclohexyl group has little effect on PGI2, but converts PGD2 to a long lasting hypotensive agent and increases the platelet anti-aggregatory potency of PGD2 by a factor of 8. The prostaglandin antagonist N-0164 selectively blocks the anti-aggregatory actions of PGD2, cyclohexyl-PGD2, and BW-245C; with essentially no effect on PGI2, cyclohexyl-PGI2 and PGE2 at comparably effective doses. The latter observation is contrary to an earlier report by MacIntyre (2,3), but supports the view that the anti-aggregatory effect of high doses of PGE2 () is mediated by the PGI2 receptor (4). The hydantoin acts at the platelet PGD2 receptor. 相似文献
110.
Platelet-derived growth factor stimulates chemotaxis and nucleic acid synthesis in the protozoan Tetrahymena 总被引:2,自引:0,他引:2
Platelet-derived growth factor (PDGF) is in concentrations of a few nanograms per ml a very active chemoattractant for the free-living ciliated protozoan Tetrahymena; at the same time it induces a rapid increase in incorporation of radioactive nucleic acid precursors into RNA and DNA. We find it remarkable that this lower eukaryote responds to platelet-derived growth factor in very much the same way as fibroblastic cells. 相似文献