全文获取类型
收费全文 | 2852篇 |
免费 | 240篇 |
国内免费 | 5篇 |
专业分类
3097篇 |
出版年
2023年 | 9篇 |
2022年 | 38篇 |
2021年 | 87篇 |
2020年 | 47篇 |
2019年 | 67篇 |
2018年 | 78篇 |
2017年 | 87篇 |
2016年 | 90篇 |
2015年 | 157篇 |
2014年 | 158篇 |
2013年 | 217篇 |
2012年 | 202篇 |
2011年 | 213篇 |
2010年 | 134篇 |
2009年 | 132篇 |
2008年 | 180篇 |
2007年 | 221篇 |
2006年 | 191篇 |
2005年 | 160篇 |
2004年 | 155篇 |
2003年 | 123篇 |
2002年 | 134篇 |
2001年 | 22篇 |
2000年 | 13篇 |
1999年 | 13篇 |
1998年 | 25篇 |
1997年 | 19篇 |
1996年 | 10篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 13篇 |
1992年 | 9篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1983年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1971年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1966年 | 3篇 |
1965年 | 4篇 |
排序方式: 共有3097条查询结果,搜索用时 15 毫秒
11.
G P Akulov Iu L Kaminski? A D Shestakov V G Kaiumov L F Chernysheva 《Bioorganicheskaia khimiia》1988,14(1):37-42
Chemical and enzymatic syntheses of [5'-3H]adenosine, [5'-3H]guanosine, and [5'-3H]uridine have been developed. The reduction of beta-D-ribo-pentadialdo-1,4-furanosyl derivatives of corresponding bases is used in the chemical synthesis. The maximum molar activity of the labelled products was 220 TBk/mol in reactions with [3H]NaBH4 and 370-740 TBk/mol in reactions with gaseous tritium. The enzymatic synthesis was performed by the rebosylation of heterocyclic bases with nucleoside phosphorylase and [5'-3H]uridine as a ribosyl donor. Nucleoside phosphorylase is proposed to be used in the immobilized form to avoid the decrease of molar activity. Nucleosides labelled with tritium both in ribosyl and heterocyclic moieties were synthesised enzymatically. 相似文献
12.
13.
Annelie Eklund Sarron Randall‐Demllo Sergey Shabala Nuri Guven Anthony L Cook Rajaraman D Eri 《Cell biochemistry and function》2013,31(7):603-611
Endoplasmic reticulum (ER) stress and oxidative stress have recently been linked to the pathogenesis of inflammatory bowel diseases. Under physiological conditions, intestinal epithelial cells are exposed to ER and oxidative stress affecting the cellular ionic homeostasis. However, these altered ion flux ‘signatures’ during these stress conditions are poorly characterized. We investigated the kinetics of K+, Ca2+ and H+ ion fluxes during ER and oxidative stress in a colonic epithelial cell line LS174T using a non‐invasive microelectrode ion flux estimation technique. ER and oxidative stress were induced by cell exposure to tunicamycin (TM) and copper ascorbate (CuAsc), respectively, from 1 to 24 h. Dramatic K+ efflux was observed following acute ER stress with peak K+ efflux being ?30·6 and ?138·7 nmolm?2 s?1 for 10 and 50 µg ml?1, respectively (p < 0·01). TM‐dependent Ca2+ uptake was more prolonged with peak values of 0·85 and 2·68 nmol m?2 s?1 for 10 and 50 µg ml?1 TM, respectively (p < 0·02). Ion homeostasis was also affected by the duration of ER stress. Increased duration of TM treatment from 0 to 18 h led to increases in both K+ efflux and Ca2+ uptake. While K+ changes were significantly higher at each time point tested, Ca2+ uptake was significantly higher only after prolonged treatment (18 h). CuAsc also led to an increased K+ efflux and Ca2+ uptake. Functional assays to investigate the effect of inhibiting K+ efflux with tetraethylammonium resulted in increased cell viability. We conclude that ER/oxidative stress in colonic epithelial cells cause dramatic K+, Ca2+ and H+ ion flux changes, which may predispose this lineage to poor stress recovery reminiscent of that seen in inflammatory bowel diseases. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
14.
We describe the enantioseparation of functionalized derivatives of the Tr?ger base by HPLC on commercially available chiral stationary phases. Cellulose-derived Chiralcel OJ and brush-type Whelk O1 are demonstrated to be complementary to each other in their scope. On the basis of the results obtained, the separation of selected compounds was successfully transferred onto semipreparative columns. We believe that the availability of enantiopure functionalized derivatives of the Tr?ger base will stimulate the further use of this interesting molecular scaffold in molecular recognition, asymmetric catalysis, and related areas of research and technology. 相似文献
15.
Bioremediation of oil polluted aquatic systems and soils with novel preparation `Rhoder' 总被引:4,自引:0,他引:4
This paper summarises the experience accumulated duringthe field application of biopreparation `Rhoder' (solely or in a combinationwith preliminary mechanical collection of free oil) for remediation of oil polluted aquatic systems and soils in the Moscow region and Western Siberia during 1994–1999.It was demonstrated that `Rhoder' had a very high efficiency (>99%) for bioremediation of the open aquatic surfaces (100 m2 bay of the River Chernaya, two 5,000 m2 lakes in Vyngayakha) at initial level of oil pollution of 0.4–19.1 g/l. During remediation of the wetland (2,000 m2) in Urai (initial level of oil pollution of 10.5 g/l), a preliminary mechanical collection of oil was applied (75% removal) followed by a triple treatment with `Rhoder'. It resulted in an overall treatment efficiency of 94%. Relatively inferior results of bioremediation of the 10,000 m2 wetland in Vyngayakha (65% removal) and the 1,000 m2 marshy peat soil in Nizhnevartovsk (19% removal) can be attributed to the very high initial level of oil pollution (24.3 g/l and >750 g/g dry matter, respectively) aggravated by the fact that it was impossible to apply a preliminary mechanical collection of oil on these sites. A possible strategy for remediation of such heavily polluted sitesis discussed. 相似文献
16.
17.
MT1-MMP, a prototypic member of a membrane-type metalloproteinase subfamily, is an invasion promoting protease and an activator of MMP-2. In addition, MT1-MMP proteolysis regulates the functionality of cell-surface adhesion/signaling receptors including tissue transglutaminase (tTG). tTG is known to serve as an adhesion coreceptor for beta1/beta3 integrins and as an enzyme that catalyzes the cross-linking of proteins and the conjugation of polyamines to proteins. Here, we report that MMP-2, functioning in concert with MT1-MMP, hydrolyzes cell-surface-associated tTG, thereby further promoting the effect initiated by the activator of MMP-2. tTG, in return, preferentially associates with the activation intermediate of MMP-2. This event decreases the rate of MMP-2 maturation and protects tTG against proteolysis by MMP-2. Our cell culture, in vitro experiments, and in silico modeling indicate that the catalytic domain of MMP-2 directly associates with the core enzymatic domain II of tTG (the K(d) = 380 nM). The follow-up cleavage of the domain II eliminates both the receptor and the enzymatic activity of tTG. Our data illuminate the coordinated interplay involving the MT1-MMP/MMP-2 protease tandem in the regulation of the cell receptors and explain the underlying biochemical mechanisms of the extensive tTG proteolysis that exists at the normal tissue/tumor boundary. Our findings also suggest that neoplasms, which express functionally active MT1-MMP and, therefore, activate soluble MMP-2, can contribute to the degradation of tTG expressed in neighboring host cells. The loss of adhesive and enzymatic activities of tTG at the interface between tumor and normal tissue will decrease cell-matrix interactions and inhibit matrix cross-linking, causing multiple pathological alterations in host cell adhesion and locomotion. 相似文献
18.
19.
Plants,microorganisms, and soil temperatures contribute to a decrease in methane fluxes on a drained Arctic floodplain 下载免费PDF全文
Min Jung Kwon Felix Beulig Iulia Ilie Marcus Wildner Kirsten Küsel Lutz Merbold Miguel D. Mahecha Nikita Zimov Sergey A. Zimov Martin Heimann Edward A. G. Schuur Joel E. Kostka Olaf Kolle Ines Hilke Mathias Göckede 《Global Change Biology》2017,23(6):2396-2412
As surface temperatures are expected to rise in the future, ice‐rich permafrost may thaw, altering soil topography and hydrology and creating a mosaic of wet and dry soil surfaces in the Arctic. Arctic wetlands are large sources of CH4, and investigating effects of soil hydrology on CH4 fluxes is of great importance for predicting ecosystem feedback in response to climate change. In this study, we investigate how a decade‐long drying manipulation on an Arctic floodplain influences CH4‐associated microorganisms, soil thermal regimes, and plant communities. Moreover, we examine how these drainage‐induced changes may then modify CH4 fluxes in the growing and nongrowing seasons. This study shows that drainage substantially lowered the abundance of methanogens along with methanotrophic bacteria, which may have reduced CH4 cycling. Soil temperatures of the drained areas were lower in deep, anoxic soil layers (below 30 cm), but higher in oxic topsoil layers (0–15 cm) compared to the control wet areas. This pattern of soil temperatures may have reduced the rates of methanogenesis while elevating those of CH4 oxidation, thereby decreasing net CH4 fluxes. The abundance of Eriophorum angustifolium, an aerenchymatous plant species, diminished significantly in the drained areas. Due to this decrease, a higher fraction of CH4 was alternatively emitted to the atmosphere by diffusion, possibly increasing the potential for CH4 oxidation and leading to a decrease in net CH4 fluxes compared to a control site. Drainage lowered CH4 fluxes by a factor of 20 during the growing season, with postdrainage changes in microbial communities, soil temperatures, and plant communities also contributing to this reduction. In contrast, we observed CH4 emissions increased by 10% in the drained areas during the nongrowing season, although this difference was insignificant given the small magnitudes of fluxes. This study showed that long‐term drainage considerably reduced CH4 fluxes through modified ecosystem properties. 相似文献
20.