首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1082篇
  免费   104篇
  国内免费   3篇
  2021年   13篇
  2020年   9篇
  2019年   9篇
  2018年   20篇
  2017年   15篇
  2016年   29篇
  2015年   37篇
  2014年   41篇
  2013年   60篇
  2012年   60篇
  2011年   68篇
  2010年   38篇
  2009年   29篇
  2008年   51篇
  2007年   55篇
  2006年   56篇
  2005年   46篇
  2004年   45篇
  2003年   53篇
  2002年   45篇
  2001年   36篇
  2000年   36篇
  1999年   24篇
  1998年   18篇
  1997年   6篇
  1996年   18篇
  1995年   11篇
  1994年   4篇
  1993年   12篇
  1992年   26篇
  1991年   29篇
  1990年   14篇
  1989年   26篇
  1988年   13篇
  1987年   7篇
  1986年   16篇
  1985年   9篇
  1984年   9篇
  1983年   8篇
  1982年   9篇
  1981年   8篇
  1980年   7篇
  1979年   7篇
  1978年   6篇
  1974年   4篇
  1971年   4篇
  1969年   9篇
  1968年   4篇
  1967年   5篇
  1966年   4篇
排序方式: 共有1189条查询结果,搜索用时 203 毫秒
101.
The cause of differences in overwintering success between the sexes in Orius sauteri (Poppius) and O. minutus (Linnaeus) (Heteroptera: Anthocoridae) was investigated in the laboratory. The survival rate of adults was examined in a screen house outdoors during the winters of 1995–1996 and 1996–1997. None of the males of either species survived until spring in either year. However, 63.9% and 40.5% of the females in O. sauteri and O. minutus, respectively, survived the winter in 1995–1996, and 54.5% and 43.2%, respectively, in 1996–1997. Most of the males died by early winter. In both species, the adult females reared under short days (= L11:D13) survived for a long period at 5 or 0°C, while the males reared under the same photoperiod died shortly after transfer to 5 or 0°C from 22°C. The males and females kept at same temperature conditions under long days (= L16:D8) died early. When nymphs were kept under long days at 22°C, the lipid content in the female and male adults of O. sauteri was 27.9% and 17.7%, respectively. When nymphs were kept under short days, their lipid content was significantly higher (41.1%) than that of those reared under long days for females, but lipid content was comparable in the males regardless of photoperiod (15.6%). In O. minutus, the mean lipid content was 27.2% in females and 19.0% in males under long days at 22°C, and 40.8% in females and 19.8% in males under short days. Thus, a large amount of lipid was accumulated only in females kept under short days in both species. These results suggest that short days induced diapause only in adult females but not in males. Due to the lack of diapause and shortage of lipid accumulation, males may not be able to survive the winter.  相似文献   
102.
Newly synthesized membrane proteins are transported by fast axonal flow to their targets such as the plasma membrane and synaptic vesicles. However, their transporting vesicles have not yet been identified. We have successfully visualized the transporting vesicles of plasma membrane proteins, synaptic vesicle proteins, and the trans-Golgi network residual proteins in living axons at high resolution using laser scan microscopy of green fluorescent protein-tagged proteins after photobleaching. We found that all of these proteins are transported by tubulovesicular organelles of various sizes and shapes that circulate within axons from branch to branch and switch the direction of movement. These organelles are distinct from the endosomal compartments and constitute a new entity of membrane organelles that mediate the transport of newly synthesized proteins from the trans-Golgi network to the plasma membrane.  相似文献   
103.
104.
Cell suspensions derived from Acer nikoense callus, not containing (S)-rhododendrol, converted 4-(p-hydroxyphenyl)-2- butanone into (R)-, (S)-rhododendrol and their glycosides. (R)- and (S)-rhododendrol formed was only detected in the culture medium and their glycosides only in the cells. The former compound disappeared within a short time and the latter one also tended to decrease during prolonged culture. Quantitative analysis of rhododendrol glycosides in the callus showed that most of them were (S)-rhododendrol 2-O--D-glucopyranoside and its content was much lower than that of the original plants. © Rapid Science Ltd. 1998  相似文献   
105.
Assimilation of nitrogen dioxide in response to fumigation with 15N-labelled nitrogen dioxide was studied in 217 plant taxa. The taxa included 50 wild herbaceous plants collected from roadsides (42 genera, 15 families), 60 cultivated herbaceous plants (55 genera, 30 families) and 107 cultivated woody plants (74 genera, 45 families). Two parameters, the 'NO2-N content', or NO2-derived reduced nitrogen content in fumigated plant leaves (mg N g–1 dry weight), and the 'NO2-utilization index', or percentage of the NO2-derived reduced nitrogen in the total reduced nitrogen, were determined. The NO2-N content differed 657-fold between the highest ( Eucalyptus viminalis ; 6·57) and lowest ( Tillandsia ionantha and T. caput-medusae ; 0·01) values in the 217 taxa; 62-fold in a family (Theaceae) and 26-fold in a species ( Solidago altissima ). Nine species had NO2-utilization indices greater than 10%, of which Magnolia kobus , Eucalyptus viminalis , Populus nigra , Nicotiana tabacum and Erechtites hieracifolia had NO2-N contents > 4·9. These plants can be considered 'NO2-philic' because in them NO2-nitrogen has an important function(s). The Compositae and Myrtaceae had high values for both parameters, whereas the monocots and gymnosperms had low ones. These findings suggest that the metabolic pathway of NO2-nitrogen differs among plant species. The information presented here will be useful for creating a novel vegetation technology to reduce the atmospheric concentration of nitrogen dioxide.  相似文献   
106.
The MLK (mixed lineage) ser/thr kinases are most closely related to the MAP kinase kinase kinase family. In addition to a kinase domain, MLK1, MLK2 and MLK3 each contain an SH3 domain, a leucine zipper domain and a potential Rac/Cdc42 GTPase-binding (CRIB) motif. The C-terminal regions of the proteins are essentially unrelated. Using yeast two-hybrid analysis and in vitro dot-blots, we show that MLK2 and MLK3 interact with the activated (GTP-bound) forms of Rac and Cdc42, with a slight preference for Rac. Transfection of MLK2 into COS cells leads to strong and constitutive activation of the JNK (c-Jun N-terminal kinase) MAP kinase cascade, but also to activation of ERK (extracellular signal-regulated kinase) and p38. When expressed in fibroblasts, MLK2 co-localizes with active, dually phosphorylated JNK1/2 to punctate structures along microtubules. In an attempt to identify proteins that affect the activity and localization of MLK2, we have screened a yeast two-hybrid cDNA library. MLK2 and MLK3 interact with members of the KIF3 family of kinesin superfamily motor proteins and with KAP3A, the putative targeting component of KIF3 motor complexes, suggesting a potential link between stress activation and motor protein function.  相似文献   
107.
IntroductionSeveral cytotoxic anticancer drugs inhibit DNA replication and/or mitosis, while EGFR tyrosine kinase inhibitors inactivate EGFR signalling in cancer cell. Both types of anticancer drugs improve the overall survival of the patients with non-small-cell lung cancer (NSCLC), although tumors often become refractory to this treatment. Despite several mechanisms by which the tumors become resistant having been described the effect of these compounds on anti-tumor immunity remains largely unknown.MethodsThis study examines the effect of the cytotoxic drug Gemcitabine and the EGFR tyrosine kinase inhibitor Gefitinib on the expression of NK group 2 member D (NKG2D) ligands as well as the sensitivity of NSCLC cells to the NK-mediated lysis.ResultsWe demonstrate that Gemcitabine treatment leads to an enhanced expression, while Gefitinib downregulated the expression of molecules that act as key ligands for the activating receptor NKG2D and promote NK cell-mediated recognition and cytolysis. Gemcitabine activated ATM and ATM- and Rad-3-related protein kinase (ATR) pathways. The Gemcitabine-induced phosphorylation of ATM as well as the upregulation of the NKG2D ligand expression could be blocked by an ATM-ATR inhibitor. In contrast, Gefitinib attenuated NKG2D ligand expression. Silencing EGFR using siRNA or addition of the PI3K inhibitor resulted in downregulation of NKG2D ligands. The observations suggest that the EGFR/PI3K pathway also regulates the expression of NKG2D ligands. Additionally, we showed that both ATM-ATR and EGFR regulate MICA/B via miR20a.ConclusionIn keeping with the effect on NKG2D expression, Gemcitabine enhanced NK cell-mediated cytotoxicity while Gefitinib attenuated NK cell killing in NSCLC cells.  相似文献   
108.
We assessed vaccine effectiveness (VE) against medically attended, laboratory-confirmed influenza in children 6 months to 15 years of age in 22 hospitals in Japan during the 2013–14 season. Our study was conducted according to a test-negative case-control design based on influenza rapid diagnostic test (IRDT) results. Outpatients who came to our clinics with a fever of 38°C or over and had undergone an IRDT were enrolled in this study. Patients with positive IRDT results were recorded as cases, and patients with negative results were recorded as controls. Between November 2013 and March 2014, a total of 4727 pediatric patients (6 months to 15 years of age) were enrolled: 876 were positive for influenza A, 66 for A(H1N1)pdm09 and in the other 810 the subtype was unknown; 1405 were positive for influenza B; and 2445 were negative for influenza. Overall VE was 46% (95% confidence interval [CI], 39–52). Adjusted VE against influenza A, influenza A(H1N1)pdm09, and influenza B was 63% (95% CI, 56–69), 77% (95% CI, 59–87), and 26% (95% CI, 14–36), respectively. Influenza vaccine was not effective against either influenza A or influenza B in infants 6 to 11 months of age. Two doses of influenza vaccine provided better protection against influenza A infection than a single dose did. VE against hospitalization influenza A infection was 76%. Influenza vaccine was effective against influenza A, especially against influenza A(H1N1)pdm09, but was much less effective against influenza B.  相似文献   
109.
The establishment of new approaches to control chewing insects has been sought not only for direct use in reducing crop loss but also in managing resistance to the pesticides already in use. Engineered formation of calcium oxalate crystals is a potential strategy that could be developed to fulfill both these needs. As a step toward this development, this study investigates the effects of transforming a non-calcium oxalate crystal accumulating plant, Arabidopsis thaliana, into a crystal accumulating plant. Calcium oxalate crystal accumulating A. thaliana lines were generated by ectopic expression of a single bacterial gene encoding an oxalic acid biosynthetic enzyme. Biochemical and cellular studies suggested that the engineered A. thaliana lines formed crystals of calcium oxalate in a manner similar to naturally occurring crystal accumulating plants. The amount of calcium oxalate accumulated in leaves also reached levels similar to those measured in the leaves of Medicago truncatula in which the crystals are known to play a defensive role. Visual inspection of the different engineered lines, however, suggested a phenotypic consequence on plant growth and development with higher calcium oxalate concentrations. The restoration of a near wild-type plant phenotype through an enzymatic reduction of tissue oxalate supported this observation. Overall, this study is a first to provide initial insight into the potential consequences of engineering calcium oxalate crystal formation in non-crystal accumulating plants.  相似文献   
110.
The achiral hydrocarbon tetraphenylethylene crystallizes in enantiomorphous forms (chiral space group: P21) to afford right- and left-handed hemihedral crystals, which can be recognized by solid-state circular dichroism spectroscopic analysis. Chiral organic crystals of tetraphenylethylene mediated enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde to give, in conjunction with asymmetric autocatalysis with amplification of chirality, almost enantiomerically pure (S)- and (R)-5-pyrimidyl alkanols whose absolute configurations were controlled efficiently by the crystalline chirality of the tetraphenylethylene substrate. Tetrakis(p-chlorophenyl)ethylene and tetrakis(p-bromophenyl)ethylene also show chirality in the crystalline state, which can also act as a chiral substrate and induce enantioselectivity of diisopropylzinc addition to pyrimidine-5-carbaldehyde in asymmetric autocatalysis to give enantiomerically enriched 5-pyrimidyl alkanols with the absolute configuration correlated with that of the chiral crystals. Highly enantioselective synthesis has been achieved using chiral crystals composed of achiral hydrocarbons, tetraphenylethylenes, as chiral inducers. This chemical system enables significant amplification of the amount of chirality using spontaneously formed chiral crystals of achiral organic compounds as the seed for the chirality of asymmetric autocatalysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号