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   372篇
  免费   25篇
  2023年   4篇
  2022年   9篇
  2021年   12篇
  2020年   16篇
  2019年   14篇
  2018年   7篇
  2017年   18篇
  2016年   19篇
  2015年   14篇
  2014年   22篇
  2013年   22篇
  2012年   37篇
  2011年   24篇
  2010年   17篇
  2009年   17篇
  2008年   20篇
  2007年   13篇
  2006年   18篇
  2005年   19篇
  2004年   8篇
  2003年   12篇
  2002年   7篇
  2001年   11篇
  2000年   7篇
  1999年   4篇
  1998年   2篇
  1997年   1篇
  1996年   3篇
  1992年   4篇
  1991年   1篇
  1989年   1篇
  1986年   1篇
  1985年   1篇
  1981年   3篇
  1979年   3篇
  1978年   1篇
  1975年   1篇
  1974年   1篇
  1973年   3篇
排序方式: 共有397条查询结果,搜索用时 31 毫秒
101.

Background  

The forkhead box/winged helix family members FOXA1, FOXA2, and FOXA3 are of high importance in development and specification of the hepatic linage and the continued expression of liver-specific genes.  相似文献   
102.
Recently, greenhouse gas emissions have been of great concern globally. Ruminant livestock due to production of methane during normal fermentation in the rumen contributes substantially to the greenhouse effects. During the recent decade, a paradigm shift has been initiated whether plant secondary metabolites (PSM) could be exploited as natural safe feed additives alternative to chemical additives to inhibit enteric methanogenesis. More than 200,000 defined structures of PSM have been known. Some plants or their extracts with high concentrations of bioactive PSM such as saponins, tannins, essential oils, organosulphur compounds, flavonoids and many other metabolites appear to have potential to inhibit methane production in the rumen. The possible mechanisms and effects of many PSM on rumen methanogenesis are not clearly understood. Saponins may decrease methanogenesis through the inhibition of rumen protozoa and in turn may suppress the numbers and activity of methanogens. Although the direct effect of saponins on methanogens has not been demonstrated, saponins might inhibit methanogens at high doses. Tannins may inhibit the methanogenesis directly and also via inhibition of protozoal growth. Essential oils, organosulphur compounds and flavonoids appear to have direct effects against methanogens, and a reduction of protozoa associated methanogenesis probably plays a minor role for these metabolites. The chemical structure and molecular weight of the PSM and chemical composition of diets dependent upon the different feeding regimes may influence the effects of PSM on methane production. Although PSM may negatively affect nutrient utilization, there is evidence that methanogenesis could be suppressed without adversely affecting rumen fermentation, which could be exploited to mitigate methane emission in ruminants.  相似文献   
103.
Immunomodulatory effect of ethanolic extract (50%) of M. oleifera leaves (MOE) has been studied in normal and immunosuppressed mice models. Different doses of MOE i.e. 125, 250 and 500 mg/kg body weight of mice were administered orally for 15 days. Cyclophosphamide at a dose of 30 mg/kg body weight was administered orally for the next 3 days. On day 16 and 19, hematological parameters like white blood cell (WBC) count, red blood cell (RBC) count, haemoglobin level (Hb), percent neutrophils and organ weight were recorded. Effect of MOE on phagocytic activity of mice macrophages was determined by carbon clearance test. MOE showed significant dose dependent increase in WBC, percent neutrophils, weight of thymus and spleen along with phagocytic index in normal and immunosuppressed mice. The results indicate that MOE significantly reduced cyclophosphamide induced immunosuppression by stimulating both cellular and humoral immunity.  相似文献   
104.
Metalloreceptors containing ruthenium(II) bipyridine unit as fluorophore and pendant macrocyclic units as ionophore have been synthesized and their luminescence and electrochemical properties have been investigated. Ion-binding study of these fluoroionophore with the anions F, Cl, Br, I, , , , , CH3COO, and and cations Na+, K+, Mg2+, Ca2+, Zn2+, Ba2+, Sr2+ Cd2+, Hg2+, Pb2+ and Cu2+, monitored by luminescence and 1H NMR spectral changes, reveal strong interactions of and F for 2 and 3 and of Cu2+ only for 3. Luminescence titrations for 2 and 3 have been carried out to determine binding constants (Ks), and the calculated values are in the range 2.85 × 102 to 4.48 × 104 M−1. The 1H NMR spectral changes for 2 and 3 with the addition of increasing concentration of F and exhibit substantial low-field shift of the CONH proton indicating its involvement in complex formation with the anions. The adduct of 2 and 3 have been isolated and characterized by 1H and 31P NMR, mass and IR spectroscopy. The results are discussed in light of selectivity, structures of the anion bound complexes and their luminescence property.  相似文献   
105.
Management by combined grazing and mowing events is commonly used in grasslands, which influences the activity and composition of soil bacterial communities. Whether observed effects are mediated by management-induced disturbances, or indirectly by changes in the identity of major plant species, is still unknown. To address this issue, we quantified substrate-induced respiration (SIR), and the nitrification, denitrification and free-living N(2)-fixation enzyme activities below grass tufts of three major plant species (Holcus lanatus, Arrhenatherum elatius and Dactylis glomerata) in extensively or intensively managed grasslands. The genetic structures of eubacterial, ammonia oxidizing, nitrate reducing, and free-living N(2)-fixing communities were also characterized by ribosomal intergenic spacer analysis, and denaturing gradient gel electrophoresis (DGGE) or restriction fragment length polymorphism (RFLP) targeting group-specific genes. SIR was not influenced by management and plant species, whereas denitrification enzyme activity was influenced only by plant species, and management-plant species interactions were observed for fixation and nitrification enzyme activities. Changes in nitrification enzyme activity were likely largely explained by the observed changes in ammonium concentration, whereas N availability was not a major factor explaining changes in denitrification and fixation enzyme activities. The structures of eubacterial and free-living N(2)-fixing communities were essentially controlled by management, whereas the diversity of nitrate reducers and ammonia oxidizers depended on both management and plant species. For each functional group, changes in enzyme activity were not correlated or were weakly correlated to overall changes in genetic structure, but around 60% of activity variance was correlated to changes in five RFLP or DGGE bands. Although our conclusions should be tested for other ecosystems and seasons, these results show that predicting microbial changes induced by management in grasslands requires consideration of management-plant species interactions.  相似文献   
106.
We report the first use of inorganic fluorescent lanthanide (europium and terbium) ortho phosphate [LnPO4·H2O, Ln = Eu and Tb] nanorods as a novel fluorescent label in cell biology. These nanorods, synthesized by the microwave technique, retain their fluorescent properties after internalization into human umbilical vein endothelial cells (HUVEC), 786-O cells, or renal carcinoma cells (RCC). The cellular internalization of these nanorods and their fluorescence properties were characterized by fluorescence spectroscopy (FS), differential interference contrast (DIC) microscopy, confocal microscopy, and transmission electron microscopy (TEM). At concentrations up to 50 μg/ml, the use of [3H]-thymidine incorporation assays, apoptosis assays (TUNEL), and trypan blue exclusion illustrated the non-toxic nature of these nanorods, a major advantage over traditional organic dyes  相似文献   
107.
Cationic lipid membranes are known to form compact complexes with DNA and to be effective as gene delivery agents both in vitro and in vivo. Here we employ molecular dynamics simulations for a detailed atomistic study of lipid bilayers consisting of a mixture of cationic dimyristoyltrimethylammonium propane (DMTAP) and zwitterionic dimyristoylphosphatidylcholine (DMPC). Our main objective is to examine how the composition of the DMPC/DMTAP bilayers affects their structural and electrostatic properties in the liquid-crystalline phase. By varying the mole fraction of DMTAP, we have found that the area per lipid has a pronounced nonmonotonic dependence on the DMTAP concentration, with a minimum around the point of equimolar DMPC/DMTAP mixture. We show that this behavior has an electrostatic origin and is driven by the interplay between positively charged TAP headgroups and the zwitterionic phosphatidylcholine (PC) heads. This interplay leads to considerable reorientation of PC headgroups for an increasing DMTAP concentration, and gives rise to major changes in the electrostatic properties of the lipid bilayer, including a significant increase of total dipole potential across the bilayer and prominent changes in the ordering of water in the vicinity of the membrane. Moreover, chloride counterions are bound mostly to PC nitrogens implying stronger screening of PC heads by Cl ions compared to TAP headgroups. The implications of these findings are briefly discussed.  相似文献   
108.
Induction and persistence of adaptive response by aluminium (Al), 1 or 10 microM, and paraquat (PQ), 5 or 10 microM, against genotoxicity of methyl mercuric chloride (MMCl), 1.26 microM, a standard environmental genotoxin, was investigated in root meristem cells of Allium cepa. Subsequently, three metabolic inhibitors, namely, 3-aminobezamide (3-AB, 10 or 100 microM), an inhibitor of poly(ADP-ribose) polymerase (PARP) implicated in DNA repair and/or apoptosis, cycloheximide (CH, 0.1 or 1 microM), an inhibitor of protein synthesis, and buthionine sulfoximine (BSO, 100 microM or 1mM), an inhibitor of glutathione synthesis were tested for their ability to prevent the adaptive response induced by conditioning doses of Al, 10 or 100 microM; and PQ, 5 or 100 microM, against MMCl-challenge, 1.26 or 100 microM, in root meristems of A. cepa or embryonic shoots of Hordeum vulgare, respectively. The findings demonstrated that once triggered, the Al- or PQ-adaptive response to MMCl could persist for at least 48h in root meristems of A. cepa. Furthermore, the adaptive response could effectively be prevented by 3-AB, to a lesser degree by CH, and the least by BSO, suggesting primarily the involvement of PARP and implicating DNA repair in the underlying mechanisms of adaptive response in plant cells in vivo.  相似文献   
109.
The synthesis of isoxazole-based derivatives utilizing Baylis-Hillman chemistry and results of their preliminary bioevaluation as hypolipidemic agents in triton model are described.  相似文献   
110.
Probiotics and Antimicrobial Proteins - The aim of this study was to determine the effects of early-life bovine lactoferrin and host specific probiotic interventions on growth performance,...  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