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   764篇
  免费   57篇
  国内免费   1篇
  2023年   3篇
  2022年   12篇
  2021年   24篇
  2020年   15篇
  2019年   17篇
  2018年   25篇
  2017年   27篇
  2016年   26篇
  2015年   35篇
  2014年   42篇
  2013年   50篇
  2012年   77篇
  2011年   54篇
  2010年   26篇
  2009年   21篇
  2008年   36篇
  2007年   31篇
  2006年   33篇
  2005年   34篇
  2004年   32篇
  2003年   11篇
  2002年   23篇
  2001年   11篇
  2000年   5篇
  1999年   7篇
  1998年   7篇
  1997年   5篇
  1996年   11篇
  1995年   9篇
  1994年   7篇
  1993年   4篇
  1992年   12篇
  1991年   9篇
  1990年   2篇
  1989年   12篇
  1988年   9篇
  1987年   5篇
  1986年   4篇
  1985年   2篇
  1984年   3篇
  1983年   2篇
  1982年   3篇
  1981年   3篇
  1980年   6篇
  1977年   6篇
  1974年   4篇
  1973年   3篇
  1972年   2篇
  1971年   3篇
  1970年   3篇
排序方式: 共有822条查询结果,搜索用时 15 毫秒
711.
Fly ash was characterized for the leaching potential of some major and minor constituents and then added to soil at 20, 40, 60 and 80 t/ha with N and P fertilizer to evaluate its effect on nutrient uptake and soybean yield singly as well as in combination with an efficient phosphate solubilizer Pseudomonas striata. The application of fly ash at 40 t/ha in conjunction with P. striata inoculation improved the bean yield and P uptake by grain. The available phosphorus of soil also showed an upward trend. The fly ash did not exert any detrimental effect on the population of inoculated bacteria. However, the uptake of trace elements did not improve significantly.  相似文献   
712.
Fly ash was co-composted with wheat straw and 2% rock phosphate (w/w) for 90 days and different chemical and microbiological parameters monitored to evaluate its effect on the composting process. Fly ash addition at 20% level resulted in the lowest C/N of 16.4:1 and highest available and total phosphorus. Increasing the addition of fly ash from 40 to 60% (w/w) did not exert any detrimental effect on either C:N or the microbial population.  相似文献   
713.
Helicases are enzymes that use energy from nucleoside triphosphate hydrolysis to unwind double-stranded (ds) DNA, a process vital to virtually every phase of DNA metabolism. Helicases have been classified as either 5'-to-3' or 3'-to-5' on the basis of their ability to unwind duplex DNA adjacent to either a 5' or 3' single-stranded (ss) DNA overhang. However, there has been debate as to whether this substrate preference is indicative of unidirectional translocation on ssDNA. We developed an assay that monitors the ability of a helicase to displace streptavidin from biotinylated oligonucleotides [Morris, P. D., and Raney, K. D. (1999) Biochemistry 38, 5164-5171]. Two helicases identified as having 5'-to-3' polarity displaced streptavidin from the 3'-end of biotinylated oligonucleotides but not from the 5'-end. We performed similar experiments using the 3'-to-5' helicases from the hepatitis C virus (NS3) and SV40 virus (SV40 T antigen). NS3 and SV40 T antigen were able to displace streptavidin from a 5'-biotinylated oligonucleotide but not from a 3'-biotinylated oligonucleotide. NS3 and SV40 T antigen enhanced the spontaneous rate of dissociation of streptavidin from biotin 340-fold and 1700-fold, respectively. The ssDNA binding protein, gp32, did not enhance dissociation of streptavidin from either end of an oligonucleotide. For NS3, the rate of displacement was faster from a 5'-biotinylated 60mer than from a 5'-biotinylated 30mer. The strong directional bias in streptavidin displacement activity exhibited by each helicase is consistent with a directional bias in translocation on ssDNA. The dependence of the reaction with NS3 on the oligonucleotide length suggests that multiple NS3 monomers are necessary for optimal activity.  相似文献   
714.
Evidence is now in favor of protein-facilitated mechanisms for the intestinal cholesterol absorption. Here we report that the unesterified cholesterol uptake by rat jejunal brush border membrane vesicles (BBMVs) is efficient, saturable, and protein-mediated. The human apolipoproteins biliary anionic peptide factor (APF) and A-I (apoA-I) up-regulate micellar cholesterol uptake in a dose-dependent manner, but for all tested concentrations (0.1-20 microM), the lipid-free APF was more efficient than apoA-I. This uptake stimulation was suppressed after addition of Pabs directed to the external lipid-binding domain of the CLA-1/SR-BI and reduced by Pabs directed to the external loop of CD36. Thus, CLA-1/SR-BI and to a lesser extent CD36 are involved in the regulation of intestinal cholesterol uptake. APF, the main protein bound to biliary lipids, is likely one of their physiological effectors. As APF is an unesterified cholesterol carrier, it could facilitate the intestinal absorption of biliary cholesterol.  相似文献   
715.
To determine whether liquid ventilation (LV) causes less cell injury and improves lung function compared with conventional gas ventilation (GV), we analyzed pulmonary physiological profiles, lung histology, and cell death in 110- and 120-day preterm lambs, which were randomized to receive either ventilation modality on FI(O(2)) = 1. LV lungs were well expanded with adequate pulmonary function, whereas GV animals exhibited marked atelectasis, poor pulmonary function, and increased mortality. Both ventilatory strategies induced marked lung cell apoptosis, but with distinct patterns of distribution. Although GV induced apoptosis of epithelium primarily in the lining and within the lumina of bronchioles, LV induced significant apoptosis much more homogeneously throughout lung parenchyma including alveoli and interstitial spaces. These studies suggest that although both forms of ventilation cause regional apoptosis, LV more effectively delivers oxygen and recruits the lung more homogeneously than GV.  相似文献   
716.
The present study was designed to investigate anti-ulcerogenic property of ethanolic extract of Desmodium gangeticum (DG) against cold restraint (CRU, 2 hr cold restraint stress), aspirin (ASP, 150 mg/kg orally), alcohol (AL, absolute alcohol 1 ml/200gm) and pyloric ligation (PL, 4 hr pylorus ligation) induced gastric ulcer models in Sprague Dawley rats, and histamine (HST, 0.25 mg/kg) induced duodenal ulcer in guinea pigs. We found that DG at a dose of 200mg/kg, (orally), markedly decreased the incidence of ulcers in all the above models. DG showed significant protection against CRU (68.37%), AL (88.87%), ASP (38.2%), PL (40.63%) and HST (63.15%) induced ulcer models, whereas standard drug omeprazole (OMZ) showed protection index of 83.86, 56.35, 70.31 and 84.21%, respectively in CRU, ASP, PL and HST models. Sucralfate as standard drug showed 92.64% protection in AL model. DG significantly reduced acid secretion 41.61%, whereas OMZ produced 43.13% reduction. Treatment with DG showed increase in mucin secretion by 56.17%, whereas OMZ showed 12.45% increase. Anti-ulcer effect of DG may be due to its cytoprotective effect along with antisecretory activity and could act as a potent therapeutic agent against peptic ulcer disease.  相似文献   
717.
718.
Rice is an excellent system for plant genomics as it represents a modest size genome of 430 Mb. It feeds more than half the population of the world. Draft sequences of the rice genome, derived by whole-genome shotgun approach at relatively low coverage (4-6 X), were published and the International Rice Genome Sequencing Project (IRGSP) declared high quality (> 10 X), genetically anchored, phase 2 level sequence in 2002. In addition, phase 3 level finished sequence of chromosomes 1, 4 and 10 (out of 12 chromosomes of rice) has already been reported by scientists from IRGSP consortium. Various estimates of genes in rice place the number at >50,000. Already, over 28,000 full-length cDNAs have been sequenced, most of which map to genetically anchored genome sequence. Such information is very useful in revealing novel features of macroand micro-level synteny of rice genome with other cereals. Microarray analysis is unraveling the identity of rice genes expressing in temporal and spatial manner and should help target candidate genes useful for improving traits of agronomic importance. Simultaneously, functional analysis of rice genome has been initiated by marker-based characterization of useful genes and employing functional knock-outs created by mutation or gene tagging. Integration of this enormous information is expected to catalyze tremendous activity on basic and applied aspects of rice genomics.  相似文献   
719.
A study of the effects of elevated levels of Cu2+ and Zn2+ on NO3- uptake and nitrate reductase (NR) activity in Scenedesmus sp. was carried out. The two metals inhibited NR and NO3- uptake in a concentration-dependent manner, with the latter process being inhibited more strongly than the former. After withdrawal of metal stress, NR activity and NO3- uptake recovered in a metal ion concentration-dependent manner. Dark pretreatment of the alga enhanced the toxic effects of the metal ions on NR activity and NO3- uptake. The recovery from metal stress was slower in the dark-pretreated cells in comparison to the light-pretreated cells. No recovery of NR and NO3- uptake occurred in the presence of the photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), suggesting that photosynthesis was required for the recovery from metal stress. Cycloheximide blocked the recovery of NR activity in metal-treated alga, suggesting that new enzyme synthesis was required for the recovery from metal stress.  相似文献   
720.
Earlier works on polyglycine I suffer from several infirmities, such as the dynamic methylene group being replaced by a mass unit and the use of poorly resolved inelastic neutron spectra, which have resulted in wrong assignments and imprecise profile of dispersion curves. In addition, the density-of-states and heat capacity variation as a function of temperature are being reported for the first time. The heat capacity is in good agreement with the measurements reported earlier by Roles and Wunderlich within a certain range (230-350 K). Deviations set in beyond this could be due to the presence of two crystalline states (I and II) in the sample used for the heat capacity measurements.  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