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   141836篇
  免费   4850篇
  国内免费   839篇
  2022年   307篇
  2021年   773篇
  2020年   457篇
  2019年   576篇
  2018年   12391篇
  2017年   11179篇
  2016年   8396篇
  2015年   2351篇
  2014年   2234篇
  2013年   2842篇
  2012年   7392篇
  2011年   15919篇
  2010年   13950篇
  2009年   10007篇
  2008年   12332篇
  2007年   13975篇
  2006年   3005篇
  2005年   3138篇
  2004年   3476篇
  2003年   3411篇
  2002年   3072篇
  2001年   790篇
  2000年   604篇
  1999年   632篇
  1998年   738篇
  1997年   569篇
  1996年   478篇
  1995年   427篇
  1994年   417篇
  1993年   472篇
  1992年   497篇
  1991年   451篇
  1990年   413篇
  1989年   399篇
  1988年   382篇
  1987年   341篇
  1986年   350篇
  1985年   375篇
  1984年   463篇
  1983年   390篇
  1982年   480篇
  1981年   491篇
  1980年   391篇
  1979年   320篇
  1978年   335篇
  1977年   293篇
  1976年   328篇
  1974年   277篇
  1972年   436篇
  1971年   417篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Cerebrospinal Fluid Nitrite/Nitrate Levels in Neurologic Diseases   总被引:5,自引:0,他引:5  
Abstract: Nitric oxide has been proposed to mediate cytotoxic effects in inflammatory diseases. To investigate the possibility that overproduction of nitric oxide might play a role in the neuropathology of inflammatory and noninflammatory neurological diseases, we compared levels of the markers of nitric oxide, nitrite plus nitrate, in the CSF of controls with those in patients with various neurologic diseases, including Huntington's and Alzheimer's disease, amyotrophic lateral sclerosis, and HIV infection. We found that there were no significant increases in the CSF levels of these nitric oxide metabolites, even in patients infected with HIV or in monkeys infected with poliovirus, both of which have significantly elevated levels of the neurotoxin quinolinic acid and the marker of macrophage activation, neopterin. However, CSF quinolinic acid, neopterin, and nitrite/nitrate levels were significantly increased in a small group of patients with bacterial and viral meningitis.  相似文献   
992.
Abstract: Tyrosine hydroxylase activity was measured under optimal and suboptimal assay conditions in hippocampal extracts from young (2 month), mature (12 month), and old (24 month) Fischer 344 male rats 72 h after the infusion of 200 µg of the neurotoxin 6-hydroxydopamine or vehicle into the lateral ventricle. The lesion resulted in a 45–55% decrease of tyrosine hydroxylase activity measured under optimal conditions (pH 6.1, 3.0 m M 6-methyl-5,6,7,8-tetrahydropterin) and an ∼35% decrease in the relative concentration of immunoreactive tyrosine hydroxylase. When measured under suboptimal conditions (pH 6.6, 0.7 m M 6-methyl-5,6,7,8-tetrahydropterin), tyrosine hydroxylase activity in 2- and 12-month-old lesioned animals was twice that measured in vehicle-treated animals. However, in the old lesioned animals, tyrosine hydroxylase activity measured under suboptimal conditions was not different from that measured in age-matched vehicle-treated animals. Isoforms of tyrosine hydroxylase were identified on immunoblots after two-dimensional gel electrophoresis using enhanced chemiluminescence. The relative proportion of lower pl isoforms of tyrosine hydroxylase in the 2-month-old lesioned animals was greater than that observed in vehicle-treated controls. In contrast, no difference was seen in the relative proportion of tyrosine hydroxylase isoforms in the 24-month-old lesioned versus control animals. These data indicate that the ability of locus ceruleus neurons to rapidly respond to and compensate for insult is attenuated in 24-month-old Fischer 344 rats due to a deficit in stimulus-evoked enzyme phosphorylation.  相似文献   
993.
An NADH oxidase activity of animal and plant plasma membrane is described that is stimulated by hormones and growth factors. In plasma membranes of cancer cells and tissues, the activity appears to be constitutively activated and no longer hormone responsive. With drugs that inhibit the activity, cells are unable to grow although growth inhibition may be more related to a failure of the cells to enlarge than to a direct inhibition of mitosis. The hormone-stimulated activity in plasma membranes of plants and the constitutively activated NADH oxidase in tumor cell plasma membranes is inhibited by thiol reagents whereas the basal activity is not. These findings point to a thiol involvement in the action of the activated form of the oxidase. NADH oxidase oxidation by Golgi apparatus of rat liver is inhibited by brefeldin A plus GDP. Brefeldin A is a macrolide antibiotic inhibitor of membrane trafficking. A model is presented where the NADH oxidase functions as a thiol-disulfide oxidoreductase activity involved in the formation and breakage of disulfide bonds. The thiol-disulfide interchange is postulated as being associated with physical membrane displacement as encountered in cell enlargement or in vesicle budding. The model, although speculative, does provide a basis for further experimentation to probe a potential function for this enzyme system which, under certain conditions, exhibits a hormone- and growth factor-stimulated oxidation of NADH.  相似文献   
994.
995.
Cold tolerance of field grown plants and shoot cultures of a commercial sugarbeet cultivar, Hilma, was compared with that of two cultivars bred for improved cold tolerance, Monofeb and Winter Hybrid 88619. Leaves of Monofeb and Winter Hybrid 88619 showed an increase in frost tolerance compared to Hilma, as assessed by electrolyte leakage measurements, in both July, and November. However, all varieties exhibited acclimation in the latter month. Similar qualitative differences between cultivars were detected in shoot cultures only when maintained on low (1%) sucrose medium, without added plant growth regulators. The use of high (3%) sucrose and benzyladenine, which releases apical dominance producing multiple shoots, each contributed to a substantial lowering of the temperature at which cold-induced damage occurred in leaves. Under these conditions varietal differences were masked. The implications of these findings in regard to in vitro selection for improved cold tolerance in organized cultures are discussed.Abbreviations BA benzyladenine - MS Murashige & Skoog (1962)  相似文献   
996.
Soybean is believed to be a diploidized tetraploid generated from an allotetraploid ancestor. In this study, we used hypomethylated genomic DNA as a source of probes to investigate the genomic structure and methylation patterns of duplicated sequences. Forty-five genomic clones from Phaseolus vulgaris and 664 genomic clones from Glycine max were used to examine the duplicated regions in the soybean genome. Southern analysis of genomic DNA using probes from both sources revealed that greater than 15% of the hypomethylated genomic regions were only present once in the soybean genome. The remaining ca. 85% of the hypomethylated regions comprise duplicated or middle repetitive DNA sequences. If only the ratio of single to duplicate probe patterns is considered, it appears that 25% of the single-copy sequences have been lost. By using a subset of probes that only detected duplicated sequences, we examined the methylation status of the homeologous genomes with the restriction enzymes MspI and HpaII. We found that in all cases both copies of these regions were hypomethylated, although there were examples of low-level methylation. It appears that duplicate sequences are being eliminated in the diploidization process. Our data reveal no evidence that duplicated sequences are being silenced by inactivation correlated with methylation patterns.  相似文献   
997.
998.
Xylem-tapping mistletoes transpire large volumes of water (E) while conducting photosynthesis (A) at low rates, thus maintaining low instantaneous wateruse efficiency (A/E). These gas-exchange characteristics have been interpreted as a means of facilitating assimilation of nitrogen dissolved at low concentration in host xylem water; however, low A/E also results in substantial heterotrophic carbon gain. In this study, host trees (Juniperus osteosperma) were fertilized and gas exchange of mistletoe (Phoradendron juniperinum) and host were monitored to determine whether mistletoe A/E would approach that of the host if mistletoes were supplied with abundant nitrogen. Fertilization significantly increased foliar N concentrations (N), net assimilation rates, and A/E in both mistletoe and host. However, at any given N concentration, mistletoes maintained lower A and lower A/E than their hosts. On the other hand, when instantaneous water-use efficiency and A/N were calculated to include heterotrophic assimilation of carbon dissolved in the xylem sap of the host, both water-use efficiency and A/N converged on host values. A simple model of Phoradendron carbon and nitrogen budgets was constructed to analyze the relative benefits of nitrogen- and carbonparasitism. The model assumes constant E and includes feedbacks of tissue nitrogen concentration on photosyn-thesis. These results, combined with our earlier observation that net assimilation rates of mistletoes and their hosts are approximately matched (Marshall et al. 1994), support part of the nitrogen-parasitism hypothesis: that high rates of transpiration benefit the mistletoe primarily through nitrogen gain. However, the low ratio of A/E is interpreted not as a means of acquiring nitrogen, but as an inevitable consequence of an imbalance in C and N assimilation.This research was supported by the National Science Foundation (grants BSR-8706772 and 8847942).  相似文献   
999.
The purpose of this paper is to describe the effects of CO2 and N treatments on soil pCO2, calculated CO2 efflux, root biomass and soil carbon in open-top chambers planted with Pinus ponderosa seedlings. Based upon the literature, it was hypothesized that both elevated CO2 and N would cause increased root biomass which would in turn cause increases in both total soil CO2 efflux and microbial respiration. This hypothesis was only supported in part: both CO2 and N treatments caused significant increases in root biomass, soil pCO2, and calculated CO2 efflux, but there were no differences in soil microbial respiration measured in the laboratory. Both correlative and quantitative comparisons of CO2 efflux rates indicated that microbial respiration contributes little to total soil CO2 efflux in the field. Measurements of soil pCO2 and calculated CO2 efflux provided inexpensive, non-invasive, and relatively sensitive indices of belowground response to CO2 and N treatments.  相似文献   
1000.
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