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   12766篇
  免费   923篇
  国内免费   5篇
  2023年   55篇
  2022年   69篇
  2021年   165篇
  2020年   117篇
  2019年   136篇
  2018年   222篇
  2017年   198篇
  2016年   341篇
  2015年   548篇
  2014年   666篇
  2013年   688篇
  2012年   946篇
  2011年   869篇
  2010年   581篇
  2009年   438篇
  2008年   638篇
  2007年   662篇
  2006年   543篇
  2005年   595篇
  2004年   540篇
  2003年   468篇
  2002年   448篇
  2001年   264篇
  2000年   298篇
  1999年   229篇
  1998年   149篇
  1997年   101篇
  1996年   110篇
  1995年   98篇
  1994年   86篇
  1993年   78篇
  1992年   148篇
  1991年   147篇
  1990年   137篇
  1989年   92篇
  1988年   127篇
  1987年   109篇
  1986年   96篇
  1985年   106篇
  1984年   107篇
  1983年   70篇
  1982年   82篇
  1981年   67篇
  1980年   51篇
  1979年   84篇
  1978年   56篇
  1977年   76篇
  1976年   51篇
  1975年   65篇
  1973年   58篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
71.
Five brain-derived and 17 urinary oligomannose-type oligosaccharides were isolated by ion-exchange chromatography on Mono Q or Dowex, followed by HPLC on Lichrosorb-NH2 from a Persian cat suffering from -mannosidosis. The structures ofthe carbohydrate chains were determined by 500- or 600-MHz1H-NMR spectroscopy. Different oligosaccharide patterns were found in brain and urine. 99% of the urinary oligosaccharides possess an (1-6)-linked mannose residue attached to -mannose, whereas only 5% of the brain-derived oligosaccharides contain such a residue. Furthermore, of the urinary carbohydrate chains 71% end with Man1-4GlcNAc1-4GlcNAc and 29% end with Man1-4GlcNAc, whereas the corresponding amounts are 23% and 77%, respectively, for the brain-derived oligosaccharides.Abbreviations MLEV-17 composite pulse devised by M. Levitt - HOHAHA homonuclear Hartman-Hahn spectroscopy - TPPI time-proportional phase incrementation - 2D two dimensional - GlcNAc N-acetylglucosamine - Man mannose - Fuc fucose  相似文献   
72.
Influences of the salt concentration on the fatty acid composition of Ectothiorhodospira species and other phototrophic purple bacteria have been analysed. Major fatty acids in bacteria of the genera Rhodobacter, Rhodopseudomonas, Chromatium, and Ectothiorhodospira were straight chain saturated and monounsaturated C-16 and C-18 fatty acids. Salt-dependent responses of all investigated bacteria revealed relations to their salt optima. Minimum values of C-16 and saturated fatty acids and maximum values of C-18 and unsaturated fatty acids were found at or close to the salt optima. Responses of Ectothiorhodospira mobilis upon changes in salinity were nearly identical, whether cells were grown in batch culture or in continuous culture with identical dilution rates at all salt concentrations. With increasing temperature, the fatty acid composition of Ectothiorhodospira mobilis and Ectothiorhodospira halophila strains showed decreasing portions of C-18 and of unsaturated fatty acids, while the contents of C-16 and saturated fatty acids increased. The results are discussed with respect to bilayer stabilisation and membrane fluidity.Abbreviations PC phosphatidylcholine - PG phosphatidylglycerol - CL cardiolipin - PE phosphatidylethanolamine  相似文献   
73.
Summary As part of a larger prospective study of the influence of environmental factors on pregnancy, birth and the fetus, chromosome examinations have been made in 34910 newborn children in Århus over a 13-year period. Klinefelter's syndrome was found in 1 per 576 boys, XYY in 1 per 851 boys, triple-X in 1 per 947 girls and Turner's syndrome in 1 per 1893 girls. Other sex chromosome aberrations were found in 1 per 11637 children. The total incidence of sex chromosome abnormalities was 1 per 426 children or 2.34 per 1000. The most frequent autosomal abnormalities were that of Down's syndrome with 1 per 592 children, and reciprocal translocations with 1 per 712 children. The total incidence of autosomal abnormalities was 1 per 164 children. Chromosome abnormalities were found in 276 liveborn children and in 19 fetuses, who were aborted after prenatal chromosome examination. The combined incidence of sex chromosomal and autosomal abnormalities was 1 per 118 children or 8.45 per 1000 children.  相似文献   
74.
Summary In tethered flying houseflies (Musca domestica), the yaw torque produced by the wings is accompanied by postural changes of the abdomen and hindlegs. In free flight, these body movements would jointly lead to turning manoeuvres of the animal. By recording the yaw torque together with the lateral deflections of either the abdomen or the hindlegs, it is shown that these motor output systems act in a highly synergistic way during two types of visual orientation behavior, compensatory optomotor turning reactions and orientation turns elicited by moving objects. This high degree of coordination is particularly conspicuous for the pathway activated by moving objects. Here, orientation responses either may be induced or may fail to be generated always simultaneously in all three motor output systems. This suggests that the pathway mediating orientation turns towards objects is gated before it segregates into the respective motor control systems of the wings, the abdomen and the hindlegs.  相似文献   
75.
Bacteriophage P008 revealed irreversible and uniform adsorption to cell walls of L. lactis subsp. 'diacetylactis' F7/2, whereas phage P127 adsorbed reversibly to a limited number of receptor sites on cell walls of L. lactis subsp. cremoris Wg2-1. Neither extraction of lipids, cell wall- and membrane-teichoic acids nor enzymatic degradation of proteins altered the binding efficiencies of both cell wall fractions. However, phage binding was inhibited, when cell walls were subjected to lysozyme, metaperiodate, or acid treatments. This reflects that a carbohydrate component embedded in the peptidoglycan matrix is part of the phage receptors of strains F7/2 and Wg2-1.  相似文献   
76.
When calf aortic tissue, preincubated under organ culture conditions in the presence of [35S]sulfate, was submitted to a sequential collagenase and elastase digestion and guanidinium chloride extraction, the bulk of proteoheparan sulfate was obtained in the elastase fraction. Ion-exchange chromatography on DEAE-cellulose of the elastase digest under dissociative conditions yielded a proteoglycan fraction that contained heparan sulfate as the sole glycosaminoglycan. The proteoheparan sulfate fraction was resolved into a high-molecular-mass (P-HS 1) and a low-molecular-mass (P-HS 2) fraction by gel filtration on Sephacryl S-400. P-HS 1 has a Mr of 175,000 and possesses four heparan sulfate side-chains (Mr 32,000) covalently bound to the protein core via a galactose- and xylose-containing polysaccharide-protein binding region. The protein core (Mr 38,000), which was obtained after deglycosylation of PG-HS 1 with trifluormethane sulfonic acid, contained in addition a few N-glycosidically linked oligosaccharide units representing a complex type with terminal neuraminic acid residues. P-HS 2 is a single-chain peptidoheparan sulfate of Mr of 38,000 containing one heparan sulfate chain (Mr 32,000) linked to a polypeptide (Mr 6000). The ratio of specific radioactivities of P-HS 1 and P-HS 2 was 1:0.66.  相似文献   
77.
From a screening of several Kluyveromyces strains, the yeast Kluyveromyces marxianus CBS 6556 was selected for a study of the parameters relevant to the commercial production of inulinase (EC 3.2.1.7). This yeast exhibited superior properties with respect to growth at elevated temperatures (40 to 45°C), substrate specificity, and inulinase production. In sucrose-limited chemostat cultures growing on mineral medium, the amount of enzyme decreased from 52 U mg of cell dry weight−1 at D = 0.1 h−1 to 2 U mg of cell dry weight−1 at D = 0.8 h−1. Experiments with nitrogen-limited cultures further confirmed that synthesis of the enzyme is negatively controlled by the residual sugar concentration in the culture. High enzyme activities were observed during growth on nonsugar substrates, indicating that synthesis of the enzyme is a result of a derepression/repression mechanism. A substantial part of the inulinase produced by K. marxianus was associated with the cell wall. The enzyme could be released from the cell wall via a simple chemical treatment of cells. Results are presented on the effect of cultivation conditions on the distribution of the enzyme. Inulinase was active with sucrose, raffinose, stachyose, and inulin as substrates and exhibited an S/I ratio (relative activities with sucrose and inulin) of 15 under standard assay conditions. The enzyme activity decreased with increasing chain length of the substrate.  相似文献   
78.
Biochemistry of metallothionein   总被引:70,自引:0,他引:70  
J H K?gi  A Sch?ffer 《Biochemistry》1988,27(23):8509-8515
  相似文献   
79.
It is increasingly recognized that macrophages play a crucial role in the development of chronic inflammatory states such as alcoholic liver disease. These cells can metabolize free arachidonic acid in the absence of a discernible trigger. The present study was undertaken to examine the short-term effects of ethanol on the generation of these exogenous arachidonate-derived extracellular mediators. Ethanol caused a dose-dependent decrease in the production of both cyclooxygenase and lipoxygenase metabolites. Similar effects were observed on the esterification of exogenous arachidonate into cellular lipids. To characterize further the effects of ethanol on exogenous arachidonic acid metabolism, we studied the short-term responses displayed by macrophages challenged with another soluble stimulus; the tumor-promoting agent phorbol myristate acetate. We observed an inhibition by ethanol of the superoxide anion response triggered by phorbol myristate acetate similar to that observed for exogenous arachidonate oxygenation. Our results show that ethanol can inhibit these soluble stimuli-elicited responses, possibly through its disorganizing effect on plasma membrane.  相似文献   
80.
Between 1982 and 1986 51 patients were treated with ciclosporin a (CSA) to prevent graft versus host disease (GvHD) after bone marrow transplantation (BMT). Major side effects of the drug were tremor, hypertension, hepatotoxicity and nephrotoxicity. Acute GvHD 0 degree to II degree occurred in 80% of our patients, and GvHD III degree and IV degree in 20% despite the use of CSA. Two to four days before the onset of GvHD, CSA serum levels were significantly lower on the average in patients who developed GvHD III degree and IV degree compared to the others. Our data indicate that plasma CSA concentrations higher than 250 ng/ml should be achieved to reduce the severity of GvHD after BMT.  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