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   13825篇
  免费   413篇
  国内免费   17篇
  2021年   97篇
  2020年   111篇
  2019年   99篇
  2018年   279篇
  2017年   241篇
  2016年   348篇
  2015年   369篇
  2014年   468篇
  2013年   598篇
  2012年   599篇
  2011年   614篇
  2010年   389篇
  2009年   309篇
  2008年   459篇
  2007年   499篇
  2006年   457篇
  2005年   444篇
  2004年   410篇
  2003年   443篇
  2002年   406篇
  2001年   297篇
  2000年   245篇
  1999年   194篇
  1998年   105篇
  1992年   245篇
  1991年   226篇
  1990年   184篇
  1989年   216篇
  1988年   235篇
  1987年   247篇
  1986年   212篇
  1985年   251篇
  1984年   202篇
  1983年   167篇
  1982年   86篇
  1979年   245篇
  1978年   159篇
  1977年   141篇
  1976年   136篇
  1975年   165篇
  1974年   270篇
  1973年   263篇
  1972年   247篇
  1971年   245篇
  1970年   259篇
  1969年   245篇
  1968年   186篇
  1967年   189篇
  1966年   166篇
  1965年   124篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
111.
An average follow-up period of 16 patients was 28 months following an implantation of the artificial aortic valve for its insufficiency. In 10 operated patients who were able to continue their occupation exercise tolerance increased by two classes, according to NYHA. Blood pressure gradient decreased significantly from 61.8 to 37.5 mmHg, cardiac volume index decreased from 639 to 602 ml/m2. Echocardiographically measured muscle mass of the left ventricle, end-diastolic and end-systolic volumes, and the left atrial dimensions decreased significantly following surgery. A significance of the relation of the left ventricle volume to its mass <4 as a prognostic factor in aortic valve replacement has also been confirmed.  相似文献   
112.
Recurrence of duodenal ulcer was diagnosed in 15 patients who underwent highly selective vagotomy before 6-13 years, i.e. in 12.2%. Factors possibly contributing to such a recurrence were analysed. Patients with ulcer recurrence were: 1) non-qualified workers, 2) tobacco smokers, 3) heavy drinkers, 4) users of ulcerogenic medicines, and 5) were involved into conflict situations.  相似文献   
113.
Summary Microbial transformations of neutral fraction (NF) and upgraded neutral fraction (UNF) of Polish tall oil byMycobacterium sp. MB 3683 were performed. Final metabolites and yields were compared to bioconversion of pure -sitosterol. Additionally, origin of a new metabolite —5-androsta-3,6,17-trione was proved by transformation of UNF in the presence of labeled -sitosterol.  相似文献   
114.
Summary The endogenous tryptophan metabolite quinolinic acid elicits in rodent brain a pattern of neuronal degeneration which resembles that caused by L-glutamate. Its qualities as a neurotoxic agent raised the hypothesis that quinolinic acid might be involved in the pathogenesis of human neurodegenerative disorders. Kynurenic acid, another endogenous tryptophan metabolite and preferential N-methyl-D-aspartate (NMDA) antagonist, has been shown to block quinolinic acid neurotoxicity. Here we report that microinjections of aminooxyacetic acid (AOAA), an inhibitor of kynurenine transaminase and of other pyridoxal phosphate-dependent enzymes, into the rat striatum produce neuronal damage resembling that caused by quinolinic acid. AOAA-induced striatal lesions can be prevented by kynurenic acid and the selective NMDA antagonist 2-amino-7-phosphonoheptanoic acid. These results suggest that AOAA produces excitotoxic lesions by depleting brain concentrations of kynurenic acid (inhibition of synthetic enzyme) or due to impairment of intracellular energy metabolism (depletion of cell energy resources). The concept of deficient neuroprotection due to metabolic defects might help to clarify the pathogenesis of human neurodegenerative disorders and to develop strategies that may be useful in their treatment.This work was supported by research grant from the Polish Academy of Sciences.These data have been communicated to the International Congress on Amino Acid Research held in Vienna in August 7–12, 1989.  相似文献   
115.
Protoplasts ofCunninghamella elegans, showing 11-, and 11-hydroxylating ability of Substance S, preserved high transformation activity when dispersed in glucose-enriched, organic osmotic stabilizers. A joint action of polyoxins and 2-deoxy-d-glucose was necessary to prevent regeneration of the cell wall in long-lasting experiments. Stabilized and active, dispersed protoplasts may be an alternative research model for studying the function of the cell wall and intracellular metabolic pool constituents in steroid hydroxylation.  相似文献   
116.
Maize cobs withFusarium ear rot were collected at 1986 season and five infected byFusarium graminearum were analyzed for presence of triohothecenes and zearalenone. Collected material was subsampled forFusarium damaged kernels and corresponding axial stems and healthy looking kernels. All investigated cobs contained deoxynivalenol (DON) (range 18.0–131.5 mg/kg) and zearalenone (ZEA) (range 0.38–2.17 mg/kg), in four cobs 15-acetyl-deoxynivalenol (15-AcDON) (range 5.2–6.2 mg/kg) was present and two cobs besides three all metabolites contained 3-acetyl-deoxynivalenol (3-AcD0N) (range 0.5–0.8 mg/kg).The average of individual toxins amount in axial stems: in mg/kg was equal to: DON — 110.36, ZEA — 4.57, 15-AcD0N — 16.66, and 3-AcD0N — 1.32.Fusarium damaged kernels contained in average the following amount (mg/kg) of: DON 77.00, ZEA 0.98, 15-AcD0N 3.78 and 3-AcD0N 0.06. Healthy looking kernels contained DON 1.96 mg/kg and ZEA 0.07 mg/kg only. Cooccurrence of 3-AcDON and 15-AcDON in two samples was an interesting finding. The amount of DON in total cob was highly correlated (r = 0.94) with percentage ofFusarium damaged kernels in given ear.  相似文献   
117.
Starting from peracetylated chloro- or bromo-glycosyl donors ofN-acetylneurmainic acid,N-acetylglucosamine, glucose and lactose, the correspondingp-formylphenyl glycosides were synthesized stereospecifically under phase transfer catalysed conditions at room temperature in yields of 38–67%. After Zemplén de-O-acetylation, the formyl groups were directly and chemoselectively coupled to the lysine residues of bovine serum albumin by reductive amination using sodium cyanoborohydride. The conjugation reactions were followed as a function of time and under a series of different molar ratios of the reactants to provide glycoconjugates of varying degree of antigenicities. Thus, carbohydrate protein conjugates were made readily available using essentially two key reactions.Presented in part at the 15th International Carbohydrate Symposium, Yokohama, Japan, August 12–17, 1990.  相似文献   
118.
1. In 102 laboratory rats fed with (a) the krill standardized meal, (b) the krill meal with low chitin content, (c) the casein diet with D,L-methionine, (d) the casein diet with D,L-methionine supplemented with the krill carapace meal, (e) the casein diet with D,L-methionine supplemented with ash from the krill standardized meal and (f) the control diet--"Murigran" standard pelleted feed; the different blood indices were investigated. 2. The mean values of following indices: the number of erythrocytes and leucocytes, the percentage of leucocytes, the corpuscular haemoglobin concentration and red blood cell diameter were similar in all experimental and control groups. 3. The mean values of haematocrit and haemoglobin levels, the mean corpuscular thickness and volume were lower in rats fed with the casein diet with D,L-methionine supplemented with the krill carapace meal than in other groups. 4. All kinds of investigated indices were similar in rats fed with krill meal with low chitin contents, whose parents received the standardized krill meal and no sex differences have been shown here.  相似文献   
119.
Kraft pulps, prepared from softwoods, and small chips of birch wood were treated with heme and tert-butyl hydroperoxide in aqueous solutions at reflux temperature. Analyses of treated pulps showed decreases in kappa number (a measure of lignin content) from about 36 to less than 2, with concomitant increases in brightness (80% increase in the better samples). Analyses of treated wood chips revealed selective delignification and removal of hemicelluloses. After 48 h of treatment, lignin losses from the wood chips approached 40%, and xylose/mannose (hemicellulose) losses approached 70%, while glucose (cellulose) losses were less than 10%. Examination of delignified chips by transmission electron microscopy showed that the removal of lignin occurred in a manner virtually indistinguishable from that seen after decay by white rot fungi. Various metalloporphyrins, which act as biomimetic catalysts, were compared to horseradish peroxidase and fungal manganese peroxidase in their abilities to oxidize syringaldazine in an organic solvent, dioxane. The metalloporphyrins and peroxidases behaved similarly, and it appeared that the activities of the peroxidases resulted from the extraction of heme into the organic phase, rather than from the activities of the enzymes themselves. We concluded that heme-tert-butyl hydroperoxide systems in the absence of a protein carrier mimic the decay of lignified tissues by white rot fungi.  相似文献   
120.
CTP-dependent lipid kinases of yeast   总被引:1,自引:0,他引:1  
Membrane fractions from yeast Saccharomyces cerevisiae catalyzed a transfer of gamma-phosphate from [gamma-32P]CTP into membranous lipids. Phosphorylated compounds were identified as phosphatidic acid and dolichyl phosphate (DolP). The membrane fraction also catalyzed phosphorylation of the exogenous dolichol. The activity of the phosphorylating enzymes could be modified by the yeast growing conditions; i.e., the enzyme from yeast grown aerobically favored the synthesis of phosphatidate over dolichyl phosphate in the ratio of 3:1, whereas the membrane fraction from anaerobically grown yeast synthesized PA and DolP in the ratio of 0.5:1. The activity of the phosphorylating enzymes could also be modified by divalent cations and the concentration of detergents. Phosphorylation of lipids does not occur in the presence of [gamma-32P]ATP and is not influenced by the presence of UTP or GTP. This result points to the specific role of CTP as a gamma-phosphate donor for the synthesis of phosphatidate and dolichyl phosphates in the yeast system.  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