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   609207篇
  免费   51743篇
  国内免费   7723篇
  668673篇
  2021年   7745篇
  2020年   5239篇
  2019年   6437篇
  2018年   17861篇
  2017年   15589篇
  2016年   15429篇
  2015年   12955篇
  2014年   14968篇
  2013年   18982篇
  2012年   24791篇
  2011年   31880篇
  2010年   24063篇
  2009年   19385篇
  2008年   24902篇
  2007年   26327篇
  2006年   15093篇
  2005年   13820篇
  2004年   13915篇
  2003年   13365篇
  2002年   12610篇
  2001年   19246篇
  2000年   19053篇
  1999年   15700篇
  1998年   5989篇
  1997年   6437篇
  1996年   6017篇
  1995年   5634篇
  1994年   5565篇
  1993年   5326篇
  1992年   13162篇
  1991年   12888篇
  1990年   12685篇
  1989年   12385篇
  1988年   11651篇
  1987年   10917篇
  1986年   10200篇
  1985年   10612篇
  1984年   8656篇
  1983年   7517篇
  1982年   5761篇
  1981年   5145篇
  1979年   8201篇
  1978年   6392篇
  1977年   6010篇
  1976年   5746篇
  1975年   6199篇
  1974年   6772篇
  1973年   6625篇
  1972年   6418篇
  1971年   5844篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
841.
Sixteen patients with an early dense hemiplegia due to cerebrovascular accidents were shown to have a greater degree of hypoxia than 16 matched control patients. The patients with hemiplegia had a reflex compensatory fall in arterial carbon dioxide tensions (PaCO2) with possible reduction in cerebral blood flow. Oxygen treatment led to an increase in PaCO2 in the patients with hemiplegia, but the increase in oxygen tensions in these patients was significantly less than that in the control group, suggesting increased pulmonary shunting as the cause for the hypoxia. Oxygen treatment may improve cerebral blood flow and oxygenation and have a useful role in the early management of patients with a dense hemiplegia.  相似文献   
842.
843.
844.
845.
846.
An evaluation of methods for studying bone marrow obtained from patients with acute lymphoblastic leukemia indicates that both directly prepared and cultured cells are necessary for complete karyotypic analysis, but that both synchronized and unsynchronized cultures may not be necessary.  相似文献   
847.
A new method for the analysis of NMR data in terms of the solution structure of proteins has been developed. The method consists of two steps: first a systematic search of the conformational space to define the region allowed by the initial set of experimental constraints, and second, the narrowing of this region by the introduction of additional constraints and optional refinement procedures. The search of the conformational space is guided by heuristics to make it computationally feasible. The method is therefore called the heuristic refinement method and is coded in an expert system called PROTEAN. The paper describes the validation of the first step of the method using an artificial NMR data set generated from the known crystal structure of sperm whale carbon monoxymyoglobin. It is shown that the initial search procedure yields a low-resolution structure of the myoglobin molecule, accurately reproducing its main topological features, and that the precision of the structure depends on the quality of the initial data set.  相似文献   
848.
The beta subunits of equine lutropin and equine chorionic gonadotropin were incubated in 0.013 N HCl for 30 min at 110 degrees C and separated into two fragments by reverse-phase high performance liquid chromatography. The amino acid and carbohydrate compositions of both fragments from each subunit were analyzed. The results demonstrated that equine lutropin-beta has a glycosylated COOH-terminal extension that differs only in carbohydrate composition from the COOH-terminal portion of equine chorionic gonadotropin-beta. This is the first demonstration of a glycosylated COOH-terminal extension in a pituitary glycoprotein hormone.  相似文献   
849.
Biomass, relative growth rate (RGR), organic and inorganic solute contents in control and NaCl (50–100 mM) affected roots or calli of the wild tomato genotypeLycopersicon pennellii and theLycopersicon esculentum wilty mutantflacca were compared. Under NaCl-stress, the RGR of calli fromL. pennellii was higher than that of the mutantflacca, while the root biomass of the former was lower than that of the latter. Constant water contents were found in calli and roots, irrespective of the genotypes and NaCl concentrations. Taking into account the solute contents of the apoplasm, Na+ accumulation was similar in the sensitive tissues (calli fromL. flacca, roots ofL. pennellii) and the tolerant ones (calli fromL. pennellii, flacca roots). Decreased K+ and Mg2+ and increased proline contents were found in both sensitive tissues. In comparison with sensitiveL. pennellii roots, salt sensitiveflacca calli showed increased total organic acid and amino acid contents.  相似文献   
850.
From a genomic library of Xenopus laevis, two genes coding for different preprocaeruleins have been isolated and sequenced. These correspond to the type I and type III precursors analyzed previously at the cDNA level [Richter, K., Egger, R. and Kreil, G. (1986) J. Biol. Chem. 261, 3676-3680]. The type III gene comprises eight exons; the type I apparently contains eight exons as well, of which six have been sequenced. The genetic information for the dekapeptide caerulein is present on small exons of 45 base pairs. The two genes are highly homologous in their 5'-flanking region, the exon/intron boundaries, and long stretches of intron sequences. A possible scheme for the evolution of this small family of genes through exon and gene duplications is presented. In the type I gene, in place of one of the caerulein exons, a potential exon with conserved splice sites was discovered. If expressed in some frog cells, this exon would code for a new peptide 60% homologous to caerulein.  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