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   322964篇
  免费   37600篇
  国内免费   135篇
  360699篇
  2018年   2856篇
  2016年   3901篇
  2015年   5274篇
  2014年   6250篇
  2013年   8934篇
  2012年   9960篇
  2011年   10136篇
  2010年   6811篇
  2009年   6053篇
  2008年   8764篇
  2007年   9214篇
  2006年   8607篇
  2005年   8334篇
  2004年   8377篇
  2003年   8083篇
  2002年   7958篇
  2001年   14342篇
  2000年   14338篇
  1999年   11501篇
  1998年   4060篇
  1997年   4149篇
  1996年   3943篇
  1995年   3819篇
  1994年   3810篇
  1993年   3704篇
  1992年   9813篇
  1991年   9393篇
  1990年   9221篇
  1989年   8947篇
  1988年   8357篇
  1987年   7999篇
  1986年   7213篇
  1985年   7325篇
  1984年   6049篇
  1983年   5306篇
  1982年   4178篇
  1981年   3699篇
  1980年   3517篇
  1979年   6096篇
  1978年   4547篇
  1977年   4278篇
  1976年   3974篇
  1975年   4295篇
  1974年   4654篇
  1973年   4718篇
  1972年   4355篇
  1971年   4065篇
  1970年   3437篇
  1969年   3335篇
  1968年   2964篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The olfactory lamellae of the catfish H. fossilis (Bl.) was studied in the scanning electron microscope. The olfactory lamellae are composed of sensory and non-sensory epithelium. The sensory epithelium contains large numbers of ciliated receptor cells, whereas the non-sensory raphe epithelium is covered with a dense mat of non-sensory cilia. It is not known whether the olfactory cilia possess receptor sites.  相似文献   
132.
Functional differentiation of long bones in lorises   总被引:2,自引:0,他引:2  
The external dimensions of the limb bones and the geometry of their midshaft cross-sections were determined for Loris tardigradus and Nycticebus coucang. Relative cortical thickness, cortical area, and second moment of area were calculated and contrasted with locomotor stresses. The difference in shape-related strength of the bones between the smaller- and the larger-bodied species is more pronounced than can be expected from stresses acting during normal locomotion. The Nycticebus skeleton has a much higher safety margin overall and seems to be dimensioned for infrequent but critical stresses of high magnitude. Lorisine gaits in general are characterized by low ground reaction forces, great mobility in all joints, and a nearly equal share in propulsion and weight-bearing by the fore- and hindlimb. Accordingly, the long bones of lorises (especially those of L. tardigradus) tend to be less rigid than those of other mammalian species (including other primates), they lack a preferential plane of higher bending strength, and femur and humerus do not differ markedly in their capacity to withstand mechanical stresses. External dimensions of the humerus and femur of the two African lorisine species parallel and corroborate these results. Some more general implications for the relationships between bone shape and locomotor stresses are also discussed.  相似文献   
133.
Viability measurements in mammalian cell systems   总被引:7,自引:0,他引:7  
  相似文献   
134.
Publisher's announcement  相似文献   
135.
A procedure for enzymatic production of dihydroneopterin triphosphate is described that allows GTP cyclohydrolase I to be reused repetitively. The reaction takes place in an ultrafiltration cell, and the product is collected in the filtrate, whereas the enzyme remains in the cell to be reused with additional substrate. This is repeated until the enzyme activity drops below a desirable level. The purity of the dihydroneopterin triphosphate is satisfactory for utilization of this compound for studies on enzymes involved in the synthesis of tetrahydrobiopterin and drosopterin. A procedure for purification of dihydroneopterin triphosphate is described that uses C18-silica and silica cartridges.  相似文献   
136.
An amino acid analysis method using a commercially available analyzer that accurately quantitates protein-derived amino acids in the 10-100 pmol range is described. The method utilizes the robotic capability of the analyzer's autosampler to perform precolumn derivatization of both primary and secondary amino acids with o-phthalaldehyde and 9-fluorenylmethyl chloroformate, respectively. The derivatized amino acids are then separated on a C-18 reverse-phase amino acid column and quantitated in a single run by fluorescence detection. The characterization of beta-lactoglobulin and two tryptic peptides from the bacterial enzyme diaminopimelic acid epimerase is used to demonstrate the sensitivity and utility of this method.  相似文献   
137.
Human ejaculated spermatozoa are heterogeneous and can be separated into two distinct populations according to their respective buoyant densities. In order to investigate the functional differences between these two types of spermatozoa, we have searched for the presence of galactosyltransferase. A Western blot of sperm proteins following their electrophoresis was probed with an anti-galactosyltransferase serum revealing that this enzyme is present in human spermatozoa. Furthermore, galactosyltransferase is detectable only in those proteins isolated from the head of high density spermatozoa. These results suggest that ejaculated spermatozoa consist of two populations that are functionally different.  相似文献   
138.
A light plasma membrane fraction corresponding to a buoyant density of 1.087 +/- 0.005 g/cm3 and devoid of chlorophyll was prepared and purified from Anacystis nidulans according to a recently published procedure (G.A.Peschek, V.Molitor, M.Trnka, M.Wastyn and W.Erber (1988) Methods Enzymol. 167, 437-449). Besides major amounts of carotenoids the plasma membranes contained a small but significant pool of chlorophyllide a and protochlorophyllide a as verified by room temperature and 77K spectrofluorimetry and analytical separation and identification by high performance liquid chromatography using authentic standards. Incubation of the plasma membranes in strict darkness in the presence of NADPH was accompanied by the gradual and stoichiometric replacement of protochlorophyllide by chlorophyllide, NADP+ effecting the reverse transition. The reaction was completely insensitive to illumination (5-20 w/m2 tungsten light) but abolished after heating of the membranes (90 degrees C, 5 min) or in the presence of 10 mM EGTA, and was specifically stimulated by calcium ions. Our results indicate the occurrence of light-independent NADPH:protochlorophyllide oxidoreductase activity in the plasma membrane of Anacystis nidulans.  相似文献   
139.
140.
Two mutants of Lactobacillus casei dihydrofolate reductase, Trp 21----Leu and Asp 26----Glu, have been prepared by using site-directed mutagenesis methods, and their ligand binding and structural properties have been compared with those of the wild-type enzyme. 1H, 13C, and 31P NMR studies have been carried out to characterize the structural changes in the complexes of the mutant and wild-type enzymes. Replacement of the conserved Trp 21 by a Leu residue causes a decrease in activity of the enzyme and reduces the NADPH binding constant by a factor of 400. The binding of substrates and substrate analogues is only slightly affected. 1H NMR studies of the Trp 21----Leu enzyme complexes have confirmed the original resonance assignments for Trp 21. In complexes formed with methotrexate and the mutant enzyme, the results indicate some small changes in conformation occurring as much as 14 A away from the site of substitution. For the enzyme-NADPH complexes, the chemical shifts of nuclei in the bound coenzyme indicate that the nicotinamide ring binds differently in complexes with the mutant and the wild-type enzyme. There are complexes where the wild-type enzyme has been shown to exist in solution as a mixture of conformations, and studies on the corresponding complexes with the Trp 21----Leu mutant indicate that the delicately poised equilibria can be perturbed. For example, in the case of the ternary complex formed between enzyme, trimethoprim, and NADP+, two almost equally populated conformations (forms I and II) are seen with the wild-type enzyme but only form II (the one in which the nicotinamide ring of the coenzyme is extended away from the enzyme structure and into the solvent) is observed for the mutant enzyme complex. It appears that the Trp 21----Leu substitution has a major effect on the binding of the nicotinamide ring of the coenzyme. For the Asp 26----Glu enzyme there is a change in the bound conformation of the substrate folate. Further indications that some conformational adjustments are required to allow the carboxylate of Glu 26 to bind effectively to the N1 proton of inhibitors such as methotrexate and trimethoprim come from the observation of a change in the dynamics of the bound trimethoprim molecule as seen from the increased rate of the flipping of the 13C-labeled benzyl ring and the increased rate of the N1-H bond breaking.  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