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   1243篇
  免费   68篇
  2015年   16篇
  2014年   14篇
  2013年   28篇
  2012年   20篇
  2011年   25篇
  2010年   39篇
  2009年   32篇
  2008年   42篇
  2007年   52篇
  2006年   23篇
  2005年   23篇
  2004年   16篇
  2002年   19篇
  2000年   17篇
  1999年   20篇
  1997年   22篇
  1996年   17篇
  1993年   20篇
  1992年   15篇
  1991年   25篇
  1990年   18篇
  1989年   15篇
  1988年   17篇
  1987年   21篇
  1986年   26篇
  1985年   19篇
  1984年   18篇
  1982年   17篇
  1980年   17篇
  1979年   28篇
  1978年   13篇
  1977年   13篇
  1976年   25篇
  1975年   17篇
  1974年   32篇
  1973年   40篇
  1972年   28篇
  1971年   20篇
  1970年   18篇
  1969年   18篇
  1968年   19篇
  1965年   38篇
  1964年   20篇
  1963年   32篇
  1962年   23篇
  1961年   33篇
  1960年   21篇
  1959年   27篇
  1958年   27篇
  1957年   12篇
排序方式: 共有1311条查询结果,搜索用时 15 毫秒
1.
Abstract

Microorganisms capable of aerobic respiration on ferrous ions are spread throughout eubacterial and archaebacterial phyla. Phylogenetically distinct organisms were shown to express spectrally distinct redox‐active biomolecules during autotrophic growth on soluble iron. A new iron‐oxidizing eubacterium, designated as strain Funis, was investigated. Strain Funis was judged to be different from other known iron‐oxidizing bacteria on the bases of comparative lipid analyses, 16S rRNA sequence analyses, and cytochrome composition studies. When grown autotrophically on ferrous ions, Funis produced conspicuous levels of a novel acid‐stable, acid‐soluble yellow cytochrome with a distinctive absorbance peak at 579 nm in the reduced state.

Stopped‐flow spectrophotometric kinetic studies were conducted on respiratory chain components isolated from cell‐free extracts of Thiobacillus ferrooxidans. Experimental results were consistent with a model where the primary oxidant of ferrous ions is a highly aggregated c‐type cytochrome that then reduces the periplasmic rusticyanin. The Fe(II)‐dependent, cytochrome c‐catalyzed reduction of the rusticyanin possessed three kinetic properties in common with corresponding intact cells that respire on iron: the same anion specificity, a similar dependence of the rate on the concentration of ferrous ions, and similar rates at saturating concentrations of ferrous ions  相似文献   
2.
Evidence for the non-indolic nature of the new citrus auxinis presented on the basis of fluorometric properties, thin-layerchromatography, Ehrlich's colour reaction, paper electrophoresis,and the infra-red spectra determinations. Citrus auxin had alower Rf in TLC than IAA, did not give the typical indole reactionwith Ehrlich's reagent, and behaved differently in electrophoresis.The infra-red spectra also provided preliminary informationconcerning chemical structure. The hypothesis that indolic compoundsconstitute the only natural auxins in higher plants should berevised in view of this evidence that a non-indole auxin existsin higher plants.  相似文献   
3.
4.
The influence of the major histocompatibility (B) complex and sex on the phytohaemagglutinin (PHA) wattle response was studied in 136 segregants (B2/B2, B2/B5 and B5/B5) of a fourth generation cross between inbred lines 6(1) and 15(1). At 6 weeks of age, chickens were injected with 100 micrograms purified PHA-P. Wattle thickness measurements were taken 4, 24, 48, 72 and 96 h after injection. Analysis of variance showed that 4 h after injection, males had a significantly higher response than females but the sex-genotype interaction was also significant. Females had higher responses than males 24 and 48 h after injection as a consequence of more rapid development and earlier resolution of the reaction in males. B2/B2 chickens had significantly lower responses than B5/B5 chickens 72 and 96 h after injection, signifying a faster late resolution phase in the B2/B2 genotype. The developmental and early resolution phases of the PHA wattle response were influenced by sex while the late resolution phase was influenced by B genotype.  相似文献   
5.
Root morphology, shoot morphology, and water uptake for Agavedeserti and Ferocactus acanthodes of various sizes were studiedusing allometric relationships (y = axb) and a previously developedwater uptake model. Shoot surface area increased with shootvolume with an exponent b of 0.75 for both species. Root lengthand the ground area explored by the roots increased with shootsurface area with b's of 0.72 for A. deserti and 0.92 for F.acanthodes. Various sized individuals had about the same ratioof root length to explored ground area, with higher values occurringfor A. deserti. Predicted water uptake averaged over the exploredground area was approximately constant over a 104-fold rangein shoot surface area, suggesting that shoot size confers nointraspecific competitive advantage for water uptake. For theroot lengths per explored ground area observed in the field,water uptake was predicted to be 85 per cent of maximal; wateruptake could be increased by the production of more rain roots.When differences in shoot volume were accounted for by allometry,small plants had relatively less shoot surface area and relativelymore root length per shoot volume than did large plants, whichmay be important for the water relations of seedling establishment. Agave deserti, Ferocactus acanthodes, allometry, desert succulents, root distribution, root length, seedling growth, seedling establishment, shoot surface area, shoot volume, water uptake  相似文献   
6.
The relationship between computational SAR studies and relevant data gathering and generation activities is complex. First, the chemical class to be studied is selected on the basis of information requirements for hazard identification and assessment. Membership in the class is determined by consideration of chemical structure and reactivity. Compilation of the existing bioassay data for this chemical class follows immediately from the specification of the class. Bioassay data, qualitative knowledge of general chemical reactivities in this class, and knowledge concerning potential interactions with biomolecular targets all contribute to the derivation of possible mechanisms for biological activity. Computational studies based on modeling the proposed mechanism of action and/or the existing data base can provide a quantitative basis for the differentiation between chemicals. There is the opportunity for continuing feedback between the quantitative computational studies and the development of a relevant bioassay data base for this chemical class. The qualitative and quantitative information on the potential biological responses obtained will provide a rational basis for extrapolation from the extant data base to the chemicals of interest, and to biological responses significant to the assessment for which complete data are unavailable. Knowledge concerning possible mechanisms of action and preexisting data determine the type of computational study that will be most useful.  相似文献   
7.
C1-tetrahydrofolate (THF) synthase is a trifunctional protein possessing the activities 10-formyl-THF synthetase, 5,10-methenyl-THF cyclohydrolase, and 5,10-methylene-THF dehydrogenase. The current model divides this protein into two functionally independent domains with dehydrogenase/cyclohydrolase activities sharing an overlapping active site on the N-terminal domain and synthetase activity associated with the C-terminal domain. Previous chemical modification studies on C1-THF synthase from the yeast Saccharomyces cerevisiae indicated at least two cysteinyl residues involved in the dehydrogenase/cyclohydrolase reactions [Appling, D. R., & Rabinowitz, J. C. (1985) Biochemistry 24, 3540-3547]. In the present work, site-directed mutagenesis of the S. cerevisiae ADE3 gene, which encodes C1-THF synthase, was used to individually change each cysteine contained within the dehydrogenase/cyclohydrolase domain (Cys-11, Cys-144, and Cys-257) to serine. The resulting proteins were overexpressed in yeast and purified for kinetic analysis. Site-specific mutations in the dehydrogenase/cyclohydrolase domain did not affect synthetase activity, consistent with the proposed domain structure. The C144S and C257S mutations result in 7- and 2-fold increases, respectively, in the dehydrogenase Km for NADP+. C144S lowers the dehydrogenase maximal velocity roughly 50% while C257S has a maximal velocity similar to that of the wild type. Cyclohydrolase catalytic activity is reduced 20-fold by the C144S mutation but increased 2-fold by the C257S mutation. Conversion of Cys-11 to serine has a negligible effect on dehydrogenase/cyclohydrolase activity. A double mutant, C144S/C257S, results in catalytic properties roughly multiplicative of the individual mutations.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
8.
9.
10.
To test the hypothesis that gibberellic acid (GA) sensitivityaffects the length of the extension zone (LEZ) of leaf No. 1of wheat seedlings, we performed a gene dosage experiment usingRht dwarfing genes that condition GA insensitivity. We utilizednearly isogenic lines, at Rht-dosage levels of 0, 2 and 4 alleles.Anatomical markers (distances between successive stomates) wereused to infer the distribution of growth along the axis of theleaf. Interstomatal distance (ISD) and LEZ were inverse linearfunctions of Rht-dosage. The number of stomates matured perhour was independent of Rht-dosage. The relationship betweenISD and distance along the axis within the extension zone (EZ)was indistinguishable from linear. Rht-dosage did not affectthe slope of the regression of ISD against distance along theEZ. A-REST (AR; ancymidol, a potent GA synthesis inhibitor)reduced LEZ. Wild type was more sensitive to AR than doubledwarf. AR affected growth of leaf No. 1 more than length ofthe coleoptile, regardless of Rht-dosage. AR-dosage affectedcell division, whereas Rht-dosage did not. Extension zone, elongation, gibberellic acid, Rht, wheat, Triticum aesiivum L.  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