全文获取类型
收费全文 | 263篇 |
免费 | 12篇 |
国内免费 | 5篇 |
出版年
2024年 | 1篇 |
2022年 | 3篇 |
2021年 | 6篇 |
2020年 | 5篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 6篇 |
2016年 | 9篇 |
2015年 | 5篇 |
2014年 | 12篇 |
2013年 | 45篇 |
2012年 | 5篇 |
2011年 | 9篇 |
2010年 | 11篇 |
2009年 | 7篇 |
2008年 | 8篇 |
2007年 | 9篇 |
2006年 | 7篇 |
2005年 | 12篇 |
2004年 | 12篇 |
2003年 | 11篇 |
2002年 | 5篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 4篇 |
1998年 | 2篇 |
1997年 | 2篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 1篇 |
1992年 | 3篇 |
1991年 | 4篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 5篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 3篇 |
1972年 | 1篇 |
1971年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有280条查询结果,搜索用时 31 毫秒
1.
About 80% of the evolutionary history of life on Earth is restricted to microorganisms which have had several billion years to speciate. The reasons for the origin (self-assembly) of life on Earth, bacterial cell division and why there are so many different bacteria and their global dispersal are discussed from an evolutionary perspective. 相似文献
2.
Romano Gellini Filippo Bussotti Davide Bettini Paolo Grossoni Alessandro Bottacci 《Plant biosystems》2013,147(3-4):481-504
Abstract We propose a study of the main species belonging to the genus Quercus in Italy, characterized and identified by means of leaf surface observation, with special attention devoted to waxes, trichomes and stomata. Comparing our results with the classification proposed by SCHWARZ (1984), we find that species belonging to Schwarz's subgenus Quercus are recognizable because their waxes are structured in vertical scales; the two other subgenera (Sclerophyllodrys and Cerris) present smooth wax structures, their distinctive feature being the shape of the stomatal rima, which is roundish in Sclerophyllodrys and elliptical in Cerris. The study characterizes Quercus pubescens Willd. and Quercus petraea Liebl. by analyzing some morphometric traits; but the authors feel that further research is needed on these critical taxonomic entities. Lastly, the study examines forms of was degeneration correlated to the phenomenon known as oak decline. 相似文献
3.
The chlorellavorous bacterium . "Vampirovibrio chlorellavorus" Gromov et Mamkaeva, grows only by killing and consuming the cell contents of species of the green alga Chlorella Beijerinck. Of the 76 algal strains examined in this study, the bacterium attacks all 31 strains of the species C. vulgaris. C. sorokiniana, and C. kessleri, but attacks only two of 39 strains of nine other Chlorella species. Neither of two Prototheca strains was susceptible to attack. This narrow host specificity may be related to cell surface properties . 相似文献
4.
Five strains of pink-pigmented, facultatively methylotrophic bacteria (PPFM) isolated from plant surfaces and grown in different carbon sources, were fixed, embedded and sectioned for examination with the electron microscope. The strains studied represented the two mainsub-groups, those that can use carbohydrates and those that can not use carbohydrates as a sole carbon and energy source. All of the isolates examined, produced crystalloid inclusions, internal membranous vesicles and internal membranous sheets although the number of cells with inclusions, varied with the carbon source and specific strain. Polybetahydroxybutyrate and polyphosphate bodies were observed in all strains, with all carbon sources used for growing cells which includes methanol, formate and glycerol. Isolates that could use glucose accumulated polyglucoside granules but not when other carbon sources were provided. The relationship of these inclusions to growth conditions is discussed. 相似文献
5.
Refinement of distance geometry (DG) structures of EETI-II (Heitz et al.: Biochemistry 28:2392-2398, 1989), a member of the squash family trypsin inhibitor, have been carried out by restrained molecular dynamics (RMD) in water. The resulting models show better side chain apolar/polar surface ratio and estimated solvation free energy than structures refined "in vacuo." The consistent lower values of residual NMR constraint violations, apolar/polar surface ratio, and solvation free energy for one of these refined structures allowed prediction of the 3D folding and disulfide connectivity of EETI-II. Except for the few first residues for which no NMR constraints were available, this computer model fully agreed with X-ray structures of CMTI-I (Bode et al.: FEBS Lett. 242:285-292, 1989) and EETI-II complexed with trypsin that appeared after the RMD simulation was completed. Restrained molecular dynamics in water is thus proved to be highly valuable for refinement of DG structures. Also, the successful use of apolar/polar surface ratio and of solvation free energy reinforce the analysis of Novotny et al. (Proteins 4:19-30, 1988) and shows that these criteria are useful indicators of correct versus misfolded models. 相似文献
6.
The adsorption of 5′-AMP onto precipitated calcium phosphate (CaPi) requires the presence of soluble calcium and this dependence
exhibits a Michaelian-like behavior. This result suggests that the formation of a complex between 5′-AMP and free Ca2+ (CaAMP) is a prelude to the adsorption of the nucleotide in the solid matrix. At concentrations one order of magnitude higher,
Mn2+ and Mg2+ can substitute for soluble Ca2+ in the adsorption of 5′-AMP onto solid CaPi. However, when added simultaneously with 5′-AMP to a heterogeneous mixture that
contains CaPi and soluble Ca2+, Mn2+ and Mg2+ inhibit the adsorption of 5′-AMP in a concentration-dependent manner. This suggests the formation of complexes that are much
less effective for 5′-AMP adsorption than the CaAMP complex. On the other hand, Mn2+ and Mg2+ cannot promote desorption of the nucleotide attached to the precipitate in the presence of soluble Ca2+ if they are added after adsorption has attained equilibrium. Although desorption of 5′-AMP can be obtained by a sequential
dilution of the soluble phase with buffer and no nucleotide in a process that obeys a Langmuir equation, the lack of effect
of Mn2+ or Mg2+ when adsorption has attained its maximal value suggests strong interactions between the CaAMP complex and the solid matrix
when adsorption equilibrium is reached. The divalent cations present in the matrix also participate with different selectivity
in the attachment of the CaAMP complex, indicating that a cation-exchange mechanism could have acted in the modulation of
adsorptive/desorptive processes involving biomonomers and phosphate surfaces in primitive aqueous environments.
Received: 11 December 1995 / Accepted: 5 April 1996 相似文献
7.
Joel S. Brown Thomas L. Vincent 《Evolution; international journal of organic evolution》1992,46(5):1269-1283
We consider a simple predator-prey model of coevolution. By allowing coevolution both within and between trophic levels the model breaks the traditional dichotomy between coevolution among competitors and coevolution between a prey and its predator. By allowing the diversity of prey and predator species to emerge as a property of the evolutionarily stable strategies (ESS), the model breaks another constraint of most approaches to coevolution that consider as fixed the number of coevolving species. The number of species comprising the ESS is influenced by a parameter that determines the predator's niche breadth. Depending upon the parameter's value the ESS may contain: 1) one prey and one predator species, 2) two prey and one predator, 3) two prey and two predators, 4) three prey and two predators, 5) three prey and three predators, etc. Evolutionarily, these different ESSs all emerge from the same model. Ecologically, however, these ESSs result in very different patterns of community organization. In some communities the predator species are ecologically keystone in that their removal results in extinctions among the prey species. In others, the removal of a predator species has no significant impact on the prey community. These varied ecological roles for the predator species contrasts sharply with the essential evolutionary role of the predators in promoting prey species diversity. The ghost of predation past in which a predator's insignificant ecological role obscures its essential evolutionary role may be a frequent property of communities of predator and prey. 相似文献
8.
Summary Techniques for the isolation of ahhighly pure population of viable osteoclasts are limited. For this reason, we developed
an isolation procedure that results in a high yield of osteoclast-like cells, up to 92% pure, from 3-wk-old chicken tibias.
The unique feature of the method is the migration of cells from marrow-free endosteal surfaces to vitronectin-coated plates.
The cells retain the osteoclast phenotype and remain viable in culture for a minimum of 1 wk. The cells were characterized
and compared to two populations of authentic avian osteoclasts, which were isolated on the basis of association with fibronectin-coated
plates. The cells contained substantial amounts of tartrate-resistant acid phosphatase. Alkaline phosphatase levels were negligible,
suggesting little contamination by osteoblasts. Response to parathyroid hormone, dibutyryl cyclic adenosine monophosphate,
calcitonin, acetazolamide, 17β-estradiol, and prostaglandin E2 was evident, as detected by measuring acid production. The vitronectin-associating cells contained numerous mitochondria,
had the ability to resorb bone in anin vitro bone slice assay, and specifically bound biotinylated vitronectin. At 5 d of culture, the cells demonstrated marginal multinuclearity,
having two to three nuclei. A large number (∼1×106 cells/tibia) of viable cells that exhibit characteristics of authentic osteoclasts can be obtained by the method described.
Potentially, this method could be applied to other species. 相似文献
9.
Calvet S Clapés P Vigo JP Xaus N Jorba X Mas RM Torres JL Valencia G Serralheiro ML Cabral JM Empis JM 《Biotechnology and bioengineering》1992,39(5):539-549
The influence of five different N-terminal protecting groups (For, Ac, Boc, Z, and Fmoc) and reaction conditions (temperature and dimethylformamide content) on the alpha-chymotrypsin-catalyzed synthesis of the dipeptide derivative X-Phe-Leu-NH(2) was studied. Groups such as For, Ac, Boc, and Z always rendered good peptide yields (82% to 85%) at low reaction temperatures and DMF concentrations, which depended on the N-alpha protection choice. Boc and Z were the most reactive N-alpha groups and, in addition, the most suitable for peptide synthesis. On the other hand, the use of empirical design methodologies allowed, with minimal experimentation and by multiple regression, to deduce an equation, which correlates the logarithm of the first order kinetic constant (log k') with reaction temperature, DMF concentration, and hydrophobicity (log P values) of the different protecting groups. The predictive value of the equation was tested by comparing the performance of another protective group, such as Aloc, with the performance predicted by said equation. Experimental and calculated k' values were found to be in good agreement. 相似文献
10.
Summary In a screening program with 7 FITC-labeled lectins as probes, ConA receptors were identified in all of the 28 members of theConjugatophyceae, being under investigation. In nearly all of them RCA120 receptors, too, are expressed. In 3 species only, PNA receptors, and in 2 species UEA receptors have been detected. No binding of DBA, SBA, and WGA was observed. The receptors for ConA, RCA120, and UEA were shown to be associated with different molecules. Each lectin exhibits a unique and specific binding pattern, both chemically, as well as with regard to the topographic distribution on cell surfaces. While ConA receptors predominantly are associated with constituents of the cell wall, RCA120 receptors mostly form part of the surrounding mucilage; the same holds for UEA receptors. Besides a variability of topographic distribution and species-to-species variation, a cell-to-cell variation exists in many species, suggesting that the expression of a lectin receptor is due to the developmental state of the cell and/or depends on external stimuli. In conclusion, we may point out, that FITC-labeled lectins turned out to be extremely useful probes for the investigation of the molecular architecture of cell walls. Calcofluor white ST binding to fibrillar polysaccharides (most probably cellulose) was shown to be inhibited by external incrustations of the cell wall. One species does not show any reaction with calcofluor white ST at all. 相似文献