全文获取类型
收费全文 | 1474篇 |
免费 | 126篇 |
国内免费 | 2篇 |
出版年
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 13篇 |
2021年 | 29篇 |
2020年 | 17篇 |
2019年 | 21篇 |
2018年 | 35篇 |
2017年 | 27篇 |
2016年 | 49篇 |
2015年 | 87篇 |
2014年 | 95篇 |
2013年 | 94篇 |
2012年 | 112篇 |
2011年 | 95篇 |
2010年 | 84篇 |
2009年 | 58篇 |
2008年 | 96篇 |
2007年 | 88篇 |
2006年 | 92篇 |
2005年 | 96篇 |
2004年 | 93篇 |
2003年 | 78篇 |
2002年 | 69篇 |
2001年 | 19篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1998年 | 16篇 |
1997年 | 9篇 |
1996年 | 9篇 |
1995年 | 10篇 |
1994年 | 12篇 |
1993年 | 8篇 |
1992年 | 4篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 1篇 |
1988年 | 6篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 1篇 |
排序方式: 共有1602条查询结果,搜索用时 15 毫秒
21.
Abstract: Quantification of the size of epiphytic bacterial populations and characterization of their composition involves definition of a sampling strategy in time and space, the choice of methods for liberating the bacteria from the leaf surface and for recovering them for subsequent determination of the number of viable or culturable cells. This literature review focuses on some of the issues related to these choices. After briefly reviewing the different types of epiphytic colonizers we consider the biological, methodological, and statistical consequences of the choice of the sampling unit and of the spatial and temporal variability of population size and composition for epiphytic bacteria. The different methods available for the detection and enumeration of naturally occurring microorganisms in the phyllosphere are discussed. Advantages and drawbacks of each are described in this review designed as a 'hands-on' guide. 相似文献
22.
Screening and characterization of microorganisms with glutaryl-7 ADCA acylase activity 总被引:2,自引:0,他引:2
G. Franzosi E. Battistel I. Gagliardi W. Van der Goes 《Applied microbiology and biotechnology》1995,43(3):508-513
A screening of microorganisms producing glutaryl-7 ADCA acylase, an enzyme able to hydrolyse glutaric acid selectively from glutaryl-3-deacetoxy-7-aminocephalosporanic acid (glutaryl-7 ADCA), has been carried out in soil samples. Five microorganisms expressing acylase activity were isolated and classified as Bacillus cereus, Achromobacter xylosooxidans, Bacillus sp., Pseudomonas sp. and Pseudomonas paucimobilis. The screening was carried out by preparing enrichment cultures containing glutaryl-7-ADCA or cephalosporin C as the selective carbon source. Four model compounds (adipoyl-, glutamyl- and glutaryl-p-nitroanilide and glutarylcoumarin), mimicking the glutaryl-7 ADCA -lactam moiety, were synthesized as substrates suitable for the rapid screening of the microorganisms (2500) isolated from the enrichment cultures. A total of 300 strains were active on the model substrates and only 5 displayed acylase activity on glutaryl-7 ADCA. The fermentation parameters, such as pH and inducer concentration, for the optimal acylase expression and acylase specificity towards the model substrates were different for each strain. 相似文献
23.
Preparation and structural characterization of large heparin-derived oligosaccharides 总被引:4,自引:2,他引:2
Pervin Azra; Gallo Cindy; Jandik Kenneth A; Han Xue-Jun; Linhardt Robert J. 《Glycobiology》1995,5(1):83-95
Porcine mucosal heparin was partially depolymerized with heparinlyase I and then fractionated into low-molecularweight (<5000)and high-molecular-weight (>5000) oligosaccharides by pressurefiltration. The high-molecular-weight oligosaccharide mixture({small tilde}50 wt% of the starting heparin) also containedintact heparin. This intact polymer complicates oligosacsharidepurification. Thus, the low-molecular-weight fraction was usedto prepare homogeneous oligosaccharides for structural characterization.The low-molecular-weight oligosaccharide mixture was first fractionatedby low pressure gel permeation chromatography into size-uniformmixtures of disaccharides, tetrasaccharides, hexasaccharides,octasaccharides, decasaccharides, dodecasaccharides, tetradecasaccharidesand higher oligosaccharides. Each size-fractionated mixturewas then purified on the basis of charge by repetitive semi-preparativestrong-anion-exchange high-performance liquid chromatography.This approach has led to the isolation of 14 homogeneous oligosaccharidesfrom disaccharide to tetradecasaccharide. The purity of theseheparin-derived oligosaccharides was determined by gradientpolyacrylamide gel electrophoresis, analytical strong-anion-exchangehigh-performance liquid chromatography, capillary electrophoresisand one-dimensional nuclear resonance spectroscopy. The structureof these oligosaccharides was established using 600 MHz two-dimensionalnuclear resonance spectroscopy . The spectral methods used includedhomonuclear correlation spectroscopy, nuclear Overhauser effectspectroscopy and heteronuclear multiple quantum coherence spech-clscopy.The 1H/1H connectivities of the protons of each sugar residuein an oligosaccharide were established by two-dimensional homonuclearcorrelation spectroscopy, while 1H/13C assignments were madeusing 1H inverse detection. One- and two-dimensional nuclearresonance spectroscopic analysis of these heparin oligosaccharidesshowed two closely related groups of heparin-oligosaccharidesare afforded by enzymatic depolymerization of heparin. One groupis fully sulphated, having the structures 相似文献
24.
Cindy Lloyd John R. Kennedy Joseph Mendicino 《In vitro cellular & developmental biology. Plant》1984,20(5):416-432
Summary Swine tracheal epithelium has been cultured as explants in a chemically defined medium for periods of up to 2 wk. The viability
of the explants was shown by the preservation of the ultrastructural features of cells in the epithelial layer and by the
active incorporation of radioactive glucosamine and sulfate into secreted mucin glycoproteins. The rate of secretion of mucin
glycoprotein was about 0.035 mg per cm2 per d. After initial 24 h lag period was shown to be due to the equilibration of intracellular mucin glycoprotein pools with
radioactive precursors. The rate of secretion of glycoprotein showed a linear dependence on the area of the explant, and maximal
incorporation was observed at 200 μM glucosamine. A higher concentration of35SO4, 1000 μM, was required for maximal incorporation of the precursor. Insulin at 0.1 to 1 μg/ml increased the rate of secretion twofold,
whereas 0.1 to 100 μg/ml of hydrocortisone and 0.1 to 100 μg/ml of epinephrine significantly decreased the rate of secretion.
Vitamin A had little or no effect of normal trachea explants at low concentrations, and, at higher concentrations, 10−5
M, it decreased the secretion of mucin glycoproteins. Vitamin A, at a concentration of 10−9
M, increased the rate of synthesis of glycoprotein at least fourfold in trachea explants from vitamin A-deficient rats.
Mucus secretions collected from the surface of swine trachea and from the culture medium of trachea explants were purified.
The mucus was solubilized by reduction and carboxymethylation, and the high molecular weight mucin glycoproteins were purified
by chromatography on Sepharose CL-6B columns under dissociating conditions in 2M guanidine HCl. The mucin glycoproteins purified from swine trachea and from the culture medium of trachea explants were virtually
indistingushable. They showed the same properties when examined by gel electrophoresis and immunoprecipitation. The purified
glycoproteins contained about 25% protein, and serine, threonine, and proline were the principal amino acids present. More
than 80% of the carbohydride chains in both samples were released by treatment with alkaline borohydride. Nearly the same
molar ratio ofN-acetylgalactosamine,N-acetylglucosamine, galactose, fucose, sulfate, and sialic acid was found in both preparations.
This investigation was supported by U.S. Public Health Service Grants HL 20868, HL 24688, and HL 24718 from the National Heart,
Lung and Blood Institute, Bethesda, MD, and AM 28187 from the National Institute of Arthritis, Diabetes and Digestive and
Kidney Diseases, Bethesda, MD. 相似文献
25.
Ronald S. Oremland Cindy Umberger Charles W. Culbertson Richard L. Smith 《Applied microbiology》1984,47(5):1106-1112
The acetylene block technique was employed to study denitrification in intertidal estuarine sediments. Addition of nitrate to sediment slurries stimulated denitrification. During the dry season, sediment-slurry denitrification rates displayed Michaelis-Menten kinetics, and ambient NO3− + NO2− concentrations (≤26 μM) were below the apparent Km (50 μM) for nitrate. During the rainy season, when ambient NO3− + NO2− concentrations were higher (37 to 89 μM), an accurate estimate of the Km could not be obtained. Endogenous denitrification activity was confined to the upper 3 cm of the sediment column. However, the addition of nitrate to deeper sediments demonstrated immediate N2O production, and potential activity existed at all depths sampled (the deepest was 15 cm). Loss of N2O in the presence of C2H2 was sometimes observed during these short-term sediment incubations. Experiments with sediment slurries and washed cell suspensions of a marine pseudomonad confirmed that this N2O loss was caused by incomplete blockage of N2O reductase by C2H2 at low nitrate concentrations. Areal estimates of denitrification (in the absence of added nitrate) ranged from 0.8 to 1.2 μmol of N2 m−2 h−1 (for undisturbed sediments) to 17 to 280 μmol of N2 m−2 h−1 (for shaken sediment slurries). 相似文献
26.
27.
The oxidation of 3-hydroxy[3-14C]butyrate to CO2 and its incorporation into cerebral lipids by cortex slices from one-week old rats were markedly inhibited by methylmalonate. However, methylmalonate had no effect on the metabolism of labelled aceto- acetate, glucose and acetate by brain slices. Addition of propionate in the incubation medium reduced cerebral lipogenesis from labelled 3-hydroxybutyrate and acetate. Acute methylmalonic acidemia induced in one-week old pups by injecting 3% methylmalonate solution caused a reduction in the incorporation of labelled 3-hydroxybutyrate into cerebral lipids. However, acute methylmalonic acidemia had no effect on cerebral lipogensis from labelled acetate. These findings show (i) the conversion of 3-hydroxybutyrate to acetoacetate by 3-hydroxybutyrate dehydrogenase in the brain is inhibited by methylmalonate, and (ii) an inhibition of cerebral lipid synthesis by propionate, which also accumulates in patients with methylmalonic aciduria. 相似文献
28.
Tadeusz J. Kawecki Berra Erkosar Cindy Dupuis Brian Hollis R. Craig Stillwell Martin Kapun 《Molecular biology and evolution》2021,38(7):2732
Periods of nutrient shortage impose strong selection on animal populations. Experimental studies of genetic adaptation to nutrient shortage largely focus on resistance to acute starvation at adult stage; it is not clear how conclusions drawn from these studies extrapolate to other forms of nutritional stress. We studied the genomic signature of adaptation to chronic juvenile malnutrition in six populations of Drosophila melanogaster evolved for 150 generations on an extremely nutrient-poor larval diet. Comparison with control populations evolved on standard food revealed repeatable genomic differentiation between the two set of population, involving >3,000 candidate SNPs forming >100 independently evolving clusters. The candidate genomic regions were enriched in genes implicated in hormone, carbohydrate, and lipid metabolism, including some with known effects on fitness-related life-history traits. Rather than being close to fixation, a substantial fraction of candidate SNPs segregated at intermediate allele frequencies in all malnutrition-adapted populations. This, together with patterns of among-population variation in allele frequencies and estimates of Tajima’s D, suggests that the poor diet results in balancing selection on some genomic regions. Our candidate genes for tolerance to larval malnutrition showed a high overlap with genes previously implicated in acute starvation resistance. However, adaptation to larval malnutrition in our study was associated with reduced tolerance to acute adult starvation. Thus, rather than reflecting synergy, the shared genomic architecture appears to mediate an evolutionary trade-off between tolerances to these two forms of nutritional stress. 相似文献
29.
Laurent Pothuaud Bert Van Rietbergen Cindy Charlot Eugene Ozhinsky 《Computer methods in biomechanics and biomedical engineering》2013,16(4):205-213
Micro-finite element (FE) analysis is a well established technique for the evaluation of the elastic properties of trabecular bone, but is limited in its application due to the large number of elements that it requires to represent the complex internal structure of the bone. In this paper, we present an alternative FE approach that makes use of a recently developed 3D-Line Skeleton Graph Analysis (LSGA) technique to represent the complex internal structure of trabecular bone as a network of simple straight beam elements in which the beams are assigned geometrical properties of the trabeculae that they represent. Since an enormous reduction of cputime can be obtained with this beam modeling approach, ranging from approximately 1,200 to 3,600 for the problems investigated here, we think that the FE modeling technique that we introduced could potentially constitute an interesting alternative for the evaluation of the elastic mechanical properties of trabecular bone. 相似文献
30.
Shannon B. Johnson Anders Warén Verena Tunnicliffe Cindy Van Dover C. Geoffrey Wheat Thomas F. Schultz 《分类学与生物多样性》2013,11(3):278-295
Large symbiont-hosting snails of the genus Alviniconcha (Gastropoda: Abyssochrysidae) are among the dominant inhabitants of hydrothermal vents in the Western Pacific and Indian oceans. The genus was originally described as monotypic, but unique DNA sequences for mitochondrial genes revealed six distinct evolutionary lineages that we could not distinguish based on external morphology. Subsumed under the name Alviniconcha hessleri Okutani & Ohta, the distinct allopatric and sympatric lineages have been assigned placeholder epithets that complicate scientific communications. Based on the present multi-gene sequence data, we hereby describe five Alviniconcha species (in the order of their discovery) – A. kojimai sp. nov., A. boucheti sp. nov., A. marisindica sp. nov., A. strummeri sp. nov. and A. adamantis sp. nov. Thus, we restrict application of the name A. hessleri to specimens that are genetically similar (≥95% for COI) to those found at localities in the Mariana Trough. Single distinct Alviniconcha species inhabit vent fields along the Central Indian Ridge, the Mariana volcanic arc, and the Mariana back-arc basin, whereas vents in the Manus, Fiji and Lau back-arc basins may host two or three additional species. Formal recognition of these species facilitates future attempts to assess their physiological differences and symbiont associations. Furthermore, their reported distributions have significant biogeographic implications, affecting estimates of the diversity within and overlap among Indo-Pacific vent localities.http://zoobank.org/urn:lsid:zoobank.org:pub:1E4B2E71-9F1D-479E-9A9A-22A9E303AAE5 相似文献