全文获取类型
收费全文 | 2650篇 |
免费 | 289篇 |
专业分类
2939篇 |
出版年
2022年 | 17篇 |
2021年 | 27篇 |
2020年 | 18篇 |
2019年 | 20篇 |
2018年 | 30篇 |
2017年 | 35篇 |
2016年 | 57篇 |
2015年 | 84篇 |
2014年 | 94篇 |
2013年 | 127篇 |
2012年 | 137篇 |
2011年 | 154篇 |
2010年 | 102篇 |
2009年 | 90篇 |
2008年 | 124篇 |
2007年 | 137篇 |
2006年 | 105篇 |
2005年 | 104篇 |
2004年 | 103篇 |
2003年 | 111篇 |
2002年 | 90篇 |
2001年 | 80篇 |
2000年 | 107篇 |
1999年 | 89篇 |
1998年 | 33篇 |
1997年 | 45篇 |
1996年 | 35篇 |
1995年 | 38篇 |
1994年 | 39篇 |
1993年 | 37篇 |
1992年 | 58篇 |
1991年 | 57篇 |
1990年 | 60篇 |
1989年 | 47篇 |
1988年 | 33篇 |
1987年 | 35篇 |
1986年 | 48篇 |
1985年 | 34篇 |
1984年 | 18篇 |
1983年 | 20篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 16篇 |
1976年 | 18篇 |
1975年 | 14篇 |
1973年 | 16篇 |
1972年 | 21篇 |
1970年 | 15篇 |
排序方式: 共有2939条查询结果,搜索用时 15 毫秒
51.
Somatic embryos in liquid culture can serve as a mass cloning system in a plant propagation program. A quantitative formulation of embryo development obtained from cell suspension cultures is used to develop a segregated kinetic model. The model is based on standard classification schemes as previously developed by plant physiologists. Dependent variables include carbohydrate concentrations (sucrose, fructose, and glucose) and biomass apportioned among the inoculum (free single cells, cell clusters), normal developmental stages, and aberrant cell and embryo types. Good agreement between the model and experimental results is indicated and allows for a rigorous approach to media optimization and reactor scaleup for embryo formation. 相似文献
52.
Formation of (4R)- and (4S)-4-hydroxyochratoxin A from ochratoxin A by liver microsomes from various species. 总被引:7,自引:6,他引:1 下载免费PDF全文
F C Strmer C E Hansen J I Pedersen G Hvistendahl A J Aasen 《Applied microbiology》1981,42(6):1051-1056
Two metabolic products were formed from ochratoxin A by human, pig, and rat liver microsomal fractions in the presence of reduced nicotinamide adenine dinucleotide phosphate. They were isolated from the incubation mixture in the presence of pig liver microsomes by extraction, thin-layer chromatography, and high-pressure liquid chromatography Their structures are suggested to be (4R)- and (4S)-4-hydroxyochratoxin A on the basis of mass and nuclear magnetic resonance spectroscopy. Km and the maximum velocity for the formation of the two metabolites by human, pig, and rat microsomes were determined. Their formation was inhibited by carbon monoxide and metyrapone. The results indicate that the microsomal hydroxylation system is a cytochrome P-450 and that different species are involved in the formation of the two epimeric forms of 4-hydroxyochratoxin A. 相似文献
53.
Analysis of the genome of an endogenous, ecotropic retrovirus of the AKR strain of mice: micromethod for detailed characterization of high-molecular-weight RNA. 总被引:18,自引:13,他引:5 下载免费PDF全文
A detailed characterization of the genome of an endogenous, ecotropic type C virus, the Akv virus, is presented. Approximately 100 RNase T1-resistant oligonucleotides characteristic of the Akv genome were identified by two-dimensional gel electrophoresis, and the complete nucleotide sequence is presented for 75 of these oligonucleotides. A correspondence between the sequence of some of these oligonucleotides and the amino acid sequence of some virus-coded gag gene proteins is reported. For this study we developed methods suitable for the analysis of high-molecular-weight RNA species in nanogram quantities. The in vitro labeling procedures used here led to uniform labeling of the unique oligonucleotides. 相似文献
54.
Summary Aqueous solutions of lanthanum nitrate may be used as electron microscopic tracers in vivo to study vascular permeability in the experimental animal. However, with this technique the size of the tracer particles is not known. To gain information about the tracer size, we injected lanthanum nitrate into the blood circulation of living rabbits. The plasma obtained from such animals 30 min later, was studied with the electron microscope. The plasma contained an electron-dense material, readily visible in the electron microscope. A precipitate obtained after centrifugation of the whole blood to separate the cells, also contained the tracer. Lanthanum was found in large amounts in the fibrin clot obtained after treating the plasma with thrombin. The tracer was not detected in the serum (i.e. thrombin-treated plasma). The study indicates that ionic lanthanum injected into the blood circulation of living rabbits, is to a great extent bound to fibrinogen, and that the smallest possible size of the tracer is that of the fibrinogen,molecule (m. w. 330,000). Larger particles are present as well. 相似文献
55.
R C Shearman W L Pedersen R V Klucas E J Kinbacher 《Canadian journal of microbiology》1979,25(10):1197-1200
Associative nitrogen fixation in Kentucky bluegrass (Poa pratensis L.) turfs inoculated with five nitrogen-fixing bacterial isolates was evaluated using the acetylene reduction assay and nitrogen accumulation as indicators of fixation. 'Park' and 'Nugget' Kentucky bluegrass turfs were grown in controlled environment chambers and inoculated with Klebsiella pneumoniae (W-2, W-6, and W-14), Erwinia herbicola (W-8), and Enterobacter cloacae (W-11). 'Park' inoculated with K. pneumoniae (W-6) had significant acetylene reduction activity using undisturbed turfs. Other treatments including turfs treated with heat-killed cells had no significant difference in acetylene reduction. In a second study, "Park' and 'South Dakota Certified' turfs were grown in a greenhouse and inoculated with K. pneumoniae (W-6) and E. herbicola (W-8). 'Park' inoculated with K. pneumoniae (W-6) had increased acetylene reduction activity rates and also a greater nitrogen accumulation in aerial tissues when compared to controls. Acetylene reduction activity was correlated (r = 0.92) to nitrogen accumulation. Other treatments did not effectively increase acetylene reduction activity or nitrogen accumulation. 相似文献
56.
Nitrogen fixation (acetylene reduction) associated with roots of winter wheat and sorghum in Nebraska. 总被引:13,自引:11,他引:2 下载免费PDF全文
Root segments and root-soil cores (6.5-cm diameter) from fields and nurseries of winter wheat and sorghum were tested for N2 fixation by using the acetylene reduction assay. Wheat samples (approximately 1,200) from 109 sites generally had low or no activity (0 to 3.1 nmol of C2H4 produced per h per g [dry weight] of root segments), even after 24 h of incubation. However, a commercial field of Scout 66, located in western Nebraska, exhibited appreciable activity (290 nmol of C2H4 produced per h per g [dry weight] of root segments). Of 400 sorghum lines and crosses, grain sorghums (i.e., CK-60A, Wheatland A, B517, and NP-16) generally exhibited higher nitrogenase activity than forage sorghums or winter wheats. CK-60A, a male sterile grain sorghum, was sampled at four locations and had the most consistent activity of 24 to 1,100 nmol of C2H4 produced per h per core. The maximum rate extrapolated to 2.5 g of N per hectare per day. Numerous N2-fixing bacterial isolates were obtained from wheat and sorghum roots that exhibited high nitrogenase activity. Most isolates were members of the Enterobacteriacae, i.e., Klebsiella pneumoniae, Enterobacter cloacae, and Erwinia herbicola. 相似文献
57.
Steen Pedersen Solveig V. Reeh Jack Parker Robert J. Watson James D. Friesen Niels P. Fiil 《Molecular & general genetics : MGG》1976,144(3):339-343
Summary The presence of EF-Tu, RNA polymerase subunit , and EF-G on the dfus-3 genome and EF-Tu, ribosomal proteins L7/L12, and RNA polymerase subunit on the drif
d
18 genome has been confirmed using a two-dimensional gel electrophoresis technique sensitive to changes in isoelectric point and molecular weight. In this system two EF-Tu gene products could not be resolved. Following infection of ultraviolet light-irradiated Escherichia coli with either dfus-3 or drif
d
18, the EF-Tu gene, tufA, near 65 minutes on the genetic map is expressed as 3–4 copies per EF-G molecule. The EF-Tu gene, tufB, near 79 minutes on the genetic map, is expressed at about one-third of this rate. is expressed as 1 copy per EF-G molecule, as 0.14 per EF-G molecule and L7/L12 as 2.5 per EF-G. These figures compare well with the relative amounts found in exponentially-growing cells, in which the ratio of EF-Tu to EF-G is approximately 5. Almost 90% of the total number of proteins (calculated on a molecular weight basis) which theoretically can be encoded on the drif
d
18 have been identified on the two-dimensional gel. 相似文献
58.
59.
Cobinding of bilirubin and sulfonamide and of two bilirubin molecules to human serum albumin: a site model 总被引:1,自引:0,他引:1
Differential light absorption spectra of the bilirubin-albumin 1:1 complex, obtained on addition of 20 different sulfonamides, differ with respect to shape and amplitude. This finding seems to indicate that the sulfonamide molecule is bound in direct touch with the bilirubin. The light absorption spectrum of bilirubin-albumin 1:1 undergoes changes on cobinding of a fatty acid anion, laurate, and on variation of pH, previously explained by a change of dihedral angle between the two chromophores of the bilirubin molecule. In bilirubin-albumin 2:1, binding of laurate and variation of pH cause little change of the spectrum. This is best explained by binding of the two bilirubin molecules in close proximity, preventing conformational changes in the complex. From measurements of fluorescence of the lone tryptophan group in albumin and quenching on binding of bilirubin, we calculated the distance of 22 A from tryptophan to the first bound bilirubin molecule, and of 18 A to the second. Mutual quenching of the bilirubin fluorescence from two bound bilirubin molecules seemed to indicate that the two are bound closely together. A model of bilirubin-albumin with a binding site capable of accommodating one bilirubin and one sulfonamide molecule, or two molecules of bilirubin, is compatible with our findings. 相似文献
60.
Chronic Helicobacter pylori Infection Does Not Significantly Alter the Microbiota of the Murine Stomach 总被引:1,自引:0,他引:1 下载免费PDF全文
Mai Ping Tan Maria Kaparakis Maja Galic John Pedersen Martin Pearse Odilia L. C. Wijburg Peter H. Janssen Richard A. Strugnell 《Applied microbiology》2007,73(3):1010-1013
We examined the impact of Helicobacter pylori infection on the murine gastric microbiota by culture and terminal-restriction fragment length polymorphism and found that neither acute nor chronic H. pylori infection substantially affected the gastric microbial composition. Interestingly, the total H. pylori burden detected by real-time PCR was significantly higher than that revealed by viable counts, suggesting that the antigenic load sustaining H. pylori-induced gastritis could be considerably higher than previously believed. 相似文献