全文获取类型
收费全文 | 6588篇 |
免费 | 448篇 |
国内免费 | 1篇 |
出版年
2023年 | 22篇 |
2022年 | 42篇 |
2021年 | 91篇 |
2020年 | 77篇 |
2019年 | 86篇 |
2018年 | 130篇 |
2017年 | 102篇 |
2016年 | 185篇 |
2015年 | 271篇 |
2014年 | 358篇 |
2013年 | 499篇 |
2012年 | 575篇 |
2011年 | 504篇 |
2010年 | 339篇 |
2009年 | 288篇 |
2008年 | 432篇 |
2007年 | 378篇 |
2006年 | 377篇 |
2005年 | 337篇 |
2004年 | 352篇 |
2003年 | 344篇 |
2002年 | 319篇 |
2001年 | 78篇 |
2000年 | 66篇 |
1999年 | 67篇 |
1998年 | 67篇 |
1997年 | 58篇 |
1996年 | 61篇 |
1995年 | 48篇 |
1994年 | 61篇 |
1993年 | 46篇 |
1992年 | 44篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 19篇 |
1987年 | 16篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 18篇 |
1983年 | 22篇 |
1982年 | 16篇 |
1981年 | 27篇 |
1980年 | 21篇 |
1979年 | 14篇 |
1978年 | 6篇 |
1977年 | 17篇 |
1976年 | 6篇 |
1975年 | 7篇 |
1974年 | 9篇 |
排序方式: 共有7037条查询结果,搜索用时 0 毫秒
31.
A M Bolognani Fantin A Franchini E Ottaviani L Benedetti 《Basic and applied histochemistry》1985,29(4):377-387
The effect of lead on ganglia of Viviparus ater were studied by histochemical and histomorphological procedures. The pollution experiment should be considered a "short-time static bioassay" because of its experimental characteristics. There was considerable accumulation of lead in the ganglia as determined by atomic absorbance (A.A.S.). The cytological damage principally affected the neuronal cell bodies which undergo degenerative processes. The most serious cytopathological changes occurred in the following sequence: nuclear damage leading to pyknosis; nucleolar damage until disappearance; changes in Nissl bodies, at times forming a uniform mass. These cytological disorders led to markedly altered protein synthesis. Nerve fibers and neuroglia did not appear affected by lead exposure, even at higher doses. Membrane enzymes, phosphorylase, NADHDH, NADPHDH and SDH activities were decreased, whereas D-LDH, G-6-PDH, G-6-Pase and MAO activities increased. GDH was unchanged. Changes in polar lipid composition were also observed with an increase of phospholipids and a decrease of sulpholipids and cerebrosides. 相似文献
32.
Estimation of the size of freshwater ciliate populations by a sub-sampling technique 总被引:4,自引:4,他引:0
Paolo Madoni 《Hydrobiologia》1984,111(3):201-206
A technique is described for the estimation of the size of ciliate populations based on replicated counts. Sample drops were
taken by automatic pipettes of different volume from activated sludge and from a small oligotrophic stream. Specific aims
were: 1) the estimation of the number of species; 2) the estimation of the number of individual in each species; 3) the selection
of a suitable sub-sample size and number of replicates. For each volume the cumulative increase in species taken in successive
sub-samples was determined. For each species the minimum permissible sub-sample volume and the number of replicates required
for an allowable error of 10% and 25% were determined. Lastly, the relationship between the mean number of individuals counted
for each species and the relative coefficient of variation was visualized. 相似文献
33.
Dion tomasellii sp. nov. occurs with an interrupted distribution from Guerrero to central Sonora. It is characterized by falcate to subfalcate leaflets. The populations from Sonora and northern Sinaloa are segregated asD. tomasellii var.sonorense on the basis of their narrower and glaucous leaflets. 相似文献
34.
Giorgio M. Hanozet Barbara Giordana Paolo Parenti Andrea Guerritore 《The Journal of membrane biology》1984,81(3):233-240
Summary In brush border membrane vesicles from the midgut ofPhilosamia cynthia larvae (Lepidoptera) thel- andd-alanine uptake is dependent on a potassium gradient and on transmembrane electrical potential difference. Each isomer inhibits the uptake of the other form: inhibition ofl-alanine uptake byd-alanine is competitive, whereas inhibition ofd-alanine uptake byl-alanine is noncompetitive. Transstimulation experiments as well as the different pattern of specificity to cations suggest the existence of two transport systems. Kinetic parameters for the two transporters have been calculated both when Kout>Kin and Kout=Kin.d-alanine is actively transported also by the whole midgut, but it is not metabolized by the intestinal tissue. 相似文献
35.
The isomerization of poly-L -proline in different solvents has been studied by NMR spectroscopy. Different resonance signals for the CHα protons have been obtained for the two different helical conformations of thus compound, namely form I and form II. 相似文献
36.
Persistent infection of rhesus macaques with a molecular clone of human immunodeficiency virus type 2: evidence of minimal genetic drift and low pathogenetic effects. 总被引:14,自引:13,他引:1
下载免费PDF全文
![点击此处可从《Journal of virology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
G Franchini P Markham E Gard K Fargnoli S Keubaruwa L Jagodzinski M Robert-Guroff P Lusso G Ford F Wong-Staal et al. 《Journal of virology》1990,64(9):4462-4467
In an attempt to generate a suitable animal model to study the infectivity and possible pathogenicity of human immunodeficiency viruses, we intravenously inoculated juvenile rhesus macaques and African green monkeys with a molecularly cloned virus, human immunodeficiency virus type 2 HIV-2sbl/isy, as well as with the uncloned HIV-2nih-z virus. Infection was monitored by virus recovery from the peripheral blood cells and by seroconversion against HIV-2 antigens measured by Western immunoblot, radioimmunoprecipitation, and enzyme-linked immunosorbent assay. We successfully infected two out of two macaques with the molecularly cloned virus and one macaque out of two with the HIV-2nih-z. No evidence of infection was seen in the African green monkeys with either virus. We followed the infected animals for 2 years. The animals remained healthy, although we observed intermittent lymphadenopathy and a transient decrease in the absolute number of circulating CD4+ T lymphocytes in both animals infected with the molecularly cloned virus. Virus isolation from the peripheral blood cells of the infected animals was successful only within the first few months after inoculation. Evidence of persistent infection was provided by the detection of proviral DNA by polymerase chain reaction analysis of the blood cells of the inoculated animals and by the stability of antiviral antibody titers. To evaluate the genetic drift of the proviral DNA, we molecularly cloned viruses which were reisolated 1 and 5 months postinoculation from one of these animals. Comparison of the DNA sequences of the envelope genes of both these isolates indicated that a low degree of variation (0.2%) in the envelope protein had occurred in vivo during the 5-month period. These data suggest that the use of HIV-2sbl/isy in rhesus macaques may represent a good animal model system to study prevention of viral infection. In particular, molecularly cloned virus can be manipulated for functional studies of viral genes in the pathogenesis of acquired immune deficiency syndrome and provides a reproducible source of virus for vaccine studies. 相似文献
37.
38.
The effect of tryptophan on the biosynthesis of proline has been investigated. Cells of Daucus carota grown in B5 medium supplemented with 5×10–4M tryptophan acquired the ability to grow in the presence of inhibitory concentrations of azetidine-2-carboxylic acid, an analog of proline. When trp was added to carrot cell cultures at sub-growth inhibiting concentrations, overproduction of intracellular free proline was observed. An increase was also observed for lys, his, ala, leu and phe. Likewise, the addition of asparagine, glutamic acid and phenylalanine to the medium stimulated the intracellular increase of free proline and other amino acids.Abbreviations A2CA
azetidine-2-carboxylic acid
- 2,4-D
2,4-dichlorophenoxyacetic acid
- 5MT
5-methyltryptophan
- P5C
pyrroline-5-carboxylic acid
- f.wt.
fresh weight
- d.wt.
dry weight 相似文献
39.
Analyses of wheat/rye addition lines by Southern blotting confirmed the presence of sequences related to theSec 1, Sec 2, andSec 3 loci on chromosomes 1R and 2R. Comparison of the 1R and 2R addition lines allowed the identification of -secalin genes atSec 1 andSec 2, respectively, while -secalin and -secalin genes atSec 1 were discriminated by comparative hybridization with three probes: -secalin, total -secalin, and 3 -secalin. The high molecular weight (HMW) secalin genes atSec 3 were identified using a homologous HMW subunit probe from wheat. Gene copy numbers were estimated as about 40–60 for -secalins, 5–10 for -secalins, and 2 for HMW secalins. Comparison of individual plants of cv. Gazelle showed a high degree of polymorphism, particularly for sequences related to -secalins and HMW secalins. 相似文献