全文获取类型
收费全文 | 5232篇 |
免费 | 387篇 |
专业分类
5619篇 |
出版年
2023年 | 16篇 |
2022年 | 39篇 |
2021年 | 94篇 |
2020年 | 41篇 |
2019年 | 59篇 |
2018年 | 82篇 |
2017年 | 64篇 |
2016年 | 140篇 |
2015年 | 253篇 |
2014年 | 265篇 |
2013年 | 323篇 |
2012年 | 381篇 |
2011年 | 349篇 |
2010年 | 238篇 |
2009年 | 199篇 |
2008年 | 286篇 |
2007年 | 284篇 |
2006年 | 256篇 |
2005年 | 276篇 |
2004年 | 274篇 |
2003年 | 229篇 |
2002年 | 211篇 |
2001年 | 62篇 |
2000年 | 54篇 |
1999年 | 50篇 |
1998年 | 70篇 |
1997年 | 60篇 |
1996年 | 52篇 |
1995年 | 64篇 |
1994年 | 55篇 |
1993年 | 50篇 |
1992年 | 36篇 |
1991年 | 43篇 |
1990年 | 32篇 |
1989年 | 33篇 |
1988年 | 39篇 |
1987年 | 36篇 |
1986年 | 30篇 |
1985年 | 26篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 18篇 |
1981年 | 27篇 |
1980年 | 21篇 |
1979年 | 30篇 |
1977年 | 29篇 |
1976年 | 20篇 |
1973年 | 13篇 |
1971年 | 15篇 |
1959年 | 14篇 |
排序方式: 共有5619条查询结果,搜索用时 0 毫秒
11.
Observations on territoriality and the associated odour-marking and foraging behaviours in male Pachymelus limbarus Saussure, a Malagasy endemic anthophorid bee, are reported. Data indicated a monandrous mating system where virgin females are attracted, probably by pheromonal cues, to male-patrolled territories associated with flowering, nectariferous Plectran- rhussensu lato species (Lamiaceae). One male observed in detail applied odour marks every 30–80 minutes on twigs around a food-resource area that contained c. 850 flowers. This marking behaviour always preceded territory residence. The male spent c. 80% of his time patrolling. Meanwhile, any intruding flying insect was inspected or chased, which caused conspecific males, as well as often females, to leave the site. The resident male fed frequently on flowers within his territory (c. 3,600 flower-visits on fine days), except immediately following odour-marking bouts. His flower-visiting was often released by interactions with intruding insects entering the territory. The observations provide the first example of male territoriality based on resource defence polygyny in the bee family Anthophoridae. The importance of resource distribution in the evolution of male territoriality in bees is discussed. 相似文献
12.
The structure of human erythrocytic carbonic anhydrase II has been refined by constrained and restrained structure–factor least-squares refinement at 2.0 Å resolution. The conventional crystallographic R value is 17.3%. Of 167 solvent molecules associated with the protein, four are buried and stabilize secondary structure elements. The zinc ion is ligated to three histidyl residues and one water molecule in a nearly tetrahedral geometry. In addition to the zinc-bound water, seven more water molecules are identified in the active site. Assuming that Glu-106 is deprotonated at pH 8.5, some of the hydrogen bond donor–acceptor relations in the active site can be assigned and are described here in detail. The Oγ1 atom of Thr-199 donates its proton to the Oε1 atom of Glu-106 and can function as a hydrogen bond acceptor only in additional hydrogen bonds. 相似文献
13.
Maria Anvret Margareta Blombäck Monika Lindstedt Elisabeth Söderlind Margareta Tapper-Persson Ann-Christine Thelander 《Human genetics》1992,89(2):147-154
Summary Twenty-five patients with von Willebrand's disease (vWD) type III were analysed with regard to blood coagulation variables and possible deletions. Nine of the probands and their families were further investigated with DNA linkage analyses. Different patterns of heredity can be suggested in our families with vWD type III, on the basis of blood coagulation analyses. The findings suggest homozygosity in five families and the possibility of compound heterozygosity or a new mutation in the proband in three families. The linkage analyses confirm the results of the coagulation analyses. The segregation of the von Willebrand factor (vWF) gene can be followed in the families, and carrier diagnosis can be made in several of the probands' relatives. The possibility of large deletions in the vWF gene of the probands and their parents was investigated with probes representing the whole vWF cDNA. No deletions were found.This study was approved by the Ethics Committee of the Karolinska Hospital (No. 84:1) 相似文献
14.
A coiling-inducing factor was isolated from tendrils of Bryonia dioica Jacq. and identified by infrared, 1H-, 13C-nuclear magnetic resonance and mass spectrometry as -linolenic acid. When applied to detached tendrils, exogenous -linolenic acid, but not linoleic acid or oleic acid, induced tendril coiling. Further investigations showed that metabolites of -linolenic acid, jasmonic acid and, even more so, methyljasmonate, are highly effective inducers of tendril coiling in B. dioica. Methyljasmonate was most active when administered by air and, in atmospheric concentrations as low as 40–80 nM, induced a full free-coiling response with kinetics similar to mechanical stimulation. Even atmospheric levels as low as 4–5 nM methyljasmonate were still found to be significantly active. Methyljasmonate could be one of the endogenous chemical signals produced in mechanically stimulated parts of a tendril and, being highly volatile, act as a diffusible gaseous mediator spreading through the intracellular spaces to trigger free coiling of tendrils.Abbreviations EI-MS
electron impact-mass spectrometry
- HPLC
high-performance liquid chromatography
- IAA
indole-3-acetic acid
- NMR
nuclear magnetic resonance
- TFA
trifluoroacetic acid
We are indebted to the Deutsche Forschungsgemeinschaft, Bonn and the Fonds der Chemischen Industrie, Frankfurt (literature provision) for their support and to Dr. C. Brückner, Halle, for jasmonic-acid determinations. 相似文献
15.
Enhancement of Oxygen-Evolution in Photosystem II-Phycobilisome Particles from Porphyridium cruentum
Oxygen-evolving photosystem II-phycobilisome particles froma red alga were inhibited 5080% by aging, dilution, lowpH and salt-washing. Bovine serum albumin, and dithiothreitolwere found to stimulate activity in all but salt-washed particles.CaCl2 and MnCl2 partially restored activity lost after agingor dilution.
1Current address: Waksman Institute of Microbiology, RutgersUniversity, Piscataway, New Jersey 08854-0759, U.S.A. (Received October 5, 1985; Accepted March 31, 1986) 相似文献
16.
Synthesis and phosphorylation of cytoskeleton components in foetal,regenerating and adult normal rat hepatocytes during culture 总被引:1,自引:0,他引:1
Georges Baffet Annie Ruelland Bruno Clement Elisabeth Le Rumeur Siegmund Fischer 《Molecular and cellular biochemistry》1985,68(2):97-105
Summary Detergent insoluble material (DIM) was prepared by gentle treatment with detergent from foetal, regenerating and adult normal rat hepatocytes cultured for various times. It retained to some degree the morphology of the cells. After incubation of intact cells with 35S-methionine, most of the labelled DIM proteins were found to be components of the cytoskeleton. They included several cytokeratins, vimentin and actin. The synthesis rate varied with the age of animals and culture conditions. The high synthetic rate of vimentin in foetal and regenerating hepatocytes could be associated with cell proliferation. No correlation was found between cytokeratin synthesis and hepatocyte growth. Most of the cytoskeleton proteins could be phosphorylated in intact cells and in DIM from cultured hepatocytes. However the degree of phosphorylation of these proteins was not related to their synthetic rate. The decreased phosphorylation level in cultured adult rat hepatocytes could be related to the rapid loss of specific functions. 相似文献
17.
Lidia Sautebin Hans Kindahl Maria Kumlin Elisabeth Granstrm 《Prostaglandins & other lipid mediators》1985,30(3):435-456
Conjugates of prostaglandins and thromboxanes with tritium labeled amino acids were prepared and employed as labeled ligands in porstaglandin and thromboxane radioimmunoassays. Assays for PGF2α, 15-keto-13, 14-dihydro-PGF2α, TXB2 and 15-keto-13, 14-dihydro-TXB2 were evaluated in comparative studies using either these heterologous ligands or the corresponding homologus tritiated eicosanoid as tracers. Binding properties for the respective antibodies were found to be similar using either tracer.Three biological studies were also conducted, viz. study of the release of TXB2 during collagen induced platelet aggregation, of 15-keto-13, 14-dihydro-TXB2 during guinea pig pulmonary anaphylaxis, and of PGF2α (measured as 15-keto-13, 14-dihydro-PGF2α in peripheral plasma) during bovine luteolysis. The analyses gave comparable results using either the heterologous or the homologous assay.Thus, this type of labeled prostanoid conjugates may serve as a convenient alternative to homologous tracers in radioimmunoassay. Heterologous tracers may even in certain cases provide the only simple solution to the problem of preparing a labeled ligand of high specific activity. 相似文献
18.
Jean-Claude Wissocq Catherine Heurteaux Elisabeth Hennequin Michel Thellier 《Development genes and evolution》1985,194(7):433-435
Summary The quantitative imaging of lithium distribution, in histological sections of 15-days old mouse embryos (whose mother had been submitted to Li-treatment), was performed using6Li isotope as tracer,6Li(n,)3H nuclear reaction for detection, and dielectric track detectors. Despite the particular difficulties of cryosectioning the embryos without disturbing the lithium distribution, the Li regionalization appeared to be very clear-cut. The ectomesodermic tissues were significantly more loaded with lithium than the endodermic ones. This is probably related to the ectomesodermic tissues being also those most sensitive to the teratogenic effect of lithium. The Li-distribution in the embryo brain was almost homogeneous, instead of being heterogeneous as in adult brain. The mean Li-concentration in the embryo brain was not much below the Li concentration in the grey matter of the mother brain, but it was significantly larger than that in the white matter of the mother brain. Our results are discussed in the context of teratogenic effects observed in situ during mammalian development. 相似文献
19.
Summary DNA haplotype constellations of the β-globin gene cluster have been analyzed in German families with hemoglobinopathies (Hb
Freiburg, Hb K?ln, Hb Presbyterian) and β-thalassemias. The polymorphis patterns obtained were compared to those found in
families from Greece, Italy, and Turkey affected by β-thalassemia syndromes. With the combined analysis of seven restriction
site polymorphisms a DNA-diagnostic prediction for additional offspring could be made with an overall frequency of 75% in
the four ethnic groups. 相似文献
20.
4-Trifluoroacetamidoaniline was reacted with reducing oligosaccharides in the presence of sodium cyanoborohydride to give aminoalditol derivatives, useful for linkage to proteins or solid matrices. A mixture of reducing oligosaccharides, difficult to separate by HPLC, was treated in the same way. The resulting derivatives were easily separated by HPLC.Abbreviations TFAN
4-trifluoroacetamidoaniline
- LcOse4
lacto-N-tetraose
- IV2Fuc-LcOse4
lacto-N-fucopentaose l
- III4Fuc-LcOse4
lacto-N-fucopentaose II
- III3Fuc-nLcOse4
lacto-N-fucopentaose III
- IV2Fuc, III4Fuc-LcOse4
lacto-N-difucohexaose I
- II6Galß1-4GlcNAc-LcOse4
lacto-N-hexaose
- II3NeuAc-Lac
3-sialyllactose
- GlcNAcß1-4GlcNAcß1-4GlcNAc
chitotriose
- GalNac1-3|Fuc1-2|Galß1-4Glc
A-tetrasaccharide 相似文献