全文获取类型
收费全文 | 438篇 |
免费 | 47篇 |
国内免费 | 9篇 |
专业分类
494篇 |
出版年
2022年 | 4篇 |
2021年 | 5篇 |
2020年 | 4篇 |
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 6篇 |
2015年 | 22篇 |
2014年 | 23篇 |
2013年 | 18篇 |
2012年 | 33篇 |
2011年 | 30篇 |
2010年 | 27篇 |
2009年 | 18篇 |
2008年 | 11篇 |
2007年 | 12篇 |
2006年 | 19篇 |
2005年 | 11篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 9篇 |
2001年 | 9篇 |
2000年 | 9篇 |
1999年 | 11篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 5篇 |
1995年 | 4篇 |
1994年 | 10篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1982年 | 4篇 |
1979年 | 8篇 |
1977年 | 5篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 9篇 |
1971年 | 5篇 |
1969年 | 5篇 |
1955年 | 3篇 |
1954年 | 3篇 |
1942年 | 3篇 |
1937年 | 5篇 |
排序方式: 共有494条查询结果,搜索用时 15 毫秒
101.
William J. Kersten Robert R. Brooks Roger D. Reeves Anguy Jaffré 《Phytochemistry》1980,19(9):1963-1965
The nature of nickel complexes in various Ni accumulating plant species, mainly from New Caledonia, was investigated by techniques including gel chromatography, ion-exchange chromatography, high performance liquid chromatography, and a combination of gas-liquid chromatography and mass spectrometry. Psychotria douarrei contained Ni complexed mainly (63%) as a negatively-charged malate complex balanced by a cationic aquo complex. Phyllanthus serpentinus had anionic Ni bound as 42% citrate and 4O% malate. All other species studied, contained Ni as an anionic citrate complex. 相似文献
102.
Sven Dnicke Erik Bannert Tanja Tesch Susanne Kersten Jana Frahm Susanne Bühler Helga Sauerwein Solvig Grs Stefan Kahlert Hermann-Josef Rothktter Cornelia C Metges Jeannette Kluess 《Innate immunity》2020,26(8):716
The sensitivity of pigs to deoxynivalenol (DON) might be influenced by systemic inflammation (SI) which impacts liver. Besides following acute-phase proteins, our aim was to investigate both the hepatic fractional albumin (ALB) synthesis rate (FSR) and the ALB concentration as indicators of ALB metabolism in presence and absence of SI induced by LPS via pre- or post-hepatic venous route. Each infusion group was pre-conditioned either with a control diet (CON, 0.12 mg DON/kg diet) or with a DON-contaminated diet (DON, 4.59 mg DON/kg diet) for 4 wk. A depression of ALB FSR was observed 195 min after LPS challenge, independent of feeding group or LPS application route, which was not paralleled by a down-regulated ALB mRNA expression but by a reduced availability of free cysteine. The drop in ALB FSR only partly explained the plasma ALB concentrations which were more depressed in the DON-pre-exposed groups, suggesting that ALB levels are influenced by further mechanisms. The abundances of haptoglobin, C-reactive protein, serum amyloid A, pig major acute-phase protein, fibrinogen and LPS-binding protein mRNA were up-regulated upon LPS stimulation but not accompanied by increases in the plasma concentrations of these proteins, pointing at an imbalance between synthesis and consumption. 相似文献
103.
The modulation of the Ca2+- (or Pb2+-)activated K+ permeability in human erythrocytes by vanadate, menadione and chloro-substituted menadione analogs was investigated by measurements of K+ fluxes and single-channel currents. Vanadate and menadione stimulate the K+ permeability by increasing the probability of channel openings; the menadione analogs, on the other hand, inhibit the K+ permeability by increasing the probability of channel closings. The compounds used in these experiments also interact with oxidoreductases; it is demonstrated that menadione analogs in contrast to menadione strongly inhibit the membrane-bound dehydrogenase in the erythrocytes. Concentrations of Pb2+ above 10 mumol/l, but not of Ca2+, inhibit the enzyme activity as well as the K+ permeability. The parallel effects on dehydrogenase activity and the K+ channels suggest a direct relationship between these two systems in the membrane of erythrocytes. 相似文献
104.
Sven Dänicke Janine Winkler Ulrich Meyer Susanne Kersten Kerstin Wernike Martin Beer Jana Frahm 《Mycotoxin Research》2018,34(2):123-139
The Fusarium toxin deoxynivalenol (DON) is a frequent contaminant of feedstuffs and is supposed to interfere with immune responses. As the relevance for growing bulls is less clear than for other livestock, the trial was designed according to the dose-response principal with a control group fed a diet with background contamination (CON, 0.36 mg DON per kilogram dry matter [DM]) and three groups with increasing concentrations of DON (mg/kg DM); FUS I, 3.01; FUS II, 5.66; FUS III, 8.31. Half of each treatment group was vaccinated against BVDV at days 1 and 21 of the 70 days lasting experiment. Sequential blood samples were collected for determination of antibody titers to BVDV and for hematological and clinical-chemical traits. Antibody response was strongest in group FUS II while group FUS III responded weakest. This group showed the lowest proportion of CD4+ T cells, but also the highest levels of liver lesion indicating enzyme activities in blood. BVDV-vaccination induced a pronounced decrease in red blood count indices, which occurred dose-dependently at a higher level in the FUS-fed groups. The obvious interactions between DON exposure and BVDV-vaccination require further elucidation. 相似文献
105.
Tyrosine phosphorylation of Munc18‐1 inhibits synaptic transmission by preventing SNARE assembly 下载免费PDF全文
Marieke Meijer Bernhard Dörr Hanna CA Lammertse Chrysanthi Blithikioti Jan RT van Weering Ruud FG Toonen Thomas H Söllner Matthijs Verhage 《The EMBO journal》2018,37(2):300-320
Tyrosine kinases are important regulators of synaptic strength. Here, we describe a key component of the synaptic vesicle release machinery, Munc18‐1, as a phosphorylation target for neuronal Src family kinases (SFKs). Phosphomimetic Y473D mutation of a SFK phosphorylation site previously identified by brain phospho‐proteomics abolished the stimulatory effect of Munc18‐1 on SNARE complex formation (“SNARE‐templating”) and membrane fusion in vitro. Furthermore, priming but not docking of synaptic vesicles was disrupted in hippocampal munc18‐1‐null neurons expressing Munc18‐1Y473D. Synaptic transmission was temporarily restored by high‐frequency stimulation, as well as by a Munc18‐1 mutation that results in helix 12 extension, a critical conformational step in vesicle priming. On the other hand, expression of non‐phosphorylatable Munc18‐1 supported normal synaptic transmission. We propose that SFK‐dependent Munc18‐1 phosphorylation may constitute a potent, previously unknown mechanism to shut down synaptic transmission, via direct occlusion of a Synaptobrevin/VAMP2 binding groove and subsequent hindrance of conformational changes in domain 3a responsible for vesicle priming. This would strongly interfere with the essential post‐docking SNARE‐templating role of Munc18‐1, resulting in a largely abolished pool of releasable synaptic vesicles. 相似文献
106.
107.
铁皮石斛的离体开花 总被引:9,自引:0,他引:9
铁皮石斛(Dendrobium candidum),为一种野生兰科植物,在栽培条件下,从种子萌发到开花通常需要3~4a.研究了多种植物激素和多胺对该种石斛组织培养中花芽形成的影响,结果表明在培养基中加入合适浓度的亚精胺(spermidine)或BA(6-苄基腺嘌呤),或同时加入NAA(萘乙酸)和BA均可诱导原球茎或由之形成的无根小苗在3~6个月开花,频率在31.6%~45.8%.当将原球茎在加有ABA(脱落酸)的培养基上预培养后再移到加有BA的培养基上,花芽形成的频率可提高到平均达82.8%(个别实验中可达100%),这种诱导提早开花的现象也与实验材料的发育阶段(原球茎、无根小苗、已生根的小苗)有关,通常发生在根的形成受到完全或部分抑制的情况中. 相似文献
108.
109.
Efficiencies of different genes and different tree-building methods in recovering a known vertebrate phylogeny 总被引:24,自引:6,他引:18
The relative efficiencies of different protein-coding genes of the
mitochondrial genome and different tree-building methods in recovering a
known vertebrate phylogeny (two whale species, cow, rat, mouse, opossum,
chicken, frog, and three bony fish species) was evaluated. The
tree-building methods examined were the neighbor joining (NJ), minimum
evolution (ME), maximum parsimony (MP), and maximum likelihood (ML), and
both nucleotide sequences and deduced amino acid sequences were analyzed.
Generally speaking, amino acid sequences were better than nucleotide
sequences in obtaining the true tree (topology) or trees close to the true
tree. However, when only first and second codon positions data were used,
nucleotide sequences produced reasonably good trees. Among the 13 genes
examined, Nd5 produced the true tree in all tree-building methods or
algorithms for both amino acid and nucleotide sequence data. Genes Cytb and
Nd4 also produced the correct tree in most tree-building algorithms when
amino acid sequence data were used. By contrast, Co2, Nd1, and Nd41 showed
a poor performance. In general, large genes produced better results, and
when the entire set of genes was used, all tree-building methods generated
the true tree. In each tree-building method, several distance measures or
algorithms were used, but all these distance measures or algorithms
produced essentially the same results. The ME method, in which many
different topologies are examined, was no better than the NJ method, which
generates a single final tree. Similarly, an ML method, in which many
topologies are examined, was no better than the ML star decomposition
algorithm that generates a single final tree. In ML the best substitution
model chosen by using the Akaike information criterion produced no better
results than simpler substitution models. These results question the
utility of the currently used optimization principles in phylogenetic
construction. Relatively simple methods such as the NJ and ML star
decomposition algorithms seem to produce as good results as those obtained
by more sophisticated methods. The efficiencies of the NJ, ME, MP, and ML
methods in obtaining the correct tree were nearly the same when amino acid
sequence data were used. The most important factor in constructing reliable
phylogenetic trees seems to be the number of amino acids or nucleotides
used.
相似文献
110.
Biosynthetic pathway of ribothymidine in B. subtilis and M. lysodeikticus involving different coenzymes for transfer RNA and ribosomal RNA. 总被引:3,自引:1,他引:2 下载免费PDF全文
Ribothymidine (m5u) in tRNAs of M. lysodeikticus is not derived from methionine. The results indicate that as in tRNAs of B. subtilis a tetrahydrofolate derivative is involved in the formation of m5U, whereas methionine serves as precursor in the biosynthesis of m7G, m1A and m6A. Ribothymidine also occurs in 23S rRNA of B. subtilis and M. lysodeikticus. Approximately 2-3 moles of m5U residues were found per mole of 23S rRNA. In contrast to m5U residues present in tRNAs of B. subtilis and M. lysodeikticus, ribothymidine in 23S rRNA of these organisms and of E. coli is synthesized via S-adenosylmethionine. m6A and m1G, present in E. coli rRNAs, were not detected in rRNAs of (methyl-14C) methionine labeled B. subtilis and M. lysodeikticus. 相似文献