全文获取类型
收费全文 | 5999篇 |
免费 | 437篇 |
国内免费 | 3篇 |
出版年
2021年 | 61篇 |
2020年 | 33篇 |
2019年 | 53篇 |
2018年 | 73篇 |
2017年 | 59篇 |
2016年 | 105篇 |
2015年 | 179篇 |
2014年 | 257篇 |
2013年 | 287篇 |
2012年 | 333篇 |
2011年 | 342篇 |
2010年 | 227篇 |
2009年 | 232篇 |
2008年 | 295篇 |
2007年 | 344篇 |
2006年 | 300篇 |
2005年 | 267篇 |
2004年 | 269篇 |
2003年 | 263篇 |
2002年 | 249篇 |
2001年 | 94篇 |
2000年 | 95篇 |
1999年 | 90篇 |
1998年 | 92篇 |
1997年 | 61篇 |
1996年 | 62篇 |
1995年 | 60篇 |
1994年 | 64篇 |
1993年 | 75篇 |
1992年 | 93篇 |
1991年 | 91篇 |
1990年 | 67篇 |
1989年 | 68篇 |
1988年 | 71篇 |
1987年 | 37篇 |
1986年 | 62篇 |
1985年 | 78篇 |
1984年 | 50篇 |
1983年 | 51篇 |
1982年 | 53篇 |
1981年 | 45篇 |
1980年 | 42篇 |
1979年 | 35篇 |
1978年 | 49篇 |
1976年 | 42篇 |
1975年 | 41篇 |
1974年 | 31篇 |
1973年 | 34篇 |
1971年 | 34篇 |
1964年 | 28篇 |
排序方式: 共有6439条查询结果,搜索用时 172 毫秒
851.
Sommer A Geist B Da Ines O Gehwolf R Schäffner AR Obermeyer G 《The New phytologist》2008,180(4):787-797
To investigate the role of aquaporin-mediated water transport during pollen grain germination and tube growth, Arabidopsis thaliana plasma membrane intrinsic proteins (PIPs) were expressed in pollen of Lilium longiflorum (lily). Successful expression of AtPIPs in particle-bombarded lily pollen grains was monitored by co-expression with fluorescent proteins and single-cell RT-PCR, and by measuring the water permeability coefficient (P(os)) in swelling assays using protoplasts prepared from transformed pollen grains and tubes. Expression of AtPIP1;1 and AtPIP1;2 in pollen grains resulted in P(os) values similar to those measured in nontransformed pollen grain protoplasts (6.65 +/- 2.41 microm s(-1)), whereas expression of AtPIP2 significantly increased P(os) (AtPIP2;1, 13.79 +/- 6.38; AtPIP2;2, 10.16 +/- 3.30 microm s(-1)). Transformation with combinations of AtPIP1 and AtPIP2 did not further enhance P(os). Native pollen tube protoplasts showed higher P(os) values (13.23 +/- 4.14 microm s(-1)) than pollen grain protoplasts but expression of AtPIP2;1 (18.85 +/- 7.60 microm s(-1)) did not significantly increase their P(os) values. Expression of none of the tested PIPs had any effect on pollen tube growth rates. The ectopic expression of AtPIP2s in lily pollen increased the water permeability of the plasma membrane in pollen grains, but not in pollen tubes. The measured endogenous water permeability does not limit water uptake during tube growth, but has to be regulated to prevent tube bursting. 相似文献
852.
Hauser R Marczak M Karaszewski B Wiergowski M Kaliszan M Penkowski M Kernbach-Wighton G Jankowski Z Namieśnik J 《Lab animal》2008,37(2):76-80
In stressful situations, many animals release alarm pheromones to warn conspecifics of impending danger. The authors sought to establish experimental conditions for a larger study aimed at identifying alarm pheromones emitted by the rat. They placed rats in a specially designed chamber and exposed them to aversive tactile, visual and acoustic stimuli over the course of a few days. The researchers observed rats' behavior and analyzed air samples taken from their immediate environment under the following conditions: (i) when rats were unstressed; (ii) immediately after rats were exposed to aversive stimuli; and (iii) when rats were left alone in the chamber after being conditioned to fear imminent aversive stimuli. Stressed rats emitted several substances that are known to function as alarm pheromones in insects. When previously unstressed control rats were exposed to these same substances, they had a distinct behavioral fear response. 相似文献
853.
Lehembre F Yilmaz M Wicki A Schomber T Strittmatter K Ziegler D Kren A Went P Derksen PW Berns A Jonkers J Christofori G 《The EMBO journal》2008,27(19):2603-2615
Loss of expression of the cell-cell adhesion molecule E-cadherin is a hallmark of epithelial-mesenchymal transition (EMT) in development and in the progression from epithelial tumours to invasive and metastatic cancers. Here, we demonstrate that the loss of E-cadherin function upregulates expression of the neuronal cell adhesion molecule (NCAM). Subsequently, a subset of NCAM translocates from fibroblast growth factor receptor (FGFR) complexes outside lipid rafts into lipid rafts where it stimulates the non-receptor tyrosine kinase p59(Fyn) leading to the phosphorylation and activation of focal adhesion kinase and the assembly of integrin-mediated focal adhesions. Ablation of NCAM expression during EMT inhibits focal adhesion assembly, cell spreading and EMT. Conversely, forced expression of NCAM induces epithelial cell delamination and migration, and high NCAM expression correlates with tumour invasion. These results establish a mechanistic link between the loss of E-cadherin expression, NCAM function, focal adhesion assembly and cell migration and invasion. 相似文献
854.
Myriam Charpentier Rolf Bredemeier Gerhard Wanner Naoya Takeda Enrico Schleiff Martin Parniske 《The Plant cell》2008,20(12):3467-3479
The mechanism underlying perinuclear calcium spiking induced during legume root endosymbioses is largely unknown. Lotus japonicus symbiosis-defective castor and pollux mutants are impaired in perinuclear calcium spiking. Homology modeling suggested that the related proteins CASTOR and POLLUX might be ion channels. Here, we show that CASTOR and POLLUX form two independent homocomplexes in planta. CASTOR reconstituted in planar lipid bilayers exhibited ion channel activity, and the channel characteristics were altered in a symbiosis-defective mutant carrying an amino acid replacement close to the selectivity filter. Permeability ratio determination and competition experiments reveled a weak preference of CASTOR for cations such as potassium over anions. POLLUX has an identical selectivity filter region and complemented a potassium transport–deficient yeast mutant, suggesting that POLLUX is also a potassium-permeable channel. Immunogold labeling localized the endogenous CASTOR protein to the nuclear envelope of Lotus root cells. Our data are consistent with a role of CASTOR and POLLUX in modulating the nuclear envelope membrane potential. They could either trigger the opening of calcium release channels or compensate the charge release during the calcium efflux as counter ion channels. 相似文献
855.
Lopandic K Tiefenbrunner W Gangl H Mandl K Berger S Leitner G Abd-Ellah GA Querol A Gardner RC Sterflinger K Prillinger H 《FEMS yeast research》2008,8(7):1063-1075
The aim of the present study was to evaluate the autochthonous yeast population during spontaneous fermentations of grape musts in Austrian wine-producing areas. Investigation of genomic and genetic variations among wine yeasts was a first step towards a long-term goal of selecting strains with valuable enological properties typical for this geographical region. An approach, combining sequences of the D1/D2 domain of the 26S rRNA gene and random amplified polymorphic DNA fingerprinting, was used to characterize yeasts at the species level, whereas the differentiation of Saccharomyces strains was accomplished by amplified fragment length polymorphism fingerprinting. At the beginning of fermentation, representatives of nine genera were identified, with Hanseniaspora and Metschnikowia species characterized most frequently. Saccharomyces cerevisiae and Saccharomyces bayanus var. uvarum strains, which were identified throughout the entire fermentation process, showed a high level of genetic diversity. A number of S. cerevisiae strains were common at multiple wineries, but a wide range of strains with characteristic profiles were characterized at individual locations. This biodiversity survey represents a contribution to the investigation and preservation of genetic diversity of biotechnologically relevant yeasts in Austrian wine-making areas. 相似文献
856.
Phenylanthraquinones belong to the quite rare class of fully unsymmetric biaryls, consisting of two different molecular portions,
an anthraquinone part, chrysophanol, and a phenyl part, 2,4-dihydroxy-6-methoxyacetophenone, linked together by phenol-oxidative
coupling. The biosynthesis of these two moieties, from eight and four acetate units, respectively, bears some unique features:
Chrysophanol is the first example of an acetogenic natural product that is, in an organism-specific manner, formed via more
than one folding mode: In eukaryotes, like, e.g., in fungi, in higher plants, and in insects, it is formed via folding mode
F, while in prokaryotes it originates through mode S. It has, more recently, even been found to be synthesized by a third
pathway, which we have named mode S′. It is thus the first example of biosynthetic convergence in polyketide biosynthesis.
The monocyclic “southern” portion of the molecule, which is much simpler (arising from only four acetate units and without
decarboxylation), unexpectedly does not show the anticipated randomization of the C2-labeling in the aromatic ring, but has clearly localized C2 units, excluding any symmetric intermediate like, e.g., 2,4,6-trihydroxyacetophenone. This is confirmed by competitive feeding
experiments with specifically 13C2-labeled acetophenones, showing the O-methylation to be the decisive symmetry-preventing step, which hints at a close collaboration of the participating enzymes.
The results make knipholone an instructive example of structure, function, and evolution of polyketide synthases and O-methyltransferases, and their cooperation.
相似文献
Gerhard BringmannEmail: |
857.
High-impact sulfur compounds: constitutional and configurational assignment of sulfur-containing heterocycles 总被引:1,自引:0,他引:1
Krafft C Brennecke S Ott F Backes M Salzer R Grunenberg J Ley JP Krammer GE Weber B 《化学与生物多样性》2008,5(6):1204-1212
To unambiguously identify their structures and to evaluate their organoleptic properties, several constitutional und configurational isomers of dialkyl-tetrathianes and dialkyl-pentathiepanes were synthesized by two different synthetic protocols, and separated by preparative gas chromatography. Raman and NMR spectroscopy were used to differentiate between the constitutional 3,6-dialkyl-1,2,4,5-tetrathiane and the 4,6-dialkyl-1,2,3,5-tetrathiane isomers. Furthermore, cis- and trans-isomers of 3,6-dialkyl-1,2,4,5-tetrathianes were distinguished by temperature-dependent NMR experiments. Static, quantum-chemical simulations of the NMR spectra for these cis- and trans-isomers were calculated in the gas layer in order to confirm our experimental assignments. In addition, the assignment of 4,7-alkyl-1,2,3,5,6-pentathiepanes were deducted from their Raman spectra. Dialkyl-tetrathianes and dialkyl-pentathiepanes are interesting components to be used in flavor applications due to their unique olfactory impact and facets. 相似文献
858.
859.
Chang S Mâsse LC Moser RP Dodd KW Arganaraz F Fuemmler BF Jemal A 《Obesity (Silver Spring, Md.)》2008,16(7):1636-1650
Objective: Given links between obesity and cancer, we estimated incident cancer burden due to overweight and obesity at the state level in the United States. Methods and Procedures: Using state rankings by per capita burden of incident cancer cases diagnosed in 2003 that were related to overweight and obesity, we examined the frequency with which states ranked in the highest and lowest quintiles of weight‐related burden for cancers of the postmenopausal breast, endometrium, kidney, colon, and prostate. In this study, data from the Behavioral Risk Factor Surveillance System (BRFSS), US Census, US Mortality Public Use Data Tapes, and National Cancer Institute Surveillance, Epidemiology, and End Results (SEER) Program were used. Results: Western states had the lowest weight‐related cancer burden for both sexes. Iowa, South Dakota, and West Virginia had the highest burden for all three types of male cancers. West Virginia is the only state that ranked in the quintile of highest weight‐related burden for all four cancers considered in women. Discussion: For certain cancers, including endometrial, postmenopausal breast, and colon cancers, states with high burdens clustered in geographic regions, warranting further inquiry. Although state ranks for the total cancer burden and the prevalence of overweight and obesity correlated with state ranks for weight‐related incident cancer burden, they often served poorly as its proxy. Such a finding cautions against simply targeting states with high overweight and obesity or high total burdens of cancers for which overweight and obesity are risk factors, as this approach may not reach areas of unrecognized burden. 相似文献
860.
Blanco YC Farias AS Goelnitz U Lopes SC Arrais-Silva WW Carvalho BO Amino R Wunderlich G Santos LM Giorgio S Costa FT 《PloS one》2008,3(9):e3126