全文获取类型
收费全文 | 537篇 |
免费 | 46篇 |
出版年
2023年 | 5篇 |
2022年 | 12篇 |
2021年 | 16篇 |
2020年 | 14篇 |
2019年 | 7篇 |
2018年 | 11篇 |
2017年 | 13篇 |
2016年 | 19篇 |
2015年 | 20篇 |
2014年 | 29篇 |
2013年 | 30篇 |
2012年 | 31篇 |
2011年 | 32篇 |
2010年 | 32篇 |
2009年 | 23篇 |
2008年 | 31篇 |
2007年 | 14篇 |
2006年 | 17篇 |
2005年 | 18篇 |
2004年 | 22篇 |
2003年 | 17篇 |
2002年 | 11篇 |
2001年 | 21篇 |
2000年 | 20篇 |
1999年 | 11篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1993年 | 8篇 |
1992年 | 8篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1970年 | 4篇 |
1967年 | 3篇 |
1960年 | 2篇 |
排序方式: 共有583条查询结果,搜索用时 328 毫秒
101.
A new natural product, 3β-acetoxydrimenin was isolated from the petrol extract of the leaves of Drimys winteri which also contains the known compounds safrol, drimenol and polygodial. The structure of the new compound was determined by chemical and spectroscopic methods. 相似文献
102.
Stability of alkaloid production in cell suspension cultures of Tabernaemontana divaricata during long-term subculture 总被引:1,自引:0,他引:1
Marta I. Sierra Robert van der Heijden Theo van der Leer Robert Verpoorte 《Plant Cell, Tissue and Organ Culture》1992,28(1):59-68
Two cell lines of Tabernaemontana divaricata cell suspension culture with different growth and alkaloid production profiles were transferred to the same medium. During 30 subcultures the changes in growth and alkaloid production were followed and compared to those of the original cell lines. The presence of NAA and BAP in the medium resulted in an increase of biomass and alkaloid yield. The effect on the growth proved to be stable during these 30 subcultures. Alkaloid production showed a maximum in the 4th subculture after the change of the medium, and stabilized on a higher level than found in the original cell lines. During some growth cycles also the activities of tryptophan decarboxylase (TDC), strictosidine synthase (SSS), and phenylalanineammonia-lyase (PAL) were measured. In both the original cell lines and the derived cell lines, growth and alkaloid production proved to be stable all through the experiment, although the derived cell lines had a period of adaptation to the new medium with increased productivity.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- NAA
naphthaleneacetic acid
- BAP
benzylaminopurine
- DW
dry weight
- TDC
tryptophan decarboxylase
- SSS
strictosidine synthase
- PAL
phenylalanineammonia-lyase
- PAT
phenylalanineammonia-transaminase 相似文献
103.
Mercedes Casado Cristina Sierra Marta Batllori Rafael Artuch Aida Ormazabal 《Metabolomics : Official journal of the Metabolomic Society》2018,14(6):76
Introduction
Amino acid analysis in biological fluids is essential for the study of inborn errors of metabolism (IEM) and other diseases.Objectives
Our aim was to develop a UPLC-MS/MS procedure for the analysis of 25 amino acids and identification of 17 related compounds.Methods
Sample treatment conditions were optimized for plasma, urine, cerebrospinal fluid (CSF) and dried blood spots. Amino acids and related compounds were analyzed on a Waters ACQUITY UPLC H-class instrument with a reversed-phase C-18 column using water and acetonitrile with 0.1% formic acid as the mobile phases (run time?=?9 min). The detection was performed with a Waters Xevo TQD triple-quadrupole mass spectrometer using positive electrospray ionization in the multiple reaction monitoring mode.Results
The method linearity, intra-assay and inter-assay precision, detection limit, quantification limit and trueness analysis displayed adequate results in both physiological and pathological conditions. Method comparison was performed between UPLC-MS/MS and ion exchange chromatography (IEC) with ninhydrin derivatization, and the methods showed good agreement, except for 4-hydroxyproline, aspartate and citrulline. Paediatrics age-related reference values in plasma, urine and CSF were established and patients with different IEM were easily identified.Conclusion
We report a modified UPLC-MS/MS procedure for the analysis of 42 amino acids and related compounds in different specimens. The method is fast, sensitive and robust, and it has been validated to be an alternative to the traditional IEC procedure as the routine method used in metabolic laboratories. The method greatly decreases the run time of the analysis while displaying good metrological results.104.
D. M. Waldner N. C. Giraldo Sierra S. Bonfield L. Nguyen I. S. Dimopoulos Y. Sauvé 《Channels (Austin, Tex.)》2018,12(1):17-33
Congenital stationary night blindness 2A (CSNB2A) is an X-linked retinal disorder, characterized by phenotypically variable signs and symptoms of impaired vision. CSNB2A is due to mutations in CACNA1F, which codes for the pore-forming α1F subunit of a L-type voltage-gated calcium channel, Cav1.4. Mouse models of CSNB2A, used for characterizing the effects of various Cacna1f mutations, have revealed greater severity of defects than in human CSNB2A. Specifically, Cacna1f-knockout mice show an apparent lack of visual function, gradual retinal degeneration, and disruption of photoreceptor synaptic terminals. Several reports have also noted cone-specific disruptions, including axonal abnormalities, dystrophy, and cell death. We have explored further the involvement of cones in our ‘G305X’ mouse model of CSNB2A, which has a premature truncation, loss-of-function mutation in Cacna1f. We show that the expression of genes for several phototransduction-related cone markers is down-regulated, while that of several cellular stress- and damage-related markers is up-regulated; and that cone photoreceptor structure and photopic visual function – measured by immunohistochemistry, optokinetic response and electroretinography – deteriorate progressively with age. We also find that dystrophic cone axons establish synapse-like contacts with rod bipolar cell dendrites, which they normally do not contact in wild-type retinas – ectopically, among rod cell bodies in the outer nuclear layer. These data support a role for Cav1.4 in cone synaptic development, cell viability, and synaptic transmission of cone-dependent visual signals. Although our novel finding of cone-to-rod-bipolar cell contacts in this mouse model of a retinal channelopathy may challenge current views of the role of Cav1.4 in photopic vision, it also suggests a potential new target for restorative therapy. 相似文献
105.
106.
107.
Three satellite DNA families were identified in three species of burying
beetles, Nicrophorus orbicollis, N. marginatus, and N. americanus. Southern
hybridization and nucleotide sequence analysis of individual randomly
cloned repeats shows that these satellite DNA families are highly abundant
in the genome, are composed of unique repeats, and are species-specific.
The repeats do not have identifiable core elements or substructures that
are similar in all three families, and most interspecific sequence
similarity is confined to homopolymeric runs of A and T. Satellite DNA from
N. marginatus and N. americanus show single-base-pair indels among repeats,
but single-nucleotide substitutions characterize most of the repeat
variability. Although the repeat units are of similar lengths (342, 350,
and 354 bp) and A + T composition (65%, 71%, and 71%, respectively), the
average nucleotide divergence among sequenced repeats is very low (0.18%,
1.22%, and 0.71%, respectively). Transition/transversion ratios from the
consensus sequence are 0.20, 0.69, and 0.70, respectively.
相似文献
108.
Mauro Maciel de Arruda Fabiano Borges Figueiredo Andreza Pain Marcelino José Ronaldo Barbosa Guilherme Loureiro Werneck Elza Ferreira Noronha Gustavo Adolfo Sierra Romero 《Memórias do Instituto Oswaldo Cruz》2016,111(3):168-173
In Brazil, human and canine visceral leishmaniasis (CVL) caused byLeishmania
infantum has undergone urbanisation since 1980, constituting a public
health problem, and serological tests are tools of choice for identifying infected
dogs. Until recently, the Brazilian zoonoses control program recommended
enzyme-linked immunosorbent assays (ELISA) and indirect immunofluorescence assays
(IFA) as the screening and confirmatory methods, respectively, for the detection of
canine infection. The purpose of this study was to estimate the accuracy of ELISA and
IFA in parallel or serial combinations. The reference standard comprised the results
of direct visualisation of parasites in histological sections, immunohistochemical
test, or isolation of the parasite in culture. Samples from 98 cases and 1,327
noncases were included. Individually, both tests presented sensitivity of 91.8% and
90.8%, and specificity of 83.4 and 53.4%, for the ELISA and IFA, respectively. When
tests were used in parallel combination, sensitivity attained 99.2%, while
specificity dropped to 44.8%. When used in serial combination (ELISA followed by
IFA), decreased sensitivity (83.3%) and increased specificity (92.5%) were observed.
Serial testing approach improved specificity with moderate loss in sensitivity. This
strategy could partially fulfill the needs of public health and dog owners for a more
accurate diagnosis of CVL. 相似文献
109.
From the ground up: biotic and abiotic features that set the course from genes to ecosystems
下载免费PDF全文
![点击此处可从《Ecology and evolution》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Spatial variation in cone serotiny in Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) across Yellowstone National Park influences initial pine recruitment after stand‐replacing fire with tremendous population, community, and ecosystem consequences. A previous study showed that much of the spatial variation in serotiny results from the balance of selection arising from high frequencies of fire favoring serotiny countered by opposing selection exerted by American red squirrels (Tamiasciurus hudsonicus) as seed predators. This earlier study, however, assumed stable local red squirrel densities over multiple generations of pines. Here, we examine environmental properties that might contribute to long‐term stability in the densities of red squirrels among sites. We found that the amount of clay in the soil, an indicator of plant and fungal growth—the latter an important food resource for red squirrels—and the coefficient of variation (CV) in diameter at breast height (DBH) of forest trees together account for a substantial amount of variation in red squirrel density. Soil development occurs over very long time scales, and thus, intersite variation in the amount of clay is unlikely to shift across pine generations. However, CV of DBH and squirrel density increase with stand age, which acts to amplify selection against serotiny with increasing interfire interval. Regardless, much of the variation in the CV of DBH is accounted for by soil bulk density, mean annual temperature, and surface curvature, which are unlikely to vary in their relative differences among sites over time. Consequently, these soil and abiotic attributes could contribute to consistent spatial patterns of red squirrel densities from one pine generation to the next, resulting in consistent local and spatial variation in selection exerted by red squirrels against serotiny. 相似文献
110.
Membrane resistance change of the frog taste cells in response to water and Nacl 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《The Journal of general physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The electrical properties of the frog taste cells during gustatory stimulations with distilled water and varying concentrations of NaCl were studied with intracellular microelectrodes. Under the Ringer adaptation of the tongue, two types of taste cells were distinguished by the gustatory stimuli. One type, termed NaCl-sensitive (NS) cells, responded to water with hyperpolarizations and responded to concentrated NaCl with depolarizations. In contrast, the other type of cells, termed water-sensitive (WS) cells, responded to water depolarizations and responded to concentrated NaCl with hyperpolarizations. The membrane resistance of both taste cell types increased during the hyperpolarizing receptor potentials and decreased during the depolarizing receptor potentials, Reversal potentials for the depolarizing and hyperpolarizing responses in each cell type were a few millivolts positive above the zero membrane potential. When the tongue was adapted with Na-free Ringer solution for 30 min, the amplitude of the depolarizing responses in the NS cells reduced to 50% of the control value under normal Ringer adaptation. On the basis of the present results, it is concluded (a) that the depolarizing responses of the NS and WS cells under the Ringer adaptation are produced by the permeability increase in some ions, mainly Na+ ions across the taste cell membranes, and (b) that the hyperpolarizing responses of both types of taste cells are produced by a decrease in the cell membrane permeability to some ions, probably Na+ ions, which is slightly enhanced during the Ringer adaptation. 相似文献