全文获取类型
收费全文 | 465篇 |
免费 | 52篇 |
出版年
2021年 | 6篇 |
2020年 | 4篇 |
2018年 | 3篇 |
2017年 | 5篇 |
2016年 | 9篇 |
2015年 | 15篇 |
2014年 | 12篇 |
2013年 | 11篇 |
2012年 | 27篇 |
2011年 | 23篇 |
2010年 | 14篇 |
2009年 | 21篇 |
2008年 | 23篇 |
2007年 | 22篇 |
2006年 | 20篇 |
2005年 | 15篇 |
2004年 | 18篇 |
2003年 | 24篇 |
2002年 | 22篇 |
2001年 | 14篇 |
2000年 | 18篇 |
1999年 | 12篇 |
1998年 | 17篇 |
1997年 | 8篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 11篇 |
1992年 | 13篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 5篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1974年 | 3篇 |
1972年 | 2篇 |
1971年 | 6篇 |
1969年 | 5篇 |
1967年 | 2篇 |
1966年 | 3篇 |
1965年 | 2篇 |
排序方式: 共有517条查询结果,搜索用时 27 毫秒
391.
We have performed quantum mechanical calculations for retinal model compounds to establish the rotational energy barriers for the C5-, C9-, and C13-methyl groups known to play an essential role in rhodopsin activation. Intraretinal steric interactions as well as electronic effects lower the rotational barriers of both the C9- and C13-methyl groups, consistent with experimental 2H NMR data. Each retinal methyl group has a unique rotational behavior which must be treated individually. These results are highly relevant for the parameterization of molecular mechanics force fields which form the basis of molecular dynamics simulations of retinal proteins such as rhodopsin. 相似文献
392.
The functional and structural adaptations to temperature have been addressed in homologous chloride-dependent α-amylases from a psychrophilic Antarctic bacterium, the ectothermic fruit fly, the homeothermic pig and from a thermophilic actinomycete. This series covers nearly all temperatures encountered by living organisms. We report a striking continuum in the functional properties of these enzymes coupled to their structural stability and related to the thermal regime of the source organism. In particular, thermal stability recorded by intrinsic fluorescence, circular dichroism and differential scanning calorimetry appears to be a compromise between the requirement for a stable native state and the proper structural dynamics to sustain the function at the environmental/physiological temperatures. The thermodependence of activity, the kinetic parameters, the activations parameters and fluorescence quenching support these activity-stability relationships in the investigated α-amylases. 相似文献
393.
394.
The objective of this study was to assess effects of different light intensities on shoot growth, root development and allocation of root-borne solutes via the transpiration stream to various shoot parts of young wheat plants (Triticum aestivum L.). Hydroponic culture allowed direct access to the roots and shoots throughout the experiment. Under low light intensity (100?μmol photons m?2?s?1), shoot growth was restricted, less (but larger) leaves were produced at the main shoot and only a few tillers became visible as compared to plants under high light intensity (380?μmol photons m?2?s?1). The root system was indirectly also affected by the illumination of the aerial parts. A larger number of shorter roots were produced under high light leading to a denser root system, while only a small number of longer roots were present under low light. The distribution of 54Mn (xylem-mobile, but essentially phloem-immobile in wheat) from the roots to the shoot lead to the conclusion that light regime strongly influences the distribution of root-borne solutes within the shoots. Labels introduced into the roots may allow a deeper insight into the transfer of solutes from the root system to the various shoot parts under different light regimes. 相似文献
395.
ABSTRACT: Cancer-initiating cells display aberrant functional and phenotypic characteristics of normal stem cells from which they evolved by accumulation of multiple cytogenetic and/or epigenetic alterations. Signal transduction pathways which are essential for normal stem cell function are abnormally expressed by cancer cells, with a cancer cell phenotype playing an essential role in cancerization and metastasis.Local tumour progression, metastasis and metastatic tumour growth are mediated by direct cell-to-cell and paracrine reciprocal interactions between cancer cells and various stromal cells including fibroblasts, macrophages, bone marrow derived stem cells and progenitor cells. These interactions mediate breakdown of basement membrane barriers and angiogenesis both locally at the invasive front of the primary tumour and at the distant metastatic site; attract primary tumour cells to the candidate metastatic site; and promote proliferation, survival and growth of primary tumour cells and of metastatic cells at their distant site.It is the purpose of this article to highlight the analogies between some of the genetic programs of normal stem cells, and of cancer cells participating in the process of metastasis. 相似文献
396.
ABSTRACT: A considerable number of soluble proteins in cells that biochemists try to analyze are difficult to handle because they seem to behave like sponges that 'suck up' many other proteins. We argue here that this behavior is commonly an artifact introduced by the experimenting scientist and that we need to study proteins like animals in the wild: they will only reveal many of their secrets when carefully observed in their largely undisturbed, natural environment. Computational studies that attempt to realistically model cellular protein networks must also factor in the diverse protein habitats to be found in cells. 相似文献
397.
Testing the generality of above‐ground biomass allometry across plant functional types at the continent scale
下载免费PDF全文
![点击此处可从《Global Change Biology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Keryn I. Paul Stephen H. Roxburgh Jerome Chave Jacqueline R. England Ayalsew Zerihun Alison Specht Tom Lewis Lauren T. Bennett Thomas G. Baker Mark A. Adams Dan Huxtable Kelvin D. Montagu Daniel S. Falster Mike Feller Stan Sochacki Peter Ritson Gary Bastin John Bartle Dan Wildy Trevor Hobbs John Larmour Rob Waterworth Hugh T.L. Stewart Justin Jonson David I. Forrester Grahame Applegate Daniel Mendham Matt Bradford Anthony O'Grady Daryl Green Rob Sudmeyer Stan J. Rance John Turner Craig Barton Elizabeth H. Wenk Tim Grove Peter M. Attiwill Elizabeth Pinkard Don Butler Kim Brooksbank Beren Spencer Peter Snowdon Nick O'Brien Michael Battaglia David M Cameron Steve Hamilton Geoff McAuthur Jenny Sinclair 《Global Change Biology》2016,22(6):2106-2124
Accurate ground‐based estimation of the carbon stored in terrestrial ecosystems is critical to quantifying the global carbon budget. Allometric models provide cost‐effective methods for biomass prediction. But do such models vary with ecoregion or plant functional type? We compiled 15 054 measurements of individual tree or shrub biomass from across Australia to examine the generality of allometric models for above‐ground biomass prediction. This provided a robust case study because Australia includes ecoregions ranging from arid shrublands to tropical rainforests, and has a rich history of biomass research, particularly in planted forests. Regardless of ecoregion, for five broad categories of plant functional type (shrubs; multistemmed trees; trees of the genus Eucalyptus and closely related genera; other trees of high wood density; and other trees of low wood density), relationships between biomass and stem diameter were generic. Simple power‐law models explained 84–95% of the variation in biomass, with little improvement in model performance when other plant variables (height, bole wood density), or site characteristics (climate, age, management) were included. Predictions of stand‐based biomass from allometric models of varying levels of generalization (species‐specific, plant functional type) were validated using whole‐plot harvest data from 17 contrasting stands (range: 9–356 Mg ha?1). Losses in efficiency of prediction were <1% if generalized models were used in place of species‐specific models. Furthermore, application of generalized multispecies models did not introduce significant bias in biomass prediction in 92% of the 53 species tested. Further, overall efficiency of stand‐level biomass prediction was 99%, with a mean absolute prediction error of only 13%. Hence, for cost‐effective prediction of biomass across a wide range of stands, we recommend use of generic allometric models based on plant functional types. Development of new species‐specific models is only warranted when gains in accuracy of stand‐based predictions are relatively high (e.g. high‐value monocultures). 相似文献
398.
Srimathi S Jayaraman G Feller G Danielsson B Narayanan PR 《Extremophiles : life under extreme conditions》2007,11(3):505-515
The halotolerance of a cold adapted α-amylase from the psychrophilic bacterium Pseudoalteromonas haloplanktis (AHA) was investigated. AHA exhibited hydrolytic activity over a broad range of NaCl concentrations (0.01–4.5 M). AHA showed
28% increased activity in 0.5–2.0 M NaCl compared to that in 0.01 M NaCl. In contrast, the corresponding mesophilic (Bacillus amyloliquefaciens) and thermostable (B. licheniformis) α-amylases showed a 39 and 46% decrease in activity respectively. Even at 4.5 M NaCl, 80% of the initial activity was detected
for AHA, whereas the mesophilic and thermostable enzymes were inactive. Besides an unaltered fluorescence emission and secondary
structure, a 10°C positive shift in the temperature optimum, a stabilization factor of >5 for thermal inactivation and a ΔT
m of 8.3°C for the secondary structure melting were estimated in 2.7 M NaCl. The higher activation energy, half-life time and
T
m indicated reduced conformational dynamics and increased rigidity in the presence of higher NaCl concentrations. A comparison
with the sequences of other halophilic α-amylases revealed that AHA also contains higher proportion of small hydrophobic residues
and acidic residues resulting in a higher negative surface potential. Thus, with some compromise in cold activity, psychrophilic
adaptation has also manifested halotolerance to AHA that is comparable to the halophilic enzymes.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
This article is dedicated to Late Dr. P. V. Sundaram. 相似文献
399.
Yacoub-George E Hell W Meixner L Wenninger F Bock K Lindner P Wolf H Kloth T Feller KA 《Biosensors & bioelectronics》2007,22(7):1368-1375
The automated 10-channel capillary chip immunodetector (10K-IDWG) is a prototype, which has been developed for automatically operated biological agents (BA) point detection. The current technology uses a chemiluminescence capillary immunoassay (EIA) technique in combination with integrated microfluidics and allows the highly sensitive and rapid detection and preliminary identification of multiple BA in aqueous solutions in the laboratory. The chemiluminescence capillary EIA are performed within a disposable capillary chip containing 10 fused-silica capillaries arranged in parallel coated with selected capture antibodies. A multianode-photomultiplier array is used to detect chemiluminescence intensity in each capillary. Reservoirs for reagents and buffers and a waste disposal reservoir are integrated. This paper describes the technology of the 10K-IDWG and its evaluation with three different BA, the toxin staphylococcal enterotoxin B (SEB), the bacterial analyte Escherichia coli (E. coli) O157:H7 as a model for bacterial pathogens, and the bacteriophage M13 as a model for virus pathogens. The 10K-IDWG is able to detect the above mentioned three BA in an aqueous sample within 29 min (single analyte-detection and multiplexing). Limits of detection (LOD) are 0.1 ng/ml for SEB, 10(4)cfu/ml for E. coli O157:H7, and 5x10(5) pfu/ml for M13. Cross reactivities between the three assays were not observed. 相似文献
400.
A petunia mutant affected in intracellular accommodation and morphogenesis of arbuscular mycorrhizal fungi 总被引:1,自引:0,他引:1
Reddy D M R S Schorderet M Feller U Reinhardt D 《The Plant journal : for cell and molecular biology》2007,51(5):739-750
The regulation of the arbuscular mycorrhizal (AM) symbiosis is largely under the control of a genetic programme of the plant host. This programme includes a common symbiosis signalling pathway that is shared with the root nodule symbiosis. Whereas this common pathway has been investigated in detail, little is known about the mycorrhiza-specific regulatory steps upstream and downstream of the common pathway. To get further insight in the regulation of the AM symbiosis, a transposon-mutagenized population of Petunia hybrida was screened for mutants with defects in AM development. Here, we describe a petunia mutant, penetration and arbuscule morphogenesis1 (pam1), which is characterized by a strong decrease in colonization by three different AM fungi. Penetrating hyphae are frequently aborted in epidermal cells. Occasionally the fungus can progress to the cortex, but fails to develop arbuscules. The resulting hyphal colonization of the cortex in mutant plants does not support symbiotic acquisition of phosphate and copper by the plant. Expression analysis of three petunia orthologues of the common SYM genes LjPOLLUX, LjSYMRK and MtDMI3 indicates that pam1 is not mutated in these genes. We conclude that the PAM1 gene may play a specific role in intracellular accommodation and morphogenesis of the fungal endosymbiont. 相似文献