全文获取类型
收费全文 | 1808篇 |
免费 | 145篇 |
国内免费 | 1篇 |
专业分类
1954篇 |
出版年
2023年 | 8篇 |
2022年 | 14篇 |
2021年 | 36篇 |
2020年 | 21篇 |
2019年 | 31篇 |
2018年 | 28篇 |
2017年 | 30篇 |
2016年 | 44篇 |
2015年 | 79篇 |
2014年 | 72篇 |
2013年 | 126篇 |
2012年 | 141篇 |
2011年 | 153篇 |
2010年 | 103篇 |
2009年 | 93篇 |
2008年 | 128篇 |
2007年 | 95篇 |
2006年 | 89篇 |
2005年 | 99篇 |
2004年 | 105篇 |
2003年 | 99篇 |
2002年 | 101篇 |
2001年 | 18篇 |
2000年 | 18篇 |
1999年 | 27篇 |
1998年 | 22篇 |
1997年 | 13篇 |
1996年 | 16篇 |
1995年 | 15篇 |
1994年 | 10篇 |
1993年 | 11篇 |
1992年 | 14篇 |
1990年 | 4篇 |
1989年 | 12篇 |
1988年 | 9篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1979年 | 5篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1972年 | 4篇 |
1971年 | 2篇 |
1968年 | 2篇 |
1967年 | 2篇 |
排序方式: 共有1954条查询结果,搜索用时 15 毫秒
111.
Wahl M Link H Alexandridis N Thomason JC Cifuentes M Costello MJ da Gama BA Hillock K Hobday AJ Kaufmann MJ Keller S Kraufvelin P Krüger I Lauterbach L Antunes BL Molis M Nakaoka M Nyström J bin Radzi Z Stockhausen B Thiel M Vance T Weseloh A Whittle M Wiesmann L Wunderer L Yamakita T Lenz M 《PloS one》2011,6(5):e19514
Species richness is the most commonly used but controversial biodiversity metric in studies on aspects of community stability such as structural composition or productivity. The apparent ambiguity of theoretical and experimental findings may in part be due to experimental shortcomings and/or heterogeneity of scales and methods in earlier studies. This has led to an urgent call for improved and more realistic experiments. In a series of experiments replicated at a global scale we translocated several hundred marine hard bottom communities to new environments simulating a rapid but moderate environmental change. Subsequently, we measured their rate of compositional change (re-structuring) which in the great majority of cases represented a compositional convergence towards local communities. Re-structuring is driven by mortality of community components (original species) and establishment of new species in the changed environmental context. The rate of this re-structuring was then related to various system properties. We show that availability of free substratum relates negatively while taxon richness relates positively to structural persistence (i.e., no or slow re-structuring). Thus, when faced with environmental change, taxon-rich communities retain their original composition longer than taxon-poor communities. The effect of taxon richness, however, interacts with another aspect of diversity, functional richness. Indeed, taxon richness relates positively to persistence in functionally depauperate communities, but not in functionally diverse communities. The interaction between taxonomic and functional diversity with regard to the behaviour of communities exposed to environmental stress may help understand some of the seemingly contrasting findings of past research. 相似文献
112.
Coenzyme A-linked acetaldehyde dehydrogenase (ACDH) of ethanol-grown cells of Acetobacterium woodii was purified to apparent homogeneity; a 28-fold purification was achieved with 13% yield. The enzyme proved to be oxygen-sensitive and was inactive in the absence of dithioerythritol. During the purification procedure addition of 1 mM MgCl2 was necessary to maintain enzyme activity. Alcohol dehydrogenase (ADH) activity was separated from ACDH during anion exchange chromatography using DEAE Sephacel. A part of the ACDH activity coeluted with ADH, but both could be separately eluted from a Cibacron Blue 3GA-Agarose column, revealing the same subunit structure and activity band for ACDH as found before and, thus, indicating an aggregation of the enzyme. The remaining ADH activity could be separated by gel filtration. For the native ACDH a molecular mass of 255 kDa was determined by polyacrylamide gel electrophoresis and of 272 kDa by gel filtration using Superose 12. The enzyme subunit sizes were 28 kDa and 40 kDa, respectively, indicating a 44 structure for the active form. The enzyme catalyzed the oxidation of several straight chain aldehydes although it was most active with acetaldehyde. NADH strongly inhibited oxidation of acetaldehyde whereas NADPH had no effect. The inhibition was noncompetitive.Non-standard abbrevations ACDH
acetaldehyde dehydrogenase
- ADH
alcohol dehydrogenase
- CHES
2-(N-cyclohexylamino)-ethanesulfonate
- DTE
dithioerythritol
- KP-buffer
25 mM K-PO4, pH 7.5, containing, 4 mM DTE
- MES
2-(N-morpholino)-ethanesulfonate
- TAPS
N-Tris-(hydroxymethyl)-methyl-3-aminopropa-nesulfonate 相似文献
113.
Jörg Müller Oliver Mitesser Marc W. Cadotte Fons van der Plas Akira S. Mori Christian Ammer Anne Chao Michael Scherer-Lorenzen Petr Baldrian Claus Bässler Peter Biedermann Simone Cesarz Alice Claßen Benjamin M. Delory Heike Feldhaar Andreas Fichtner Torsten Hothorn Claudia Kuenzer Marcell K. Peters Kerstin Pierick Thomas Schmitt Bernhard Schuldt Dominik Seidel Diana Six Ingolf Steffan-Dewenter Simon Thorn Goddert von Oheimb Martin Wegmann Wolfgang W. Weisser Nico Eisenhauer 《Global Change Biology》2023,29(6):1437-1450
Intensification of land use by humans has led to a homogenization of landscapes and decreasing resilience of ecosystems globally due to a loss of biodiversity, including the majority of forests. Biodiversity–ecosystem functioning (BEF) research has provided compelling evidence for a positive effect of biodiversity on ecosystem functions and services at the local (α-diversity) scale, but we largely lack empirical evidence on how the loss of between-patch β-diversity affects biodiversity and multifunctionality at the landscape scale (γ-diversity). Here, we present a novel concept and experimental framework for elucidating BEF patterns at α-, β-, and γ-scales in real landscapes at a forest management-relevant scale. We examine this framework using 22 temperate broadleaf production forests, dominated by Fagus sylvatica. In 11 of these forests, we manipulated the structure between forest patches by increasing variation in canopy cover and deadwood. We hypothesized that an increase in landscape heterogeneity would enhance the β-diversity of different trophic levels, as well as the β-functionality of various ecosystem functions. We will develop a new statistical framework for BEF studies extending across scales and incorporating biodiversity measures from taxonomic to functional to phylogenetic diversity using Hill numbers. We will further expand the Hill number concept to multifunctionality allowing the decomposition of γ-multifunctionality into α- and β-components. Combining this analytic framework with our experimental data will allow us to test how an increase in between patch heterogeneity affects biodiversity and multifunctionality across spatial scales and trophic levels to help inform and improve forest resilience under climate change. Such an integrative concept for biodiversity and functionality, including spatial scales and multiple aspects of diversity and multifunctionality as well as physical and environmental structure in forests, will go far beyond the current widely applied approach in forestry to increase resilience of future forests through the manipulation of tree species composition. 相似文献
114.
Dietmar Weinert Heike Eimert Hans G. Erkert Ulrich Schneyer 《Chronobiology international》1994,11(4):222-231
Most of the extensive literature concerning the resynchronization of circadian rhythms after a Zeitgeber shift is devoted to the dependence of resynchronization on the mode of the shift and the strength of the Zeitgeber, as well as on the circadian function investigated. Ontogenetic influences have rarely been investigated. Therefore, we studied the resynchronization of several circadian rhythms in juvenile and adult female laboratory mice. We present here the results concerning the corticosterone rhythm. The daily rhythms were determined as transverse profiles (2-h intervals) before as well as 3, 7, and 14 days after an 8-h phase delay of the light/dark cycle produced by a single prolongation of dark time. The corticosterone concentration in serum was determined radioimmunologically. In the control animals the daily patterns were bimodal, with main maxima at the end of the light time and secondary ones just after lights on. Ontogenetic differences were small. In adult mice the amplitude was slightly increased due to an increase in the maximum values, and the time of highest hormone concentrations was slightly phase advanced. In juvenile mice, a distinct daily pattern with a phase position in relation to the light/dark cycle corresponding to that of control animals was present on the 3rd day after the Zeitgeber shift. The daily mean as well as the minimum and maximum values increased initially and reached the values of control animals during the second week. In adult animals, a pronounced daily rhythm with the normal phase position was present only at the 7th postshift day. The amplitude, daily mean, and maximum values were decreased, and the minimum values were increased. The initial values were not reached even after 2 weeks. The results show that resynchronization was faster in juvenile mice compared with adult mice. As a possible cause for the observed age-related differences, a not yet stabilized phase-coupling between various circadian rhythms is supposed. 相似文献
115.
David A. Hughes Anke Hinney Harald Brumm Anne-Kathrin Wermter Heike Biebermann Johannes Hebebrand Mark Stoneking 《Human genetics》2009,124(6):633-647
The melanocortin 4 receptor (MC4R) is routinely investigated for the role it plays in human obesity, as mutations in MC4R are the most common dominantly inherited form of the disease. As little is known about the evolutionary history of this locus,
we investigated patterns of variation at MC4R in a worldwide sample of 1,015 humans from 51 populations, and in 8 central chimpanzees. There is a significant paucity of
diversity at MC4R in humans, but not in chimpanzees. The spectrum of mutations in humans, combined with the overall low level of diversity,
suggests that most (if not all) of the observed non-synonymous polymorphisms are likely to be transient deleterious mutations.
The MC4R coding region was resequenced in 12 primate species and sequences from an additional 29 vertebrates were included in molecular
evolutionary analyses. MC4R is highly conserved throughout vertebrate evolution, and has apparently been subject to high levels of continuous purifying
selection that increased approximately threefold during primate evolution. Furthermore, the strong selection extends to codon
usage bias, where most silent mutations are expected to be either quickly fixed or removed from the population, which may
help explain the unusually low levels of silent polymorphisms in humans. Finally, there is a significant tendency for non-synonymous
mutations that impact MC4R function to occur preferentially at sites that are identified by evolutionary analyses as being subject to very strong purifying
selection. The information from this study should help inform future epidemiological investigations of MC4R.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
116.
Susannetom Dieck Lydia Sanmartí-Vila Kristina Langnaese Karin Richter Stefan Kindler Antje Soyke Heike Wex Karl-Heinz Smalla Udo K?mpf Jürgen-Theodor Fr?nzer Markus Stumm Craig C. Garner Eckart D. Gundelfinger 《The Journal of cell biology》1998,142(2):499-509
The molecular architecture of the cytomatrix of presynaptic nerve terminals is poorly understood. Here we show that Bassoon, a novel protein of >400,000 M
r, is a new component of the presynaptic cytoskeleton. The murine bassoon gene maps to chromosome 9F. A comparison with the corresponding rat cDNA identified 10 exons within its protein-coding region. The Bassoon protein is predicted to contain two double-zinc fingers, several coiled-coil domains, and a stretch of polyglutamines (24 and 11 residues in rat and mouse, respectively). In some human proteins, e.g., Huntingtin, abnormal amplification of such poly-glutamine regions causes late-onset neurodegeneration. Bassoon is highly enriched in synaptic protein preparations. In cultured hippocampal neurons, Bassoon colocalizes with the synaptic vesicle protein synaptophysin and Piccolo, a presynaptic cytomatrix component. At the ultrastructural level, Bassoon is detected in axon terminals of hippocampal neurons where it is highly concentrated in the vicinity of the active zone. Immunogold labeling of synaptosomes revealed that Bassoon is associated with material interspersed between clear synaptic vesicles, and biochemical studies suggest a tight association with cytoskeletal structures. These data indicate that Bassoon is a strong candidate to be involved in cytomatrix organization at the site of neurotransmitter release. 相似文献
117.
118.
119.
The slow Wallerian degeneration protein, WldS, binds directly to VCP/p97 and partially redistributes it within the nucleus 下载免费PDF全文
Laser H Conforti L Morreale G Mack TG Heyer M Haley JE Wishart TM Beirowski B Walker SA Haase G Celik A Adalbert R Wagner D Grumme D Ribchester RR Plomann M Coleman MP 《Molecular biology of the cell》2006,17(3):1075-1084
Slow Wallerian degeneration (Wld(S)) mutant mice express a chimeric nuclear protein that protects sick or injured axons from degeneration. The C-terminal region, derived from NAD(+) synthesizing enzyme Nmnat1, is reported to confer neuroprotection in vitro. However, an additional role for the N-terminal 70 amino acids (N70), derived from multiubiquitination factor Ube4b, has not been excluded. In wild-type Ube4b, N70 is part of a sequence essential for ubiquitination activity but its role is not understood. We report direct binding of N70 to valosin-containing protein (VCP; p97/Cdc48), a protein with diverse cellular roles including a pivotal role in the ubiquitin proteasome system. Interaction with Wld(S) targets VCP to discrete intranuclear foci where ubiquitin epitopes can also accumulate. Wld(S) lacking its N-terminal 16 amino acids (N16) neither binds nor redistributes VCP, but continues to accumulate in intranuclear foci, targeting its intrinsic NAD(+) synthesis activity to these same foci. Wild-type Ube4b also requires N16 to bind VCP, despite a more C-terminal binding site in invertebrate orthologues. We conclude that N-terminal sequences of Wld(S) protein influence the intranuclear location of both ubiquitin proteasome and NAD(+) synthesis machinery and that an evolutionary recent sequence mediates binding of mammalian Ube4b to VCP. 相似文献
120.
Sebastian Schuerlein Thomas Schwarz Steffan Krziminski Sabine Gätzner Anke Hoppensack Ivo Schwedhelm Matthias Schweinlin Heike Walles Jan Hansmann 《Biotechnology journal》2017,12(2)
Tissue Engineering (TE) bears potential to overcome the persistent shortage of donor organs in transplantation medicine. Additionally, TE products are applied as human test systems in pharmaceutical research to close the gap between animal testing and the administration of drugs to human subjects in clinical trials. However, generating a tissue requires complex culture conditions provided by bioreactors. Currently, the translation of TE technologies into clinical and industrial applications is limited due to a wide range of different tissue‐specific, non‐disposable bioreactor systems. To ensure a high level of standardization, a suitable cost‐effectiveness, and a safe graft production, a generic modular bioreactor platform was developed. Functional modules provide robust control of culture processes, e.g. medium transport, gas exchange, heating, or trapping of floating air bubbles. Characterization revealed improved performance of the modules in comparison to traditional cell culture equipment such as incubators, or peristaltic pumps. By combining the modules, a broad range of culture conditions can be achieved. The novel bioreactor platform allows using disposable components and facilitates tissue culture in closed fluidic systems. By sustaining native carotid arteries, engineering a blood vessel, and generating intestinal tissue models according to a previously published protocol the feasibility and performance of the bioreactor platform was demonstrated. 相似文献