全文获取类型
收费全文 | 4374篇 |
免费 | 387篇 |
国内免费 | 2篇 |
出版年
2023年 | 17篇 |
2022年 | 33篇 |
2021年 | 89篇 |
2020年 | 54篇 |
2019年 | 53篇 |
2018年 | 66篇 |
2017年 | 65篇 |
2016年 | 104篇 |
2015年 | 239篇 |
2014年 | 190篇 |
2013年 | 255篇 |
2012年 | 392篇 |
2011年 | 317篇 |
2010年 | 206篇 |
2009年 | 144篇 |
2008年 | 240篇 |
2007年 | 265篇 |
2006年 | 213篇 |
2005年 | 217篇 |
2004年 | 237篇 |
2003年 | 205篇 |
2002年 | 192篇 |
2001年 | 43篇 |
2000年 | 31篇 |
1999年 | 39篇 |
1998年 | 91篇 |
1997年 | 34篇 |
1996年 | 49篇 |
1995年 | 38篇 |
1994年 | 45篇 |
1993年 | 40篇 |
1992年 | 38篇 |
1991年 | 28篇 |
1990年 | 34篇 |
1989年 | 41篇 |
1988年 | 25篇 |
1987年 | 21篇 |
1986年 | 23篇 |
1985年 | 27篇 |
1984年 | 30篇 |
1983年 | 32篇 |
1982年 | 38篇 |
1981年 | 28篇 |
1980年 | 31篇 |
1979年 | 23篇 |
1978年 | 14篇 |
1977年 | 21篇 |
1976年 | 17篇 |
1974年 | 14篇 |
1973年 | 12篇 |
排序方式: 共有4763条查询结果,搜索用时 62 毫秒
211.
Troy E. Sandberg Christopher P. Long Jacqueline E. Gonzalez Adam M. Feist Maciek R. Antoniewicz Bernhard O. Palsson 《PloS one》2016,11(3)
13C-Metabolic flux analysis (13C-MFA) traditionally assumes that kinetic isotope effects from isotopically labeled compounds do not appreciably alter cellular growth or metabolism, despite indications that some biochemical reactions can be non-negligibly impacted. Here, populations of Escherichia coli were adaptively evolved for ~1000 generations on uniformly labeled 13C-glucose, a commonly used isotope for 13C-MFA. Phenotypic characterization of these evolved strains revealed ~40% increases in growth rate, with no significant difference in fitness when grown on either labeled (13C) or unlabeled (12C) glucose. The evolved strains displayed decreased biomass yields, increased glucose and oxygen uptake, and increased acetate production, mimicking what is observed after adaptive evolution on unlabeled glucose. Furthermore, full genome re-sequencing revealed that the key genetic changes underlying these phenotypic alterations were essentially the same as those acquired during adaptive evolution on unlabeled glucose. Additionally, glucose competition experiments demonstrated that the wild-type exhibits no isotopic preference for unlabeled glucose, and the evolved strains have no preference for labeled glucose. Overall, the results of this study indicate that there are no significant differences between 12C and 13C-glucose as a carbon source for E. coli growth. 相似文献
212.
The gills of euryhaline fish are the ultimate ionoregulatory tissue, achieving ion homeostasis despite rapid and significant changes in external salinity. Cellular handling of sodium is not only critical for salt and water balance but is also directly linked to other essential functions such as acid–base homeostasis and nitrogen excretion. However, although measurement of intracellular sodium ([Na+]i) is important for an understanding of gill transport function, it is challenging and subject to methodological artifacts. Using gill filaments from a model euryhaline fish, inanga (Galaxias maculatus), the suitability of the fluorescent dye CoroNa Green as a probe for measuring [Na+]i in intact ionocytes was confirmed via confocal microscopy. Cell viability was verified, optimal dye loading parameters were determined, and the dye–ion dissociation constant was measured. Application of the technique to freshwater- and 100% seawater-acclimated inanga showed salinity-dependent changes in branchial [Na+]i, whereas no significant differences in branchial [Na+]i were determined in 50% seawater-acclimated fish. This technique facilitates the examination of real-time changes in gill [Na+]i in response to environmental factors and may offer significant insight into key homeostatic functions associated with the fish gill and the principles of sodium ion transport in other tissues and organisms. 相似文献
213.
214.
Megan B. Machmuller Jacqueline E. Mohan Jeffrey M. Minucci Carly A. Phillips Nina Wurzburger 《Biogeochemistry》2016,129(3):255-272
Climate change may affect the microbial production and temperature sensitivity of extracellular enzymes that release carbon (C) and nutrients from soil organic matter. We measured the response of six hydrolytic enzymes involved in C, nitrogen (N), and phosphorus (P) degradation to experimental warming in a mixed-deciduous forest persisting on highly-weathered Ultisols (Whitehall Forest, Georgia, US). We found that warming produced no consistent enzyme response. However, we observed significant seasonal variation in enzyme activities, temperature sensitivities and elemental enzyme ratios (C:N and C:P). Seasonal differences in enzyme activity and temperature sensitivity were best explained by soil moisture and temperature. Our results suggest that seasonal dynamics in soil microclimate, organic matter supply, and microbial demand exert more control on enzyme dynamics than does a uniform increase in soil temperature. 相似文献
215.
216.
Long‐term response of temperate canopy trees to removal of browsing from an invasive arboreal herbivore in New Zealand
下载免费PDF全文
![点击此处可从《Austral ecology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Peter J. Sweetapple Graham Nugent Jacqueline Whitford M. Cecilia Latham Kees Pekelharing 《Austral ecology》2016,41(5):538-548
Defoliation of forest tree canopies by herbivores and other agents, leading to tree mortality and reduced productivity, threatens the ecological stability of forests globally. This study shows that long‐term control of a mammalian arboreal folivore (brushtail possums; Trichosurus vulpecula Phalangeridae) reduces crown dieback and increases foliage cover in browsing‐damaged canopy trees. We monitored indices of possum density, possum browsing, tree foliage cover and crown dieback for 20 years following initiation of possum control in 1994 that repeatedly reduced possum densities to near zero every 5–6 years and kept the population below 35% of pre‐control levels over the entire period. Observable possum browsing was recorded on 20–49% of individuals of three palatable tree species at the time of first control. Those percentages fell to zero after control and never exceeded 2–10% for individual species over the next 19 years. We recorded significant increases in foliage cover attributable to recovery from defoliation by possums for all three species during the first 10 years. Large increases in foliage cover occurred on individuals that were heavily browsed in 1994 (mean increases: 36–89%), but mean population increases were modest (3–19%) because only 10–19% of trees were initially heavily browsed. Twenty‐year mortality rates were similar for plants with, or without, initial possum browsing, indicating no residual impact of pre‐control browsing on tree mortality. Times for full recovery of crown foliage cover varied from 10 years for the youngest trees and faster growing species to more than 20 years for mature individuals of the slowest growing species. 相似文献
217.
Differential foraging success across a light level spectrum explains the maintenance and spatial structure of colour morphs in a polymorphic bird
下载免费PDF全文
![点击此处可从《Ecology letters》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Detectability of different colour morphs under varying light conditions has been proposed as an important driver in the maintenance of colour polymorphism via disruptive selection. To date, no studies have tested whether different morphs have selective advantages under differing light conditions. We tested this hypothesis in the black sparrowhawk, a polymorphic raptor exhibiting a discrete white and dark morph, and found that prey provisioning rates differ between the morphs depending on light condition. Dark morphs delivered more prey in lower light conditions, while white morphs provided more prey in brighter conditions. We found support for the role of breeding season light level in explaining the clinal pattern of variation in morph ratio across the species range throughout South Africa. Our results provide the first empirical evidence supporting the hypothesis that polymorphism in a species, and the spatial structuring of morphs across its distribution, may be driven by differential selective advantage via improved crypsis, under varying light conditions. 相似文献
218.
219.
Geert P. M. Mommen Fabio Marino Hugo D. Meiring Martien C. M. Poelen Jacqueline A. M. van Gaans-van den Brink Shabaz Mohammed Albert J. R. Heck Cécile A. C. M. van Els 《Molecular & cellular proteomics : MCP》2016,15(4):1412-1423
Comprehensive analysis of the complex nature of the Human Leukocyte Antigen (HLA) class II ligandome is of utmost importance to understand the basis for CD4+ T cell mediated immunity and tolerance. Here, we implemented important improvements in the analysis of the repertoire of HLA-DR-presented peptides, using hybrid mass spectrometry-based peptide fragmentation techniques on a ligandome sample isolated from matured human monocyte-derived dendritic cells (DC). The reported data set constitutes nearly 14 thousand unique high-confident peptides, i.e. the largest single inventory of human DC derived HLA-DR ligands to date. From a technical viewpoint the most prominent finding is that no single peptide fragmentation technique could elucidate the majority of HLA-DR ligands, because of the wide range of physical chemical properties displayed by the HLA-DR ligandome. Our in-depth profiling allowed us to reveal a strikingly poor correlation between the source proteins identified in the HLA class II ligandome and the DC cellular proteome. Important selective sieving from the sampled proteome to the ligandome was evidenced by specificity in the sequences of the core regions both at their N- and C- termini, hence not only reflecting binding motifs but also dominant protease activity associated to the endolysosomal compartments. Moreover, we demonstrate that the HLA-DR ligandome reflects a surface representation of cell-compartments specific for biological events linked to the maturation of monocytes into antigen presenting cells. Our results present new perspectives into the complex nature of the HLA class II system and will aid future immunological studies in characterizing the full breadth of potential CD4+ T cell epitopes relevant in health and disease.Human Leukocyte Antigen (HLA)1 class II molecules on professional antigen presenting cells such as dendritic cells (DC) expose peptide fragments derived from exogenous and endogenous proteins to be screened by CD4+ T cells (1, 2). The activation and recruitment of CD4+ T cells recognizing disease-related peptide antigens is critical for the development of efficient antipathogen or antitumor immunity. Furthermore, the presentation of self-peptides and their interaction with CD4+ T cells is essential to maintain immunological tolerance and homeostasis (3). Knowledge of the nature of HLA class II-presented peptides on DC is of great importance to understand the rules of antigen processing and peptide binding motifs (4), whereas the identity of disease-related antigens may provide new knowledge on immunogenicity and leads for the development of vaccines and immunotherapy (5, 6).Mass spectrometry (MS) has proven effective for the analysis HLA class II-presented peptides (4, 7, 8). MS-based ligandome studies have demonstrated that HLA class II molecules predominantly present peptides derived from exogenous proteins that entered the cells by endocytosis and endogenous proteins that are associated with the endo-lysosomal compartments (4). Yet proteins residing in the cytosol, nucleus or mitochondria can also be presented by HLA class II molecules, primarily through autophagy (9–11). Multiple studies have mapped the HLA class II ligandome of antigen presenting cells in the context of infectious pathogens (12), autoimmune diseases (13–17) or cancer (14, 18, 19), or those that are essential for self-tolerance in the human thymus (3, 20). Notwithstanding these efforts, and certainly not in line with the extensive knowledge on the HLA class I ligandome (21), the nature of the HLA class II-presented peptide repertoire and particular its relationship to the cellular source proteome remains poorly understood.To advance our knowledge on the HLA-DR ligandome on activated DC without having to deal with limitations in cell yield from peripheral human blood (12, 21, 22) or tissue isolates (3), we explored the use of MUTZ-3 cells. This cell line has been used as a model of human monocyte-derived DCs. MUTZ-3 cells can be matured to act as antigen presenting cells and express then high levels of HLA class II molecules, and can be propagated in vitro to large cell densities (23–25). We also evaluated the performance of complementary and hybrid MS fragmentation techniques electron-transfer dissociation (ETD), electron-transfer/higher-energy collision dissociation (EThcD) (26), and higher-energy collision dissociation (HCD) to sequence and identify the HLA class II ligandome. Together this workflow allowed for the identification of an unprecedented large set of about 14 thousand unique peptide sequences presented by DC derived HLA-DR molecules, providing an in-depth view of the complexity of the HLA class II ligandome, revealing underlying features of antigen processing and surface-presentation to CD4+ T cells. 相似文献
220.
The order Passeriformes comprises the majority of extant avian species. Analyses of molecular data have provided important insights into the evolution of this diverse order. However, molecular estimates of the evolutionary and demographic timescales of passerine species have been hindered by a lack of reliable calibrations. This has led to a reliance on the application of standard substitution rates to mitochondrial DNA data, particularly rates estimated from analyses of the gene encoding cytochrome b (CYTB). To investigate patterns of rate variation across passerine lineages, we used a Bayesian phylogenetic approach to analyse the protein‐coding genes of 183 mitochondrial genomes. We found that the most commonly used mitochondrial marker, CYTB, has low variation in rates across passerine lineages. This lends support to its widespread use as a molecular clock in birds. However, we also found that the patterns of among‐lineage rate variation in CYTB are only weakly related to the evolutionary rate of the mitochondrial genome as a whole. Our analyses confirmed the presence of mutational saturation at third codon positions across the protein‐coding genes of the mitochondrial genome, reinforcing the view that these sites should be excluded in studies of deep passerine relationships. The results of our analyses have provided information that will be useful for molecular‐clock studies of passerine evolution. 相似文献