首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1899篇
  免费   203篇
  国内免费   1篇
  2023年   8篇
  2022年   12篇
  2021年   44篇
  2020年   26篇
  2019年   44篇
  2018年   48篇
  2017年   35篇
  2016年   49篇
  2015年   92篇
  2014年   89篇
  2013年   103篇
  2012年   157篇
  2011年   130篇
  2010年   89篇
  2009年   83篇
  2008年   112篇
  2007年   84篇
  2006年   104篇
  2005年   78篇
  2004年   88篇
  2003年   74篇
  2002年   77篇
  2001年   26篇
  2000年   29篇
  1999年   33篇
  1998年   26篇
  1997年   26篇
  1996年   10篇
  1995年   12篇
  1994年   16篇
  1993年   17篇
  1992年   17篇
  1991年   23篇
  1990年   16篇
  1989年   17篇
  1988年   16篇
  1987年   14篇
  1986年   17篇
  1985年   17篇
  1984年   13篇
  1983年   10篇
  1982年   9篇
  1981年   11篇
  1980年   10篇
  1979年   15篇
  1978年   10篇
  1975年   8篇
  1974年   10篇
  1973年   9篇
  1972年   10篇
排序方式: 共有2103条查询结果,搜索用时 223 毫秒
951.
952.
Understanding the environmental factors that shape microbial communities is crucial, especially in extreme environments, like Antarctica. Two main forces were reported to influence Antarctic soil microbes: birds and plants. Both birds and plants are currently undergoing relatively large changes in their distribution and abundance due to global warming. However, we need to clearly understand the relationship between plants, birds and soil microorganisms. We therefore collected rhizosphere and bulk soils from six different sampling sites subjected to different levels of bird influence and colonized by Colobanthus quitensis and Deschampsia antarctica in Admiralty Bay, King George Island, Maritime Antarctic. Microarray and qPCR assays targeting 16S rRNA genes of specific taxa were used to assess microbial community structure, composition and abundance and analyzed with a range of soil physico-chemical parameters. The results indicated significant rhizosphere effects in four out of the six sites, including areas with different levels of bird influence. Acidobacteria were significantly more abundant in soils with little bird influence (low nitrogen) and in bulk soil. In contrast, Actinobacteria were significantly more abundant in the rhizosphere of both plant species. At two of the sampling sites under strong bird influence (penguin colonies), Firmicutes were significantly more abundant in D. antarctica rhizosphere but not in C. quitensis rhizosphere. The Firmicutes were also positively and significantly correlated to the nitrogen concentrations in the soil. We conclude that the microbial communities in Antarctic soils are driven both by bird and plants, and that the effect is taxa-specific.  相似文献   
953.
954.
A chemical investigation of the Glyphaea brevis leaves and of the Monodora myristica fruits led to the identification of thirteen compounds, seven linear long‐chain aliphatic compounds, 1, 2, 4, 6 , and 9 – 11 , three steroids, 3a, 3b , and 7 , two triterpenes, 5a and 5b , and one polyol, 8 . The compounds 2 and 8 , previously mentioned in the literature, are here characterized by their complete 1H‐ and 13C‐NMR assignments. This is the first report of a full NMR assignment for linear fatty acid esters of aliphatic primary alcohols and for meso‐erythritol. Compound 5b and 8 were isolated for the first time from plant extracts of the Tiliaceae family, and compounds 9 – 11 from the Annonaceae plant family. Our results constitute the basis for further chemotaxonomic studies on the two species.  相似文献   
955.

Background

Staphylococcus epidermidis orthopedic device infections are caused by direct inoculation of commensal flora during surgery and remain rare, although S. epidermidis carriage is likely universal. We wondered whether S. epidermidis orthopedic device infection strains might constitute a sub-population of commensal isolates with specific virulence ability. Biofilm formation and invasion of osteoblasts by S. aureus contribute to bone and joint infection recurrence by protecting bacteria from the host-immune system and most antibiotics. We aimed to determine whether S. epidermidis orthopedic device infection isolates could be distinguished from commensal strains by their ability to invade osteoblasts and form biofilms.

Materials and Methods

Orthopedic device infection S. epidermidis strains (n = 15) were compared to nasal carriage isolates (n = 22). Osteoblast invasion was evaluated in an ex vivo infection model using MG63 osteoblastic cells co-cultured for 2 hours with bacteria. Adhesion of S. epidermidis to osteoblasts was explored by a flow cytometric approach, and internalized bacteria were quantified by plating cell lysates after selective killing of extra-cellular bacteria with gentamicin. Early and mature biofilm formations were evaluated by a crystal violet microtitration plate assay and the Biofilm Ring Test method.

Results

No difference was observed between commensal and infective strains in their ability to invade osteoblasts (internalization rate 308+/−631 and 347+/−431 CFU/well, respectively). This low internalization rate correlated with a low ability to adhere to osteoblasts. No difference was observed for biofilm formation between the two groups.

Conclusion

Osteoblast invasion and biofilm formation levels failed to distinguish S. epidermidis orthopedic device infection strains from commensal isolates. This study provides the first assessment of the interaction between S. epidermidis strains isolated from orthopedic device infections and osteoblasts, and suggests that bone cell invasion is not a major pathophysiological mechanism in S. epidermidis orthopedic device infections, contrary to what is observed for S. aureus.  相似文献   
956.
Etienne Rajon  Joanna Masel 《Genetics》2013,193(4):1209-1220
Cryptic genetic sequences have attenuated effects on phenotypes. In the classic view, relaxed selection allows cryptic genetic diversity to build up across individuals in a population, providing alleles that may later contribute to adaptation when co-opted—e.g., following a mutation increasing expression from a low, attenuated baseline. This view is described, for example, by the metaphor of the spread of a population across a neutral network in genotype space. As an alternative view, consider the fact that most phenotypic traits are affected by multiple sequences, including cryptic ones. Even in a strictly clonal population, the co-option of cryptic sequences at different loci may have different phenotypic effects and offer the population multiple adaptive possibilities. Here, we model the evolution of quantitative phenotypic characters encoded by cryptic sequences and compare the relative contributions of genetic diversity and of variation across sites to the phenotypic potential of a population. We show that most of the phenotypic variation accessible through co-option would exist even in populations with no polymorphism. This is made possible by a history of compensatory evolution, whereby the phenotypic effect of a cryptic mutation at one site was balanced by mutations elsewhere in the genome, leading to a diversity of cryptic effect sizes across sites rather than across individuals. Cryptic sequences might accelerate adaptation and facilitate large phenotypic changes even in the absence of genetic diversity, as traditionally defined in terms of alternative alleles.  相似文献   
957.
Katanin is an evolutionarily conserved microtubule (MT)-severing complex implicated in multiple aspects of MT dynamics. In Caenorhabditis elegans, the katanin homologue MEI-1 is required for meiosis, but must be inactivated before mitosis. Here we show that PPFR-1, a regulatory subunit of a trimeric protein phosphatase 4 complex, enhanced katanin MT-severing activity during C. elegans meiosis. Loss of ppfr-1, similarly to the inactivation of MT severing, caused a specific defect in meiosis II spindle disassembly. We show that a fraction of PPFR-1 was degraded after meiosis, contributing to katanin inactivation. PPFR-1 interacted with MEL-26, the substrate recognition subunit of the CUL-3 RING E3 ligase (CRL3MEL-26), which also targeted MEI-1 for post-meiotic degradation. Reversible protein phosphorylation of MEI-1 may ensure temporal activation of the katanin complex during meiosis, whereas CRL3MEL-26-mediated degradation of both MEI-1 and its activator PPFR-1 ensure efficient katanin inactivation in the transition to mitosis.  相似文献   
958.
The dynamic equilibrium model of species diversity predicts that ecosystem productivity interacts with disturbance to determine how many species coexist. However, a robust test of this model requires manipulations of productivity and disturbance over a sufficient timescale to allow competitive exclusion, and such long-term experimental tests of this hypothesis are rare. Here we use long-term (27 years), large-scale (8 × 50-m plots), factorial manipulations of soil resource availability and sheep grazing intensity (disturbance) in grasslands to test the dynamic equilibrium model. As predicted by the model, increased productivity not only reduced plant species richness, but also moderated the effects of grazing intensity, shifting them from negative to neutral with increasing productivity. Reductions in species richness with productivity were associated with dominance by faster growing (i.e. high specific leaf area) and taller plants. Conversely, grazing favoured shorter plants and this effect became stronger with greater productivity, consistent with the view that grazing can lead to weaker asymmetric competition for light. Our study shows that the dynamic equilibrium model can help to explain changes in plant species richness following long-term increases in soil resource availability and grazing pressure, two fundamental drivers of change in grasslands worldwide.  相似文献   
959.
ABSTRACT

Daily pattern of locomotor activity (LA), one of the most studied rhythms in humans and rodents, has not been widely investigated in large mammals. This is partly due to the high cost and breakability of used automatic devices. Since last decade, smartphones are becoming ubiquitous. Meanwhile, several applications detecting activity by using internal sensors were made available. In this study, we assumed that this device could be a cheaper and easier way to measure the LA rhythm in humans and large mammals, like camel and goat. A smartphone application (Nokia Mate Health), normally used to quantify physical activities in humans, was chosen for the study. To validate the rhythm data obtained from the smartphone, LA rhythm was simultaneously recorded using an automatic device, the Actiwatch-Mini®. Results showed that the smartphone provided a clear and significant daily rhythm of LA. The visual assessment of the superimposed LA rhythm’s curves in all three species showed that the smartphone application displayed similar rhythms as those recorded by the Actiwatch-Mini. Highly significant positive correlation (p≤ 0.0001) exists between the two recording rhythms. The daily periods were both the same at 24.0 h. Acrophases were also significantly similar and occurring around mid-day: 11:40 ± 0.35 h vs 11:41 ± 0.35 h for the camel, 11:25 ± 0.19 h vs 11:37 ± 0.25 h for the goat and 13:04 ± 0.11 h vs 13:51 ± 0.28 h for humans using smartphone and Actiwatch, respectively. The related mesor and amplitude were also close between the two recording devices. Results indicate clearly that using smartphones constitutes a reliable cheap tool to study LA rhythm for chronobiology studies, especially in laboratories facing lack of funding.  相似文献   
960.

Purpose

This study provides a general methodology to integrate LCA into a single- or multi-objective process design optimization context. It uses specific weightings for foreground emissions, for preventable background emissions and for unpreventable background emissions, for each impact category. It is illustrated for a natural gas combined cycle power plant with three scenarios to reduce its carbon dioxide emissions: CO2 capture and sequestration, fuel substitution with biogas or fuel substitution with synthetic gas from wood.

Methods

Assuming that the opportunity to prevent emissions elsewhere is an implicit part of the process design decision space, the optimal solution cannot waste such opportunities and is shown to minimize total life cycle costs, including emission avoidance costs based on the optimal combination of prevention and compensation measures in the background system. In the case study, background emissions are inventoried from the ecoinvent database, their compensation costs are derived from the Ecocosts 2007 impact assessment method and their prevention costs are estimated from the literature. The calculated avoidance costs (weightings) then show how the background system affects the final choice of CO2 reduction scenario.

Results and discussion

In the case study, all three options partially shift environmental burdens to the background system, which can be prevented or compensated. The corresponding minimum avoidance cost is highest overall for the biogas option, thus putting it at a disadvantage. For a vast majority of ecoinvent processes, energy efficiency is important to minimize total avoidance costs because they are dominated by background CO2. Furthermore, prevention cost data gathering can be simplified in some cases, without distorting design decisions, using a CO2-only background inventory. The non-CO2 background inventory is more useful after process design, for procurement decisions.

Conclusions

Over-investing in design modifications cannot achieve the same background impact reductions as a sensible green procurement policy. Thus, the proposed weighting methodology ensures that all types of design decisions integrate LCA without incorrectly assuming that emissions are necessarily unavoidable when in the background. Within a context of future emission taxes or tradable permits, the weightings can also anticipate the after-tax cost passed on by suppliers—a marketable benefit of LCA.

Recommendations

Since many LCA studies are equivalent to design optimization problems, the proposed weighting methodology provides a single-score impact method relevant to decision-making as well as a straightforward approach to LCA interpretation in terms of detailing the optimal combination of applicable design modifications, prevention measures and compensation measures.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号