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   262篇
  免费   23篇
  2020年   2篇
  2019年   4篇
  2018年   4篇
  2017年   4篇
  2016年   4篇
  2015年   11篇
  2014年   8篇
  2013年   15篇
  2012年   11篇
  2011年   28篇
  2010年   15篇
  2009年   7篇
  2008年   10篇
  2007年   17篇
  2006年   10篇
  2005年   13篇
  2004年   5篇
  2003年   4篇
  2002年   12篇
  2001年   7篇
  2000年   10篇
  1999年   3篇
  1998年   2篇
  1994年   3篇
  1993年   4篇
  1992年   7篇
  1991年   5篇
  1990年   4篇
  1989年   4篇
  1988年   4篇
  1987年   3篇
  1985年   2篇
  1983年   4篇
  1981年   1篇
  1980年   3篇
  1978年   1篇
  1977年   1篇
  1975年   1篇
  1974年   2篇
  1973年   1篇
  1972年   5篇
  1970年   1篇
  1969年   6篇
  1968年   2篇
  1967年   2篇
  1966年   2篇
  1965年   2篇
  1963年   1篇
  1942年   1篇
  1940年   1篇
排序方式: 共有285条查询结果,搜索用时 15 毫秒
131.
The origin of species-rich insect-plant food webs has traditionally been explained by diversifying antagonistic coevolution between plant defences and herbivore counter-defences. However, recent studies combining paleoclimatic reconstructions with time-calibrated phylogenies suggest that variation in global climate determines the distribution, abundance and diversity of plant clades and, hence, indirectly influences the balance between speciation and extinction in associated herbivore groups. Extant insect communities tend to be richest on common plant species that have many close relatives. This could be explained either by climate-driven diffuse cospeciation between plants and insects, or by elevated speciation and reduced extinction in herbivore lineages associated with expanding host taxa (resources). Progress in paleovegetation reconstructions in combination with the rapidly increasing availability of fossil-calibrated phylogenies provide means to discern between these alternative hypotheses. In particular, the 'Diffuse cospeciation' scenario predicts closely matching main diversification periods in plants and in the insects that feed upon them, while the 'Resource abundance-dependent diversification' hypothesis predicts that both positive and negative responses of insect diversity are lagged in relation to host-plant availability. The dramatic Cenozoic changes in global climate provide multiple possibilities for studying the mechanisms by which climatic shifts may drive diversity dynamics in plants and insect herbivores.  相似文献   
132.
Long-term colonic inflammation promotes carcinogenesis and histological abnormalities of the liver, and colorectal tumours frequently arise in a background of dysplasia, a precursor of adenomas. Altered colonic microbiota with an increased proportion of bacteria with pro-inflammatory characteristics, have been implicated in neoplastic progression. The composition of the microbiota can be modified by dietary components such as probiotics, polyphenols and dietary fibres. In the present study, the influence of probiotics in combination with blueberry husks on colorectal carcinogenesis and subsequent liver damage was evaluated.Colorectal tumours were induced in rats by cyclic treatment with dextran sulphate sodium (DSS). Blueberry husks and a mixture of three probiotic strains (Bifidobacterium infantis DSM 15159, Lactobacillus gasseri, DSM 16737 and Lactobacillus plantarum DSM 15313) supplemented a basic diet fortified with oats. The condition of the rats was monitored using a disease activity index (DAI). A qualitative and quantitative histological judgement was performed on segments of distal colon and rectum and the caudate lobe of the liver. The formation of short-chain fatty acids, bacterial translocation, the inflammatory reaction and viable count of lactobacilli and Enterobaceriaceae were addressed.Blueberry husks with or without probiotics significantly decreased DAI, and significantly reduced the number of colonic ulcers and dysplastic lesions. With a decreased proportion of blueberry husk in the diet, the probiotic supplement was needed to achieve a significant decrease in numbers of dysplastic lesions. Probiotics decreased faecal viable count of Enterobacteriaceae and increased that of lactobacilli. Blueberry husks with or without probiotics lowered the proportion of butyric acid in distal colon, and decreased the haptoglobin levels. Probiotics mitigated hepatic injuries by decreasing parenchymal infiltration and the incidence of stasis and translocation. The results demonstrate a dietary option for use of blueberry husks and probiotics to delay colonic carcinogenesis and hepatic injuries in the rat model.  相似文献   
133.
A series of potent antagonists of the ion channel transient receptor potential A1 (TRPA1) was developed by modifying lead structure 16 that was discovered by high-throughput screening. Based on lead compound 16, a SAR was established, showing a narrow region at the nitro-aromatic R(1) moiety and at the warhead, while the R(2) side had a much wider scope including ureas and carbamates. Compound 16 inhibits Ca(2+)-activated TRPA1 currents reversibly in whole cell patch clamp experiments, indicating that under in vivo conditions, it does not react covalently, despite its potentially electrophilic ketone.  相似文献   
134.
135.

Background  

Footrot is a world-wide contagious disease in sheep and goats. It is an infection of the epidermis of the interdigital skin, and the germinal layers of the horn tissue of the feet. The first case of footrot in Swedish sheep was diagnosed in 2004. Due to difficulties in distinguishing benign footrot from early cases of virulent footrot and because there is no possibility for virulence testing of strains of Dichelobacter nodosus in Sweden, the diagnosis is based of the presence or absence of clinical signs of footrot in sheep flocks. Ever since the first diagnosed case the Swedish Animal Health Service has worked intensively to stop the spread of infection and control the disease at flock level. However, to continue this work effectively it is important to have knowledge about the distribution of the disease both nationally and regionally. Therefore, the aims of this study were to estimate the prevalence of footrot in Swedish lambs at abattoirs and to assess the geographical distribution of the disease.  相似文献   
136.
Recent advancements in magnetic resonance imaging (MRI) have enabled clinical imaging of human cortical bone, providing a potentially powerful new means for assessing bone health with molecular-scale sensitivities unavailable to conventional X-ray-based diagnostics. To this end, 1H nuclear magnetic resonance (NMR) and high-resolution X-ray signals from human cortical bone samples were correlated with mechanical properties of bone. Results showed that 1H NMR signals were better predictors of yield stress, peak stress, and pre-yield toughness than were the X-ray derived signals. These 1H NMR signals can, in principle, be extracted from clinical MRI, thus offering the potential for improved clinical assessment of fracture risk.  相似文献   
137.
Evasion of apoptosis is recognized as a characteristic of malignant growth. Anti-apoptotic B-cell lymphoma-2 (Bcl-2) family members have therefore emerged as potential therapeutic targets due to their critical role in proliferating cancer cells. Here, we present the crystal structure of Bfl-1, the last anti-apoptotic Bcl-2 family member to be structurally characterized, in complex with a peptide corresponding to the BH3 region of the pro-apoptotic protein Bim. The structure reveals distinct features at the peptide-binding site, likely to define the binding specificity for pro-apoptotic proteins. Superposition of the Bfl-1:Bim complex with that of Mcl-1:Bim reveals a significant local plasticity of hydrophobic interactions contributed by the Bim peptide, likely to be the basis for the multi specificity of Bim for anti-apoptotic proteins.  相似文献   
138.
Groundwater near the S3 ponds at the US Department of Energy's Y-12 site in Oak Ridge, Tennessee, is contaminated by high levels of nitrate (up to 160 mM) and U(VI) (∼0.3 mM). To minimize nitrate inhibition, the authors proposed extraction of contaminated groundwater, nitrate removal in a denitrifying fluidized bed bioreactor (FBR), and return of nitrate-free effluent to the aquifer to stimulate in situ microbial reduction of U(VI). In the presence of carbonate, U(VI) sorption to biomass was negligible, but in its absence, sorption was significant. Biomass reduced U(VI) to U(IV), exhibiting slow first-order removal with respect to U(VI). Addition of electron donor increased rates. Addition of an inhibitor of sulfate reduction (molybdate) slowed the rate and inhibited sulfate reduction. Denitrifying β-Proteobacteria dominated clone libraries of SSU rRNA and dsrA gene sequences. Approximately 10% were low-G+C microorganisms that had 90% to 92% sequence identity with Sporomusa, Acetonema, and Propionispora. The dsrA sequences were dominated by a single clone with ∼80% nucleotide identity to dsrA of Desulfovibrio vulgaris sub sp. oxamicus. The authors conclude that some members of this denitrifyng community reduce uranium, and that sulfate-reducing bacteria likely contribute to this capability.  相似文献   
139.
The ‘Symphyta’ is a paraphyletic assemblage at the base of the order Hymenoptera, comprising 14 families and about 8750 species. All have phytophagous larvae, except for the Orussidae, which are parasitoids. This study presents and evaluates the results of DNA barcoding of approximately 5360 specimens of ‘Symphyta’, mainly adults, and 4362 sequences covering 1037 species were deemed of suitable quality for inclusion in the analysis. All extant families are represented, except for the Anaxyelidae. The majority of species and specimens are from Europe, but approximately 38% of the species and 13% of the specimens are of non‐European origin. The utility of barcoding for species identification and taxonomy of ‘Symphyta’ is discussed on the basis of examples from each of the included families. A significant level of cryptic species diversity was apparent in many groups. Other attractive applications include the identification of immature stages without the need to rear them, community analyses based on metabarcoding of bulk samples and association of the sexes of adults.  相似文献   
140.
Printing technologies were recently introduced to the pharmaceutical field for manufacturing of drug delivery systems. Printing allows on demand manufacturing of flexible pharmaceutical doses in a personalized manner, which is critical for a successful and safe treatment of patient populations with specific needs, such as children and the elderly, and patients facing multimorbidity. Printing of pharmaceuticals as technique generates new demands on the quality control procedures. For example, rapid quality control is needed as the printing can be done on demand and at the point of care. This study evaluated the potential use of a handheld colorimetry device for quality control of printed doses of vitamin Bs on edible rice and sugar substrates. The structural features of the substrates with and without ink were also compared. A multicomponent ink formulation with vitamin B1, B2, B3, and B6 was developed. Doses (4 cm2) were prepared by applying 1–10 layers of yellow ink onto the white substrates using thermal inkjet technology. The colorimetric method was seen to be viable in detecting doses up to the 5th and 6th printed layers until color saturation of the yellow color parameter (b*) was observed on the substrates. Liquid chromatography mass spectrometry was used as a reference method for the colorimetry measurements plotted against the number of printed layers. It was concluded that colorimetry could be used as a quality control tool for detection of different doses. However, optimization of the color addition needs to be done to avoid color saturation within the planned dose interval.  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