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   825篇
  免费   43篇
  国内免费   1篇
  869篇
  2024年   1篇
  2023年   12篇
  2022年   29篇
  2021年   41篇
  2020年   38篇
  2019年   71篇
  2018年   53篇
  2017年   32篇
  2016年   54篇
  2015年   48篇
  2014年   57篇
  2013年   69篇
  2012年   71篇
  2011年   55篇
  2010年   38篇
  2009年   25篇
  2008年   31篇
  2007年   26篇
  2006年   25篇
  2005年   16篇
  2004年   21篇
  2003年   9篇
  2002年   10篇
  2001年   2篇
  2000年   1篇
  1999年   2篇
  1997年   1篇
  1996年   2篇
  1995年   4篇
  1994年   1篇
  1993年   3篇
  1992年   4篇
  1989年   2篇
  1986年   1篇
  1982年   3篇
  1979年   2篇
  1978年   2篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
  1973年   2篇
  1968年   1篇
  1966年   1篇
排序方式: 共有869条查询结果,搜索用时 0 毫秒
101.
Summary The bioconversion of hydrocortisone by a locally isolated strain of cyanobacterium Fischerella ambigua PTCC 1635 was investigated. Fischerella ambigua had not been previously examined for this potential. The fermentation led to production of 11β,17α, 20β, 21-tetrahydroxypregn-4-en-3-one and 11β-hydroxyandrost-4-en-3,17-dione. The metabolites were isolated and purified by chromatographic methods and identified using instrumental analyses.  相似文献   
102.
103.
Summary At high salinities and high temperatures, the growth rate of Pterocladia was high, the tips of branch initials were dilated and holdfasts broadened. By the end of the fourth week bleaching of the whole plant was complete.Under low salinities and low temperatures, the rate of vegetative growth was decreased, the cuticle was thin and food reserve was centrally accumulated in the cells.Under high salinities and low temperatures, the rate of growth was greatly reduced and cells were devoid of food reserve. Defoliation of lateral branches was frequent.Under low salinities and high temperatures, the rate of growht was rather high and cells were relatively large.  相似文献   
104.
This study integrates ichnological and sedimentological data to refine depositional sequences and interpretations of sea-level dynamics for the shallow marine, Albian–Cenomanian Aitamir Formation in northeastern Iran. Three ichnofabrics are present in a succession of glauconitic mudstone and sandstone. This is a sequence that grades upward from a lower glauconitic sandstone unit with trough cross-stratification, hummocky and ripple cross-lamination into a fining-up unit of mudstone with intercalated sandstone beds. The lower unit contains an ichoassemblage of the OphiomorphaPalaeophycus ichnofabric (upper shoreface), whereas the upper unit bears ichnoassemblages of the Thalassinoides ichnofabric (in a distinctive level at the top cycle which demarcates the base of the next cycle) (middle shoreface) and the ChondritesPlanolites ichnofabric (lower shoreface). An upper shoreface–lower shoreface trend from the OphiomorphaPalaeophycus ichnofabric to the ChondritesPlanolites ichnofabric represents a deepening-upward sequence. An integrated sedimentological and ichnological approach has allowed the recognition of the internal organization of the sequence and the characterization of significant discontinuity surfaces at sequence scales. Thalassinoides ichnofabric reveals colonization of firmgrounds during prolonged times between erosion and deposition related to transgressive surfaces. Transgressive surfaces (sequence boundaries) are generally well-cemented and marked by increased glauconite content, and densely crowded, predominantly vertical or oblique, relatively large, very distinct, unlined, and uncompacted burrows (omission suite) and are associated with rare highly abraded and fragmented shell remains.  相似文献   
105.
Plant-based methods such as rhizodegradation are very promising for the remediation of petroleum-contaminated soils. Associations of plants with endophytes can further enhance their phytoremediation potential. In this study, a rhizobox experiment was conducted to investigate whether inoculation with the root-colonizing fungus Piriformospora indica could further enhance the degradation of petroleum hydrocarbons in the root zone of maize (Zea mays L.). The rhizoboxes were subdivided into compartments in accordance with distance from the plants. After filling the boxes with soil from a petroleum-contaminated site, seedlings that had either been inoculated with P. indica or not were grown in the middle compartments of the rhizoboxes and grown for 64 days. A plant-free treatment was included for control. The presence of roots strongly increased the counts of total and petroleum-degrading soil bacteria, respiration, dehydrogenase activity, water-soluble phenols and petroleum degradation. All these effects were also found in the soil adjacent to the middle compartments of the rhizoboxes, but strongly decreased further away from it. Inoculation with P. indica further enhanced all the recorded parameters without changing the spatial pattern of the effects. Inoculated plants also produced around 40% more root and shoot biomass than noninoculated plants and had greener leaves. Together, the results indicate that the treatment effects on the recorded soil microbial and biochemical parameters including petroleum hydrocarbon degradation were primarily due to increased root exudation. Irrespectively of this, they show that maize can be used to accelerate the rhizodegradation of petroleum hydrocarbons in soil and that inoculation with P. indica can substantially enhance the phytoremediation performance of maize.  相似文献   
106.

Background

Neutralization of vascular endothelial growth factor receptor 1 (VEGFR1) and/or VEGFR2 is a widely used means of inhibiting tumor angiogenesis.

Methods

Based on the complex X-ray structures of VEGFA/VEGFR1, VEGFA/VEGFR2, and VEGFB/VEGFR1, a peptide (referred to as VGB) was designed to simultaneously bind to VEGFR1 and VEGFR2, and binding, antiangiogenic and antitumor properties of the peptide was investigated in vitro.

Results

VGB bound to both VEGFR1 and VEGFR2 in human umbilical vein endothelial cells (HUVECs) and 4?T1 mammary carcinoma tumor (MCT) cells, and inhibited the proliferation of HUVE, 4?T1 MCT, and U87 glioblastoma cells. Through abrogation of AKT and ERK1/2 phosphorylation, VEGFA-stimulated proliferation, migration, and two- and three-dimensional tube formation in HUVECs were inhibited more potently by VGB than by bevacizumab. In a murine 4?T1 MCT model, VGB strongly inhibited tumor growth without causing weight loss, accompanied by inhibition of AKT and ERK1/2 phosphorylation, a significant decrease in tumor cell proliferation (Ki-67 expression), angiogenesis (CD31 and CD34 expression), an increase in apoptosis index (increased TUNEL staining and p53 expression and decreased Bcl-2 expression), and the suppression of systematic spreading of the tumor (reduced NF-κB and MMP-9 and increased E-cadherin expression).

Conclusion

The dual specificity of VGB for VEGFR1 and VEGFR2, through which the PI3K/AKT and MAPK/ERK1/2 signaling pathways can be abrogated and, subsequently, angiogenesis, tumor growth, and metastasis are inhibited.

General significance

This study demonstrated that simultaneous blockade of VEGFR1 and VEGFR2 downstream cascades is an effective means for treatment of various angiogenic disorders, especially cancer.  相似文献   
107.
Ficus religiosa is known as a long-lived multipurpose forest tree. The tree plays an important role for religious, medicinal, and ornamental purposes. However, the propagation rate of Ficus religiosa is low in natural habitat so the plant tissue culture techniques are an applicable method for multiplication of this valuable medicinal plants. Thus, the aim of this study is to understand the effect of different auxin/cytokinin ratios on indirect shoot organogenesis of this plant. According to our results, the maximum callus induction frequency (100%) was obtained on Murashige and Skoog (MS) medium supplemented with 0.5?mg/l 2,4-dichlorophenoxyacetic acid (2,4-D) plus 0.05?mg/l 6-benzylaminopurine (BAP) from petiole segments. For shoot induction purpose, the yellow-brownish, friable, organogenic calli were inoculated on shoot induction medium. On MS medium supplemented with 1.5?mg/l BAP and 0.15?mg/l Indole-3-butyric acid (IBA), 96.66% of the petiole-derived calli responded with an average number of 3.56 shoots per culture. The highest root formation frequency (96.66%), root number (5.5), and root length (4.83?cm) were achieved on MS medium containing 2.0?mg/l IBA plus 0.1?mg/l Naphthaleneacetic acid (NAA). The rooted shoots were successfully transferred to field condition and the substrate with the mixture of cocopeat and perlite (1:1) had the highest survival rate (96.66%). This is the first report of an effective in vitro organogenesis protocol for F. religiosa by indirect shoot organogenesis through axenic seedling derived petiole explants, which can be efficiently employed for conservation of this important medicinal plant species as well as the utilization of active biomolecules.  相似文献   
108.

Background

Tuberculosis (TB) is a contagious infectious disease caused by Mycobacterium tuberculosis (Mtb). This disease with two million deaths per year has the highest mortality rate among bacterial infections. The only available vaccine against TB is BCG vaccine. BCG is an effective vaccine against TB in childhood, however, due to some limitations, has not proper efficiency in adults. Also, BCG cannot produce an adequately protective response against reactivation of latent infections.

Objective

In the present study we will review the most recent findings about contribution of HspX protein in the vaccines against tuberculosis.

Methods

Therefore, many attempts have been made to improve BCG or to find its replacement. Most of the subunit vaccines for TB in various phases of clinical trials were constructed as prophylactic vaccines using Mtb proteins expressed in the replicating stage. These vaccines might prevent active TB but not reactivation of latent tuberculosis infection (LTBI). A literature search was performed on various online databases (PubMed, Scopus, and Google Scholar) regarding the roles of HspX protein in tuberculosis vaccines.

Results

Ideal subunit post-exposure vaccines should target all forms of TB infection, including active symptomatic and dormant (latent) asymptomatic forms. Among these subunit vaccines, HspX is the most important latent phase antigen of M. tuberculosis with a strong immunological response. There are many studies that have evaluated the immunogenicity of this protein to improve TB vaccine.

Conclusion

According to the studies, HspX protein is a good candidate for development of subunit vaccines against TB infection.
  相似文献   
109.
Bioprocess and Biosystems Engineering - In this study, we synthesized super magnetic Fe(OH)3@Fe3O4 nanoparticles (SPIONs) by the co-precipitation method and introduction of amine groups via...  相似文献   
110.
Antimicrobial peptides (AMPs) have attracted attentions as a novel antimicrobial agent because of their unique activity against microbes. In the present study, we described a new, previously unreported AMP, moronecidin-like peptide, from Hippocampus comes and compared its antimicrobial activity with moronecidin from hybrid striped bass. Antibacterial assay indicated that gram-positive bacteria were more sensitive to moronecidin and moronecidin-like compared with gram-negative bacteria. Furthermore, both AMPs were found to exhibit effective antifungal activity. Comparative analysis of the antimicrobial activity revealed that moronecidin-like peptide has higher activity against Acinetobacter baumannii and Staphylococcus epidermidis relative to moronecidin. Both moronecidin-like and moronecidin peptides retained their antibacterial activity in physiological pH and salt concentration. The time-killing assay showed that the AMPs completely killed A. baumannii and S. epidermidis isolates after 1 and 5 h at five- and tenfold above their corresponding MICs, respectively. Anti-biofilm assay demonstrated that peptides were able to inhibit 50% of biofilm formation at sub-MIC of 1/8 MIC. Furthermore, moronecidin-like significantly inhibited biofilm formation more than moronecidin at 1/16 MIC. Collectively, our results revealed that antimicrobial and anti-biofilm activities of moronecidin-like are comparable to moronecidin. In addition, the hemolytic and cytotoxic activities of moronecidin-like were lower than those of moronecidin, suggesting it as a potential novel therapeutic agent, and a template to design new therapeutic AMPs.  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