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   127篇
  免费   10篇
  国内免费   25篇
  2023年   7篇
  2022年   9篇
  2021年   7篇
  2020年   8篇
  2019年   7篇
  2018年   6篇
  2017年   9篇
  2016年   7篇
  2015年   6篇
  2014年   12篇
  2013年   6篇
  2012年   13篇
  2011年   6篇
  2010年   8篇
  2009年   11篇
  2008年   7篇
  2007年   5篇
  2006年   6篇
  2005年   3篇
  2004年   3篇
  2003年   4篇
  2002年   5篇
  2001年   1篇
  2000年   1篇
  1998年   1篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1990年   1篇
排序方式: 共有162条查询结果,搜索用时 15 毫秒
61.
Vibration is a common mechanical phenomenon and possesses mechanical energy in ambient environment, which can serve as a sustainable source of power for equipment and devices if it can be effectively collected. In the present work, a novel soft and robust triboelectric nanogenerator (TENG) made of a silicone rubber‐spring helical structure with nanocomposite‐based elastomeric electrodes is proposed. Such a spring based TENG (S‐TENG) structure operates in the contact‐separation mode upon vibrating and can effectively convert mechanical energy from ambient excitation into electrical energy. The two fundamental vibration modes resulting from the vertical and horizontal excitation are analyzed theoretically, numerically, and experimentally. Under the resonant states of the S‐TENG, its peak power density is found to be 240 and 45 mW m?2 with an external load of 10 MΩ and an acceleration amplitude of 23 m s?2. Additionally, the dependence of the S‐TENG's output signal on the ambient excitation can be used as a prime self‐powered active vibration sensor that can be applied to monitor the acceleration and frequency of the ambient excitation. Therefore, the newly designed S‐TENG has a great potential in harvesting arbitrary directional vibration energy and serving as a self‐powered vibration sensor.  相似文献   
62.
A levan-producing strain, BD1707, was isolated from Tibetan kefir and identified as Leuconostoc citreum. The effects of carbon sources on the growth of L. citreum BD1707 and levan production in tomato juice were measured. The changes in pH, viable cell count, sugar content, and levan yield in the cultured tomato juice supplemented with 15% (wt/vol) sucrose were also assayed. L. citreum BD1707 could synthesize more than 28 g/liter of levan in the tomato juice-sucrose medium when cultured at 30°C for 96 h. Based on the monosaccharide composition, molecular mass distribution, Fourier transform infrared (FTIR) spectra, and nuclear magnetic resonance (NMR) spectra, the levan synthesized by L. citreum BD1707 was composed of a linear backbone consisting of consecutive β-(2→6) linked d-fructofuranosyl units, with an estimated average molecular mass of 4.3 × 106 Da.  相似文献   
63.
Huang  Yu  Ma  Lingwei  Hou  Mengjing  Zhang  Zhengjun 《Plasmonics (Norwell, Mass.)》2016,11(5):1377-1383
Plasmonics - We have demonstrated directly that the physical origin of Fano resonances in two-dimensional (2D) plasmonic crystals (PCs) is a wave-interference phenomenon. This is achieved by...  相似文献   
64.
Large amounts of veterinary medicines are widely used as therapeutic drugs and feed additives (growth promoters) in China, the environmental presence of which possibly poses challenges to the environment and human health. Therefore, it is important to list the veterinary medicines that are considered to be of relatively high priority in China for environmental management. In this study, a three-stage prioritization scheme was applied to veterinary medicines in China. In Stage I, exposure assessment was conducted based on usage amounts and the possibility of entering the environment. In Stage II, the ecotoxicity and human health effects of compounds having a high potential to enter the environment were assessed. In Stage III, considering both the results of Stages I and II, veterinary medicines were assigned into four priority classifications. Using the approach, 38 compounds were assigned to “H,” 7 compounds to “M,” 2 compounds to “L,” and 22 compounds to “VL.” Among the top-ranked compounds, antibiotics, endoparasiticides, and aquacultural medicines accounted for 57.9%, 28.9%, and 10.5%, respectively. Insecticides used widely in China's aquaculture need to be taken into account due to their high priority rank. This is the first study on the prioritization of veterinary pharmaceuticals in China.  相似文献   
65.
3,8-Divinyl (proto)chlorophyll(ide) a 8-vinyl reductase (DVR) catalyzes the reduction of 8-vinyl group on the tetrapyrrole to an ethyl group, which is indispensable for monovinyl chlorophyll (Chl) synthesis. So far, three 8-vinyl reductase genes (DVR, bciA, and slr1923) have been characterized from Arabidopsis (Arabidopsis thaliana), Chlorobium tepidum, and Synechocystis sp. PCC6803. However, no 8-vinyl reductase gene has yet been identified in monocotyledonous plants. In this study, we isolated a spontaneous mutant, 824ys, in rice (Oryza sativa). The mutant exhibited a yellow-green leaf phenotype, reduced Chl level, arrested chloroplast development, and retarded growth rate. The phenotype of the 824ys mutant was caused by a recessive mutation in a nuclear gene on the short arm of rice chromosome 3. Map-based cloning of this mutant resulted in the identification of a gene (Os03g22780) showing sequence similarity with the Arabidopsis DVR gene (AT5G18660). In the 824ys mutant, nine nucleotides were deleted at residues 952 to 960 in the open reading frame, resulting in a deletion of three amino acid residues in the encoded product. High-performance liquid chromatography analysis of Chls indicated the mutant accumulates only divinyl Chl a and b. A recombinant protein encoded by Os03g22780 was expressed in Escherichia coli and found to catalyze the conversion of divinyl chlorophyll(ide) a to monovinyl chlorophyll(ide) a. Therefore, it has been confirmed that Os03g22780, renamed as OsDVR, encodes a functional DVR in rice. Based upon these results, we succeeded to identify an 8-vinyl reductase gene in monocotyledonous plants and, more importantly, confirmed the DVR activity to convert divinyl Chl a to monovinyl Chl a.Chlorophyll (Chl) is the main component of the photosynthetic pigments. Chl molecules universally exist in photosynthetic organisms and perform essential processes of harvesting light energy in the antenna systems and by driving electron transfer in the reaction centers (Fromme et al., 2003). In higher plants, there are two Chl species, Chl a and Chl b. The photosynthetic reaction centers contain only Chl a, and the peripheral light-harvesting antenna complexes contain Chl a and Chl b (Grossman et al., 1995). Chl a is synthesized from glutamyl-tRNA, and Chl b is synthesized from Chl a at the last step of Chl biosynthesis (Beale, 1999). So far, genes for all 15 steps in the Chl biosynthetic pathway have been identified in higher plants, at least in angiosperms represented by Arabidopsis (Arabidopsis thaliana; Beale, 2005; Nagata et al., 2005). Analysis of the complete genome of Arabidopsis showed that it has 15 enzymes encoded by 27 genes for Chl biosynthesis from glutamyl-tRNA to Chl b (Nagata et al., 2005). However, only six genes encoding three enzymes involved in Chl biosynthesis have been identified in rice (Oryza sativa). Magnesium chelatase comprises three subunits (ChlH, ChlD, and ChlI) and catalyzes the insertion of Mg2+ into protoporphyrin IX, the last common intermediate precursor in both Chl and heme biosyntheses. Jung et al. (2003) characterized OsCHLH gene for the OsChlH subunit of magnesium chelatase, and Zhang et al. (2006) cloned Chl1 and Chl9 genes encoding the OsChlD and OsChlI subunits of magnesium chelatase. Chl synthase catalyzes esterification of chlorophyllide (Chlide), resulting in the formation of Chl a. Wu et al. (2007) identified the YGL1 gene encoding the Chl synthase. Chl b is synthesized from Chl a by Chl a oxygenase; Lee et al. (2005) identified OsCAO1 and OsCAO2 genes for Chl a oxygenase.According to the number of vinyl side chains, Chls of oxygenic photosynthetic organisms are classified into two groups: 3,8-divinyl Chl (DV-Chl) and 3-vinyl Chl (monovinyl Chl [MV-Chl]). Almost all of the oxygenic photosynthetic organisms contain MV-Chls, regardless of the variation in their indigenous environments (Porra, 1997). The exceptions are species of Prochlorococcus marinus, marine picophytoplanktons that contain DV-Chls as their photosynthetic pigments (Chisholm et al., 1992).Chl biosynthetic heterogeneity is assumed to originate mainly in parallel DV- and MV-Chl biosynthetic routes interconnected by 8-vinyl reductases that convert DV-tetrapyrroles to MV-tetrapyrroles by conversion of the vinyl group at position 8 of ring B to the ethyl group (Parham and Rebeiz, 1995; Rebeiz et al., 2003). Most of Chls carry an ethyl group or, less frequently, a vinyl group. For example, Chl a and b occur as the MV-derivatives in green plants, but Chl precursors sometimes accumulate as DV-intermediates, and the ratio between the two forms can vary depending on the species, tissue, and growth conditions (Shioi and Takamiya, 1992; Kim and Rebeiz, 1996). So far, five 8-vinyl reductase activities have been detected at the levels of DV Mg-protoporphyrin IX (Kim and Rebeiz, 1996), Mg-protomonomethyl ester (Kolossov et al., 2006), protochlorophyllide (Pchlide) a (Tripathy and Rebeiz, 1988), Chlide a (Kolossov and Rebeiz, 2001; Nagata et al., 2005), and Chl a (Adra and Rebeiz, 1998). What is not clear at this stage is whether the various 8-vinyl reductase activities are catalyzed by one enzyme of broad specificity or by a family of enzymes of narrow specificity encoded by one gene or multiple genes, as is the case for NADPH Pchlide oxidoreductases (Rebeiz et al., 2003). The issue could be settled by purification of the various putative reductases and comparison of their properties.Nagata et al. (2005) and Nakanishi et al. (2005) independently identified the AT5G18660 gene of Arabidopsis as a divinyl reductase (DVR) that has sequence similarity to isoflavone reductase. Chew and Bryant (2007) demonstrated that BciA (CT1063), which is an ortholog of the Arabidopsis gene, encodes a DVR of the green sulfur bacterium Chlorobium tepidum TLS. They also considered that BchJ, which had been reported to be a vinyl reductase (Suzuki and Bauer, 1995), is not the enzyme, but it may play an important role in substrate channeling and/or regulation of bacteriochlorophyll biosynthesis. Islam et al. (2008) and Ito et al. (2008) independently identified a novel 8-vinyl reductase gene (Slr1923) in DVR-less cyanobacterium Synechocystis sp. PCC6803. However, no DVR gene has yet been identified in monocotyledonous plants.In this study, we isolated a spontaneous mutant, 824ys, from indica rice cv 824B. The mutant exhibited a yellow-green leaf phenotype throughout the growth stage, reduced level of Chls, arrested development of chloroplasts, and retarded growth rate. Map-based cloning of the mutant resulted in the identification of the OsDVR gene, showing sequence similarity to the DVR gene of Arabidopsis. In the 824ys mutant, nine nucleotides were deleted at residues 952 to 960 in the open reading frame (ORF), resulting in three amino acid deletion in the encoded protein. HPLC analysis of Chls indicated the mutant accumulates only DV-Chls. Enzymatic analysis demonstrated that the recombinant protein expressed in Escherichia coli is able to catalyze the conversion of DV-Chl(ide) a to MV-Chl(ide) a. Therefore, this study has confirmed that the OsDVR gene encodes a functional DVR in rice.  相似文献   
66.
Yiming L  Zhengjun W  Duncan RP 《Oecologia》2006,148(1):129-136
Islands are often considered easier to invade than mainland locations because of lower biotic resistance, but this hypothesis is difficult to test. We compared invasion success (the probability of establishing a wild reproducing population) for bullfrogs (Rana catesbeiana) introduced to enclosures on 26 farms on islands in the Zhoushan archipelago and 15 farms in neighboring mainland China. Bullfrogs were more likely to invade farms located on islands with lower native frog species richness than mainland farms, consistent with the biotic resistance hypothesis. However, human frog hunting pressure also differed between islands and the mainland and, along with the number of bullfrogs raised in enclosures, was a stronger predictor of invasion success than native frog richness in multiple regression. Variation in hunting pressure was also able to account for the difference in invasion success between islands and mainlands: islands had lower hunting pressure and thus higher invasion probability. We conclude that the ease with which bullfrogs have invaded islands of the Zhoushan archipelago relative to the mainland has little to do with biotic resistance but results from variation in factors under human control.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.  相似文献   
67.
Colorectal cancer (CRC), is mostly derived from normal colon epithelial cells, and has been reported to be one of most common gastrointestinal malignancies globally. An increasing number of researchers have claimed that long noncoding RNAs (lncRNAs) exert significant functions in tumor progression. Nevertheless, the function of MAGI2-AS3 remains uncertain in CRC. The expression of MAGI2-AS3, miR-3163, and transmembrane protein 106B (TMEM106B) messenger RNA was examined by quantitative real-time polymerase chain reaction. Cell apoptosis was measured by caspase-3 activity test. Cell proliferation was tested by cell-counting kit 8 and 5-ethynyl-2′-deoxyuridine assays. Cell migration was detected by transwell assay. Western blot analysis examined the protein expression of TMEM106B. The expression of Ki-67 was evaluated by immunohistochemistry assay. The binding capacity between miR-3163 and MAGI2-AS3 (or TMEM106B) was studied by radioimmunoprecipitation and luciferase reporter assays. The expression of MAGI2-AS3 and TMEM106B was conspicuously upregulated whereas miR-3163 presented lower expression in CRC cells. MAGI2-AS3 deficiency facilitated cell apoptosis but hampered cell proliferation and migration. MAGI2-AS3 combined with miR-3163 and negatively regulated miR-3163 expression. In addition, the administration of sh-MAGI2-AS3 or miR-3163 mimics suppressed CRC cell growth in vivo. Subsequently, miR-3163 targeted TMEM106B and the transfection of sh-MAGI2-AS3 or miR-3163 mimics downregulated TMEM106B expression. Rescue assays verified that TMEM106B overexpression recovered the effects of MAGI2-AS3 inhibition on cell apoptosis, proliferation, and migration in CRC. MAGI2-AS3 drives CRC progression through regulating miR-3163/TMEM106B axis. This supplies innovative insights on the investigation of molecular mechanism in CRC progression.  相似文献   
68.
The migration and proliferation of vascular smooth muscle cells (VSMCs) are essential elements during the development of atherosclerosis and restenosis. An increasing number of studies have reported that extracellular matrix (ECM) proteins, including the CCN protein family, play a significant role in VSMC migration and proliferation. CCN4 is a member of the CCN protein family, which controls cell development and survival in multiple systems of the body. Here, we sought to determine whether CCN4 is involved in VSMC migration and proliferation. We examined the effect of CCN4 using rat cultured VSMCs. In cultured VSMCs, CCN4 stimulated the adhesion and migration of VSMCs in a dose-dependent manner, and this effect was blocked by an antibody for integrin α5β1. CCN4 expression was enhanced by the pro-inflammatory cytokine tumor necrosis factor α (TNF-α). Furthermore, knockdown of CCN4 by siRNA significantly inhibited the VSMC proliferation. CCN4 also could up-regulate the expression level of marker proteins of the VSMCs phenotype. Taken together, these results suggest that CCN4 is involved in the migration and proliferation of VSMCs. Inhibition of CCN4 may provide a promising strategy for the prevention of restenosis after vascular interventions.  相似文献   
69.
Flowering is indicative of the transition from vegetative to reproductive phase, a critical event in the life cycle of plants. In soybean (Glycine max), a flowering quantitative trait locus, FT2, corresponding to the maturity locus E2, was detected in recombinant inbred lines (RILs) derived from the varieties "Misuzudaizu" (ft2/ft2; JP28856) and "Moshidou Gong 503" (FT2/FT2; JP27603). A map-based cloning strategy using the progeny of a residual heterozygous line (RHL) from the RIL was employed to isolate the gene responsible for this quantitative trait locus. A GIGANTEA ortholog, GmGIa (Glyma10g36600), was identified as a candidate gene. A common premature stop codon at the 10th exon was present in the Misuzudaizu allele and in other near isogenic lines (NILs) originating from Harosoy (e2/e2; PI548573). Furthermore, a mutant line harboring another premature stop codon showed an earlier flowering phenotype than the original variety, Bay (E2/E2; PI553043). The e2/e2 genotype exhibited elevated expression of GmFT2a, one of the florigen genes that leads to early flowering. The effects of the E2 allele on flowering time were similar among NILs and constant under high (43°N) and middle (36°N) latitudinal regions in Japan. These results indicate that GmGIa is the gene responsible for the E2 locus and that a null mutation in GmGIa may contribute to the geographic adaptation of soybean.  相似文献   
70.

Aims

This study sought to evaluate the prognostic significance of postoperative complications for colon cancer patients undergoing laparoscopic surgery.

Methods

From May 2006 to May 2009, a total 224 patients who underwent laparoscopic curative resection (R0) for colon cancer were included in our retrospective study. Postoperative complications were evaluated according to a standardized grading system. The main outcome measurements of our study were overall survival (OS) and relapse-free survival (RFS), which were then compared between the no complication and complication groups. Univariate and multivariate analysis were used to assess the correlation between complications and prognosis.

Results

Fifty-nine postoperative complications occurred in 43 patients. The overall morbidity rate was 26.3%. The 5-year OS in the complication group was 41.4% compared with 78.5% in the no complication group (P<0.001). The cumulative incidence of relapse was also more aggressive in patients with complications (5-year RFS: complication group 40.9% vs. no complication group 82.1%, P<0.001). Multivariate analysis identified complications as a significant factor increasing the risk for both OS (RR 2.737; 95% CI 1.512–4.952; P = 0.001) and RFS (RR 4.247; 95% CI 2.291–7.876; P<0.001).

Conclusion

Postoperative complications could pose a significant adverse impact not only on OS but also on RFS in patients with colon cancer even when laparoscopic R0 resection is available.  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