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   3238篇
  免费   212篇
  国内免费   3篇
  2021年   33篇
  2020年   16篇
  2019年   24篇
  2018年   23篇
  2017年   31篇
  2016年   67篇
  2015年   80篇
  2014年   110篇
  2013年   299篇
  2012年   167篇
  2011年   190篇
  2010年   127篇
  2009年   117篇
  2008年   186篇
  2007年   213篇
  2006年   200篇
  2005年   204篇
  2004年   202篇
  2003年   173篇
  2002年   183篇
  2001年   56篇
  2000年   51篇
  1999年   58篇
  1998年   34篇
  1997年   36篇
  1996年   22篇
  1995年   22篇
  1994年   23篇
  1993年   24篇
  1992年   40篇
  1991年   41篇
  1990年   38篇
  1989年   43篇
  1988年   32篇
  1987年   21篇
  1986年   19篇
  1985年   20篇
  1984年   23篇
  1983年   24篇
  1982年   20篇
  1981年   10篇
  1980年   15篇
  1979年   10篇
  1978年   15篇
  1977年   11篇
  1975年   11篇
  1972年   13篇
  1969年   8篇
  1968年   7篇
  1966年   10篇
排序方式: 共有3453条查询结果,搜索用时 203 毫秒
81.
Evidence is presented for the selective breakdown of altered tomato β-fructofuranosidase molecules by a neutral protease from Bacillus subtilis.  相似文献   
82.
83.
Because antimicrobial resistance in food-producing animals is a major public health concern, many countries have implemented antimicrobial monitoring systems at a national level. When designing a sampling scheme for antimicrobial resistance monitoring, it is necessary to consider both cost effectiveness and statistical plausibility. In this study, we examined how sampling scheme precision and sensitivity can vary with the number of animals sampled from each farm, while keeping the overall sample size constant to avoid additional sampling costs. Five sampling strategies were investigated. These employed 1, 2, 3, 4 or 6 animal samples per farm, with a total of 12 animals sampled in each strategy. A total of 1,500 Escherichia coli isolates from 300 fattening pigs on 30 farms were tested for resistance against 12 antimicrobials. The performance of each sampling strategy was evaluated by bootstrap resampling from the observational data. In the bootstrapping procedure, farms, animals, and isolates were selected randomly with replacement, and a total of 10,000 replications were conducted. For each antimicrobial, we observed that the standard deviation and 2.5–97.5 percentile interval of resistance prevalence were smallest in the sampling strategy that employed 1 animal per farm. The proportion of bootstrap samples that included at least 1 isolate with resistance was also evaluated as an indicator of the sensitivity of the sampling strategy to previously unidentified antimicrobial resistance. The proportion was greatest with 1 sample per farm and decreased with larger samples per farm. We concluded that when the total number of samples is pre-specified, the most precise and sensitive sampling strategy involves collecting 1 sample per farm.  相似文献   
84.
85.
86.
87.
It has recently been unveiled that a wide variety of microbial eukaryotes (protists) occur in chemosynthetic ecosystems, such as hydrothermal vents and methane seeps. However, there is little knowledge regarding protists associated with endemic animals inhabiting these environments. In the present study, utilizing PCR techniques, we detected fragments of the small subunit ribosomal RNA gene (SSU rRNA gene) from a particular protist from gill tissues of a significant fraction of the vesicomyid clams Calyptogena soyoae and C. okutanii complex and of the mussel Bathymodiolus platifrons and B. japonicus, all of which harbor chemosynthetic endosymbiont bacteria and dominate methane seeps in Sagami Bay, Japan. Based on the phylogeny of SSU rRNA gene, the organism in question was shown to belong to Alveolata. It is noteworthy that this protist did not affiliate with any known alveolate group, although being deeply branched within the lineage of Syndiniales, for which the monophyly was constantly recovered, but not robustly supported. In addition, the protist detected using PCR followed by sequencing was localized within gill epithelial cells of B. platifrons with whole‐mount fluorescence in situ hybridization. This protist may be an endoparasite or an endocommensal of Calyptogena spp. and Bathymodiolus spp., and possibly have physiological and ecological impacts on these bivalves.  相似文献   
88.
The inhibitory effects of various fatty acids on three hyaluronidases (h-ST, h-SH and h-SD) and four chondroitinases (c-ABC, c-B, c-ACI and c-ACII) were examined, and their structure-activity relationships and mechanism of action were studied. The fatty acids used in this experiment showed various inhibitory activities against the enzymes. None of the fatty acids did not inhibit h-ST and h-SH. The saturated fatty acids (C 10:0 to C 22:0) showed very weak or no inhibition against h-SD, c-ABC, c-B, c-ACI and c-ACII but the unsaturated fatty acids (C 14:1 to C 24:1) with one double bond strongly inhibited the enzymes, and the inhibitory potency increased with increase in carbon chain length of the fatty acids. In contrast, the increase in number of double bonds caused a decrease in inhibitory potency against the enzymes. The position of the double bond and the stereochemistry of the cis - trans form of oleic acid (C 18:1) did not influence the inhibitory potency against the enzymes. Carboxyl and hydroxyl groups in the fatty acid molecule were concerned in the inhibition of c-ACI. Among the fatty acids, eicosatrienoic acid (C 20:3) generally inhibited h-SD, c-ABC, c-B and c-ACI, and nervonic acid (C 24:1) was a potent inhibitor of c-ACII, and the fatty acids inhibited the enzymes in a noncompetitive manner.  相似文献   
89.

Introduction

The development of effective treatments for osteoarthritis (OA) has been hampered by a poor understanding of OA at the cellular and molecular levels. Emerging as a disease of the ''whole joint’, the importance of the biochemical contribution of various tissues, including synovium, bone and articular cartilage, has become increasingly significant. Bathing the entire joint structure, the proteomic analysis of synovial fluid (SF) from osteoarthritic shoulders offers a valuable ''snapshot’ of the biologic environment throughout disease progression. The purpose of this study was to identify differentially expressed proteins in early and late shoulder osteoarthritic SF in comparison to healthy SF.

Methods

A quantitative 18O labeling proteomic approach was employed to identify the dysregulated SF proteins in early (n = 5) and late (n = 4) OA patients compared to control individuals (n = 5). In addition, ELISA was used to quantify six pro-inflammatory and two anti-inflammatory cytokines.

Results

Key results include a greater relative abundance of proteins related to the complement system and the extracellular matrix in SF from both early and late OA. Pathway analyses suggests dysregulation of the acute phase response, liver x receptor/retinoid x receptor (LXR/RXR), complement system and coagulation pathways in both early and late OA. The network related to lipid metabolism was down-regulated in both early and late OA. Inflammatory cytokines including interleukin (IL) 6, IL 8 and IL 18 were up-regulated in early and late OA.

Conclusions

The results suggest a dysregulation of wound repair pathways in shoulder OA contributing to the presence of a ''chronic wound’ that progresses irreversibly from early to later stages of OA. Protease inhibitors were downregulated in late OA suggesting uncontrolled proteolytic activity occurring in late OA. These results contribute to the theory that protease inhibitors represent promising therapeutic agents which could limit proteolytic activity that ultimately leads to cartilage destruction.  相似文献   
90.
Abstract

The reaction of glycosyl isothiocyanates (la, b, c, d, e) with 5,6-diamino-1-3-dimethyluracil gave the respective 1-glycosyl-3-(6-amino-1, 3-dimethyl-2, 4-dioxopyrimidine-5-yl) thioureas (2a, b, c, d, e) in excellent yields. Treatment of these thioureas with NBS afforded the respective 5,7-dioxopyrimido-[5,4-e]-as-triazine glycosides (4a, b, c, d, e) in good yields.  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