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   5576篇
  免费   343篇
  5919篇
  2023年   13篇
  2022年   66篇
  2021年   107篇
  2020年   71篇
  2019年   82篇
  2018年   123篇
  2017年   106篇
  2016年   172篇
  2015年   263篇
  2014年   268篇
  2013年   430篇
  2012年   491篇
  2011年   427篇
  2010年   247篇
  2009年   252篇
  2008年   313篇
  2007年   321篇
  2006年   280篇
  2005年   273篇
  2004年   272篇
  2003年   267篇
  2002年   228篇
  2001年   47篇
  2000年   43篇
  1999年   51篇
  1998年   55篇
  1997年   36篇
  1996年   51篇
  1995年   33篇
  1994年   50篇
  1993年   22篇
  1992年   40篇
  1991年   31篇
  1990年   27篇
  1989年   23篇
  1988年   22篇
  1987年   18篇
  1986年   16篇
  1985年   20篇
  1984年   32篇
  1983年   18篇
  1982年   26篇
  1981年   22篇
  1980年   22篇
  1979年   11篇
  1978年   17篇
  1977年   16篇
  1976年   17篇
  1975年   15篇
  1973年   15篇
排序方式: 共有5919条查询结果,搜索用时 15 毫秒
981.
The probiotic potential of Lactobacillus plantarum and Lactobacillus fermentum strains, capable of overproducing riboflavin, was investigated. The riboflavin production was quantified in co-cultures of lactobacilli and human intestinal epithelial cells, and the riboflavin overproduction ability was confirmed. When milk and yogurt were used as carrier matrices, L. plantarum and L. fermentum strains displayed a significant ability to survive through simulated gastrointestinal transit. Adhesion was studied on both biotic and abiotic surfaces. Both strains adhered strongly on Caco-2 cells, negatively influenced the adhesion of Escherichia coli O157:H7, and strongly inhibited the growth of three reference pathogenic microbial strains. Resistance to major antibiotics and potential hemolytic activity were assayed. Overall, this study reveals that these Lactobacillus stains are endowed with promising probiotic properties and thus are candidates for the development of novel functional food which would be both enriched in riboflavin and induce additional health benefits, including a potential in situ riboflavin production, once the microorganisms colonize the host intestine.  相似文献   
982.
The collection and organization of distributional data is the first crucial stage of any conservation planning action: therefore the decline in field research has implications in both the systematic, floristic and conservation fields. The aim of this paper is to analyze the effects of data updating on conservation planning and priorities. Focusing on the short time frame ranging from 2006 to 2011, we present a case study showing the rate of increase of collected data (taxa and records) and the consequential effects on the definition of areas of priority interest for plant conservation (Important Plant Areas—IPAs). We gathered data on a total of 193 taxa and 849 records with a mean rate of increase of +97 % for taxa and +166 % for records (2006/2011). This increase caused a positive rate of change in high ranking cells (+78 %) defining IPAs, while the number of low ranking cells and no data cells slightly decreased (?12 and ?8 %, respectively). Our results suggest that specific investment to complete the knowledge on the distribution of selected taxa (e.g. 193 taxa represent the 7.5 % of the total vascular flora of Sardinia) would dramatically reduce both the Linnean and Wallacean shortfalls and would allow robust conservation programs to preserve the diversity of the island. Updating the IPAs on a regular basis is a good example of a process that has a low impact as well as a big potential gain especially when field research can only be performed with low intensity and small monetary investments.  相似文献   
983.
In many arthropods, there is a change in relative segment size during post-embryonic development, but how segment differential growth is produced is little known. A new dataset of the highest quality specimens of the 429 Myr old trilobite Aulacopleura koninckii provides an unparalleled opportunity to investigate segment growth dynamics and its control in an early arthropod. Morphometric analysis across nine post-embryonic stages revealed a growth gradient in the trunk of A. koninckii. We contrastively tested different growth models referable to two distinct hypotheses of growth control for the developing trunk: (i) a segment-specific control, with individual segments having differential autonomous growth progression, and (ii) a regional control, with segment growth depending on their relative position along the main axis. We show that the trunk growth pattern of A. koninckii was consistent with a regional growth control producing a continuous growth gradient that was stable across all developmental stages investigated. The specific posterior-to-anterior decaying shape of the growth gradient suggests it deriving from the linear transduction of a graded signal, similar to those commonly provided by morphogens. A growth control depending on a form of positional specification, possibly realized through the linear interpretation of a graded signal, may represent the primitive condition for arthropod differential growth along the main body axis, from which the diverse and generally more complex forms of growth control in subsequent arthropods have evolved.  相似文献   
984.
In humans we know 25 selenoproteins that play important roles in redox regulation, detoxification, immune-system protection and viral suppression. In particular, selenoprotein M (SelM) may function as thiol disulfide oxidoreductase that participates in the formation of disulfide bonds, and can be implicated in calcium responses. However, it presents a redox motif (CXXU), where U is a selenocysteine, and may also function as redox regulator because its decreased or increased expression regulated by dietary selenium alters redox homeostasis. No data are reported in literature about its involvement in cancer but only in neurodegenerative diseases. In this paper we evaluated the SelM expression in two hepatoma cell lines, HepG2 and Huh7, compared to normal hepatocytes. The results suggested its involvement in hepatocellular carcinoma (HCC) as well as its possible use to follow the progression of this cancer as putative marker. The aim of this study has been to analyze the structure–function relationships of SelM. Hence, firstly we studied the evolutionary history of this protein by phylogenetic analysis and GC content of genes from various species. So, we modeled the three-dimensional structure of the human SelM evaluating its energetic stability by molecular dynamics simulations. Moreover, we modeled some of its mutants to obtain structural information helpful for structure-based drug design.  相似文献   
985.
Storage conditions and duration have a considerable influence on wine quality. Optimum temperature and humidity conditions may improve wine quality through ageing, while incorrect or excessively long storage leads to negative results. In order to evaluate the global effects of storage on red wine composition, 20 Sangiovese wines were stored in two different conditions (cellar or house) for a period of 2 years and analysed every 6 months. Untargeted LC–MS analysis showed various putative markers for the type and length of conservation (i.e. pigments, flavanols, pantothenic acid etc.), while targeted LC–MS confirmed and expanded these results within specific metabolic groups. The results of multivariate analysis showed that wines stored in the cellar changed little even after 2 years of storage, while wines stored in typical domestic conditions (house) developed approximately four times faster, reaching a composition similar to wines stored in the cellar for 2 years after just 6 months. The formation of several monosulfonated flavanols during domestic ageing provided the first evidence in wine of a reaction between wine tannins—both catechins and proanthocyanidins—and the exogenous antioxidant bisulfite. Moreover, ageing in domestic conditions appeared to induce an accelerated decrease in wine pigments, while specifically promoting the formation of pinotin A-like pigments and the hydrolysis of flavonol glycosides.  相似文献   
986.
Weight gain and obesity are among the most important risk factors for post-menopausal oestrogen-dependent breast cancer (EDBC). Weight gain is associated with oxidative stress, which in turn promotes breast cancer progression. We carried out a prospective study in 216 consecutive post-menopausal breast cancer patients aiming to examine the correlations between traditional prognostic factors (tumour size, T, nodal, N, grading, G, and metastasis status, M), and body mass index (BMI), leptin, pro-inflammatory cytokines (Interleukin, IL,-6 and tumour necrosis factor-alpha, TNF-α), and oxidative stress (reactive oxygen species, ROS, glutathione peroxidase, GPx, superoxide dismutase, SOD) among patients with oestrogen receptor (ER)+ and ER− breast cancers. Distribution of T, N and M categories did not differ between ER+ and ER− breast cancer patients. ER− patients showed a higher incidence of G3 tumours. Weight, BMI, leptin, IL-6 and ROS were higher in ER+ compared with ER− patients. Among ER+ patients, BMI, leptin, IL-6 and ROS correlated with T and M. Leptin, IL-6 and ROS were positively correlated also with N. Among ER− patients, BMI and leptin did not correlate with any of prognostic parameters, whereas a positive correlation between IL-6, ROS and M was found. Multivariate regression analysis showed that BMI, leptin, IL-6 and ROS were predictive for T, N and M in ER+ patients. Weight gain, inflammation and oxidative stress are involved in EDBC prognosis. Their modulation through antidiabetic, anti-inflammatory and antioxidants drugs combined with endocrine therapy may constitute a targeted approach in post-menopausal EDBC.  相似文献   
987.
A novel enantioselective divergent route to 13‐alkyl derivatives of α‐ and γ‐ionone, important components of perfumes and fragrances, is reported. This relatively short and convenient methodology takes advantage of the use of a common intermediate, easily obtained from highly enantiomerically enriched (S)‐α‐ionone, which avoids the separate installation of the butenone side chain at C(6) for each analog. Olfactory evaluation of synthesized compounds reconfirmed the influence of the hydrophobic interactions of alkyl substituents at C(5) with olfactory receptors (ORs) in the chemoreception of ionones, and suggested that a synperiplanar orientation of C(13) and the lateral chain is the better geometry fitting OR's cavity.  相似文献   
988.
Repair and defence of genome integrity from endogenous and environmental hazard is a primary need for all organisms. Natural selection has driven the evolution of multiple cell pathways to deal with different DNA damaging agents. Failure of such processes can hamper cell functions and induce inheritable mutations, which in humans may cause cancerogenicity or certain genetic syndromes, and ultimately cell death. A special case is that of hyperthermophilic bacteria and archaea, flourishing at temperatures higher than 80 °C, conditions that favor genome instability and thus call for specific, highly efficient or peculiar mechanisms to keep their genome intact and functional. Over the last few years, numerous studies have been performed on the activity, function, regulation, physical and functional interaction of enzymes and proteins from hyperthermophilic microorganisms that are able to bind, repair, bypass damaged DNA, or modify its structure or conformation. The present review is focused on two enzymes that act on DNA catalyzing unique reactions: reverse gyrase and DNA alkyltransferase. Although both enzymes belong to evolutionary highly conserved protein families present in organisms of the three domains (Eucarya, Bacteria and Archaea), recently characterized members from hyperthermophilic archaea show both common and peculiar features.  相似文献   
989.
990.

Introduction

The association between fish consumption and rheumatoid arthritis (RA) is unclear. The aim of this paper was to summarize the available evidence on the association between fish consumption and risk of RA using a dose-response meta-analysis.

Methods

Relevant studies were identified by a search of MEDLINE and EMBASE through December 2013, with no restrictions. A random-effects dose-response meta-analysis was conducted to combine study specific relative risks. Potential non-linear relation was investigated using restricted cubic splines. A stratified analysis was conducted by study design.

Results

Seven studies (four case-controls and three prospective cohorts) involving a total of 174 701 participants and 3346 cases were included in the meta-analysis. For each one serving per week increment in fish consumption, the relative risk (RR) of RA was 0.96 (95% confidence interval (CI) 0.91 to 1.01). Results did not change when stratifying by study design. No heterogeneity or publication bias was observed. When fish consumption was modeled using restricted cubic splines, the risk of RA was 20 to 24% lower for 1 up to 3 servings per week of fish (RR =0.76, 95% CI: 0.57 to 1.02) as compared to never consumption.

Conclusions

Results from this dose-response meta-analysis showed a non-statistically significant inverse association between fish consumption and RA.  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