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   8497篇
  免费   551篇
  国内免费   1篇
  9049篇
  2023年   43篇
  2022年   92篇
  2021年   172篇
  2020年   96篇
  2019年   157篇
  2018年   213篇
  2017年   157篇
  2016年   271篇
  2015年   403篇
  2014年   470篇
  2013年   575篇
  2012年   730篇
  2011年   651篇
  2010年   425篇
  2009年   422篇
  2008年   553篇
  2007年   513篇
  2006年   451篇
  2005年   471篇
  2004年   415篇
  2003年   385篇
  2002年   366篇
  2001年   73篇
  2000年   56篇
  1999年   76篇
  1998年   92篇
  1997年   67篇
  1996年   67篇
  1995年   45篇
  1994年   66篇
  1993年   46篇
  1992年   44篇
  1991年   37篇
  1990年   28篇
  1989年   28篇
  1988年   21篇
  1987年   15篇
  1986年   11篇
  1985年   13篇
  1984年   21篇
  1983年   13篇
  1982年   20篇
  1981年   16篇
  1980年   13篇
  1979年   9篇
  1978年   17篇
  1977年   13篇
  1976年   14篇
  1975年   10篇
  1938年   8篇
排序方式: 共有9049条查询结果,搜索用时 15 毫秒
101.
The best way to appraise the size of abdominal organs remains undefined. Herein we compare the size of liver and spleen in hepatosplenic schistosomiasis using clinical and ultrasound (US) examination, and the size of the organs measured by US with their visualization below the costal margin ("palpable by US"). For this study, 411 individuals from an endemic area for schistosomiasis mansoni in Brazil have been selected. We found that palpable spleens and left liver lobes are larger than non palpable ones. Also, 23% of normal spleens measured by US were palpable on clinical examination, and 22% of spleens increased in size on US were non palpable. A total of 21% of normal spleens were "palpable by US". We also found 54% of normal sized right liver lobes palpable on clinical examination, whilst 54% of the increased livers, measured by US, were non palpable. About 76% of normal right liver lobes were "palpable by US". We conclude that the association of clinical, ultrasound and magnetic resonance imaging (MRI) examinations, in the near future, should give the investigators the necessary tools to perform a more accurate clinical diagnosis of hepatosplenic schistosomiasis mansoni.  相似文献   
102.
103.
104.
Although the considerable progress against gastric cancer, it remains a complex lethal disease defined by peculiar histological and molecular features. The purpose of the present study was to investigate pRb2/p130, VEGF, EZH2, p53, p16(INK4A), p27(KIP1), p21(WAF1), Ki-67 expressions, and analyze their possible correlations with clinicopathological factors. The expression patterns were examined by immunohistochemistry in 47 patients, 27 evaluated of intestinal-type, and 20 of diffuse-type, with a mean follow up of 56 months and by Western blot in AGS, N87, KATO-III, and YCC-2, -3, -16 gastric cell lines. Overall, stomach cancer showed EZH2 correlated with high levels of p53, Ki-67, and cytoplasmic pRb2/p130 (P < 0.05, and P < 0.01, respectively). Increased expression of EZH2 was found in the intestinal-type and correlated with the risk of distant metastasis (P < 0.05 and P < 0.01, respectively), demonstrating that this protein may have a prognostic value in this type of cancer. Interestingly, a strong inverse correlation was observed between p27(KIP1) expression levels and the risk of advanced disease and metastasis (P < 0.05), and a positive correlation between the expression levels of p21(WAF1) and low-grade (G1) gastric tumors (P < 0.05), confirming the traditionally accepted role for these tumor-suppressor genes in gastric cancer. Finally, a direct correlation was found between the expression levels of nuclear pRb2/p130 and low-grade (G1) gastric tumors that was statistically significant (P < 0.05). Altogether, these data may help shed some additional light on the pathogenetic mechanisms related to the two main gastric cancer histotypes and their invasive potentials.  相似文献   
105.
106.
CASP13 has investigated the impact of sparse NMR data on the accuracy of protein structure prediction. NOESY and 15N-1H residual dipolar coupling data, typical of that obtained for 15N,13C-enriched, perdeuterated proteins up to about 40 kDa, were simulated for 11 CASP13 targets ranging in size from 80 to 326 residues. For several targets, two prediction groups generated models that are more accurate than those produced using baseline methods. Real NMR data collected for a de novo designed protein were also provided to predictors, including one data set in which only backbone resonance assignments were available. Some NMR-assisted prediction groups also did very well with these data. CASP13 also assessed whether incorporation of sparse NMR data improves the accuracy of protein structure prediction relative to nonassisted regular methods. In most cases, incorporation of sparse, noisy NMR data results in models with higher accuracy. The best NMR-assisted models were also compared with the best regular predictions of any CASP13 group for the same target. For six of 13 targets, the most accurate model provided by any NMR-assisted prediction group was more accurate than the most accurate model provided by any regular prediction group; however, for the remaining seven targets, one or more regular prediction method provided a more accurate model than even the best NMR-assisted model. These results suggest a novel approach for protein structure determination, in which advanced prediction methods are first used to generate structural models, and sparse NMR data is then used to validate and/or refine these models.  相似文献   
107.
Phosphorus (P) use in global food and bioenergy production needs to become more efficient and sustainable to reduce environmental impacts and conserve a finite and critical resource (Carpenter & Bennett, Environmental Research Letters, 2011, 6, 014009; Springmann et al., Nature, 2018, 562, 519). Sugarcane is one crop with a large P footprint because production is centered on P‐fixing soils with low P availability (Roy et al., Nature Plants, 2016, 2, 16043; Withers et al., Scientific Reports, 2018, 8, 2537). As global demand for processed sugar and bioethanol continues to increase, we advocate that improving P efficiency could become a key sustainability goal for the sugarcane industry. Here, we applied the 5R global P stewardship framework (Withers et al., Ambio, 2015, 44, 193) to identify more sustainable options to manage P in Brazilian sugarcane production. We show that current inputs of P fertilizer to the current crop area could be reduced by over 305 Gg, or 63%, over the next three decades by reducing unnecessary P fertilizer use, better utilization of recyclable bioresources and redesigning recommendation systems. Adoption of these 5R options would save the sugarcane industry in Brazil 528 US$ million and help safeguard global food and energy security.  相似文献   
108.
Many Gram‐negative bacteria pathogenic to plants and animals deploy the type III secretion system (T3SS) to inject virulence factors into their hosts. All bacteria that rely on the T3SS to cause infectious diseases in humans have developed antibiotic resistance. The T3SS is an attractive target for developing new antibiotics because it is essential in virulence, and part of its structural component is exposed on the bacterial surface. The structural component of the T3SS is the needle apparatus, which is assembled from over 20 different proteins and consists of a base, an extracellular needle, a tip, and a translocon. This review summarizes the current knowledge on the structure and assembly of the needle, tip, and translocon.  相似文献   
109.
Cells from a mouse B lymphoma were transfected by DQ alpha and DQ beta genes derived from a DR4 haplotype. Quantitatively, the resulting expression of human class II molecules was similar to that of human B lymphoblastoid cell lines. Qualitatively, the transformant class II molecules differed from normal class II molecules in their carbohydrate moiety. As for their antigenic specificity, they were shown to carry two determinants previously identified on DQ molecules controlled by DR4 haplotypes, i. e., DQw3 and DCHON. The transformant molecules did not carry a third DR4-associated specificity, DC5 (equivalent to TA10), and must possess a structure allelic to DC5. However, no corresponding alloantigenic specificity was detected by a screening of relevant alloantisera.  相似文献   
110.
Amazonia forest plays a major role in providing ecosystem services for human and sanctuaries for wildlife. However, ongoing deforestation and habitat fragmentation in the Brazilian Amazon has threatened both. The ocelot is an ecologically important mesopredator and a potential conservation ambassador species, yet there are no previous studies on its habitat preference and spatial patterns in this biome. From 2010 to 2017, twelve sites were surveyed, totaling 899 camera trap stations, the largest known dataset for this species. Using occupancy modeling incorporating spatial autocorrelation, we assessed habitat use for ocelot populations across the Brazilian Amazon. Our results revealed a positive sigmoidal correlation between remote‐sensing derived metrics of forest cover, disjunct core area density, elevation, distance to roads, distance to settlements and habitat use, and that habitat use by ocelots was negatively associated with slope and distance to river/lake. These findings shed light on the regional scale habitat use of ocelots and indicate important species–habitat relationships, thus providing valuable information for conservation management and land‐use planning.  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