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   5643篇
  免费   358篇
  国内免费   1篇
  6002篇
  2022年   25篇
  2021年   64篇
  2020年   38篇
  2019年   47篇
  2018年   67篇
  2017年   57篇
  2016年   95篇
  2015年   157篇
  2014年   184篇
  2013年   349篇
  2012年   261篇
  2011年   256篇
  2010年   151篇
  2009年   159篇
  2008年   233篇
  2007年   262篇
  2006年   256篇
  2005年   266篇
  2004年   251篇
  2003年   235篇
  2002年   226篇
  2001年   214篇
  2000年   228篇
  1999年   182篇
  1998年   66篇
  1997年   58篇
  1996年   50篇
  1995年   54篇
  1994年   59篇
  1993年   68篇
  1992年   137篇
  1991年   110篇
  1990年   111篇
  1989年   127篇
  1988年   95篇
  1987年   102篇
  1986年   81篇
  1985年   73篇
  1984年   64篇
  1983年   51篇
  1982年   32篇
  1981年   36篇
  1980年   29篇
  1979年   52篇
  1978年   26篇
  1977年   38篇
  1976年   21篇
  1975年   21篇
  1974年   26篇
  1973年   22篇
排序方式: 共有6002条查询结果,搜索用时 15 毫秒
141.
Calyculin A and okadaic acid, potent and cell permeable inhibitors of type 1 and type 2A protein phosphatases, inhibit platelet aggregation and secretion. However, the relationship between phosphatase inhibition and inhibition of platelet function is not well understood. We found that in unstimulated platelets, talin (P235) was phosphorylated at threonine residues by calyculin A. Furthermore, the extent of talin phosphorylation by calyculin A was closely correlated with its inhibition of thrombin-induced platelet aggregation. Since the binding of talin to platelet glycoprotein IIb/IIIa complex has been shown to be affected by its phosphorylation, these results suggest that type 1 and/or type 2A protein phosphatases may play a role in the regulation of membrane-cytoskeleton interaction through dephosphorylation of talin.  相似文献   
142.
Summary Analysis of lactate and malate dehydrogenase zymograms of rodent salivary glands showed species and organ specific patterns.Lactate dehydrogenase isoenzyme patterns occupied the middle positions in relation to those of skeletal and heart muscle. Activities of the major salivary glands were in the order submaxillary gland>parotid>sublingual gland. Zymogram of the mouse and rat showed LDH4 and LDH5 high activity patterns, while that of the rabbit was the fast moving active one. Hamster salivary gland exhibited a neutral type of the former and the latter.Malate dehydrogenase isoenzyme exhibited very similar patterns for the mouse, rat and hamster. Malate dehydrogenase zymogram of rabbit showed 3 active bands, which was different from the other rodents.  相似文献   
143.
Virchows Archiv B Cell Pathology - Paget cells from cases of mammary and extramammary Paget’s disease were examined for carcinoembryonic antigen (CEA) and CEA-related antigens by the...  相似文献   
144.
145.
The fish fin is a breathtaking repository full of evolutionary diversity, novelty, and convergence. Over 500 million years, the adaptation to novel habitats has provided landscapes of fin diversity. Although comparative anatomy of evolutionarily divergent patterns over centuries has highlighted the fundamental architectures and evolutionary trends of fins, including convergent evolution, the developmental constraints on fin evolution, which bias the evolutionary trajectories of fin morphology, largely remain elusive. Here, we review the evolutionary history, developmental mechanisms, and evolutionary underpinnings of paired fins, illuminating possible developmental constraints on fin evolution. Our compilation of anatomical and genetic knowledge of fin development sheds light on the canalized and the unpredictable aspects of fin shape in evolution. Leveraged by an arsenal of genomic and genetic tools within the working arena of spectacular fin diversity, evolutionary developmental biology embarks on the establishment of conceptual framework for developmental constraints, previously enigmatic properties of evolution.  相似文献   
146.
Kanazawa  K.  Mihashi  S.  Nishizawa  N. K.  Chino  M.  Mori  S. 《Plant and Soil》1993,155(1):103-105
A new assay system for the detection of nicotianamine amino-transferase activity was developed. The activity of nicotianamine amino-transferase which participated in biosynthetic pathway of MAs from methionine in graminaceous plants was induced by the iron deficiency treatment.  相似文献   
147.
Fluoro-Jade C (FJC) staining has been used to detect degenerating neurons in tissue sections. It is a simple and easy staining procedure and does not depend on the manner of cell death. In some experiments, double staining with FJC and fluorescent immunostaining (FI) is required to identify cell types. However, pretreatment for FJC staining contains some processes that are harsh to fluorophores, and the FI signal is greatly reduced. To overcome this issue, we improved the double staining protocol to acquire clear double-stained images by introducing the labeled streptavidin–biotin system. In addition, several studies indicate that FJC can label non-degenerating glial cells, including resting/reactive astrocytes and activated microglia. Moreover, our previous study indicated that degenerating mesenchymal cells were also labeled by FJC, but it is still unclear whether FJC can label degenerating glial cells. Acute encephalopathy model mice contained damaged astrocytes with clasmatodendrosis, and 6-aminonicotinamide-injected mice contained necrotic astrocytes and oligodendrocytes. Using our improved double staining protocol with FJC and FI, we detected FJC-labeled degenerating astrocytes and oligodendrocytes with pyknotic nuclei. These results indicate that FJC is not specific to degenerating neurons in some experimental conditions:  相似文献   
148.

Background

Pretreatment is an essential step in the enzymatic hydrolysis of biomass for bio-ethanol production. The dominant concern in this step is how to decrease the high cost of pretreatment while achieving a high sugar yield. Fungal pretreatment of biomass was previously reported to be effective, with the advantage of having a low energy requirement and requiring no application of additional chemicals. In this work, Gloeophyllum trabeum KU-41 was chosen for corn stover pretreatment through screening with 40 strains of wood-rot fungi. The objective of the current work is to find out which characteristics of corn stover pretreated with G. trabeum KU-41 determine the pretreatment method to be successful and worthwhile to apply. This will be done by determining the lignin content, structural carbohydrate, cellulose crystallinity, initial adsorption capacity of cellulase and specific surface area of pretreated corn stover.

Results

The content of xylan in pretreated corn stover was decreased by 43% in comparison to the untreated corn stover. The initial cellulase adsorption capacity and the specific surface area of corn stover pretreated with G. trabeum were increased by 7.0- and 2.5-fold, respectively. Also there was little increase in the cellulose crystallinity of pretreated corn stover.

Conclusion

G. trabeum has an efficient degradation system, and the results indicated that the conversion of cellulose to glucose increases as the accessibility of cellulose increases due to the partial removal of xylan and the structure breakage of the cell wall. This pretreatment method can be further explored as an alternative to the thermochemical pretreatment method.  相似文献   
149.
Context and objective: Plasma arachidonate (20:4) levels in patients with chronic obstructive pulmonary disease (COPD) were investigated. Methods: Plasma was extracted and free fatty acids (FFAs) were separated using column chromatography and measured by fluorescence. Plasma 20:4 levels and its percentage relative to total FFA levels (%20:4) were measured in COPD (n = 18) and control (n = 20) subjects. Results and conclusions: FFA levels were lower in COPD compared with normals. However, there was a significant increase in %20:4 levels in COPD patients (GOLD stage I/II 0.9 ± 0.4%; GOLD stage III/IV 1.1 ± 0.1%) compared with control subjects (0.6 ± 0.1, p < 0.05). %20:4 is a potential biomarker for COPD.  相似文献   
150.
This study examined the endogenous androgen regulation of the marking behavior in Mongolian gerbils (Meriones unguiculatus). In the first experiment, developmental changes of fecal testosterone levels, ventral gland growth, and the marking frequency of male gerbils were investigated. From 9 weeks of age, marking frequency increased with increases in fecal testosterone levels and ventral gland size. The ventral gland size and marking frequency were significantly correlated to the fecal testosterone level. In the second experiment, we hypothesized that reduction in the marking frequency of subordinate males after social confrontations was controlled by a decrease in the circulating testosterone level, and we followed changes in marking frequency, endocrine status, and ventral gland size after social confrontations in which two adult male gerbils established their social ranks by fighting. As expected, marking frequency and ventral gland size were significantly related to social rank, that is, marking frequency was higher among dominant gerbils and lower among subordinates. In addition, fecal corticosterone levels among subordinates were higher than those of dominant animals. However, neither the fecal and plasma testosterone levels, nor testis size, differed between dominant and subordinate gerbils. These results revealed that endogenous androgen played a role in regulating marking behavior and ventral gland size during the developmental stage and that the reductions in marking frequency and ventral gland size occurring in subordinate males after social confrontations were not directly regulated by androgen changes.  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