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   1873篇
  免费   150篇
  国内免费   1篇
  2023年   9篇
  2022年   14篇
  2021年   42篇
  2020年   27篇
  2019年   28篇
  2018年   35篇
  2017年   29篇
  2016年   39篇
  2015年   70篇
  2014年   94篇
  2013年   100篇
  2012年   136篇
  2011年   124篇
  2010年   62篇
  2009年   73篇
  2008年   111篇
  2007年   111篇
  2006年   90篇
  2005年   81篇
  2004年   65篇
  2003年   45篇
  2002年   54篇
  2001年   41篇
  2000年   48篇
  1999年   37篇
  1998年   16篇
  1997年   13篇
  1996年   15篇
  1995年   16篇
  1994年   14篇
  1993年   14篇
  1992年   44篇
  1991年   24篇
  1990年   19篇
  1989年   29篇
  1988年   18篇
  1987年   20篇
  1986年   14篇
  1985年   16篇
  1984年   14篇
  1983年   14篇
  1980年   9篇
  1979年   16篇
  1977年   14篇
  1976年   11篇
  1975年   12篇
  1972年   8篇
  1971年   8篇
  1970年   8篇
  1969年   10篇
排序方式: 共有2024条查询结果,搜索用时 31 毫秒
41.
The garnet-type phase Li7La3Zr2O12 (LLZO) attracts significant attention as an oxide solid electrolyte to enable safe and robust solid-state batteries (SSBs) with potentially high energy density. However, while significant progress has been made in demonstrating compatibility with Li metal, integrating LLZO into composite cathodes remains a challenge. The current perspective focuses on the critical issues that need to be addressed to achieve the ultimate goal of an all-solid-state LLZO-based battery that delivers safety, durability, and pack-level performance characteristics that are unobtainable with state-of-the-art Li-ion batteries. This perspective complements existing reviews of solid/solid interfaces with more emphasis on understanding numerous homo- and heteroionic interfaces in a pure oxide-based SSB and the various phenomena that accompany the evolution of the chemical, electrochemical, structural, morphological, and mechanical properties of those interfaces during processing and operation. Finally, the insights gained from a comprehensive literature survey of LLZO–cathode interfaces are used to guide efforts for the development of LLZO-based SSBs.  相似文献   
42.
43.
Glycine decarboxylase is a mitochondrial enzyme complex, which is the site of photorespiratory CO2 and NH3 release. Although the proteins that constitute the complex are located within the mitochondria, because of their intimate association with photosynthesis their expression is controlled by light. Comparisons of the kinetics of mRNA accumulation between the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase and the H-protein of glycine decarboxylase during the greening of etiolated Arabidopsis thaliana suggest that their expression is controlled in parallel. A genomic clone for the H-protein (gdcH) was isolated from Arabidopsis and sequenced. The upstream region from -856 to +62 was fused to the beta-glucuronidase (GUS) reporter gene, and this construct was transformed into tobacco. This 5' upstream regulatory region appears to control GUS expression in a manner very similar to that of the endogenous H-protein gene. Constructs with deletions in the 5' upstream region were transformed into tobacco. These deletions revealed that light-dependent and tissue-specific expression was largely controlled by a 259-bp region between -376 and -117 bp. This region contains several putative GT boxes with the GGTTAA consensus core sequence. Once these strong light-dependent elements were removed, a second level of control was revealed. In constructs in which the gdcH 5' regulatory region was shortened to -117 bp or less, there was more GUS activity in the roots than in the leaves, and in dark-grown plants than in light-grown plants. This suggests that more proximal control elements may be responsible for the constitutive low levels of gene expression noted in all nonphotosynthetic tissues.  相似文献   
44.
Nerve Growth Factor as a Mitogen for a Pancreatic Carcinoid Cell Line   总被引:1,自引:0,他引:1  
Abstract: Carcinoid tumors are a group of neuroendocrine neoplasms distributed widely throughout the body but most commonly occurring in the gut. These tumors retain many characteristics of their neural crest origin, including secretion of neuroactive peptides and responsiveness to neurotrophic substances. Nerve growth factor (NGF), a neurotrophic protein involved in maintenance and differentiation of peripheral sympathetic and sensory neurons, regulates growth of several neural tumor cells by inducing a differentiated phenotype and subsequent inhibition of cell growth rate. We examined the actions of NGF in a functioning human pancreatic carcinoid cell line (termed BON). NGF has no effect on the cytoarchitecture or constitutive secretion of bioamines in this carcinoid cell line. NGF, however, stimulates the in vitro cellular proliferation of BON cells. BON cells possess mRNA for the NGF receptors (p75LNGFR and p140trkA) and membrane-associated tyrosine kinase activity is increased in response to NGF. Both the mitogenic activity of NGF, as well as the receptor-linked tyrosine kinase activity, can be abrogated in BON cells by the trkA inhibitor K-252a and specific anti-NGF antibody. Our studies demonstrate that NGF is a mitogen for this carcinoid cell line without effect on cellular phenotype or cytoarchitecture. NGF may play a role in the development and progression of human carcinoid tumors.  相似文献   
45.
Various roles for the viral receptor, CD4, have been proposed in facilitating human immunodeficiency virus type 1 (HIV-1) entry, including virion binding to the target cell and the induction of conformational changes in the viral envelope glycoproteins required for the membrane fusion reaction. Here, we compare the structural requirements in the CDR2-like loop of CD4 domain 1, the major contact site of the gp120 envelope glycoprotein, for gp120 binding and virus entry. For every CD4 mutant examined, the level of cell surface expression and the gp120 binding affinity were sufficient to explain the relative ability to function as a viral receptor. The decrease in relative infectibility associated with decreased gp120 binding affinity was more pronounced at lower cell surface CD4 concentrations. These results imply that both receptor density and affinity determine the efficiency of HIV-1 entry and that specific structures in the CD4 residues examined are probably not required for HIV-1 entry functions other than gp120 binding.  相似文献   
46.
Summary Growth and biomass protein formation by filamentous fungi grown on pretreated tropical woods of Mesta (Hibiscus cannabinus Linn.) and Subabul [Leucaena leucocephala (Lam.) de Witt] as well as their isolated hemicellulose and cellulose fractions have been studied. Penicillium janthinellum and Penicillium funiculosum produced a biomass having 20 to 30% crude protein when grown on either hemicellulose, while growth on pretreated (autoclaved in 1% NaOH) wood or isolated cellulose fractions was comparatively poor and crude protein content only 5 to 8% in the biomass.NCL Communication no.3550  相似文献   
47.
Summary Specific glucose and xylose isomerases have been identified in cell-free culture filtrates of a Chainia species. Treatment with DEAE-cellulose selectively adsorbed xylose isomerase activity while only the glucose isomerase was adsorbed on CM-cellulose. Glucose isomerase was completely inhibited by xylose at 1.3 × 10-4 M concentration. The differential identity of the extracellular glucose and xylose isomerases, unique to Chainia, is discussed.(NCL Communication 3562)  相似文献   
48.
49.
Interleukin-1 (IL-1) plays an important role in cartilage destruction associated with inflammatory and degenerative arthritis because of its ability to induce matrix degrading enzymes. Previously, we have shown that the IL-1-induced chondrocyte protease activity was inhibited by transforming growth factor-β (TGF-β). In this paper, we show that TGF-β inhibits the IL-1-induced synthesis of collagenase and stromelysin by reducing the steady-state mRNA levels in rabbit articular chondrocytes. We further demonstrate that TGF-β-treated chondrocytes show reduced 125I-IL-1 binding that returns to a normal level when TGF-β is removed from the culture medium. The inhibitory effect of TGF-β is observed for both naturally occurring as well as fibroblast growth factor (FGF)-inducible binding sites (receptors). Scatchard analysis of receptor—ligand interactions demonstrate that the reduced binding is due to a reduction in the number of receptors for IL-1 and is not due to changes in affinity. Affinity cross-linking studies suggest that control chondrocytes contain two major cross-linked bands of Mr =116 and 80 kDa and a minor band of Mr =100 kDa. FGF-treated cells show enhanced levels of all the bands, plus an additional 200-kDa band. TGF-β treatment of chondrocytes results in the reduction of all of these bands in both control as well as FGF-induced cells. These observations suggest that the ability of TGF-β to down-regulate the IL-1 receptor may be a mechanism by which it exerts its effects in antagonizing the IL-1 activity on chondrocytes.  相似文献   
50.
Increased degradation of dermal collagen in diabetic rats.   总被引:2,自引:0,他引:2  
The effect of alloxan induced diabetes on the dermal collagen content of albino rats was studied in relation to few lysosomal enzymes. Diabetes decreased the dermal collagen content. The specific activities of the lysosomal enzymes studied in the diabetic rat skin were elevated. It has been established that lysosomal enzymes degrade the connective tissue components. Thus, it may be suggested that the increase in the lysosomal enzymes studied should have facilitated the decrease in dermal collagen content of diabetic rats by increasing the degradation of dermal collagen.  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