全文获取类型
收费全文 | 6617篇 |
免费 | 608篇 |
国内免费 | 387篇 |
专业分类
7612篇 |
出版年
2023年 | 64篇 |
2022年 | 134篇 |
2021年 | 262篇 |
2020年 | 199篇 |
2019年 | 237篇 |
2018年 | 240篇 |
2017年 | 175篇 |
2016年 | 259篇 |
2015年 | 412篇 |
2014年 | 450篇 |
2013年 | 480篇 |
2012年 | 576篇 |
2011年 | 511篇 |
2010年 | 322篇 |
2009年 | 291篇 |
2008年 | 340篇 |
2007年 | 321篇 |
2006年 | 267篇 |
2005年 | 232篇 |
2004年 | 214篇 |
2003年 | 194篇 |
2002年 | 171篇 |
2001年 | 123篇 |
2000年 | 115篇 |
1999年 | 117篇 |
1998年 | 61篇 |
1997年 | 47篇 |
1996年 | 43篇 |
1995年 | 45篇 |
1994年 | 43篇 |
1993年 | 32篇 |
1992年 | 46篇 |
1991年 | 41篇 |
1990年 | 48篇 |
1989年 | 36篇 |
1988年 | 46篇 |
1987年 | 31篇 |
1986年 | 39篇 |
1985年 | 31篇 |
1984年 | 28篇 |
1983年 | 28篇 |
1982年 | 16篇 |
1981年 | 15篇 |
1980年 | 22篇 |
1979年 | 19篇 |
1978年 | 18篇 |
1976年 | 20篇 |
1975年 | 26篇 |
1974年 | 15篇 |
1970年 | 15篇 |
排序方式: 共有7612条查询结果,搜索用时 11 毫秒
991.
人体和动物模型的体表物理信息地形图的研究 总被引:1,自引:0,他引:1
对人体头面、躯干、四肢、耳廓各局部几十个及整个人体等体表部位正、背面等210个部位进行超微弱冷光和温度测量,输入电子计算机,经特殊的自编程序处理,获得十分清晰的,由3000多数据构成的各个局部或人体整体的冷光和温度地形图。 对家兔左、右耳廓、胸腹部、背部都分别观察32个部位的冷光与体表温度,经计算机分析处理,每观察区域获得约由2000个数据构成的精确的冷光、温度地形分市图。并可见不同生理、病理状态及不同病程家兔体表冷光、温度等地形图呈有规律的改变。 此外,我们还编制了以体表左右相应对称部位差值为分析数据进行地形图分析的程序,用以人体和动物体表物理信息对称规律的研究。 本工作以图形的形式显示物理参量在体表的广泛的分布规律,以揭示机体内部的不同生理、病理状态。本方法定位准确、直观醒目,为研究体表信息及机体生命活动规律提供了与逐点直接测量方法相互补充的有益的新手段。 相似文献
992.
993.
Human mu-opioid receptor (HmuOR) with a tag of six consecutive histidines at its carboxyl terminus had been expressed in recombinant baculovirus infected Sf9 insect cells. The maximal binding capacity for the [3H] diprenorphine and [3H]ohmefentanyl (Ohm) were 9.1 +/- 0.7 and 6.52 +/- 0.23 nmol/g protein, respectively. The [3H] diprenorphine or [3H] Ohm binding to the receptor expressed in Sf9 cells was strongly inhibited by mu-selective agonists [D-Ala2, N-methyl-Phe4, glyol5]enkephalin (DAGO), Ohm, and morphine, but neither by delta nor by kappa selective agonist. Na+ (100 mM) and GTP (50 microM) could reduce HmuOR agonists etorphine and Ohm affinity binding to the overexpressed HmuOR. mu-selective agonists DAGO and Ohm effectively stimulated [35S]GTP-gammaS binding (EC50 = 2.7 nM and 6.9 nM) and inhibited forskolin- stimulated cAMP accumulation (IC50 = 0.9 nM and 0.3 nM). The agonist-dependent effects could be blocked by opioid antagonist naloxone or by pretreatment of cells with pertussis toxin (PTX). These results demonstrated that HmuOR overexpressed in Sf9 insect cells functionally coupled to endogenous G(i/o) proteins. 相似文献
994.
从养殖场污泥中筛选出菌株YP4,经16S rDNA分子发育树的同源序列比对,确定为克雷伯什菌属(Klebsiella sp.)。由NCBI数据库查编码亚硝酸还原酶(Nir)的基因nirS序列,设计引物,以铜绿假单胞菌PAOI基因组DNA为模板,应用PCR技术扩增目的片段nirS,经过双酶切、克隆和转化,得到重组质粒pYP4S,然后转化野生菌株YP4,构建反硝化基因工程菌YP4S。菌株生长曲线测定表明,工程菌株YP4S与YP4的生长特性基本一致。工程菌株YP4S对模拟污水COD、TN、NH_4^+-N和NO_3^--N具有较高的去除率,YP4S与YP4相比,对NO_2^--N积累的减少量为(32.44±3.96)%,明显减少了NO_2^--N的积累。通过正交试验获得工程菌株YP4S在C/N=10、T=30℃、r=200 r/min和pH=7.0的最佳组合条件下,对模拟污水TN去除率较高。应用工程菌株YP4S处理猪场沉淀池的实际污水,COD、TN、TP、NH_4^+-N和NO_3^--N去除率分别为(95.87±0.82)%、(76.38±3.84)%、(97.13±0.54)%和(75.35±2.57)%,NO_2^--N积累量为(3.31±1.24) mg/L,表明工程菌株YP4S具有较好反硝化作用,对含氮量高的实际污水修复具有潜在的应用前景。 相似文献
995.
Huawei Mao Wenwei Tu Gang Qin Helen Ka Wai Law Sin Fun Sia Ping-Lung Chan Yinping Liu Kwok-Tai Lam Jian Zheng Malik Peiris Yu-Lung Lau 《Journal of virology》2009,83(18):9215-9222
Influenza is an acute respiratory viral disease that is transmitted in the first few days of infection. Evasion of host innate immune defenses, including natural killer (NK) cells, is important for the virus''s success as a pathogen of humans and other animals. NK cells encounter influenza viruses within the microenvironment of infected cells and are important for host innate immunity during influenza virus infection. It is therefore important to investigate the direct effects of influenza virus on NK cells. In this study, we demonstrated for the first time that influenza virus directly infects and replicates in primary human NK cells. Viral entry into NK cells was mediated by both clathrin- and caveolin-dependent endocytosis rather than through macropinocytosis and was dependent on the sialic acids on cell surfaces. In addition, influenza virus infection induced a marked apoptosis of NK cells. Our findings suggest that influenza virus can directly target and kill NK cells, a potential novel strategy of influenza virus to evade the NK cell innate immune defense that is likely to facilitate viral transmission and may also contribute to virus pathogenesis.Influenza is an acute respiratory virus infection that continues to pose endemic, zoonotic, and pandemic threats to human health, with significant morbidity and mortality (17). At the early phase of viral infection, innate immunity plays important roles in host defense by limiting viral replication and helping to initiate an adaptive immune response. Natural killer (NK) cells are key effector cells in innate immunity and play a critical role in the first line of host defense against acute viral infections by directly destroying infected cells without the need for prior antigen stimulation (7, 20). As influenza illness and virus transmission usually occur in the first few days of infection, the virus has to devise strategies to evade host innate immune responses, including NK cell immunity (15, 21).NK cells can recognize and kill influenza virus-infected cells (2, 10, 23); to counteract this killing, however, influenza virus has developed an escape strategy that inhibits NK cell cytotoxicity by increasing the binding of two inhibitory receptors to the infected cells after infection (1). The individuals with complete NK cell deficiency developed life-threatening varicella zoster virus and cytomegalovirus infection, but no severe influenza virus infection occurred (30, 40). Indeed, the interaction between human NK cells and influenza virus remains poorly understood. After influenza virus infection, respiratory epithelial cells release inflammatory chemokines that recruit NK cells to the site of infection (12). As a lytic virus, numerous influenza virus particles are released from the infected epithelia and macrophages (5, 9, 33). In the infected microenvironment, NK cells undoubtedly encounter these infective virus particles. It is therefore important to investigate the direct interaction of NK cells with influenza virus. Patients with severe influenza virus infection were shown to have diminished NK cells in peripheral blood and an almost complete absence of pulmonary NK cells, together with marked apoptosis (13, 42). During influenza virus infection in mice, a transient increase of NK cytotoxicity is followed by a marked decrease in NK cell activity, with a virus dose-dependent effect (8, 28). These data suggest that influenza virus may directly target NK cells as part of its immunoevasion strategies. However, no reports of the direct effects of influenza virus on human NK cells have so far been available.In this study, we demonstrated that influenza virus infects and replicates in primary human NK cells. Viral infection was dependent on sialic acids on the cells. The entry was mediated by both clathrin- and caveolin-dependent endocytosis rather than macropinocytosis. Influenza virus infection induced a marked apoptosis of NK cells, which contributed to reduced NK cell cytotoxicity. This, to the best of our knowledge, is the first paper to demonstrate that influenza virus can directly infect NK cells and induce cell apoptosis. These findings suggest that influenza virus may have developed a novel strategy to evade NK cell innate immune defenses, which is likely to facilitate viral transmission and may also contribute to virus pathogenesis. 相似文献
996.
997.
Xiang Luo Chaozhi Ma Yao Yue Kaining Hu Yaya Li Zhiqiang Duan Ming Wu Jinxing Tu Jinxiong Shen Bin Yi Tingdong Fu 《BMC genomics》2015,16(1)
Background
Harvest index (HI), the ratio of grain yield to total biomass, is considered as a measure of biological success in partitioning assimilated photosynthate to the harvestable product. While crop production can be dramatically improved by increasing HI, the underlying molecular genetic mechanism of HI in rapeseed remains to be shown.Results
In this study, we examined the genetic architecture of HI using 35,791 high-throughput single nucleotide polymorphisms (SNPs) genotyped by the Illumina BrassicaSNP60 Bead Chip in an association panel with 155 accessions. Five traits including plant height (PH), branch number (BN), biomass yield per plant (BY), harvest index (HI) and seed yield per plant (SY), were phenotyped in four environments. HI was found to be strongly positively correlated with SY, but negatively or not strongly correlated with PH. Model comparisons revealed that the A–D test (ADGWAS model) could perfectly balance false positives and statistical power for HI and associated traits. A total of nine SNPs on the C genome were identified to be significantly associated with HI, and five of them were identified to be simultaneously associated with HI and SY. These nine SNPs explained 3.42 % of the phenotypic variance in HI.Conclusions
Our results showed that HI is a complex polygenic phenomenon that is strongly influenced by both environmental and genotype factors. The implications of these results are that HI can be increased by decreasing PH or reducing inefficient transport from pods to seeds in rapeseed. The results from this association mapping study can contribute to a better understanding of natural variations of HI, and facilitate marker-based breeding for HI.Electronic supplementary material
The online version of this article (doi:10.1186/s12864-015-1607-0) contains supplementary material, which is available to authorized users. 相似文献998.
999.
Lipid components in biological membranes are essential for maintaining cellular function. Phosphoinositides, the phosphorylated derivatives of phosphatidylinositol (PI), regulate many critical cell processes involving membrane signaling, trafficking, and reorganization. Multiple metabolic pathways including phosphoinositide kinases and phosphatases and phospholipases tightly control spatio-temporal concentration of membrane phosphoinositides. Metabolizing enzymes responsible for PI 4,5-bisphosphate (PI(4,5)P2) production or degradation play a regulatory role in Toll-like receptor (TLR) signaling and trafficking. These enzymes include PI 4-phosphate 5-kinase, phosphatase and tensin homolog, PI 3-kinase, and phospholipase C. PI(4,5)P2 mediates the interaction with target cytosolic proteins to induce their membrane translocation, regulate vesicular trafficking, and serve as a precursor for other signaling lipids. TLR activation is important for the innate immune response and is implicated in diverse pathophysiological disorders. TLR signaling is controlled by specific interactions with distinct signaling and sorting adaptors. Importantly, TLR signaling machinery is differentially formed depending on a specific membrane compartment during signaling cascades. Although detailed mechanisms remain to be fully clarified, phosphoinositide metabolism is promising for a better understanding of such spatio-temporal regulation of TLR signaling and trafficking. [BMB Reports 2014; 47(7): 361-368] 相似文献
1000.
Le Van Phung Tran Thi Bich Chi Hisako Hotta Eiko Yabuuchi Ikuya Yano 《Microbiology and immunology》1995,39(2):105-116
Viet nam is known as an endemic area of melioidosis but its etiologic agent originated in Viet nam was not extensively studied. For the first time, we analyzed the cellular lipid and fatty acid compositions of 15 Vietnamese isolates of Burkholderia pseudomallei, 10 from humans and 5 from the environment. Cellular lipid compositions were analyzed by two-dimensional thin-layer chromatography on silica gel G plates. Cellular fatty acid methyl esters were analyzed by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). The major lipids in all the isolates were phosphatidylglycerol (PG), two forms of phosphatidylethanolamine (PE-1 and PE-2), and two forms of ornithine-containing lipid (OL-1 and OL-2). PE-1 contained non-hydroxy fatty acids at both sn-1 and ?2 positions, while PE-2 possessed 2-hydroxy fatty acids and non-hydroxy fatty acids in a ratio of 1: 1. Since snake venom phospholipase A2 digestion of PE-2 liberated 2-hydroxy fatty acids, it was confirmed that these acids are at the sn-2 position of glycerol moiety. In both OL-1 and OL-2, amide-linked fatty acid was 3-hydroxy palmitic acid (3-OH-C16: 0), while ester-linked fatty acids were non-hydroxy acids in OL-1 and 2-hydroxy acids in OL-2. The total cellular fatty acid compositions of the test strains were characterized by the presence of 2-hydroxy palmitic (2-OH-C16: 0), 2-hydroxy hexadecenoic (2-OH-C16: 1), 2-hydroxy octadecenoic (2-OH-C18: 1), 2-hydroxy methylene octadecanoic (2-OH-C19CPA), 3-hydroxy myristic (3-OH-C14: 0) and 3-hydroxy palmitic (3-OH-C16: 0) acids. There were significant differences in the concentration of hexadecenoic (C16: 1), methylene hexadecanoic (C17CPA), octadecenoic (C18: 1) and methylene octadecanoic (C19CPA) acids among the Vietnamese isolates of B. pseudomallei. However, no significant difference was observed in cellular lipid and fatty acid components between strains of human and environmental origins. 相似文献