全文获取类型
收费全文 | 58030篇 |
免费 | 4620篇 |
国内免费 | 4559篇 |
专业分类
67209篇 |
出版年
2024年 | 143篇 |
2023年 | 790篇 |
2022年 | 1859篇 |
2021年 | 3049篇 |
2020年 | 2097篇 |
2019年 | 2507篇 |
2018年 | 2350篇 |
2017年 | 1807篇 |
2016年 | 2547篇 |
2015年 | 3633篇 |
2014年 | 4390篇 |
2013年 | 4445篇 |
2012年 | 5305篇 |
2011年 | 4790篇 |
2010年 | 2896篇 |
2009年 | 2621篇 |
2008年 | 2955篇 |
2007年 | 2647篇 |
2006年 | 2275篇 |
2005年 | 1901篇 |
2004年 | 1515篇 |
2003年 | 1424篇 |
2002年 | 1078篇 |
2001年 | 914篇 |
2000年 | 894篇 |
1999年 | 812篇 |
1998年 | 499篇 |
1997年 | 461篇 |
1996年 | 478篇 |
1995年 | 422篇 |
1994年 | 416篇 |
1993年 | 326篇 |
1992年 | 448篇 |
1991年 | 325篇 |
1990年 | 286篇 |
1989年 | 260篇 |
1988年 | 210篇 |
1987年 | 194篇 |
1986年 | 176篇 |
1985年 | 155篇 |
1984年 | 115篇 |
1983年 | 122篇 |
1982年 | 81篇 |
1981年 | 45篇 |
1980年 | 51篇 |
1979年 | 63篇 |
1976年 | 46篇 |
1974年 | 54篇 |
1973年 | 45篇 |
1972年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
851.
厦门凤林红树林湿地大型底栖动物群落 总被引:8,自引:0,他引:8
为摸清厦门集美凤林红树林湿地的大型底栖动物群落结构及其多样性现状,2002年1、4、7和10月在厦门集美凤林红树林区进行大型底栖动物调查,4个季度共获得大型底栖动物42种。生物量优势种是软体动物门的珠带拟蟹守螺(Cerithideacingulata)和节肢动物门的弧边招潮(Ucaarcuata)。密度优势种是软体动物门的短拟沼螺(Assimineabrevicula)和环节动物门的沼蚓(Limnodriloidessp.)。集美凤林红树林区大型底栖动物年平均密度和年平均生物量分别为1,990ind./m2和139.0g/m2。密度的季节变化是:1月>4月>10月>7月,生物量的季节变化是1月>10月>4月>7月。聚类分析和数量分布表明,优势种珠带拟蟹守螺、短拟沼螺、弧边招潮和沼蚓的季节变化各不相同。与2002年10月深圳湾福田红树林区大型底栖动物群落的物种多样性指数平均值(0.56)比较,厦门凤林红树林区的平均值较高(2.66)。文中分析了影响大型底栖动物多样性的环境因素。 相似文献
852.
兔肺静脉肌袖心肌细胞动作电位的特性和一些离子流机制 总被引:3,自引:0,他引:3
研究兔肺静脉肌袖心肌细胞(cardiomyocytes from rabbit pulmonary vein sleeves, PVC)动作电位的特性和一些离子流机制——内向整流钾电流(IKl)、瞬时外向钾电流(ITo)和非选择性阳离子流(I NSCC),并与左心房心肌细胞(left atrial cardiomyocytes,LAC)进行比较。采用全细胞膜片钳技术,记录动作电位和上述各离子流。发现PVC动作电位时程(action potential duration,APD)较LAC的明显延长,并可以诱发出第二平台反应。PVC上存在I NSCC. PVC的IKl、I To和I NSCC电流密度均较LAC的明显减小。PVC和LAC存在复极离子流的差异,这种差异构成了两者动作电位差异的基础,进而可能成为肺静脉肌袖致心律失常特性的重要离子流机制。 相似文献
853.
Sang-Hwal Yoon Cui Li Young-Mi Lee Sook-Hee Lee Sung-Hee Kim Myung-Suk Choi Weon-Taek Seo Jae-Kyung Yang Jae-Yeon Kim Seon-Won Kim 《Biotechnology and Bioprocess Engineering》2005,10(4):378-384
Vanillin is one of the world's principal flavoring compounds, and is used extensively in the food industry. The potential
vanillin production of the bacteria was compared to select and clone genes which were appropriate for highly productive vanillin
production byE. coli. Thefcs (feruloyl-CoA synthetase) andech (enoyl-CoA hydratase/aldolase) genes cloned fromAmycolatopsis sp. strain HR104 andDelftia acidovorans were introduced to pBAD24 vector with PBAD promoter and were named pDAHEF and pDDAEF, respectively. We observed 160 mg/L vanillin production withE. coli harboring pDAHEF, whereas 10 mg/L of vanillin was observed with pDDAEF. Vanillin production was optimized withE. coli harboring pDAHEF. Induction of thefcs andech genes from pDAHEF was optimized with the addition of 13.3 mM arabinose at 18 h of culture, from which 450 mg/L of vanillin
was produced. The feeding time and concentration of ferulic acid were also optimized by the supplementation of 0.2% ferulic
acid at 18 h of culture, from which 500 mg/L of vanillin was obtained. Under the above optimized condition of arabinose induction
and ferulic acid supplementation, vanillin production was carried out with four different types of media, M9, LB, 2YT, and
TB. The highest vanillin production, 580 mg/L, was obtained with LB medium, a 3.6 fold increase in comparison to the 160 mg/L
obtained before the optimization of vanillin production. 相似文献
854.
AMPK is an AMP-activated protein kinase that plays an important role in regulating cellular energy homeostasis. Metabolic
stress, such as heat shock and glucose starvation, causes an energy deficiency in the cell and leads to elevated levels of
intracellular AMP. This results in the phosphorylation and activation of AMPK. LKB1, a tumor suppressor, has been identified
as an upstream kinase of AMPK. We found that in response to treatment with 5-aminoimidazole-4-carboxamide-1-β-4-ribofuranoside
(AICAR), the LKB1 deficient cancer cell line, HeLa, exhibited AMPK-α phosphorylation. This indicates the existence of an LKB1-independent
AMPK-α phosphorylation pathway. ATM is a protein that is deficient in the disease ataxia telangiectasia (A-T). We measured
the activation of AMPK by AICAR in the normal mouse embryo fibroblast cell line, A29, and the mouse cell line lacking the
ATM protein, A38. In A38 cells, the level of AICAR-induced AMPK-α phosphorylation was significantly lower than that found
in A29 cells. Furthermore, phosphorylation of AMPK in HeLa and A29 cells was inhibited by an ATM specific inhibitor, KU-55933.
Our results demonstrate that AICAR treatment could lead to phosphorylation of AMPK in an ATM-dependent and LKB1-independent
manner. Thus, ATM may function as a potential AMPK kinase in response to AICAR treatment. 相似文献
855.
Molecular basis of the differences between normal and tumor tissues of gastric cancer 总被引:1,自引:0,他引:1
Yang S Shin J Park KH Jeung HC Rha SY Noh SH Yang WI Chung HC 《Biochimica et biophysica acta》2007,1772(9):1033-1040
To be able to describe the differences between the normal and tumor tissues of gastric cancer at a molecular level would be essential in the study of the disease. We investigated the gene expression pattern in the two types of tissues from gastric cancer by performing expression profiling of 86 tissues on 17K complementary DNA microarrays. To select for the differentially expressed genes, class prediction algorithm was employed. For predictor selection, samples were first divided into a training (n=58), and a test set (n=28). A group of 894 genes was selected by a t-test in a training set, which was used for cross-validation in the training set and class (normal or tumor) prediction in the test set. Smaller groups of 894 genes were individually tested for their ability to correctly predict the normal or tumor samples based on gene expression pattern. The expression ratios of the 5 genes chosen from microarray data can be validated by real time RT-PCR over 6 tissue samples, resulting in a high level of correlation, individually or combined. When a representative predictor set of 92 genes was examined, pathways of 'focal adhesion' (with gene components of THBS2, PDGFD, MAPK1, COL1A2, COL6A3), 'ECM-receptor interaction' pathway (THBS2, COL1A2, COL6A3, FN1) and 'TGF-beta signaling' (THBS2, MAPK1, INHBA) represent some of the main differences between normal and tumor of gastric cancer at a molecular level. 相似文献
856.
Yang YS Ahn TH Lee JC Moon CJ Kim SH Park SC Chung YH Kim HY Kim JC 《Birth defects research. Part B, Developmental and reproductive toxicology》2007,80(5):374-382
This study investigated the potential adverse effects of tert-butyl acetate (TBAc) on maternal toxicity and embryo-fetal development after maternal exposure of pregnant rats from gestational days 6 through 19. TBAc was administered to pregnant rats by gavage at 0, 400, 800, and 1,600 mg/kg/day. All dams were subjected to a Caesarean section on day 20 of gestation, and their fetuses were examined for any morphological abnormalities. At 1,600 mg/kg, maternal toxicity manifested as increases in the incidence of clinical signs and death, lower body weight gain and food intake, increases in the weights of adrenal glands and liver, and a decrease in thymus weight. Developmental toxicity included a decrease in fetal weight, an increase in the incidence of skeletal variation, and a delay in fetal ossification. At 800 mg/kg, only a minimal developmental toxicity, including an increase in the incidence of skeletal variation and a delay in fetal ossification, were observed. In contrast, no adverse maternal or developmental effects were observed at 400 mg/kg. These results show that a 14-day repeated oral dose of TBAc is embryotoxic at a maternally toxic dose (i.e., 1,600 mg/kg/day) and is minimally embryotoxic at a nonmaternally toxic dose (i.e., 800 mg/kg/day) in rats. However, no evidence for the teratogenicity of TBAc was noted in rats. It is concluded that the developmental findings observed in the present study are secondary effects to maternal toxicity. Under these experimental conditions, the no-observed-adverse-effect level of TBAc is considered to be 800 mg/kg/day for dams and 400 mg/kg/day for embryo-fetal development. 相似文献
857.
Apelin activates L-arginine/nitric oxide synthase/nitric oxide pathway in rat aortas 总被引:4,自引:0,他引:4
Jia YX Lu ZF Zhang J Pan CS Yang JH Zhao J Yu F Duan XH Tang CS Qi YF 《Peptides》2007,28(10):2023-2029
Apelin was recently found to be an inotropic polypeptide in isolated rat hearts, and intravenous injection of apelin can induce a transient decrease in blood pressure. To illustrate the mechanism of apelin-induced vasodilation, we observed the in vitro effects of apelin on the L-arginine (L-Arg)/nitric oxide (NO) pathway in the incubated, isolated rat aorta. Apelin stimulated vascular NO(2)(-) product and NOS activation in a concentration- and time-dependent manner. Compared with no apelin treatment, incubation with apelin (10(-9), 10(-8), and 10(-7)mol/L) increased NO(2)(-) product by 33%, 46%, and 69% (all p<0.01), respectively, and Ca(2+)-dependent constitutive NOS (cNOS) activity by 200%, 460%, and 550% (all p<0.01), respectively. However, Ca(2+)-independent NOS (iNOS) activity was not significantly altered (p>0.05). Apelin incubation (10(-9), 10(-8), and 10(-7)mol/L) increased L-Arg uptake by 130%, 180%, and 240% (all p<0.01), respectively. The mRNA level of cationic amino acid transporters, CAT-1 and CAT-2B, in rat aortic tissues treated with 10(-7)mol/L apelin was increased by 110% and 128%, respectively (both p<0.01). Incubation with 10(-7)mol/L apelin elevated eNOS mRNA and protein levels, by 53% (p<0.05) and 319% (p<0.01), respectively. Collectively, these results demonstrate that apelin directly activated the vascular L-Arg/NOS/NO pathway, which could be one of the important mechanisms of apelin-regulated vascular function. 相似文献
858.
Lijun Liu Xin Jin Shaoming Yang Zhichun Chen Xianfu Lin 《Biosensors & bioelectronics》2007,22(12):3210
The bilayer of Con A/HRP through the biospecific affinity of concanavalin A (Con A) and glycoprotein horseradish peroxidase (HRP) was prepared on the surface of an Au electrode modified by the precursor film consisted of poly(allylamine hydrochloride) poly(sodium-p-styrene-sulfonate). Atomic force microscopy and electrochemical impedance spectroscopy were adopted to monitor the uniform layer-by-layer assembly of the Con A/HRP bilayers. The amperometric measurement was based on the inhibition of reduced thiols and performed in the presence of the electron mediator hydroquinone in 0.2 M phosphate buffer of pH 6.5 at an applied potential of −0.15 V versus Ag/AgCl. Under the optimal conditions, the biosensor presented a linear response for cysteine from 0.1 to 23.5 μM, with a detection limit of 0.02 μM. The biosensor demonstrated high stability and repeatability. A series of reduced thiols were detected by this inhibition biosensor and oxidized thiols showed no effect on the current response of the biosensor. 相似文献
859.
Populations are at risk of extinction when unsuitable or when sink habitat exceeds a threshold frequency in the environment. Sinks that present cues associated with high-quality habitats, termed ecological traps, have especially detrimental effects on net population growth at metapopulation scales. Ecological traps for viruses arise naturally, or can be engineered, via the expression of viral-binding sites on cells that preclude viral reproduction. We present a model for virus population growth in a heterogeneous host community, parameterized with data from populations of the RNA bacteriophage Φ6 presented with mixtures of suitable host bacteria and either neutral or trap cells. We demonstrate that viruses can sustain high rates of population growth in the presence of neutral non-hosts as long as some host cells are present, whereas trap cells dramatically reduce viral fitness. In addition, we demonstrate that the efficacy of traps for viral elimination is frequency dependent in spatially structured environments such that population viability is a nonlinear function of habitat loss in dispersal-limited virus populations. We conclude that the ecological concepts applied to species conservation in altered landscapes can also contribute to the development of trap cell therapies for infectious human viruses. 相似文献
860.
The “unprotected” Pt nanoclusters (average size 2 nm) mixed with the nanoscale SiO2 particles (average size 13 nm) were used as a glucose oxidase immobilization carrier to fabricate the amperometric glucose biosensor. The bioactivity of glucose oxidase (GOx) immobilized on the composite was maintained and the as-prepared biosensor demonstrated high sensitivity (3.85 μA mM−1) and good stability in glucose solution. The Pt–SiO2 biosensor showed a detection limit of 1.5 μM with a linear range from 0.27 to 4.08 mM. In addition, the biosensor can be operated under wide pH range (pH 4.9–7.5) without great changes in its sensitivity. Cyclic voltammetry measurements showed a mixed controlled electrode reaction. 相似文献