全文获取类型
收费全文 | 1348篇 |
免费 | 8篇 |
国内免费 | 12篇 |
专业分类
系统科学 | 26篇 |
丛书文集 | 1篇 |
教育与普及 | 1篇 |
理论与方法论 | 13篇 |
现状及发展 | 252篇 |
研究方法 | 212篇 |
综合类 | 803篇 |
自然研究 | 60篇 |
出版年
2020年 | 5篇 |
2018年 | 14篇 |
2017年 | 17篇 |
2016年 | 7篇 |
2015年 | 12篇 |
2014年 | 16篇 |
2013年 | 18篇 |
2012年 | 99篇 |
2011年 | 188篇 |
2010年 | 37篇 |
2009年 | 9篇 |
2008年 | 101篇 |
2007年 | 102篇 |
2006年 | 95篇 |
2005年 | 119篇 |
2004年 | 93篇 |
2003年 | 72篇 |
2002年 | 87篇 |
2001年 | 21篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1994年 | 3篇 |
1992年 | 2篇 |
1991年 | 7篇 |
1990年 | 8篇 |
1989年 | 2篇 |
1988年 | 4篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 9篇 |
1977年 | 14篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 14篇 |
1973年 | 7篇 |
1972年 | 7篇 |
1971年 | 12篇 |
1970年 | 14篇 |
1969年 | 17篇 |
1968年 | 12篇 |
1967年 | 11篇 |
1966年 | 10篇 |
1965年 | 7篇 |
排序方式: 共有1368条查询结果,搜索用时 15 毫秒
91.
DU LiNa YANG JunXing RINTELEN Thomas von CHEN XiaoYong ALDRIDGE David 《科学通报(英文版)》2013,58(18):2154-2162
We investigated the phylogeny of the viviparid genus Margarya,endemic to Yunnan,China,using two mitochondrial gene fragments(COI and 16S rRNA).The molecular phylogeny based on the combined dataset indicates that Margarya is polyphyletic,as two of the three well-supported clades containing species of Margarya also comprise species from other viviparid genera.In one clade,sequences of four species of Margarya even cluster indiscriminately with those of two species of Cipangopaludina,indicating that the current state of Asian viviparid taxonomy needs to be revised.Additionally,these data suggest that shell evolution in viviparids is complex,as even the large and strongly sculptured shells of Margarya,which are outstanding among Asian viviparids,can apparently be easily converted to simple smooth shells.The molecular data also indicate that the species status of the six extant species of Margarya should be re-assessed. 相似文献
92.
Our world has been changing at an exponential rate.As a result of this rapid growth,we will be forced to make changes in not only the way we live in the environment but also in the environment itself such as designing the cities of the future to be in greater harmony with the increasing population and growing complexity.The paper contains both reflections on global awareness and comprehensive criteria and their priorities for choosing the most desirable city. 相似文献
93.
Incubation periods for paediatric AIDS patients 总被引:11,自引:0,他引:11
I Auger P Thomas V De Gruttola D Morse D Moore R Williams B Truman C E Lawrence 《Nature》1988,336(6199):575-577
A recent seroprevalence study of newborns indicates that one in 62 children born in New York City has antibodies to the human immunodeficiency virus (HIV). The distribution of incubation periods for paediatric patients is needed to estimate future AIDS case loads from these seroprevalence data. Current estimates of incubation periods for paediatric patients are based on limited data. We use parametric and non-parametric methods to analyse incubation periods for 215 paediatric patients with AIDS whose only known route of infection is maternal. We conclude that incubation periods are longer than previously reported; that there is a distinct knee in the incubation period distribution at seven months which suggests two risk populations; and that there is an increase in incidence which is consistent with exponential growth. 相似文献
94.
Bennett IM Farfano HM Bogani F Primak A Liddell PA Otero L Sereno L Silber JJ Moore AL Moore TA Gust D 《Nature》2002,420(6914):398-401
Transport of calcium ions across membranes and against a thermodynamic gradient is essential to many biological processes, including muscle contraction, the citric acid cycle, glycogen metabolism, release of neurotransmitters, vision, biological signal transduction and immune response. Synthetic systems that transport metal ions across lipid or liquid membranes are well known, and in some cases light has been used to facilitate transport. Typically, a carrier molecule located in a symmetric membrane binds the ion from aqueous solution on one side and releases it on the other. The thermodynamic driving force is provided by an ion concentration difference between the two aqueous solutions, coupling to such a gradient in an auxiliary species, or photomodulation of the carrier by an asymmetric photon flux. Here we report a different approach, in which active transport is driven not by concentration gradients, but by light-induced electron transfer in a photoactive molecule that is asymmetrically disposed across a lipid bilayer. The system comprises a synthetic, light-driven transmembrane Ca2+ pump based on a redox-sensitive, lipophilic Ca2+-binding shuttle molecule whose function is powered by an intramembrane artificial photosynthetic reaction centre. The resulting structure transports calcium ions across the bilayer of a liposome to develop both a calcium ion concentration gradient and a membrane potential, expanding Mitchell's concept of a redox loop mechanism for protons to include divalent cations. Although the quantum yield is relatively low (approximately 1 per cent), the Ca2+ electrochemical potential developed is significant. 相似文献
95.
The detection of image motion is fundamental to vision. In many species, unique classes of retinal ganglion cells selectively respond to visual stimuli that move in specific directions. It is not known which retinal cell first performs the neural computations that give rise to directional selectivity in the ganglion cell. A prominent candidate has been an interneuron called the 'starburst amacrine cell'. Using two-photon optical recordings of intracellular calcium concentration, here we find that individual dendritic branches of starburst cells act as independent computation modules. Dendritic calcium signals, but not somatic membrane voltage, are directionally selective for stimuli that move centrifugally from the cell soma. This demonstrates that direction selectivity is computed locally in dendritic branches at a stage before ganglion cells. 相似文献
96.
RhoA/Rho激酶在大鼠阴茎勃起机制中的调节作用 总被引:11,自引:1,他引:10
一般认为一氧化氮(nitric oxide,NO)释放增加促进小动脉和海绵体平滑肌舒张是导致男性阴茎勃起的主要机制。研究发现内源性血管收缩因子对维持阴茎萎状态有重要作用。应用大鼠模型活体检测使用Y-27632拮抗Rho-激酶活性后对阴茎勃起生理机制的影响;应用免疫转印技术检测 阴茎海绵体内RhoA和Rho激酶蛋白的表达。结果显示在大鼠海绵体组织内有内源性RhoA和Rho激酶蛋白的表达和存在;Y-27632海绵体内注射阻滞RhoA/Rho激酶活性使ICP和CCP/MAP比值明显升高;局部小剂量应用Y-27632对MAP没有明显影响;Y-27632可增强系列电刺激引起的由NO介导的CCP/MAP比值的增加;NO合成酶和鸟苷酸环化酶抑制剂的作用不能阻滞RhoA/Rho激酶抑制剂对大鼠阴茎海绵体平滑肌的松弛作用和CCP/MAP的增加。说明RhoA/Rho激酶信号系统发挥了维持海绵体萎软状态重要作用,这是与NO介导途径不同的阴茎勃起生理调节机制。RhoA/Rho激酶抑制剂可能是ED治疗的新领域新方法。 相似文献
97.
I. W. Althaus J. J. Chou A. J. Gonzales R. J. LeMay M. R. Deibel K. -C. Chou F. J. Kezdy D. L. Romero R. C. Thomas P. A. Aristoff W. G. Tarpley F. Reusser 《Cellular and molecular life sciences : CMLS》1994,50(1):23-28
The tetramer of ethylenesulfonic acid (U-9843) is a potent inhibitor of HIV-1 RT* and possesses excellent antiviral activity at nontoxic doses in HIV-1 infected lymphocytes grown in tissue culture. Kinetic studies of the HIV-1 RT-catalyzed RNA-directed DNA polymerase activity were carried out in order to determine if the inhibitor interacts with the template: primer or the deoxyribonucleotide triphosphate (dNTP) binding sites of the polymerase. Michaelis-Menten kinetics, which are based on the establishment of a rapid equilibrium between the enzyme and its substrates, proved inadequate for the analysis of the experimental data. The data were thus analyzed using steady-state Briggs-Haldane kinetics assuming that the template:primer binds to the enzyme first, followed by the binding of the dNTP and that the polymerase is a processive enzyme. Based on these assumptions, a velocity equation was derived which allows the calculation of all the specific forward and backward rate constants for the reactions occurring between the enzyme, its substrates and the inhibitor. The calculated rate constants are in agreement with this model and the results indicated that U-9843 acts as a noncompetitive inhibitor with respect to both the template:primer and dNTP binding sites. Hence, U-9843 exhibits the same binding affinity for the free enzyme as for the enzyme-substrate complexes and must inhibit the RT polymerase by interacting with a site distinct from the substrate binding sites. Thus, U-9843 appears to impair an event occurring after the formation of the enzyme-substrate complexes, which involves either an event leading up to the formation of the phosphoester bond, the formation of the ester bond itself or translocation of the enzyme relative to its template:primer following the formation of the ester bond. 相似文献
98.
The use of complexes of lymphocyte antigens with antibody as immunogens for the preparation of antilymphocytic sera 总被引:1,自引:0,他引:1
Résumé Des complexes antigène-anticorps préparées par réaction du sérum antilymphocytaire (ALS) avec des lymphocytes ou avec des broyats lymphocytaires furent injectées aux lapins. Les ALS obtenu ainsi des lapins furent immunodépresseurs. Cette technique peut être utilisée pour obtenir des ALS plus specifique et moins toxique.
Acknowlegments. We are indebted to our colleagues in the ALS Project Group of these Laboratories for much help and advice. 相似文献
Acknowlegments. We are indebted to our colleagues in the ALS Project Group of these Laboratories for much help and advice. 相似文献
99.
Recovery of immune system after cigarette smoking 总被引:7,自引:0,他引:7
100.