全文获取类型
收费全文 | 43417篇 |
免费 | 135篇 |
国内免费 | 157篇 |
专业分类
系统科学 | 331篇 |
丛书文集 | 869篇 |
教育与普及 | 82篇 |
理论与方法论 | 169篇 |
现状及发展 | 19255篇 |
研究方法 | 1666篇 |
综合类 | 20570篇 |
自然研究 | 767篇 |
出版年
2013年 | 306篇 |
2012年 | 591篇 |
2011年 | 1356篇 |
2010年 | 253篇 |
2008年 | 688篇 |
2007年 | 828篇 |
2006年 | 796篇 |
2005年 | 818篇 |
2004年 | 878篇 |
2003年 | 783篇 |
2002年 | 749篇 |
2001年 | 1268篇 |
2000年 | 1220篇 |
1999年 | 796篇 |
1992年 | 797篇 |
1991年 | 626篇 |
1990年 | 674篇 |
1989年 | 622篇 |
1988年 | 620篇 |
1987年 | 645篇 |
1986年 | 651篇 |
1985年 | 881篇 |
1984年 | 633篇 |
1983年 | 534篇 |
1982年 | 467篇 |
1981年 | 498篇 |
1980年 | 630篇 |
1979年 | 1383篇 |
1978年 | 1074篇 |
1977年 | 1032篇 |
1976年 | 861篇 |
1975年 | 918篇 |
1974年 | 1268篇 |
1973年 | 1076篇 |
1972年 | 1150篇 |
1971年 | 1309篇 |
1970年 | 1778篇 |
1969年 | 1390篇 |
1968年 | 1204篇 |
1967年 | 1246篇 |
1966年 | 1152篇 |
1965年 | 862篇 |
1964年 | 263篇 |
1959年 | 489篇 |
1958年 | 846篇 |
1957年 | 604篇 |
1956年 | 492篇 |
1955年 | 449篇 |
1954年 | 496篇 |
1948年 | 333篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
991.
P.A. Bretscher N. Ismail J.N. Menon C.A. Power J. Uzonna G. Wei 《Cellular and molecular life sciences : CMLS》2001,58(12-13):1879-1896
The occurrence of infectious disease represents a failure of the immune system, a failure that must be prevented by effective vaccination or remedied by treatment. Vaccination against acute diseases such as smallpox and polio are very effective, due to the rapid and increased immune response of vaccinated individuals upon natural infection. In contrast, effective vaccination against intracellular pathogens that cause chronic diseases, such as the leishmaniases, tuberculosis and AIDS, has not been achieved. Clinical observations suggest cell-mediated, Th1 responses, exclusive of antibody production and the generation of Th2 cells, are optimally protective against these intracellular pathogens. Effective vaccination must ensure the generation of such a protective response. We explore here whether understanding very broad features of the regulation of the immune response can accommodate modern findings on the immunological features of these diseases, and provide a perspective within which strategies for effective vaccination and treatment can be developed. 相似文献
992.
Michael R. Yeaman 《Cellular and molecular life sciences : CMLS》2010,67(4):525-544
Platelets interact with bacterial pathogens through a wide array of cellular and molecular mechanisms. The consequences of
this interaction may significantly influence the balance between infection and immunity. On the one hand, recent data indicate
that certain bacteria may be capable of exploiting these interactions to gain a virulence advantage. Indeed, certain bacterial
pathogens appear to have evolved specific ways in which to subvert activated platelets. Hence, it is conceivable that some
bacterial pathogens exploit platelet responses. On the other hand, platelets are now known to possess unambiguous structures
and functions of host defense effector cells. Recent discoveries emphasize critical features enabling such functions, including
expression of toll-like receptors that detect hallmark signals of bacterial infection, an array of microbicidal peptides,
as well as other host defense molecules and functions. These concepts are consistent with increased risk and severity of bacterial
infection as correlates of clinical abnormalities in platelet quantity and quality. In these respects, the molecular and cellular
roles of platelets in host defense against bacterial pathogens are explored with attention on advances in platelet immunobiology. 相似文献
993.
994.
Duane A. Tolle 《西北部美国博物学家》2011,37(4)
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} The relative abundance and habitat affinities of breeding and migratory waterfowl were documented for a 16,556 ha (40,880 acre) area of the Navajo Indian Reservation, San Juan County, New Mexico. The objective of the 1973 – 1974 study was to obtain baseline data for use as part of an environmental assessment for coal gasification plants and an associated strip mine proposed for an area not previously described in ornithological journals. Roadside breeding - bird surveys resulted in the observation of 26 species of birds, of which the horned lark was most abundant. Additional breeding - bird observations were made during afternoon reconnaissance surveys around stock ponds and arroyos, and by a helicopter survey of cliff - nesting raptors. Nine of 30 raptor nests located were active. Migratory waterfowl surveys were made in September and November at four stock ponds and a marsh on the study area. Peak waterfowl numbers were present in September, when teal and shovelers were the most abundant species. The importance of stock ponds to breeding and migratory birds and the significance of cliffs to nesting raptors were discussed. Details of observations made for five species of birds designated ";threatened"; or ";status undetermined"; were presented. 相似文献
995.
Streambed surveys were conducted along the upper Colorado River, Colorado, to describe the distributions of Claassenia sabulosa larvae in relation to current speed and to determine their diets. We also addressed diel feeding periodicity by sampling during both day and night. Claassenia sabulosa was more abundant in riffle habitats than in runs. A positive relationship existed between C. sabulosa abundance and stream current, with larval size increasing with current speed. Chironomidae, Baetidae, and Simulidae collectively accounted for 93% of the prey found in stonefly guts; however, these categories were not consumed equally by all C. sabulosa . Smaller C. sabulosa primarily ate chironomids, and larger individuals consumed more baetids. Only a slight difference existed in the percentage of empty guts between night- and day-collected stoneflies, and ranges of prey per gut at night were higher than those in the day, suggesting that these stoneflies may forage more intensively at night. 相似文献
996.
Karina Weinhold Udo Krause-Buchholz Gerhard Rödel Michael Kasper Kathrin Barth 《Cellular and molecular life sciences : CMLS》2010,67(15):2631-2642
P2X4 and P2X7 receptors are ATP-gated ion channels that are co-expressed in alveolar epithelial type I cells. Both receptors
are localized to the plasma membrane and partly associated with lipid rafts. Here we report on our study in an alveolar epithelial
cell line of the molecular organization of P2X7R and P2X4R receptors and the effect of their knockdown. Native gel electrophoresis
reveals three P2X7R complexes of ~430, ~580 and ~760 kDa. The latter two correspond exactly in size to signals of Cav-1, the
structural protein of caveolae. Interestingly knockdown of P2rx7 affects protein levels, the intracellular distribution and the supramolecular organization of Cav-1 as well as of P2X4R,
which is mainly detected in a complex of ~430 kDa. Our data suggest upregulation of P2X4R as a compensatory mechanism of P2X7R
depletion. 相似文献
997.
There is substantial evidence that the martian volatile inventory and climate have changed markedly throughout the planet's history. Clues come from areas as disparate as the history and properties of the deep interior, the composition of the crust and regolith, the morphology of the surface, composition of the present-day atmosphere, and the nature of the interactions between the upper atmosphere and the solar wind. We piece together the relevant observations into a coherent view of the evolution of the martian climate, focusing in particular on the observations that provide the strongest constraints. 相似文献
998.
夏提姑丽·阿不都热依木 《中国西部科技》2009,8(15):47-48
公路基本建设事业的快速发展,对公路养护工程提出了更高的要求,公路养护不断遇到新的难题,面对困难和挑战。文章结合本地区公路养护实际工作实践,对公路养护管理存在的问题进行了分析,并探讨了相应对策。 相似文献
999.
本文建立了同时考虑轴颈径向位移和歪斜时的滑动轴承动力学模型.对于这种模型,油膜力将扩展为四个广义力分量.文章将相应的动力系数矩阵分解为对称和反对称两部分并讨论其物理性质.然后,本文推导得出该模型的正交变换矩阵,在此基础上导得各向同性的动力系数矩阵表达式,并讨论了诸元素的物理意义.文章最后引入动力系数椭圆的概念以分析非对称动力系数矩阵的变换性质. 相似文献
1000.
Membrane depolarization causes many kinds of ion channels to open, a process termed activation. For both Na+ channels and Ca2+ channels, kinetic analysis of current has suggested that during activation the channel undergoes several conformational changes before reaching the open state. Structurally, these channels share a common motif: the central element is a large polypeptide with four repeating units of homology (repeats I-IV), each containing a voltage-sensing region, the S4 segment. This suggests that the distinct conformational transitions inferred from kinetic analysis may be equated with conformational changes of the individual structural repeats. To investigate the molecular basis of channel activation, we constructed complementary DNAs encoding chimaeric Ca2+ channels in which one or more of the four repeats of the skeletal muscle dihydropyridine receptor are replaced by the corresponding repeats derived from the cardiac dihydropyridine receptor. We report here that repeat I determines whether the chimaeric Ca2+ channel shows slow (skeletal muscle-like) or rapid (cardiac-like) activation. 相似文献