全文获取类型
收费全文 | 38474篇 |
免费 | 64篇 |
国内免费 | 95篇 |
专业分类
系统科学 | 200篇 |
丛书文集 | 862篇 |
教育与普及 | 107篇 |
理论与方法论 | 239篇 |
现状及发展 | 17162篇 |
研究方法 | 1548篇 |
综合类 | 18014篇 |
自然研究 | 501篇 |
出版年
2013年 | 253篇 |
2012年 | 515篇 |
2011年 | 1047篇 |
2010年 | 230篇 |
2008年 | 660篇 |
2007年 | 680篇 |
2006年 | 715篇 |
2005年 | 720篇 |
2004年 | 632篇 |
2003年 | 680篇 |
2002年 | 695篇 |
2001年 | 1122篇 |
2000年 | 1034篇 |
1999年 | 689篇 |
1992年 | 657篇 |
1991年 | 543篇 |
1990年 | 572篇 |
1989年 | 581篇 |
1988年 | 584篇 |
1987年 | 587篇 |
1986年 | 592篇 |
1985年 | 738篇 |
1984年 | 570篇 |
1983年 | 487篇 |
1982年 | 414篇 |
1981年 | 407篇 |
1980年 | 533篇 |
1979年 | 1202篇 |
1978年 | 1034篇 |
1977年 | 1020篇 |
1976年 | 719篇 |
1975年 | 769篇 |
1974年 | 1154篇 |
1973年 | 973篇 |
1972年 | 1014篇 |
1971年 | 1225篇 |
1970年 | 1611篇 |
1969年 | 1239篇 |
1968年 | 1214篇 |
1967年 | 1229篇 |
1966年 | 1029篇 |
1965年 | 746篇 |
1964年 | 208篇 |
1959年 | 445篇 |
1958年 | 685篇 |
1957年 | 574篇 |
1956年 | 488篇 |
1955年 | 414篇 |
1954年 | 458篇 |
1948年 | 282篇 |
排序方式: 共有10000条查询结果,搜索用时 390 毫秒
821.
Yvonne G. J. van Helden Roger W. L. Godschalk Hans J. M. Swarts Peter C. H. Hollman Frederik J. van Schooten Jaap Keijer 《Cellular and molecular life sciences : CMLS》2011,68(3):489-504
Molecular mechanisms triggered by high dietary beta-carotene (BC) intake in lung are largely unknown. We performed microarray
gene expression analysis on lung tissue of BC supplemented beta-carotene 15,15′-monooxygenase 1 knockout (Bcmo1
−/−) mice, which are—like humans—able to accumulate BC. Our main observation was that the genes were regulated in an opposite direction in male and female Bcmo1
−/− mice by BC. The steroid biosynthetic pathway was overrepresented in BC-supplemented male Bcmo1
−/− mice. Testosterone levels were higher after BC supplementation only in Bcmo1
−/− mice, which had, unlike wild-type (Bcmo1
+/+) mice, large variations. We hypothesize that BC possibly affects hormone synthesis or metabolism. Since sex hormones influence
lung cancer risk, these data might contribute to an explanation for the previously found increased lung cancer risk after
BC supplementation (ATBC and CARET studies). Moreover, effects of BC may depend on the presence of frequent human BCMO1 polymorphisms, since these effects were not found in wild-type mice. 相似文献
822.
The skin is our primary shield against microbial pathogens and has evolved innate and adaptive strategies to enhance immunity
in response to injury or microbial insult. The study of antimicrobial peptide (AMP) production in mammalian skin has revealed
several of the elegant strategies that AMPs use to prevent infection. AMPs are inducible by both infection and injury and
protect the host by directly killing pathogens and/or acting as multifunctional effector molecules that trigger cellular responses
to aid in the anti-infective and repair response. Depending on the specific AMP, these molecules can influence cytokine production,
cell migration, cell proliferation, differentiation, angiogenesis and wound healing. Abnormal production of AMPs has been
associated with the pathogenesis of several cutaneous diseases and plays a role in determining a patient’s susceptibility
to pathogens. This review will discuss current research on the regulation and function of AMPs in the skin and in skin disorders. 相似文献
823.
Good MF 《Cellular and molecular life sciences : CMLS》2011,68(7):1105-1113
Malaria presents a challenge to world health that to date has been beyond the abilities of researchers to conquer. This critique
presents some of the strategies employed by the parasite to overcome immunity and the immunological challenges that we face
to develop vaccines. A conclusion is that a vaccine must identify novel antigens or epitopes that are not normally immunogenic
and which are therefore not under immune pressure and most likely to be conserved between different strains. Such antigens
are most likely to be targets of cellular immunity. The case for a whole parasite blood stage vaccine is presented based on
these premises. 相似文献
824.
F.A.Zhang L.Chen J.Q.Ma X.Yang 《复旦学报(自然科学版)》2007,46(5)
1 Results Nanocomposite has attracted more and more interest all over the world.Polystyrene (PS) is a commercialized and mass-productive polymer,continuous research efforts have been devoted to the development of polystyrene/montmorillonite (PS/MMT) nanocomposites[1-2].But the polarity of styrene (St) is too small to intercalate the space between the clay layers.The polarity of hydrophilic monomer is so strong that it can intercalate the MMT easily,the intercalated smectic clays maybe exfoliated by usin... 相似文献
825.
826.
827.
Jérémie Gautheron Gilles Courtois 《Cellular and molecular life sciences : CMLS》2010,67(18):3101-3113
Ubiquitination has emerged over the years as the most sophisticated way to modify proteins to affect their fate and function.
In particular, it has been reported to be instrumental in regulating several steps of the NF-κB signalling pathway which controls
inflammation, immunity, adhesion and cell survival. Integrating ubiquitination into NF-κB activation requires the regulatory
subunit of IKK, NEMO, which not only displays affinity for polyubiquitin chains, but is also posttranslationally modified
by a complex set of reactions involving ubiquitin. Here, we examine how studies of the NEMO/ubiquitin relationship have provided
novel insights into the IKK activation process and have uncovered molecular mechanisms that should represent in the future
attractive targets for specifically modulating NF-κB function. 相似文献
828.
桥梁结构在各种荷载作用下产生的内力和弯矩影响着桥梁的性能.桥梁弯曲、扭转和挠度等过大时会导致结构的破坏,这在结构设计阶段就应加以分析.本文提出一种有限条简化分析方法,可以较好地分析桥梁的挠度、弯曲和扭转变形.通过对一座简支板梁桥模型的分析,与现行有限元方法的分析结果进行了比较.结果表明,本方法能够较好地处理边界条件,只需要很少的输入工作就可以得到正确的分析结果,从而为结构工程师提供了一种用于分析板梁桥在交通荷载作用下行为的有效工具. 相似文献
829.
Transthyretin (formerly called prealbumin) plays important physiological roles as a transporter of thyroxine and retinol-binding
protein. X-ray structural studies have provided information on the active conformation of the protein and the site of binding
of both ligands. Transthyretin is also one of the precursor proteins commonly found in amyloid deposits. Both wild-type and
single-amino-acid-substituted variants have been identified in amyloid deposits, the variants being more amyloidogenic. Sequencing
of the gene and the resulting production of a transgenic mouse model have resulted in progress toward solving the mechanism
of amyloid formation and detecting the variant gene in individuals at risk.
Received 23 January 2001; received after revision 4 April 2001; accepted 30 April 2001 相似文献
830.
E A Donley N R Claussen S L Cornish J L Roberts E A Cornell C E Wieman 《Nature》2001,412(6844):295-299
When atoms in a gas are cooled to extremely low temperatures, they will-under the appropriate conditions-condense into a single quantum-mechanical state known as a Bose-Einstein condensate. In such systems, quantum-mechanical behaviour is evident on a macroscopic scale. Here we explore the dynamics of how a Bose-Einstein condensate collapses and subsequently explodes when the balance of forces governing its size and shape is suddenly altered. A condensate's equilibrium size and shape is strongly affected by the interatomic interactions. Our ability to induce a collapse by switching the interactions from repulsive to attractive by tuning an externally applied magnetic field yields detailed information on the violent collapse process. We observe anisotropic atom bursts that explode from the condensate, atoms leaving the condensate in undetected forms, spikes appearing in the condensate wavefunction and oscillating remnant condensates that survive the collapse. All these processes have curious dependences on time, on the strength of the interaction and on the number of condensate atoms. Although the system would seem to be simple and well characterized, our measurements reveal many phenomena that challenge theoretical models. 相似文献