全文获取类型
收费全文 | 17286篇 |
免费 | 25篇 |
国内免费 | 67篇 |
专业分类
系统科学 | 123篇 |
丛书文集 | 288篇 |
教育与普及 | 43篇 |
理论与方法论 | 63篇 |
现状及发展 | 7398篇 |
研究方法 | 908篇 |
综合类 | 8292篇 |
自然研究 | 263篇 |
出版年
2013年 | 112篇 |
2012年 | 298篇 |
2011年 | 566篇 |
2010年 | 104篇 |
2009年 | 99篇 |
2008年 | 316篇 |
2007年 | 360篇 |
2006年 | 376篇 |
2005年 | 376篇 |
2004年 | 376篇 |
2003年 | 323篇 |
2002年 | 320篇 |
2001年 | 609篇 |
2000年 | 571篇 |
1999年 | 388篇 |
1992年 | 325篇 |
1991年 | 264篇 |
1990年 | 279篇 |
1989年 | 253篇 |
1988年 | 235篇 |
1987年 | 280篇 |
1986年 | 291篇 |
1985年 | 319篇 |
1984年 | 280篇 |
1983年 | 222篇 |
1982年 | 182篇 |
1981年 | 195篇 |
1980年 | 249篇 |
1979年 | 566篇 |
1978年 | 432篇 |
1977年 | 425篇 |
1976年 | 314篇 |
1975年 | 349篇 |
1974年 | 515篇 |
1973年 | 437篇 |
1972年 | 428篇 |
1971年 | 519篇 |
1970年 | 688篇 |
1969年 | 484篇 |
1968年 | 399篇 |
1967年 | 487篇 |
1966年 | 398篇 |
1965年 | 282篇 |
1959年 | 162篇 |
1958年 | 268篇 |
1957年 | 207篇 |
1956年 | 180篇 |
1955年 | 144篇 |
1954年 | 150篇 |
1948年 | 126篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere 总被引:15,自引:0,他引:15
Oren R Ellsworth DS Johnsen KH Phillips N Ewers BE Maier C Schäfer KV McCarthy H Hendrey G McNulty SG Katul GG 《Nature》2001,411(6836):469-472
Northern mid-latitude forests are a large terrestrial carbon sink. Ignoring nutrient limitations, large increases in carbon sequestration from carbon dioxide (CO2) fertilization are expected in these forests. Yet, forests are usually relegated to sites of moderate to poor fertility, where tree growth is often limited by nutrient supply, in particular nitrogen. Here we present evidence that estimates of increases in carbon sequestration of forests, which is expected to partially compensate for increasing CO2 in the atmosphere, are unduly optimistic. In two forest experiments on maturing pines exposed to elevated atmospheric CO2, the CO2-induced biomass carbon increment without added nutrients was undetectable at a nutritionally poor site, and the stimulation at a nutritionally moderate site was transient, stabilizing at a marginal gain after three years. However, a large synergistic gain from higher CO2 and nutrients was detected with nutrients added. This gain was even larger at the poor site (threefold higher than the expected additive effect) than at the moderate site (twofold higher). Thus, fertility can restrain the response of wood carbon sequestration to increased atmospheric CO2. Assessment of future carbon sequestration should consider the limitations imposed by soil fertility, as well as interactions with nitrogen deposition. 相似文献
172.
Hunt MC Greene S Hultenby K Svensson LT Engberg S Alexson SE 《Cellular and molecular life sciences : CMLS》2007,64(12):1558-1570
Acyl-CoA thioesterases (ACOTs) catalyze the hydrolysis of acyl-CoAs to free fatty acids and coenzyme A. Recent studies have
demonstrated that one gene named Acot7, reported to be mainly expressed in brain and testis, is transcribed in several different isoforms by alternative usage of
first exons. Strongly decreased levels of ACOT7 activity and protein in both mitochondria and cytosol was reported in patients
diagnosed with fatty acid oxidation defects, linking ACOT7 function to regulation of fatty acid oxidation in other tissues.
In this study, we have identified five possible first exons in mouse Acot7 (Acot7a–e) and show that all five first exons are transcribed in a tissue-specific manner. Taken together, these data show that the
Acot7 gene is expressed as multiple isoforms in a tissue-specific manner, and that expression in tissues other than brain and testis
is likely to play important roles in fatty acid metabolism.
Received 5 February 2007: received after revision 3 April 2007; accepted 19 April 2007 相似文献
173.
Infection of bacteria triggers innate immune defense reactions in Drosophila. So far, the only bacterial component known to be recognized by the insect innate immune system is peptidoglycan, one of
the most abundant constituents of the bacterial cell wall. Insects use peptidoglycan recognition proteins to detect peptidoglycan
and to activate innate immune responses. Such specialized peptidoglycan receptors appear to have evolved from phage enzymes
that hydrolyze bacterial cell walls. They are able to bind specific peptidoglycan molecules with distinct chemical moieties
and activate innate immune pathways by interacting with other signaling proteins. Recent X-ray crystallographic studies of
the peptidoglycan recognition proteins LCa, and LCx bound to peptidoglycan have provided structural insights into recognition
of peptidoglycan and activation of innate immunity in insects.
Received 28 December 2006; received after revision 2 February 2007; accepted 21 February 2007 相似文献
174.
Strell C Lang K Niggemann B Zaenker KS Entschladen F 《Cellular and molecular life sciences : CMLS》2007,64(24):3306-3316
The extravasation of leukocytes and tumor cells is a multi-step process with the involvement of various adhesion molecules
depending on the three steps rolling, adhesion, and diapedesis. We have developed an in vitro model, by which we investigated the rolling and adhesion of neutrophil granulocytes and MDA-MB-468 human breast carcinoma
cells to lung endothelial cells under physiological flow-conditions. We found that norepinephrine had an inhibitory function
on the fMLP-promoted adhesion of neutrophil granulocytes due to a down-regulation of β2-integrin. Furthermore, neutrophil
granulocytes serve as linking cells for the interaction of the MDA-MB-468 cells with the endothelium, which are both β2-integrin
negative, but express the β2-integrin ligand ICAM-1. In addition, we show here that N-cadherin is up-regulated on the endothelial
cells and on neutrophil granulocytes in response to fMLP. This up-regulation resulted in a significant increase of adherent
MDA-MB-468 cells, which are also N-cadherin positive.
Received 3 September 2007; received after revision 17 October 2007; accepted 22 October 2007 相似文献
175.
The study of candidate genes over the past three decades has yielded notable successes in common-disease genetics. During
this time, however, interpretation of genetic association studies has been hampered by the use of clinical cohorts of inadequate
power and insufficient information on genetic variation in candidate genes. The unavailability of highthroughput and low-cost
genotyping technologies has also limited the scope of complex-disease genetic studies. More recently, however, the sequencing
and characterization of variation within the human genome has revolutionized genetic studies and enabled full genome-wide
scans for genes associated with disease. The identification of disease-associated (causative) genes has illuminated disease
mechanisms. The translation of this knowledge into direct clinical benefit in diagnosis, prognosis and therapy for an individual’s
disease still remains a challenge.
Received 11 September 2006; received after revision 17 December 2006; accepted 18 January 2007 相似文献
176.
A variety of viral-based and immune cell therapies have been proposed for use in the treatment of cancer. One possible approach
to improve the effectiveness of these biological agents may be to combine them such that we can take advantage of natural
immune cell-pathogen relationships. Here we discuss these potential approaches with particular emphasis on the use of immune
cells as carrier vehicles to deliver viral therapies to the tumor.
Received 15 December 2006; received after revision 28 January 2007; accepted 5 March 2007 相似文献
177.
Komen JC Distelmaier F Koopman WJ Wanders RJ Smeitink J Willems PH 《Cellular and molecular life sciences : CMLS》2007,64(24):3271-3281
Refsum disease is a rare, inherited neurodegenerative disorder characterized by accumulation of the dietary branched-chain
fatty acid phytanic acid in plasma and tissues caused by a defect in the alphaoxidation pathway. The accumulation of phytanic
acid is believed to be the main pathophysiological cause of the disease. However, the exact mechanism(s) by which phytanic
acid exerts its toxicity have not been resolved. In this study, the effect of phytanic acid on mitochondrial respiration was
investigated. The results show that in digitonin-permeabilized fibroblasts, phytanic acid decreases ATP synthesis, whereas
substrate oxidation per se is not affected. Importantly, studies in intact fibroblasts revealed that phytanic acid decreases both the mitochondrial
membrane potential and NAD(P)H autofluorescence. Taken together, the results described here show that unesterified phytanic
acid exerts its toxic effect mainly through its protonophoric action, at least in human skin fibroblasts.
Received 4 August 2007; received after revision 26 September 2007; accepted 10 October 2007
J. C. Komen, F. Distelmaier: These authors contributed equally to this work. 相似文献
178.
Samuel CS Hewitson TD Unemori EN Tang ML 《Cellular and molecular life sciences : CMLS》2007,64(12):1539-1557
The peptide hormone relaxin is emerging as a multi-functional factor in a broad range of target tissues including several
non-reproductive organs, in addition to its historical role as a hormone of pregnancy. This review discusses the evidence
that collectively demonstrates the many diverse and vital roles of relaxin: the homeostatic role of endogenous relaxin in
mammalian pregnancy and ageing; its gender-related effects; the therapeutic effects of relaxin in the treatment of fibrosis,
inflammation, cardioprotection, vasodilation and wound healing (angiogenesis) amongst other pathophysiological conditions,
and its potential mechanism of action. Furthermore, translational issues using experimental models (to humans) and its use
in various clinical trials, are described, each with important lessons for the design of future trials involving relaxin.
The diverse physiological and pathological roles for relaxin have led to the search for its significance in humans and highlight
its potential as a drug of the future.
Received 12 December 2006; received after revision 12 February 2007; accepted 15 March 2007 相似文献
179.
Albert-Weissenberger C Cazalet C Buchrieser C 《Cellular and molecular life sciences : CMLS》2007,64(4):432-448
The bacterial pathogen Legionella pneumophila is found ubiquitously in fresh water environments where it replicates within protozoan hosts. When inhaled by humans it can
replicate within alveolar macrophages and cause a severe pneumonia, Legionnaires disease. Yet much needs to be learned regarding
the mechanisms that allow Legionella to modulate host functions to its advantage and the regulatory network governing its intracellular life cycle. The establishment
and publication of the complete genome sequences of three clinical L. pneumophila isolates paved the way for major breakthroughs in understanding the biology of L. pneumophila. Based on sequence analysis many new putative virulence factors have been identified foremost among them eukaryotic-like
proteins that may be implicated in many different steps of the Legionella life cycle. This review summarizes what is currently known about regulation of the Legionella life cycle and gives insight in the Legionella-specific features as deduced from genome analysis.
Received 1 September 2006; received after revision 10 October 2006; accepted 22 November 2006 相似文献
180.