全文获取类型
收费全文 | 2562篇 |
免费 | 83篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2021年 | 22篇 |
2018年 | 69篇 |
2017年 | 69篇 |
2016年 | 72篇 |
2015年 | 89篇 |
2014年 | 98篇 |
2013年 | 125篇 |
2012年 | 129篇 |
2011年 | 117篇 |
2010年 | 110篇 |
2009年 | 71篇 |
2008年 | 39篇 |
2007年 | 39篇 |
2006年 | 37篇 |
2005年 | 143篇 |
2004年 | 108篇 |
2003年 | 83篇 |
2002年 | 24篇 |
2001年 | 31篇 |
1999年 | 21篇 |
1998年 | 21篇 |
1996年 | 27篇 |
1995年 | 30篇 |
1994年 | 22篇 |
1993年 | 25篇 |
1992年 | 22篇 |
1991年 | 34篇 |
1990年 | 26篇 |
1988年 | 28篇 |
1987年 | 24篇 |
1986年 | 25篇 |
1985年 | 28篇 |
1984年 | 37篇 |
1983年 | 41篇 |
1982年 | 28篇 |
1981年 | 31篇 |
1980年 | 32篇 |
1979年 | 59篇 |
1978年 | 31篇 |
1977年 | 24篇 |
1976年 | 24篇 |
1975年 | 35篇 |
1974年 | 33篇 |
1973年 | 31篇 |
1972年 | 31篇 |
1971年 | 21篇 |
1970年 | 24篇 |
1969年 | 35篇 |
1966年 | 23篇 |
排序方式: 共有2646条查询结果,搜索用时 15 毫秒
941.
Büch TR Biebermann H Kalwa H Pinkenburg O Hager D Barth H Aktories K Breit A Gudermann T 《The Journal of biological chemistry》2008,283(29):20330-20341
Stimulation of the thyrotropin receptor (TSHR) activates G proteins of all four subfamilies (G(s), G(i/o), G(q/11), and G(12/13)). Whereas G(s)/cAMP-dependent cellular responses upon TSHR stimulation are well established, other signaling pathways are less characterized. We evaluated TSH-elicited cellular responses in human follicular thyroid carcinoma cells stably expressing the TSHR and in primary, nonneoplastic human thyrocytes. In these cellular models, stimulation with TSH caused activation of p44/42 MAPK and subsequent induction of c-Fos. MAPK stimulation occurred independently of G(s), G(i/o), and G(q/11) signaling. Dominant negative constructs of G(12) or G(13) as well as shRNA-mediated suppression of Galpha(12) or Galpha(13) revealed that MAPK activation was dependent on G(13) but not on G(12) signaling. Furthermore, G(13)-dependent transactivation of the epidermal growth factor receptor was necessary for MAPK activation in follicular carcinoma cells, whereas EGFR was not involved in MAPK activation in nonneoplastic primary thyrocytes. The use of bacterial inhibitors of monomeric GTPases revealed that MAPK activation proceeded independently of Rho proteins but was clostridial toxin B-sensitive, suggesting involvement of Cdc42 or Rac. Thus, our data shed new light on cAMP-independent TSHR signaling and identify the first G(13)-dependent TSHR signaling pathway in human thyrocytes. 相似文献
942.
The monotonous cordaitalean leaves are usually difficult to determine as leaf shape and venation can be similar in many species. Therefore cuticular analysis is an important method for distinguishing cordaitalean leaves. Pennsylvanian Cordaites schatzlarensis nov. sp. comes from the ?aclé? locality in the Intrasudetic Basin, Czech Republic. It has been found in mudstone accompanying the upper coal seams No. 9 and 10 of the Jan Šverma Coals, Lampertice Member, ?aclé? Formation and are late Duckmantian in age. The leaves of Cordaites schatzlarensis are lanceolate, amphistomatic. Stomata of the adaxial epidermis are scarce, isolated, or in very short stomatal rows. In contrast, the density of stomata on the abaxial cuticle is high and stomata are arranged into single or double stomatal rows. The cuticles of C. schatzlarensis are comparable with the Chinese Upper Permian C. baodeensis Ge. Leaves can be narrow, comparable to French Bolsovian Dorycordaites zeilleri Ledran, or relatively wide. Accompanying big seeds more than 5 cm in diameter are attributed to Samaropsis newberryi (Andrews) Seward. The pith casts Artisia Sternberg were found in sandy channel fill deposits. 相似文献
943.
Petra Matějková Michal Strouhal David Šmajs Steven J Norris Timothy Palzkill Joseph F Petrosino Erica Sodergren Jason E Norton Jaz Singh Todd A Richmond Michael N Molla Thomas J Albert George M Weinstock 《BMC microbiology》2008,8(1):1-12
Background
Probiotics such as bifidobacteria have been shown to maintain a healthy intestinal microbial balance and help protect against infections. However, despite these benefits, bifidobacteria still remain poorly understood at the biochemical, physiological and especially the genetic level. Herein we describe, for the first time, the development of a non-invasive luciferase-based reporter system for real-time tracking of Bifidobacterium species in vivo.Results
The reporter vector pLuxMC1 is based on the recently described theta-type plasmid pBC1 from B. catenatulatum [1] and the luxABCDE operon from pPL2lux [2]. Derivatives of pLuxMC1, harbouring a bifidobacterial promoter (pLuxMC2) as well as a synthetically derived promoter (pLuxMC3) [3] placed upstream of luxABCDE, were constructed and found to stably replicate in B. breve UCC2003. The subsequent analysis of these strains allowed us to assess the functionality of pLuxMC1 both in vitro and in vivo.Conclusion
Our results demonstrate the potential of pLuxMC1 as a real-time, non-invasive reporter system for Bifidobacterium. It has also allowed us, for the first time, to track the colonisation potential and persistence of this probiotic species in real time. An interesting and significant outcome of the study is the identification of the caecum as a niche environment for B. breve UCC2003 within the mouse gastrointestinal tract (GI) tract. 相似文献944.
Doubravská L Símová S Cermak L Valenta T Korínek V Andera L 《Apoptosis : an international journal on programmed cell death》2008,13(4):573-587
Wnt signaling enhances cell proliferation and the maintenance of hematopoietic cells. In contrast, cytotoxic ligand Apo2L/TRAIL
induces the apoptosis of various transformed cells. We observed that co-culture of human pre-B leukemia cells KM3 and REH
with Wnt1- or Wnt3a-producing rat embryonic fibroblasts efficiently suppressed Apo2L/TRAIL-induced apoptosis of the lymphoid
cells. This suppression occurs at the early stages of the Apo2L/TRAIL apoptotic cascade and, interestingly, the activation
of the Wnt pathway alone in human leukemia cells is not sufficient for their full anti-apoptotic protection. We hypothesize
that a stimulus emanating specifically from Wnt1- or Wnt3a-expressing rat fibroblasts is responsible for the observed resistance
to Apo2L/TRAIL. This anti-apoptotic signaling was significantly hampered by the inhibition of the MEK1/ERK1/2 or NFκB pathways
in KM3 and REH cells. Our results imply that paracrine Wnt-related signals could be important for the survival of pre-B cell-derived
malignancies.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
L. Doubravská and Š. Šímová are joint first authors. V. Kořínek and L. Anděra are senior co-authors. 相似文献
945.
Comparing the rate of invasion by Heracleum mantegazzianum at continental, regional, and local scales 总被引:1,自引:1,他引:0
Petr Pyek Vojtêch Jaroík Jana Müllerová Jan Pergl Jan Wild 《Diversity & distributions》2008,14(2):355-363
This paper compares the rate of invasion of Heracleum mantegazzianum (Apiaceae), a Caucasian species invading Europe, at three spatial scales (continental, regional, and local). The rate of invasion was evaluated using inclusion curves, by plotting the cumulative number of invaded countries against time on the continental scale of Europe, number of occupied grid cells at the regional scale of the Czech Republic, and invaded area inferred from a series of aerial photographs taken at the local scale over a period of 49 years in the Slavkovký les region, Czech Republic. Time of 50% inclusion (with 95% confidence intervals, CI) of invaded countries, occupied grid cells, and invaded area was assessed. The invasion was slowest at the continental scale (62 years, CI = 53–70) and did not differ significantly between regional (16 years, CI = 10–20) and local (22 years, CI = 19–24) scales. Our results indicate that there are two different mechanisms of spread acting together in this system, namely human influences and natural spread, and the relative influence of these mechanisms appears to change in an inverse proportion from the largest to the smallest scale. At the local scale, under suitable habitat conditions, the process is driven by biological traits of the species related to dispersal. At the continental and regional scales, humans played a crucial role in the invasion of H. mantegazzianum by planting it as a garden ornamental. At these scales, human-mediated dispersal seems to have been the major driver of spread, responsible for creating dispersal foci in the initial phases of invasion. Species traits played an important role in local spread, resulting in the colonization of new sites. 相似文献
946.
David Kub
Ondej Kaplan Veronika Elikov Miroslav Ptek Vojtch Vejvoda Kristýna Slmov Andrea Tthov Marielle Lemaire Estelle Gallienne Sabine Lutz-Wahl Lutz Fischer Marek Kuzma Helena Pelantov Sander van Pelt Jean Bolte Vladimír Ken Ludmila Martínkov 《Journal of Molecular Catalysis .B, Enzymatic》2008,50(2-4):107-113
A semi-purified nitrile hydratase from Rhodococcus erythropolis A4 was applied to biotransformations of 3-oxonitriles 1a–4a, 3-hydroxy-2-methylenenitriles 5a–7a, 4-hydroxy-2-methylenenitriles 8a–9a, 3-hydroxynitriles 10a–12a and 3-acyloxynitrile 13a into amides 1b–13b. Cross-linked enzyme aggregates (CLEAs) with nitrile hydratase and amidase activities (88% and 77% of the initial activities, respectively) were prepared from cell-free extract of this microorganism and used for nitrile hydration in presence of ammonium sulfate, which selectively inhibited amidase activity. The genes nha1 and nha2 coding for and β subunits of nitrile hydratase were cloned and sequenced. 相似文献
947.
Invasion of unsown species to artificially created assemblages of grassland species was investigated in a 3-year field experiment.
In the experiment, assemblages varying in species richness (1, 2, 4, 8, and 16 species) and functional group richness (1–4,
grasses, legumes, rosette forbs, and creeping forbs) were grown in control and fertilized plots, without any attempt to prevent
the invasion of unsown species or to weed them. The relationship between species and functional group diversity and above-ground
biomass was positive for sown species in all study years (2003, 2004 and 2005). In the latter 2 years, weed invader biomass
decreased significantly with increasing biomass of sown species and their functional group richness, but not with number of
species. However, no suppressive effect of species or functional group richness beyond that by increased biomass of residents
was found. In fact, slight but significant positive partial effect of species richness was found, suggesting that the negative
effect of the same amount of resident biomass on invaders is stronger when the biomass is composed of fewer species. The negative
relationship between the number of functional groups of residents and invader biomass suggested that better coverage of functional
trait space could be a mechanism promoting the resistance to invasion. In Addition, species composition of invaders were significantly
related to initial composition of sown residents. 相似文献
948.
Mohamed Niang Pavel Tomšík Milan Mělka Alena Stoklasová Jaroslav Cerman Ludìk Šišpera 《Central European Journal of Biology》2008,3(3):268-272
Various experiments were performed in our laboratory to define a possible role for carnitine derivatives in mitoxantrone (MX)
therapy. We report here the results of the effect of L-carnitine (LCAR) on the lethal toxicity of MX in mice. MX was administered
intravenously at doses of 7, 9, 11, 13, 15 or 17 mg kg−1 either alone or in combination with LCAR at a single intravenous dose of 200 mg kg−1. The dependence of the probability of death on various doses was evaluated for the MX-LCAR combination compared to MX alone.
From these experiments, the following lethal dose fifty (LD50) values were calculated: LD50 for MX alone was 15.2 mg kg−1, whereas in combination with LCAR it increases to 21.8 mg kg−1. The relative toxicity given as the ratio of the LD50 of both MX alone and the combination of MX-LCAR was 69.7%. The results
of our experiments unequivocally show the effect of LCAR on acute toxic doses of MX. 相似文献
949.
950.
Alena Bartoňová Vojtěch Kolář Jana Marešová Martina Šašić Jana Šlancarová Pavel Sucháček Martin Konvička 《Journal of Insect Conservation》2017,21(3):559-571
A characteristic butterfly of Asian steppes, Proterebia afra (Fabricius 1787), was studied in its two relic areas of occurrence within the Balkans—the Askion Mts (a single mountain massif in NW Greece) and Dalmatia (karstic inland S Croatia)—together with co-occurring butterfly communities during its early spring adult flight period. P. afra adults tolerate harsh continental conditions prevailing at its localities during its flight. Local densities were higher in the Askion Mts (up to 100 individuals per 50 m walk) than in Dalmatia (up to 30 individuals per 50 m). Within both areas of occurrence, it inhabits dry grasslands subject to intermediate grazing pressure that retains enough dry grass litter but still suppresses shrubs and trees. Hotter slopes are preferred in more northerly Dalmatia than in the more southerly Askion Mts. More co-occurring butterfly species and more Mediterranean species were recorded in the Askion Mts (69 vs 63; 23 vs 12), but more butterfly individuals and higher species’ richness per transect were observed in Dalmatia. In ordination analyses, the main gradients organising the grassland butterfly communities distinguished between grassy plains and rocky slopes containing more woody plants. Given the distribution extents, local densities and present land use conditions, the Balkan P. afra is not currently endangered, but this may swiftly change with grazing abandonment, especially in Dalmatia, and some conservation actions should be considered in the near future. 相似文献