全文获取类型
收费全文 | 110169篇 |
免费 | 2011篇 |
国内免费 | 815篇 |
出版年
2021年 | 263篇 |
2020年 | 196篇 |
2019年 | 273篇 |
2018年 | 12116篇 |
2017年 | 10889篇 |
2016年 | 7921篇 |
2015年 | 1400篇 |
2014年 | 1182篇 |
2013年 | 1369篇 |
2012年 | 5503篇 |
2011年 | 13930篇 |
2010年 | 12755篇 |
2009年 | 8855篇 |
2008年 | 10713篇 |
2007年 | 12213篇 |
2006年 | 1087篇 |
2005年 | 1256篇 |
2004年 | 1747篇 |
2003年 | 1671篇 |
2002年 | 1368篇 |
2001年 | 601篇 |
2000年 | 476篇 |
1999年 | 296篇 |
1998年 | 166篇 |
1997年 | 160篇 |
1996年 | 117篇 |
1995年 | 111篇 |
1994年 | 134篇 |
1993年 | 144篇 |
1992年 | 214篇 |
1991年 | 232篇 |
1990年 | 181篇 |
1989年 | 195篇 |
1988年 | 184篇 |
1987年 | 168篇 |
1986年 | 157篇 |
1985年 | 140篇 |
1984年 | 125篇 |
1983年 | 114篇 |
1982年 | 101篇 |
1981年 | 102篇 |
1980年 | 92篇 |
1979年 | 106篇 |
1978年 | 106篇 |
1976年 | 86篇 |
1975年 | 104篇 |
1974年 | 96篇 |
1973年 | 90篇 |
1972年 | 316篇 |
1971年 | 328篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
During the breeding season of 2008 a total of 24–30 female Ruddy Shelduck Tadorna ferruginea with brood were counted at two habitat types (reservoirs and natural gullies). The population was relatively stable from 1994–2002, with a likely increase afterwards. Generally, the island breeding distribution comprises the north and mid part of Fuerteventura. Breeding ducks were mainly concentrated in man-made reservoirs, the most important site was Los Molinos Reservoir with 40–42% of the island population, but single pairs also occurred in gullies with flourishing saline water and lush shrubby and arboreal fringing vegetation. We recommend that special attention should be paid by the local authorities to breeding areas outside the Important Bird Areas for this island (e.g. Catalina Garcia Reservoir, the second-most important breeding location for this Anatidae) and some other sites in gullies. 相似文献
992.
993.
994.
Jamie C. Snyder Susan K. Brumfield Keshia M. Kerchner Tessa E. F. Quax David Prangishvili Mark J. Young 《Journal of virology》2013,87(4):2186-2192
Archaeal host cells infected by Sulfolobus turreted icosahedral virus (STIV) and Sulfolobus islandicus rod-shaped virus 2 (SIRV2) produce unusual pyramid-like structures on the cell surface prior to virus-induced cell lysis. This viral lysis process is distinct from known viral lysis processes associated with bacterial or eukaryal viruses. The STIV protein C92 and the SIRV2 protein 98 are the only viral proteins required for the formation of the pyramid lysis structures of STIV and SIRV2, respectively. Since SIRV2 and STIV have fundamentally different morphotypes and genome sequences, it is surprising that they share this lysis system. In this study, we have constructed a collection of C92/P98 chimeric proteins and tested their abilities, both in the context of virus replication and alone, to form pyramid lysis structures in S. solfataricus. The results of this study illustrate that these proteins are functionally homologous when expressed as individual chimeric proteins but not when expressed in the context of complete STIV infection. 相似文献
995.
Hyo Jae Kang Young Sik Park Chang-Hoon Lee Sang-Min Lee Jae-Joon Yim Chul-Gyu Yoo Young Whan Kim Sung Koo Han Jong-Yil Chai Jinwoo Lee 《The Korean journal of parasitology》2013,51(5):569-572
Dirofilariasis is a rare disease in humans. We report here a case of a 48-year-old male who was diagnosed with pulmonary dirofilariasis in Korea. On chest radiographs, a coin lesion of 1 cm in diameter was shown. Although it looked like a benign inflammatory nodule, malignancy could not be excluded. So, the nodule was resected by video-assisted thoracic surgery. Pathologically, chronic granulomatous inflammation composed of coagulation necrosis with rim of fibrous tissues and granulations was seen. In the center of the necrotic nodules, a degenerating parasitic organism was found. The parasite had prominent internal cuticular ridges and thick cuticle, a well-developed muscle layer, an intestinal tube, and uterine tubules. The parasite was diagnosed as an immature female worm of Dirofilaria immitis. This is the second reported case of human pulmonary dirofilariasis in Korea. 相似文献
996.
Kyeong Ah Kim Ju Young Kim Young Ah Lee Arim Min Young Yil Bahk Myeong Heon Shin 《The Korean journal of parasitology》2013,51(1):61-68
Entamoeba histolytica, which causes amoebic colitis and occasionally liver abscess in humans, is able to induce host cell death. However, signaling mechanisms of colon cell death induced by E. histolytica are not fully elucidated. In this study, we investigated the signaling role of NOX in cell death of HT29 colonic epithelial cells induced by E. histolytica. Incubation of HT29 cells with amoebic trophozoites resulted in DNA fragmentation that is a hallmark of apoptotic cell death. In addition, E. histolytica generate intracellular reactive oxygen species (ROS) in a contact-dependent manner. Inhibition of intracellular ROS level with treatment with DPI, an inhibitor of NADPH oxidases (NOXs), decreased Entamoeba-induced ROS generation and cell death in HT29 cells. However, pan-caspase inhibitor did not affect E. histolytica-induced HT29 cell death. In HT29 cells, catalytic subunit NOX1 and regulatory subunit Rac1 for NOX1 activation were highly expressed. We next investigated whether NADPH oxidase 1 (NOX1)-derived ROS is closely associated with HT29 cell death induced by E. histolytica. Suppression of Rac1 by siRNA significantly inhibited Entamoeba-induced cell death. Moreover, knockdown of NOX1 by siRNA, effectively inhibited E. histolytica-triggered DNA fragmentation in HT29 cells. These results suggest that NOX1-derived ROS is required for apoptotic cell death in HT29 colon epithelial cells induced by E. histolytica. 相似文献
997.
Aims
Anethole, the major component of the essential oil of star anise, has been reported to have antioxidant, antibacterial, antifungal, anti-inflammatory, and anesthetic properties. In this study, we investigated the anti-inflammatory effects of anethole in a mouse model of acute lung injury induced by lipopolysaccharide (LPS).Main methods
BALB/C mice were intraperitoneally administered anethole (62.5, 125, 250, or 500 mg/kg) 1 h before intratracheal treatment with LPS (1.5 mg/kg) and sacrificed after 4 h. The anti-inflammatory effects of anethole were assessed by measuring total protein and cell levels and inflammatory mediator production and by histological evaluation and Western blot analysis.Key findings
LPS significantly increased total protein levels; numbers of total cells, including macrophages and neutrophils; and the production of inflammatory mediators such as matrix metalloproteinase 9 (MMP-9), tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), and nitric oxide (NO) in bronchoalveolar lavage fluid. Anethole (250 mg/kg) decreased total protein concentrations; numbers of inflammatory cells, including neutrophils and macrophages; and the inflammatory mediators MMP-9, TNF-α and NO. In addition, pretreatment with anethole decreased LPS-induced histopathological changes. The anti-inflammatory mechanism of anethole in LPS-induced acute lung injury was assessed by investigating the effects of anethole on NF-κB activation. Anethole suppressed the activation of NF-κB by blocking IκB-α degradation.Significance
These results, showing that anethole prevents LPS-induced acute lung inflammation in mice, suggest that anethole may be therapeutically effective in inflammatory conditions in humans. 相似文献998.
The combination of abiotic stress and consumer stress can have complex impacts on plant community structure. Effective conservation and management of semi-arid ecosystems requires an understanding of how different stresses interact to structure plant communities. We explored the separate and combined impacts of episodic drought, livestock grazing, and wild ungulate herbivory on species co-occurrence and diversity patterns in a relatively productive, semi-arid Acacia savanna. Specifically, we analyzed 9 years of biannual plant community data from the Kenya long-term exclosure experiment, a broad-scale manipulative experiment that has excluded different combinations of large mammalian herbivores from 18 4-ha plots since 1995. During droughts, we observed low species diversity and random species co-occurrence patterns. However, when rain followed a major drought, areas exposed to moderate cattle grazing displayed high species diversity and evidence of significant species aggregation. These patterns were not apparent in the absence of cattle, even if other large herbivores were present. To explore possible mechanisms, we examined patterns separately for common and rare species. We found that aggregation patterns were likely driven by rare species responding similarly to the availability of open micro-sites. Our results indicate that in a productive, fire-suppressed savanna, the combination of periodic drought and moderate cattle grazing can enhance plant biodiversity and fine-scale spatial heterogeneity by opening up space for species that are otherwise rare or cryptic. Our findings also emphasize that domestic herbivores can have significantly stronger impacts on plant community dynamics than wild herbivores, even in an ecosystem with a long history of grazing. 相似文献
999.
Mark T. Young Marco Brandalise De Andrade Steve Etches Brian L. Beatty 《Zoological Journal of the Linnean Society》2013,169(4):820-848
A new metriorhynchid crocodylomorph from the Lower Kimmeridge Clay Formation (Kimmeridgian, Upper Jurassic) of England is described. This specimen, a three‐dimensionally preserved skull and left mandibular ramus, is referred to a new species: T orvoneustes coryphaeus sp. nov. Within the genus Torvoneustes, T . coryphaeus sp. nov. is unique as it has a long anteromedial process of the frontal, ornamented dermatocranium, and the supraorbital notch forms a strongly acute angle. Our phylogenetic analysis confirms the placement of this specimen in Torvoneustes. The dentition of T . coryphaeus sp. nov. , like that of the type species, has a blunt apex, crown basal–mid regions with numerous tightly packed apicobasally aligned ridges, and apical region with an anastomosed pattern of ridges that interact with the carinae. Within Thalattosuchia these dental characteristics are only found in Torvoneustes and the teleosaurid Machimosaurus. The heavily ornamented dermatocranium of T . coryphaeus sp. nov. is in contrast to the unornamented (nasals and frontal)–lightly ornamented (maxillae and premaxillae) pattern seen in Torvoneustes carpenteri. Curiously, this pattern of reduction and loss of dermatocranium ornamentation is also observed in Metriorhynchus, Dakosaurus, and the subclade Rhacheosaurini. We hypothesize that the ‘smooth’ dermatocranium of Late Jurassic metriorhynchids evolved independently in each subclade (parallel evolution), and would have reduced drag, thereby making locomotion through water more energy efficient. © 2013 The Linnean Society of London 相似文献
1000.
Seung Hwan Lee Kyoung-Hee Kang Eun Young Kim Tong Un Chae Young Hoon Oh Soon Ho Hong Bong Keun Song Jonggeon Jegals Si Jae Park Sang Yup Lee 《Biotechnology letters》2013,35(10):1631-1637
We have previously analyzed the proteome of recombinant Escherichia coli producing poly(3-hydroxybutyrate) [P(3HB)] and revealed that the expression level of several enzymes in central metabolism are proportional to the amount of P(3HB) accumulated in the cells. Based on these results, the amplification effects of triosephosphate isomerase (TpiA) and fructose-bisphosphate aldolase (FbaA) on P(3HB) synthesis were examined in recombinant E. coli W3110, XL1-Blue, and W lacI mutant strains using glucose, sucrose and xylose as carbon sources. Amplification of TpiA and FbaA significantly increased the P(3HB) contents and concentrations in the three E. coli strains. TpiA amplification in E. coli XL1-Blue lacI increased P(3HB) from 0.4 to 1.6 to g/l from glucose. Thus amplification of glycolytic pathway enzymes is a good strategy for efficient production of P(3HB) by allowing increased glycolytic pathway flux to make more acetyl-CoA available for P(3HB) biosynthesis. 相似文献