全文获取类型
收费全文 | 16935篇 |
免费 | 1063篇 |
国内免费 | 6篇 |
专业分类
18004篇 |
出版年
2023年 | 144篇 |
2022年 | 251篇 |
2021年 | 455篇 |
2020年 | 326篇 |
2019年 | 371篇 |
2018年 | 548篇 |
2017年 | 471篇 |
2016年 | 649篇 |
2015年 | 892篇 |
2014年 | 987篇 |
2013年 | 1231篇 |
2012年 | 1276篇 |
2011年 | 1261篇 |
2010年 | 809篇 |
2009年 | 713篇 |
2008年 | 931篇 |
2007年 | 964篇 |
2006年 | 850篇 |
2005年 | 700篇 |
2004年 | 667篇 |
2003年 | 590篇 |
2002年 | 528篇 |
2001年 | 278篇 |
2000年 | 246篇 |
1999年 | 210篇 |
1998年 | 133篇 |
1997年 | 88篇 |
1996年 | 85篇 |
1995年 | 75篇 |
1994年 | 70篇 |
1993年 | 60篇 |
1992年 | 111篇 |
1991年 | 98篇 |
1990年 | 87篇 |
1989年 | 73篇 |
1988年 | 73篇 |
1987年 | 59篇 |
1986年 | 55篇 |
1985年 | 55篇 |
1984年 | 55篇 |
1983年 | 39篇 |
1982年 | 44篇 |
1981年 | 38篇 |
1980年 | 30篇 |
1979年 | 40篇 |
1978年 | 27篇 |
1977年 | 28篇 |
1976年 | 19篇 |
1975年 | 30篇 |
1973年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
- 1 There is growing appreciation of climatic effects on insect population dynamics at the margins of distribution limits. Climatic effects might be less important and/or involve different drivers and processes near the centre of distributions.
- 2 We evaluated the effects of interannual variation in temperatures, radiation and precipitation on populations of pine processionary moth Thaumetopoea pityocampa in Central–South Portugal, a low altitude area with a relatively mild Mediterranean climate near the centre of the north–south range of the species.
- 3 We tested for effects of climate on mortality of young larvae, growth rates, final larval mass and fecundity.
- 4 Results indicated high mortality of early instars associated with low minimum and maximum daily temperatures and low precipitation. Low minimum temperatures were further associated with high parasitism by the larval parasitoid Phryxe caudata (Rondani) (Diptera, Tachinidae). Furthermore, larval growth rates were higher with high solar radiation during December and January, which was itself negatively related to precipitation and air temperature. Slow larval growth rates led to lower final mass at pupation in the spring, and smaller egg masses and smaller initial colony sizes during the next autumn.
- 5 Thus climatic factors, and temperature in particular, apparently contributed to population dynamics of T. pityocampa in the core of its distribution, as well as at its northern limits. The most specific climatic parameters of importance, however, and the connections between climate, physiology and insect demographics in the core area were clearly different from northern areas.
52.
Seedlings of two barley genotypes (‘Maresi’ and wild form of Hordeum spontaneum) were treated with jasmonic acid (JA 5 μM and 15 μM) for 24 h, and then subjected to water stress (PEG 6000 solution of −
1.5 MPa). JA caused an increase in the content of ABA but not in that of proline and spermidine in the two studied genotypes.
The effect of the treatment did not depend on the applied JA concentration. The pre-stress treatment with JA changed plant
response to water deficit with regard to membrane injury. Treatment with a lower JA concentration (5 μM) caused a substantial
reduction of the stress-induced membrane damage in the both genotypes. A higher JA concentration (15 μM) caused the reduction
of membrane injury only in H. spontaneum and was ineffective in ‘Maresi’. JA had no influence on the leaf water status in water-stressed plants. A possible role of
JA in leaf ABA accumulation and alleviation of cell membrane injury under water deficit is discussed.
The work was partly supported by the Polish Committee For Scientific Research, grant No 5 PO6A 036 18 相似文献
53.
Babu M Beloglazova N Flick R Graham C Skarina T Nocek B Gagarinova A Pogoutse O Brown G Binkowski A Phanse S Joachimiak A Koonin EV Savchenko A Emili A Greenblatt J Edwards AM Yakunin AF 《Molecular microbiology》2011,79(2):484-502
Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs) and the associated proteins (Cas) comprise a system of adaptive immunity against viruses and plasmids in prokaryotes. Cas1 is a CRISPR-associated protein that is common to all CRISPR-containing prokaryotes but its function remains obscure. Here we show that the purified Cas1 protein of Escherichia coli (YgbT) exhibits nuclease activity against single-stranded and branched DNAs including Holliday junctions, replication forks and 5'-flaps. The crystal structure of YgbT and site-directed mutagenesis have revealed the potential active site. Genome-wide screens show that YgbT physically and genetically interacts with key components of DNA repair systems, including recB, recC and ruvB. Consistent with these findings, the ygbT deletion strain showed increased sensitivity to DNA damage and impaired chromosomal segregation. Similar phenotypes were observed in strains with deletion of CRISPR clusters, suggesting that the function of YgbT in repair involves interaction with the CRISPRs. These results show that YgbT belongs to a novel, structurally distinct family of nucleases acting on branched DNAs and suggest that, in addition to antiviral immunity, at least some components of the CRISPR-Cas system have a function in DNA repair. 相似文献
54.
Acinetobacter baumannii causes serious infections especially in immunocompromised and/or hospitalized patients. Several A. baumannii strains are multidrug resistant and infect wounds, bones, and the respiratory tract. Current studies are focused on finding new effective agents against A. baumannii. Phage therapy is a promising means to fight this bacterium and many studies on procuring and applying new phages against A. baumannii are currently being conducted. As shown in animal models, phages against multidrug-resistant A. baumannii may control bacterial infections caused by this pathogen and may be a real hope to solve this dangerous health problem. 相似文献
55.
Clumping factor A (ClfA), a fibrinogen-binding protein expressed on the Staphylococcus aureus cell surface, has previously been shown to act as a virulence factor in experimental septic arthritis. Although the interaction between ClfA and fibrinogen is assumed to be of importance for the virulence of S. aureus, this has not been demonstrated in any in vivo model of infection. Therefore, the objective of this study was to investigate the contribution of this interaction to ClfA-mediated virulence in murine S. aureus-induced arthritis. Ancrod, a serine protease with thrombin-like activity, was used to induce in vivo depletion of fibrinogen in mice. Ancrod treatment significantly aggravated septic arthritis following inoculation with a ClfA-expressing strain (Newman) compared to control treatment. Also, ancrod treatment tended to enhance the arthritis induced by a clfA mutant strain (DU5876), indicating that fibrinogen depletion exacerbates septic arthritis in a ClfA-independent manner. Most importantly, the ClfA-expressing strain was much more arthritogenic than the isogenic clfA mutant, following inoculation of fibrinogen-depleted mice. This finding indicates that the interaction between ClfA and free fibrinogen is not required for ClfA-mediated functions contributing to S. aureus virulence. It is conceivable that ClfA contributes to the virulence of S. aureus through interactions with other host ligands than fibrinogen. 相似文献
56.
Jeemeng Lao Ai Oikawa Jennifer R. Bromley Peter McInerney Anongpat Suttangkakul Andreia M. Smith‐Moritz Hector Plahar Tsan‐Yu Chiu Susana M. González Fernández‐Niño Berit Ebert Fan Yang Katy M. Christiansen Sara F. Hansen Solomon Stonebloom Paul D. Adams Pamela C. Ronald Nathan J. Hillson Masood Z. Hadi Miguel E. Vega‐Sánchez Dominique Loqué Henrik V. Scheller Joshua L. Heazlewood 《The Plant journal : for cell and molecular biology》2014,79(3):517-529
The glycosyltransferases (GTs) are an important and functionally diverse family of enzymes involved in glycan and glycoside biosynthesis. Plants have evolved large families of GTs which undertake the array of glycosylation reactions that occur during plant development and growth. Based on the Carbohydrate‐Active enZymes (CAZy) database, the genome of the reference plant Arabidopsis thaliana codes for over 450 GTs, while the rice genome (Oryza sativa) contains over 600 members. Collectively, GTs from these reference plants can be classified into over 40 distinct GT families. Although these enzymes are involved in many important plant specific processes such as cell‐wall and secondary metabolite biosynthesis, few have been functionally characterized. We have sought to develop a plant GTs clone resource that will enable functional genomic approaches to be undertaken by the plant research community. In total, 403 (88%) of CAZy defined Arabidopsis GTs have been cloned, while 96 (15%) of the GTs coded by rice have been cloned. The collection resulted in the update of a number of Arabidopsis GT gene models. The clones represent full‐length coding sequences without termination codons and are Gateway® compatible. To demonstrate the utility of this JBEI GT Collection, a set of efficient particle bombardment plasmids (pBullet) was also constructed with markers for the endomembrane. The utility of the pBullet collection was demonstrated by localizing all members of the Arabidopsis GT14 family to the Golgi apparatus or the endoplasmic reticulum (ER). Updates to these resources are available at the JBEI GT Collection website http://www.addgene.org/ . 相似文献
57.
Max M. Tilzer Anna Hillbricht-Ilkowska Andrzej Kowalczewski Irena Spodniewska Joanna Turczyska 《International Review of Hydrobiology》1977,62(2):279-289
Photosynthetic rates as measured by the oxygen light and dark bottle method were highly correlated with estimates using the 14C technique. The high O2/14C ratios found are explained by algal respiration and extracellular release which are included in photosynthetic measurements by the oxygen technique, while the 14C method yields values close to net photosynthesis. Separation of net- and nannoplankton using a 50 μm plankton net for filtration was not comparable to distinctions made by microscopic examination. Separation of both by filtration caused a significant decrease in the photosynthetic activity of nannoplankton in 24-hour incubations, but had no detectable effect after 4 hours of exposure. “Bottle effects” in 24-hour measurements of photosynthesis were similar using both methods. Asymmetric photosynthetic time-curves as well as vertical phytoplankton migrations were the main reasons for errors in estimates of daily photosynthetic rates from part-day incubations which were extrapolated to the entire day. 相似文献
58.
59.
Ana O. Fagundes Maira R. Aguiar Claudia S. Aguiar Giselli Scaini Monique U. Sachet Nayara M. Bernhardt Gislaine T. Rezin Samira S. Valvassori João Quevedo Emilio L. Streck 《Neurochemical research》2010,35(11):1675-1680
Methylphenidate is commonly used for the treatment of attention deficit/hyperactivity disorder. There are still few works
regarding the effects of methylphenidate on brain energy metabolism. Thus, in the present study we evaluated the effect of
chronic administration of methylphenidate on the activities of mitochondrial respiratory chain complexes I and III in the
brain of young rats. The effect of acute administration of methylphenidate on mitochondrial respiratory chain complexes I,
II, III and IV in the brain of young rats was also investigated. For acute administration, a single injection of methylphenidate
was given to rats on postnatal day 25. For chronic administration, methylphenidate injections were given starting at postnatal
day 25 once daily for 28 days. Our results showed that complexes I and III were not affected by chronic administration of
methylphenidate. Moreover, the acute administration of methylphenidate decreased complex I activity in cerebellum and prefrontal
cortex, whereas complexes II, III and IV were not altered. 相似文献
60.
Dinis-Oliveira RJ Remião F Duarte JA Ferreira R Sánchez Navarro A Bastos ML Carvalho F 《Free radical biology & medicine》2006,41(8):1213-1224
The widespread use of the nonselective contact herbicide paraquat (PQ) has been the cause of thousands of deaths from both accidental and voluntary ingestion. The main target organ for PQ toxicity is the lung. No antidote or effective treatment to decrease PQ accumulation in the lung or to disrupt its toxicity has yet been developed. The present study describes a procedure that leads to a remarkable decrease in PQ accumulation in the lung, together with an increase in its fecal excretion and a subsequent decrease in several biochemical and histopathological biomarkers of toxicity. The administration of dexamethasone (100 mg/kg ip) to Wistar rats, 2 h after PQ intoxication (25 mg/kg ip), decreased the lung PQ accumulation to about 40% of the group exposed to only PQ and led to an improvement in tissue healing in just 24 h as a result of the induction of de novo synthesis of P-glycoprotein (P-gp). The involvement of P-gp in these effects was confirmed by Western blot analysis and by the use of a competitive inhibitor of this transporter, verapamil (10 mg/kg ip), which, given 1 h before dexamethasone, blocked its protective effects, causing instead an increase in lung PQ concentration and an aggravation of toxicity. In conclusion, the induction of P-gp, leading to a decrease in lung levels of PQ and the consequent prevention of toxicity, seems to be a new and promising treatment for PQ poisonings that should be further clinically tested. 相似文献