全文获取类型
收费全文 | 5031篇 |
免费 | 216篇 |
国内免费 | 13篇 |
专业分类
5260篇 |
出版年
2023年 | 54篇 |
2022年 | 141篇 |
2021年 | 297篇 |
2020年 | 125篇 |
2019年 | 171篇 |
2018年 | 192篇 |
2017年 | 148篇 |
2016年 | 211篇 |
2015年 | 254篇 |
2014年 | 286篇 |
2013年 | 410篇 |
2012年 | 359篇 |
2011年 | 365篇 |
2010年 | 204篇 |
2009年 | 174篇 |
2008年 | 196篇 |
2007年 | 189篇 |
2006年 | 176篇 |
2005年 | 147篇 |
2004年 | 117篇 |
2003年 | 86篇 |
2002年 | 93篇 |
2001年 | 56篇 |
2000年 | 62篇 |
1999年 | 47篇 |
1998年 | 29篇 |
1997年 | 14篇 |
1996年 | 20篇 |
1995年 | 17篇 |
1993年 | 16篇 |
1992年 | 31篇 |
1991年 | 47篇 |
1990年 | 41篇 |
1989年 | 33篇 |
1988年 | 28篇 |
1987年 | 42篇 |
1986年 | 30篇 |
1985年 | 36篇 |
1984年 | 28篇 |
1983年 | 20篇 |
1982年 | 17篇 |
1981年 | 13篇 |
1980年 | 13篇 |
1979年 | 23篇 |
1978年 | 24篇 |
1977年 | 13篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1973年 | 20篇 |
1968年 | 12篇 |
排序方式: 共有5260条查询结果,搜索用时 15 毫秒
91.
Imran Ahmad Jay Prakash Thakur Debabrata Chanda Dharmendra Saikia Feroz Khan Shivani Dixit Amit Kumar Rituraj Konwar Arvind Singh Negi Atul Gupta 《Bioorganic & medicinal chemistry letters》2013,23(5):1322-1325
Lipophilic chalcones and their conformationally restricted analogues were synthesized and evaluated for their antitubercular efficacy against Mycobacterium tuberculosis H37Rv strain. Compounds 16, 24, 25a and 25c were found to be active MIC at 60, 30, 3.5 and 7.5 μg-mL?1. In vitro cytotoxicity of compounds 16, 24, 25a, 25c and 26 in non-cancerous human epithelial kidney cell line (HEK-293) showed that most active compound 25a was approximately 2.85 times selective towards tubercular versus healthy cells whereas compound 24 was found to be 16 times selective. 相似文献
92.
ObjectivesTo assess the prevalence of noncommunicable disease (NCD) risk factors among Saudi university employees and their families; to estimate the cardiovascular risk (CVR) amongst the study population in the following 10years.MethodsThe NCD risk factors prevalence was estimated using a cross-sectional approach for a sample of employees and their families aged ≥ 18 years old, in a Saudi university (Riyadh in Kingdom of Saudi Arabia; KSA). WHO STEPwise standardized tools were used to estimate NCD risk factors and the Framingham Coronary Heart Risk Score calculator was used to calculate the CVR.ResultsFive thousand and two hundred subjects were invited, of whom 4,500 participated in the study, providing a response rate of 87%. The mean age of participants was 39.3±13.4 years. The majority of participants reported low fruit/vegetables consumption (88%), and physically inactive (77%). More than two thirds of the cohort was found to be either overweight or obese (72%), where 36% were obese, and 59% had abdominal obesity. Of the total cohort, 22–37% were found to suffer from dyslipidaemia, 22% either diabetes or hypertension, with rather low reported current tobacco use (12%). One quarter of participants was estimated to have >10% risk to develop cardiovascular disease within the following 10-years.ConclusionThe prevalence of NCD risk factors was found to be substantially high among the university employees and their families in this study. 相似文献
93.
Amino acid transporter mutants of Arabidopsis provides evidence that a non‐mycorrhizal plant acquires organic nitrogen from agricultural soil 下载免费PDF全文
Ulrika Ganeteg Iftikhar Ahmad Sandra Jämtgård Camila Aguetoni‐Cambui Erich Inselsbacher Henrik Svennerstam Susanne Schmidt Torgny Näsholm 《Plant, cell & environment》2017,40(3):413-423
Although organic nitrogen (N) compounds are ubiquitous in soil solutions, their potential role in plant N nutrition has been questioned. We performed a range of experiments on Arabidopsis thaliana genetically modified to enhance or reduce root uptake of amino acids. Plants lacking expression of the Lysine Histidine Transporter 1 (LHT1) displayed significantly lower contents of 13C and 15N label and of U‐13C5,15N2 L‐glutamine, as determined by liquid chromatography–mass spectrometry when growing in pots and supplied with dually labelled L‐glutamine compared to wild type plants and LHT1‐overexpressing plants. Slopes of regressions between accumulation of 13C‐labelled carbon and 15N‐labelled N were higher for LHT1‐overexpressing plants than wild type plants, while plants lacking expression of LHT1 did not display a significant regression between the two isotopes. Uptake of labelled organic N from soil tallied with that of labelled ammonium for wild type plants and LHT1‐overexpressing plants but was significantly lower for plants lacking expression of LHT1. When grown on agricultural soil plants lacking expression of LHT1 had the lowest, and plants overexpressing LHT1 the highest C/N ratios and natural δ15N abundance suggesting their dependence on different N pools. Our data show that LHT1 expression is crucial for plant uptake of organic N from soil. 相似文献
94.
Superoxide anion (O
2
.–
) was photogenerated upon illumination of riboflavin in fluorescent light. The rate of O
2
.–
formation was stimulated by double stranded DNA but not by denatured DNA or RNA. Depurinated DNA, which was predominantly depleted in guanine residues, did not exhibit the stimulatory effect, indicating an interaction of riboflavin, or active oxygen species derived from it, with guanine bases. Also, the stimulation of O
2
.–
photogeneration was not observed with ethidium bromide but was seen with proflavin-intercalated DNA. Since ethidium bromide intercalates preferentially between purines and pyrimidines, and proflavin prefers dA-dT rich sites, these results were interpreted to suggest that the interaction of riboflavin with DNA is mainly with GC or CG base pairs. 相似文献
95.
96.
The root extract of Operculina turpethum (OTE) has been used as an anti-inflammatory, purgative, and hepato-protective agent. N-Nitrosodimethylamine (NDMA) is a potent hepatotoxin that induces fibrosis of the liver. In the present study, we examined the therapeutic effects of OTE root extract against NDMA-induced hepatotoxicity and clastogenicity in rats. Hepatic fibrosis was induced in adult male albino rats through serial intraperitoneal administrations of NDMA at a concentration of 10 mg/kg body weight on three consecutive days of each week over a period of three weeks. A group of rats received OTE orally in doses of 75, 150 and 200 mg/kg body weight at 5 h after the administration of NDMA. The controls and treated animals were sacrificed on days-7, 14 and 21 after the start of the administration of NDMA. The progression of hepatic fibrosis as well as the amelioration effect of OTE was evaluated through histopathologically as well as by immunohistochemical staining for the activation of hepatic stellate cells. Alterations in serum and liver biochemical parameters and LDH isoenzymes were also studied. Serial administration of NDMA resulted in well formed fibrosis in the liver and induction of micronuclei in the bone marrow cells. Staining of α-SMA demonstrated activated stellate cells from day-7 onwards which was dramatically increased on day-21. An elevation of micronuclei count, liver function enzymes, serum hydroxyproline levels and LDH isoenzymes 4 and 5 were also observed. All these changes were remarkably reduced in OTE administered animals and fibrogenesis was completely absent. Our results suggest that OTE has hepatoprotective and anti-clastogenic effects against NDMA-induced hepatic fibrosis. Therefore OTE may be used as a hepatoprotective agent against various liver diseases including toxic liver injury. 相似文献
97.
98.
Yufeng Li Rong Ni Wei Song Wenshuo Shao Sadeep Shrestha Sushma Ahmad Coleen K. Cunningham Patricia M. Flynn Bill G. Kapogiannis Craig M. Wilson Jianming Tang 《Human genetics》2009,126(5):685-696
To confirm and refine associations of human leukocyte antigen (HLA) genotypes with variable antibody (Ab) responses to hepatitis B vaccination, we have analyzed 255 HIV-1 seropositive (HIV+) youth and 80 HIV-1 seronegatives (HIV?) enrolled into prospective studies. In univariate analyses that focused on HLA-DRB1, -DQA1, and -DQB1 alleles and haplotypes, the DRB1*03 allele group and DRB1*0701 were negatively associated with the responder phenotype (serum Ab concentration ≥ 10 mIU/mL) (P = 0.026 and 0.043, respectively). Collectively, DRB1*03 and DRB1*0701 were found in 42 (53.8%) out of 78 non-responders (serum Ab <10 mIU/mL), 65 (40.6%) out of 160 medium responders (serum Ab 10–1,000 mIU/mL), and 27 (27.8%) out of 97 high responders (serum Ab >1,000 mIU/mL) (P < 0.001 for trend). Meanwhile, DRB1*08 was positively associated with the responder phenotype (P = 0.010), mostly due to DRB1*0804 (P = 0.008). These immunogenetic relationships were all independent of non-genetic factors, including HIV-1 infection status and immunodeficiency. Alternative analyses confined to HIV+ youth or Hispanic youth led to similar findings. In contrast, analyses of more than 80 non-coding, single nucleotide polymorphisms within and beyond the three HLA class II genes revealed no clear associations. Overall, several HLA-DRB1 alleles were major predictors of differential Ab responses to hepatitis B vaccination in youth, suggesting that T-helper cell-dependent pathways mediated through HLA class II antigen presentation are critical to effective immune response to recombinant vaccines. 相似文献
99.
100.
Kamran Ashraf Altaf Ahmad Anis Chaudhary Mohd. Mujeeb Sayeed Ahmad Mohd. Amir N. Mallick 《Saudi Journal of Biological Sciences》2014,21(2):159-165
The present investigation was undertaken for the assessment of 12 accessions of Zingiber officinale Rosc. collected from subcontinent of India by RAPD markers. DNA was isolated using CTAB method. Thirteen out of twenty primers screened were informative and produced 275 amplification products, among which 261 products (94.90%) were found to be polymorphic. The percentage polymorphism of all 12 accessions ranged from 88.23% to 100%. Most of the RAPD markers studied showed different levels of genetic polymorphism. The data of 275 RAPD bands were used to generate Jaccard’s similarity coefficients and to construct a dendrogram by means of UPGMA. Results showed that ginger undergoes genetic variation due to a wide range of ecological conditions. This investigation was an understanding of genetic variation within the accessions. It will also provide an important input into determining resourceful management strategies and help to breeders for ginger improvement program. 相似文献