全文获取类型
收费全文 | 19812篇 |
免费 | 1338篇 |
国内免费 | 6篇 |
出版年
2023年 | 133篇 |
2022年 | 120篇 |
2021年 | 220篇 |
2020年 | 269篇 |
2019年 | 246篇 |
2018年 | 608篇 |
2017年 | 519篇 |
2016年 | 797篇 |
2015年 | 1169篇 |
2014年 | 1100篇 |
2013年 | 1512篇 |
2012年 | 1808篇 |
2011年 | 1691篇 |
2010年 | 1012篇 |
2009年 | 801篇 |
2008年 | 1259篇 |
2007年 | 1201篇 |
2006年 | 1166篇 |
2005年 | 1004篇 |
2004年 | 1011篇 |
2003年 | 894篇 |
2002年 | 839篇 |
2001年 | 140篇 |
2000年 | 103篇 |
1999年 | 141篇 |
1998年 | 131篇 |
1997年 | 82篇 |
1996年 | 97篇 |
1995年 | 94篇 |
1994年 | 84篇 |
1993年 | 85篇 |
1992年 | 77篇 |
1991年 | 57篇 |
1990年 | 51篇 |
1989年 | 46篇 |
1988年 | 41篇 |
1987年 | 29篇 |
1986年 | 25篇 |
1985年 | 43篇 |
1984年 | 48篇 |
1983年 | 28篇 |
1982年 | 39篇 |
1981年 | 22篇 |
1980年 | 19篇 |
1979年 | 21篇 |
1978年 | 17篇 |
1977年 | 12篇 |
1976年 | 28篇 |
1975年 | 22篇 |
1967年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
The lifeform and the biological production of pure stands ofEichhornia azurea Kunth in three lakes in tropical Brazil were studied. The lifeform ofE. azurea is termed ‘semi-emergent’, because the plant has well developed trailing stems just under the water, and the aerial lamina
emerges with the thick petiole. The density of shoot apices was 9.9, 17.2 and 17.1 m−2 in Lake Dom Helvecio, Lake Jacaré and Lake Carioca, respectively. The mean daily increment of the apical shoot biomass was
between 1.8 and 4.8 g m−2 day−1. The mean leaf life-span in Lake Dom Helvecio, Lake Jacaré and Lake Carioca was estimated to be 78, 49 and 64 days in the
wet season and 73, 70 and 73 days in the dry season, respectively. The stem life-span was estimated to be about 28 months.
Starch content in the current years' stem ranged from 24 to 118 mg g−1 dry matter with fluctuations, the amplitude of which decreased with age. The differences for most of the growth parameters,
such as density of shoot apices, daily increment of biomass and leaf life-span, between dry and wet season are smaller than
those among the three lakes. Both the decrease in daily dry matter production and the increase in leaf life-span occurred
in order from Lake Dom Helvecio to Lake Jacaré and Lake Carioca. The low productivity ofE. azurea is considered to be related to a low leaf area index, a long time interval for the emergence of new leaves, long leaf life-span
and a low capacity for branching. 相似文献
132.
Mario H.J. Vogt Elisabet H. Stet Ronney A. De Abreu Jos P.M. Bökkerink Lambert H.J. Lambooy Frans J.M. Trijbels 《生物化学与生物物理学报:疾病的分子基础》1993,1181(2):189-194
The importance of methyl-thioIMP (Me-tIMP) formation for methylmercaptopurine ribonucleoside (Me-MPR) cytotoxicity was studied in Molt F4 cells. Cytotoxicity of Me-MPR is caused by Me-tIMP formation with concomitant inhibition of purine de novo synthesis. Inhibition of purine de novo synthesis resulted in decreased purine nucleotide levels and enhanced 5-phosphoribosyl-1-pyrophosphate (PRPP) levels, with concurrent increased pyrimidine nucleotide levels. The Me-tIMP concentration increased proportionally with the concentration of Me-MPR. High Me-tIMP concentration also caused inhibition of PRPP synthesis. Maximal accumulation of PRPP thus occurred at low Me-MPR concentrations. As little as 0.2 μM Me-MPR resulted already after 2 h in maximal inhibition of formation of adenine and guanine nucleotides, caused by inhibition of purine de novo synthesis by Me-tIMP. Under these circumstances increased intracellular PRPP concentrations could be demonstrated, resulting in increased levels of pyrimidine nucleotides. So, in Molt F4 cells, formation of Me-tIMP form Me-MPR results in cytotoxicity by inhibition of purine de novo synthesis. 相似文献
133.
Carlos Soccol José Rodríguez León Bernard Marin Sevastianos Roussos Maurice Raimbault 《Biotechnology Techniques》1993,7(8):563-568
Summary Growth kinetics of Rhizopus arrhizus MUCL 28168 were determined for different treatments of cassava during solid state fermentation. The best case gave a specific growth rate () of 0.24 h-1, a yield calculated on a basis that oxygen consumption (Yx/o) was 2.9 g biomass. g-1 O2 consumed and the maintenance coefficient (m) was 0.004 g O2 consumed. g-1 biomass. h-1. 相似文献
134.
da Cunha Cruz Yasmini Scarpa Ana Lívia Martins Díaz Alejandro Sandria Pereira Marcio Paulo de Castro Evaristo Mauro Pereira Fabricio José 《Journal of plant research》2023,136(5):665-678
Journal of Plant Research - Precipitation is an important climatic element that defines the hydrological regime, and its seasonal variation produces annual dry and wet periods in some areas. This... 相似文献
135.
García-Machorro Jazmin Mirzaeicheshmeh Elaheh Fragoso-Vázquez Manuel Jonathan Bello Martiniano Méndez-Luna David León-Cardona Alám Correa-Basurto José 《化学与生物多样性》2023,20(7):e202201077
Antiviral resistance has turned into a world concern nowadays. Influenza A H1N1 emerged as a problem at the world level due to the neuraminidase (NA) mutations. The NA mutants conferred resistance to oseltamivir and zanamivir. Several efforts were conducted to develop better anti-influenza A H1N1 drugs. Our research group combined in silico methods to create a compound derived from oseltamivir to be tested in vitro against influenza A H1N1. Here we show the results of a new compound derived from oseltamivir but with specific chemical modifications, with significant affinity either on NA (in silico and in vitro assays) or HA (in silico) from influenza A H1N1 strain. We include docking and molecular dynamics (MD) simulations of the oseltamivir derivative at the binding site onto NA and HA of influenza A H1N1. Additionally, the biological experimental results show that oseltamivir derivative decreases the lytic-plaque formation on viral susceptibility assays, and it does not show cytotoxicity. Finally, oseltamivir derivative assayed on viral NA showed a concentration-dependent inhibition behavior at nM, depicting a high affinity of the compound for the enzyme, corroborated with the MD simulations results, placing our designed oseltamivir derivative as a potential antiviral against influenza A H1N1. 相似文献
136.
Flavio Alexandre Carvalho Natália Valadares de Moraes Antônio Eduardo Miller Crotti Eduardo José Crevelin André Gonzaga dos Santos 《化学与生物多样性》2023,20(9):e202300492
Casearia species are found in the America, Africa, Asia, and Australia and present pharmacological activities, besides their traditional uses. Here, we reviewed the chemical composition, content, pharmacological activities, and toxicity of the essential oils (EOs) from Casearia species. The EO physical parameters and leaf botanical characteristics were also described. The bioactivities of the EOs from the leaves and their components include cytotoxicity, anti-inflammatory, antiulcer, antimicrobial, antidiabetic, antioxidant, antifungal, and antiviral activities. The main components associated with these activities are the α-zingiberene, (E)-caryophyllene, germacrene D, bicyclogermacrene, spathulenol, α-humulene, β-acoradiene, and δ-cadinene. Data on the toxicity of these EOs are scarce in the literature. Casearia sylvestris Sw. is the most studied species, presenting more significant pharmacological potential. The chemical variability of EOs components was also investigated for this species. Caseria EOs have relevant pharmacological potential and must be further investigated and exploited. 相似文献
137.
Juan José Pierella Karlusich Eric Pelletier Lucie Zinger Fabien Lombard Adriana Zingone Sébastien Colin Josep M. Gasol Richard G. Dorrell Nicolas Henry Eleonora Scalco Silvia G. Acinas Patrick Wincker Colomban de Vargas Chris Bowler 《Molecular ecology resources》2023,23(1):16-40
Phytoplankton account for >45% of global primary production, and have an enormous impact on aquatic food webs and on the entire Earth System. Their members are found among prokaryotes (cyanobacteria) and multiple eukaryotic lineages containing chloroplasts. Genetic surveys of phytoplankton communities generally consist of PCR amplification of bacterial (16S), nuclear (18S) and/or chloroplastic (16S) rRNA marker genes from DNA extracted from environmental samples. However, our appreciation of phytoplankton abundance or biomass is limited by PCR-amplification biases, rRNA gene copy number variations across taxa, and the fact that rRNA genes do not provide insights into metabolic traits such as photosynthesis. Here, we targeted the photosynthetic gene psbO from metagenomes to circumvent these limitations: the method is PCR-free, and the gene is universally and exclusively present in photosynthetic prokaryotes and eukaryotes, mainly in one copy per genome. We applied and validated this new strategy with the size-fractionated marine samples collected by Tara Oceans, and showed improved correlations with flow cytometry and microscopy than when based on rRNA genes. Furthermore, we revealed unexpected features of the ecology of these ecosystems, such as the high abundance of picocyanobacterial aggregates and symbionts in the ocean, and the decrease in relative abundance of phototrophs towards the larger size classes of marine dinoflagellates. To facilitate the incorporation of psbO in molecular-based surveys, we compiled a curated database of >18,000 unique sequences. Overall, psbO appears to be a promising new gene marker for molecular-based evaluations of entire phytoplankton communities. 相似文献
138.
Coelho-Rocha Nina Dias de Jesus Luís Cláudio Lima Barroso Fernanda Alvarenga Lima da Silva Tales Fernando Ferreira Enio Gonçalves José Eduardo dos Santos Martins Flaviano de Oliveira Carvalho Rodrigo Dias Barh Debmalya Azevedo Vasco Ariston de Carvalho 《Probiotics and antimicrobial proteins》2023,15(1):160-174
Probiotics and Antimicrobial Proteins - Beneficial effects of Lactiplantibacillus plantarum strains have been widely reported. Knowing that the effects of probiotic bacteria are strain-dependent,... 相似文献
139.
140.
Aonio Bernao Isabel Meseguer María Victorina Aguilar María Carmen Martínez Para María José González Mu?oz 《Journal of trace elements in medicine and biology》2004,18(1):33-39
This 12-day study was conducted to evaluate the effects of three different levels of dietary chromium (100, 200, and 500 microg/day) in the form of chromium picolinate (CrPic) on growth and protein use in weaned rats. No significant effect of CrPic on body weight gain, food intake, or food conversion rate was observed. Elevated doses of CrPic seemed to increase muscle mass, either by stimulating protein anabolism by activation of insulin by chromium or by lowering protein degradation. However, these effects had no repercussions on overall growth, suggesting that any anabolic effect of chromium due to the action of insulin was probably marginal. 相似文献