全文获取类型
收费全文 | 6928篇 |
免费 | 681篇 |
国内免费 | 3篇 |
出版年
2024年 | 7篇 |
2023年 | 67篇 |
2022年 | 132篇 |
2021年 | 279篇 |
2020年 | 155篇 |
2019年 | 227篇 |
2018年 | 183篇 |
2017年 | 192篇 |
2016年 | 301篇 |
2015年 | 517篇 |
2014年 | 564篇 |
2013年 | 585篇 |
2012年 | 745篇 |
2011年 | 668篇 |
2010年 | 382篇 |
2009年 | 330篇 |
2008年 | 421篇 |
2007年 | 381篇 |
2006年 | 321篇 |
2005年 | 286篇 |
2004年 | 245篇 |
2003年 | 168篇 |
2002年 | 146篇 |
2001年 | 31篇 |
2000年 | 14篇 |
1999年 | 26篇 |
1998年 | 13篇 |
1997年 | 15篇 |
1996年 | 12篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 6篇 |
1988年 | 10篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1974年 | 5篇 |
1973年 | 8篇 |
1972年 | 7篇 |
1969年 | 5篇 |
1912年 | 5篇 |
排序方式: 共有7612条查询结果,搜索用时 15 毫秒
121.
Pollinators provide an important service in many crops. Managed honey bees (Apis mellifera L.) are used to supplement pollination services provided by wild bees with the assumption that they will enhance pollination, fruit set and crop yield beyond the levels provided by the wild bees. Recent declines in managed honey bee populations have stimulated interest in finding alternative managed pollinators to service crops. In the eastern U.S., managed hives of the native common eastern bumble bee (Bombus impatiens Cresson) may be an excellent choice. To examine this issue, a comprehensive 2-yr study was conducted to compare fruit yield and bee visits to flowers in pumpkin (Cucurbita pepo L.) fields that were either supplemented with A. mellifera hives, B. impatiens hives or were not supplemented. We compared pumpkin yield, A. mellifera flower visitation frequency and B. impatiens flower visitation frequency between treatments. Results indicated that supplementing pumpkin fields with either A. mellifera or B. impatiens hives did not increase their visitation to pumpkin flowers or fruit yield compared with those that were not supplemented. Next, the relationship between frequency of pumpkin flower visitation by the most prominent bee species (Peponapis pruinosa (Say), B. impatiens and A. mellifera) and fruit yield was determined across all pumpkin fields sampled. Fruit yield increased as the frequency of flower visits by A. mellifera and B. impatiens increased in 2011 and 2012, respectively. These results suggest that supplementation with managed bees may not improve pumpkin production and that A. mellifera and B. impatiens are important pollinators of pumpkin in our system. 相似文献
122.
H. Irene Hall Jessica Halverson David P. Wilson Barbara Suligoi Mercedes Diez Stéphane Le Vu Tian Tang Ann McDonald Laura Camoni Caroline Semaille Chris Archibald 《PloS one》2013,8(11)
Background
Testing for HIV infection and entry to care are the first steps in the continuum of care that benefit individual health and may reduce onward transmission of HIV. We determined the percentage of people with HIV who were diagnosed late and the percentage linked into care overall and by demographic and risk characteristics by country.Methods
Data were analyzed from national HIV surveillance systems. Six countries, where available, provided data on two late diagnosis indicators (AIDS diagnosis within 3 months of HIV diagnosis, and AIDS diagnosis within 12 months before HIV diagnosis) and linkage to care (≥1 CD4 or viral load test result within 3 months of HIV diagnosis) for people diagnosed with HIV in 2009 or 2010 (most recent year data were available).Principal Findings
The percentage of people presenting with late stage disease at HIV diagnosis varied by country, overall with a range from 28.7% (United States) to 8.8% (Canada), and by transmission categories. The percentage of people diagnosed with AIDS who had their initial HIV diagnosis within 12 months before AIDS diagnosis varied little among countries, except the percentages were somewhat lower in Spain and the United States. Overall, the majority of people diagnosed with HIV were linked to HIV care within 3 months of diagnosis (more than 70%), but varied by age and transmission category.Conclusions
Differences in patterns of late presentation at HIV diagnosis among countries may reflect differences in screening practices by providers, public health agencies, and people with HIV. The percentage of people who received assessments of immune status and viral load within 3 months of diagnosis was generally high. 相似文献123.
do Nascimento Jessica Catarine Frutuoso Beltrão Eduardo Isidoro Carneiro Rocha Cíntia Renata Costa 《Glycoconjugate journal》2020,37(2):263-275
Glycoconjugate Journal - The complex enzyme network responsible for glycan synthesis suffers significant changes during the first steps of tumor development, leading to the early formation of... 相似文献
124.
125.
Severo Juliana Soares Morais Jennifer Beatriz Silva Beserra Jessica Batista dos Santos Loanne Rocha de Sousa Melo Stéfany Rodrigues de Sousa Gustavo Santos de Matos Neto Emídio Marques Henriques Gilberto Simeone do Nascimento Marreiro Dilina 《Biological trace element research》2020,193(1):81-88
Biological Trace Element Research - Excessive adipose tissue promotes the manifestation of endocrine disorders such as reduction of the secretion of zinc-α2-glycoprotein (ZAG), an adipokine... 相似文献
126.
García-Tuñón Ignacio Vuelta Elena Lozano Laura Herrero María Méndez Lucía Palomero-Hernandez Javier Pérez-Caro María Pérez-García Jessica González-Sarmiento Rogelio Sánchez-Martín Manuel 《Molecular biology reports》2020,47(2):1381-1391
Molecular Biology Reports - The Nomo1 gene mediates a wide range of biological processes of importance in embryonic development. Accordingly, constitutive perturbation of Nomo1 function may result... 相似文献
127.
Alexis Rodriguez Marie Ø. Pedersen Elba Villegas Bruno Rivas-Santiago Jessica Villegas-Moreno Carlos Amero Raymond S. Norton Gerardo Corzo 《Proteins》2020,88(1):175-186
The spread of multidrug resistant bacteria owing to the intensive use of antibiotics is challenging current antibiotic therapies, and making the discovery and evaluation of new antimicrobial agents a high priority. The evaluation of novel peptide sequences of predicted antimicrobial peptides from different sources is valuable approach to identify alternative antibiotic leads. Two strategies were pursued in this study to evaluate novel antimicrobial peptides from the human β-defensin family (hBD). In the first, a 32-residue peptide was designed based on the alignment of all available hBD primary structures, while in the second a putative 35-residue peptide, hBD10, was mined from the gene DEFB110. Both hBDconsensus and hBD10 were chemically synthesized, folded and purified. They showed antimicrobial activity against Escherichia coli, Staphylococcus aureus, and Mycobacterium tuberculosis, but were not hemolytic on human red blood cells. The NMR-based solution structure of hBDconsensus revealed that it adopts a classical β-defensin fold and disulfide connectivities. Even though the mass spectrum of hBD10 confirmed the formation of three disulfide bonds, it showed limited dispersion in 1H NMR spectra and structural studies were not pursued. The evaluation of different β-defensin structures may identify new antimicrobial agents effective against multidrug-resistant bacterial strains. 相似文献
128.
Melissa Holstein Jessica Hung Hasin Feroz Swarnim Ranjan Cheng Du Sanchayita Ghose Zheng Jian Li 《Biotechnology and bioengineering》2020,117(11):3591-3606
To achieve the high protein concentrations required for subcutaneous administration of biologic therapeutics, numerous manufacturing process challenges are often encountered. From an operational perspective, high protein concentrations result in highly viscous solutions, which can cause pressure increases during ultrafiltration. This can also lead to low flux during ultrafiltration and sterile filtration, resulting in long processing times. In addition, there is a greater risk of product loss from the hold-up volumes during filtration operations. From a formulation perspective, higher protein concentrations present the risk of higher aggregation rates as the closer proximity of the constituent species results in stronger attractive intermolecular interactions and higher frequency of self-association events. There are also challenges in achieving pH and excipient concentration targets in the ultrafiltration/diafiltration (UF/DF) step due to volume exclusion and Donnan equilibrium effects, which are exacerbated at higher protein concentrations. This paper highlights strategies to address these challenges, including the use of viscosity-lowering excipients, appropriate selection of UF/DF cassettes with modified membranes and/or improved flow channel design, and increased understanding of pH and excipient behavior during UF/DF. Additional considerations for high-concentration drug substance manufacturing, such as appearance attributes, stability, and freezing and handling are also discussed. These strategies can be employed to overcome the manufacturing process challenges and streamline process development efforts for high-concentration drug substance manufacturing. 相似文献
129.
Jessica R. Honorato Rachel A. Hauser-Davis Enrico M. Saggioro Fábio V. Correia Sidney F. Sales-Junior Lorena O. S. Soares Leandro da R. Lima Vivaldo Moura-Neto Giselle P. de F. Lopes Tania C. L. de S. Spohr 《Journal of cellular physiology》2020,235(4):3798-3814
The first-line chemotherapy treatment for Glioblastoma (GBM) - the most aggressive and frequent brain tumor - is temozolomide (TMZ). The Sonic hedgehog (SHH) pathway is involved with GBM tumorigenesis and TMZ chemoresistance. The role of SHH pathway inhibition in the potentiation of TMZ's effects using T98G, U251, and GBM11 cell lines is investigated herein. The combination of GANT-61 and TMZ over 72 hr suggested a synergistic effect. All TMZ-resistant cell lines displayed a significant decrease in cell viability, increased DNA fragmentation and loss of membrane integrity. For T98G cells, G2/M arrest was observed, while U251 cells presented a significant increase in reactive oxygen species production and catalase activity. All the cell lines presented acidic vesicles formation correlated to Beclin-1 overexpression. The combined treatment also enhanced GLI1 expression, indicating the presence of select resistant cells. The selective inhibition of the SHH pathway potentiated the cytotoxic effect of TMZ, thus becoming a promising in vitro strategy for GBM treatment. 相似文献
130.
Feminization of complex traits in Drosophila melanogaster via female-limited X chromosome evolution*
Katrine K. Lund-Hansen Jessica K. Abbott Edward H. Morrow 《Evolution; international journal of organic evolution》2020,74(12):2703-2713
A handful of studies have investigated sexually antagonistic constraints on achieving sex-specific fitness optima, although exclusively through male-genome-limited evolution experiments. In this article, we established a female-limited X chromosome evolution experiment, where we used an X chromosome balancer to enforce the inheritance of the X through the matriline, thus removing exposure to male selective constraints. This approach eliminates the effects of sexually antagonistic selection on the X chromosome, permitting evolution toward a single sex-specific optimum. After multiple generations of selection, we found strong evidence that body size and development time had moved toward a female-specific optimum, whereas reproductive fitness and locomotion activity remained unchanged. The changes in body size and development time are consistent with previous results, and suggest that the X chromosome is enriched for sexually antagonistic genetic variation controlling these particular traits. The lack of change in reproductive fitness and locomotion activity could be due to a number of mutually nonexclusive explanations, including a lack of sexually antagonistic variance on the X chromosome for those traits or confounding effects of the use of the balancer chromosome. This study is the first to employ female-genome-limited selection and adds to the understanding of the complexity of sexually antagonistic genetic variation. 相似文献