全文获取类型
收费全文 | 4938篇 |
免费 | 421篇 |
国内免费 | 3篇 |
专业分类
5362篇 |
出版年
2024年 | 3篇 |
2023年 | 32篇 |
2022年 | 77篇 |
2021年 | 135篇 |
2020年 | 69篇 |
2019年 | 101篇 |
2018年 | 92篇 |
2017年 | 95篇 |
2016年 | 173篇 |
2015年 | 284篇 |
2014年 | 307篇 |
2013年 | 382篇 |
2012年 | 462篇 |
2011年 | 463篇 |
2010年 | 274篇 |
2009年 | 260篇 |
2008年 | 361篇 |
2007年 | 329篇 |
2006年 | 318篇 |
2005年 | 278篇 |
2004年 | 240篇 |
2003年 | 218篇 |
2002年 | 182篇 |
2001年 | 21篇 |
2000年 | 16篇 |
1999年 | 29篇 |
1998年 | 33篇 |
1997年 | 22篇 |
1996年 | 29篇 |
1995年 | 10篇 |
1994年 | 6篇 |
1993年 | 9篇 |
1992年 | 4篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 4篇 |
1988年 | 7篇 |
1987年 | 1篇 |
1985年 | 2篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有5362条查询结果,搜索用时 0 毫秒
81.
Evaluation of Postharvest-Processed Oysters by Using PCR-Based Most-Probable-Number Enumeration of Vibrio vulnificus Bacteria 下载免费PDF全文
Anita C. Wright Victor Garrido Georgia Debuex Melissa Farrell-Evans Archana A. Mudbidri W. Steven Otwell 《Applied microbiology》2007,73(22):7477-7481
Postharvest processing (PHP) is used to reduce levels of Vibrio vulnificus in oysters, but process validation is labor-intensive and expensive. Therefore, quantitative PCR was evaluated as a rapid confirmation method for most-probable-number enumeration (QPCR-MPN) of V. vulnificus bacteria in PHP oysters. QPCR-MPN showed excellent correlation (R2 = 0.97) with standard MPN and increased assay sensitivity and efficiency. 相似文献
82.
Zhu XY Mardenborough LG Li S Khan A Zhang W Fan P Jacob M Khan S Walker L Ablordeppey SY 《Bioorganic & medicinal chemistry》2007,15(2):686-695
Isosteres of cryptolepine (1) were synthesized and evaluated for their antiinfective activities. Overall, the sulfur isostere, 5-methyl benzothieno[3,2-b]quinolinium salt (5b), was equipotent to 1 and has shown no cytotoxicity at 23.8 microg/mL. Compound 5b was also found to have a broad spectrum of activity. Both the carbon and oxygen isosteres were less potent than cryptolepine. A limited library of 2-substituted analogs of 5b has been synthesized and evaluated in antifungal screens but did not show increase in potency compared to the unsubstituted 5b. Similarly, evaluation of tricyclic benzothieno[3,2-b]pyridines while showing promise in individual screens did not produce an overall increase in potency. Overall, the evaluation of the activities of 5b compared with standard antifungal/anti-protozoal agents suggests that the benzothienoquinoline scaffold could serve as a lead for optimization. 相似文献
83.
Rhizoeconomics: Carbon costs of phosphorus acquisition 总被引:4,自引:0,他引:4
Plants display a wide array of physiological adaptations to low soil phosphorus availability. Here we discuss metabolic and
ecological costs associated with these strategies, focusing on the carbon costs of root traits related to phosphorus acquisition
in crop plants. We propose that such costs are an important component of adaptation to low phosphorus soils. In common bean,
genotypes with superior low phosphorus adaptation express traits that reduce the respiratory burden of root growth, including
greater allocation to metabolically inexpensive root classes, such as adventitious roots, and greater formation of cortical
aerenchyma, which reduces specific root respiration. Root hair formation increases phosphorus acquisition at minimal carbon
cost, but may have other unknown ecological costs. Mycorrhizas and root exudates enhance phosphorus acquisition in some taxa,
but at significant carbon cost. Root architectural patterns that enhance topsoil foraging enhance phosphorus acquisition but
appear to incur tradeoffs for water acquisition and spatial competition. A better understanding of the metabolic and ecological
costs associated with phosphorus acquisition strategies is needed for an intelligent deployment of such traits in crop improvement
programs. 相似文献
84.
85.
Divya Sitaraman Melissa Zars Troy Zars 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2010,196(4):299-305
The biogenic amines play a critical role in establishing memories. In the insects, octopamine, dopamine, and serotonin have
key functions in memory formation. For Drosophila, octopamine is necessary and sufficient for appetitive olfactory memory formation. Whether octopamine plays a general role
in reinforcing memories in the fly is not known. Place learning in the heat-box associates high temperatures with one part
of a narrow chamber, and a cool, strongly preferred temperature with the other half of the chamber. The cool-temperature-associated
chamber half could provide a rewarding stimulus to a fly, and thus a place memory is composed of an aversive and rewarded
memory component. The role of octopamine in place memory was thus tested. Using a mutation in the tyramine beta hydroxylase (TβH[M18]) and blocking of evoked synaptic transmission in the octopamine (and tyramine) neurons labeled with a tyramine decarboxylase-2 (TDC2) gene regulatory elements we found that reinforcement of place memories is independent of normal octopamine signaling.
Thus, reinforcing mechanisms in Drosophila have specialized systems in the formation of specific memory types. 相似文献
86.
Tatarkiewicz K Smith PA Sablan EJ Polizzi CJ Aumann DE Villescaz C Hargrove DM Gedulin BR Lu MG Adams L Whisenant T Roy D Parkes DG 《American journal of physiology. Endocrinology and metabolism》2010,299(6):E1076-E1086
The risk of developing pancreatitis is elevated in type 2 diabetes and obesity. Cases of pancreatitis have been reported in type 2 diabetes patients treated with GLP-1 (GLP-1R) receptor agonists. To examine whether the GLP-1R agonist exenatide potentially induces or modulates pancreatitis, the effect of exenatide was evaluated in normal or diabetic rodents. Normal and diabetic rats received a single exenatide dose (0.072, 0.24, and 0.72 nmol/kg) or vehicle. Diabetic ob/ob or HF-STZ mice were infused with exenatide (1.2 and 7.2 nmol·kg(-1)·day(-1)) or vehicle for 4 wk. Post-exenatide treatment, pancreatitis was induced with caerulein (CRN) or sodium taurocholate (ST), and changes in plasma amylase and lipase were measured. In ob/ob mice, plasma cytokines (IL-1β, IL-2, IL-6, MCP-1, IFNγ, and TNFα) and pancreatitis-associated genes were assessed. Pancreata were weighed and examined histologically. Exenatide treatment alone did not modify plasma amylase or lipase in any models tested. Exenatide attenuated CRN-induced release of amylase and lipase in normal rats and ob/ob mice but did not modify the response to ST infusion. Plasma cytokines and pancreatic weight were unaffected by exenatide. Exenatide upregulated Reg3b but not Il6, Ccl2, Nfkb1, or Vamp8 expression. Histological analysis revealed that the highest doses of exenatide decreased CRN- or ST-induced acute inflammation, vacuolation, and acinar single cell necrosis in mice and rats, respectively. Ductal cell proliferation rates were low and similar across all groups of ob/ob mice. In conclusion, exenatide did not modify plasma amylase and lipase concentrations in rodents without pancreatitis and improved chemically induced pancreatitis in normal and diabetic rodents. 相似文献
87.
Anne E. Powell Chia-Yi Shung Katherine W. Saylor Karin A. Müllendorf Joseph B. Weiss Melissa H. Wong 《Stem cell research》2010,4(1):3-9
Asymmetric stem cell division has emerged as a major regulatory mechanism for physiologic control of stem cell numbers. Reinvigoration of the cancer stem cell theory suggests that tumorigenesis may be regulated by maintaining the balance between asymmetric and symmetric cell division. Therefore, mutations affecting this balance could result in aberrant expansion of stem cells. Although a number of molecules have been implicated in regulation of asymmetric stem cell division, here, we highlight known tumor suppressors with established roles in this process. While a subset of these tumor suppressors were originally defined in developmental contexts, recent investigations reveal they are also lost or mutated in human cancers. Mutations in tumor suppressors involved in asymmetric stem cell division provide mechanisms by which cancer stem cells can hyperproliferate and offer an intriguing new focus for understanding cancer biology. Our discussion of this emerging research area derives insight from a frontier area of basic science and links these discoveries to human tumorigenesis. This highlights an important new focus for understanding the mechanism underlying expansion of cancer stem cells in driving tumorigenesis. 相似文献
88.
Background
The mechanisms by which humans regulate pro- and anti-inflammatory responses on exposure to different malaria parasites remains unclear. Although Plasmodium vivax usually causes a relatively benign disease, this parasite has been suggested to elicit more host inflammation per parasitized red blood cell than P. falciparum.Methodology/Principal Findings
We measured plasma concentrations of seven cytokines and two soluble tumor necrosis factor (TNF)-α receptors, and evaluated clinical and laboratory outcomes, in Brazilians with acute uncomplicated infections with P. vivax (n = 85), P. falciparum (n = 30), or both species (n = 12), and in 45 asymptomatic carriers of low-density P. vivax infection. Symptomatic vivax malaria patients, compared to those infected with P. falciparum or both species, had more intense paroxysms, but they had no clear association with a pro-inflammatory imbalance. To the contrary, these patients had higher levels of the regulatory cytokine interleukin (IL)-10, which correlated positively with parasite density, and elevated IL-10/TNF-α, IL-10/interferon (IFN)-γ, IL-10/IL-6 and sTNFRII/TNF-α ratios, compared to falciparum or mixed-species malaria patient groups. Vivax malaria patients had the highest levels of circulating soluble TNF-α receptor sTNFRII. Levels of regulatory cytokines returned to normal values 28 days after P. vivax clearance following chemotherapy. Finally, asymptomatic carriers of low P. vivax parasitemias had substantially lower levels of both inflammatory and regulatory cytokines than did patients with clinical malaria due to either species.Conclusions
Controlling fast-multiplying P. falciparum blood stages requires a strong inflammatory response to prevent fulminant infections, while reducing inflammation-related tissue damage with early regulatory cytokine responses may be a more cost-effective strategy in infections with the less virulent P. vivax parasite. The early induction of regulatory cytokines may be a critical mechanism protecting vivax malaria patients from severe clinical complications. 相似文献89.
The repair of DNA double-stranded breaks (DSBs) is essential for cell viability and genome stability. Aberrant repair of DSBs has been linked with cancer predisposition and aging. During the repair of DSBs by non-homologous end joining (NHEJ), DNA ends are brought together, processed and then joined. In eukaryotes, this repair pathway is initiated by the binding of the ring-shaped Ku heterodimer and completed by DNA ligase IV. The DNA ligase IV complex, DNA ligase IV/XRRC4 in humans and Dnl4/Lif1 in yeast, is recruited to DNA ends in vitro and in vivo by an interaction with Ku and, in yeast, Dnl4/Lif1 stabilizes the binding of yKu to in vivo DSBs. Here we have analyzed the interactions of these functionally conserved eukaryotic NHEJ factors with DNA by electron microscopy. As expected, the ring-shaped Ku complex bound stably and specifically to DNA ends at physiological salt concentrations. At a ratio of 1 Ku molecule per DNA end, the majority of DNA ends were occupied by a single Ku complex with no significant formation of linear DNA multimers or circular loops. Both Dnl4/Lif1 and DNA ligase IV/XRCC4 formed complexes with Ku-bound DNA ends, resulting in intra- and intermolecular DNA end bridging, even with non-ligatable DNA ends. Together, these studies, which provide the first visualization of the conserved complex formed by Ku and DNA ligase IV at juxtaposed DNA ends by electron microscopy, suggest that the DNA ligase IV complex mediates end-bridging by engaging two Ku-bound DNA ends. 相似文献
90.