全文获取类型
收费全文 | 8582篇 |
免费 | 761篇 |
国内免费 | 3篇 |
专业分类
9346篇 |
出版年
2024年 | 7篇 |
2023年 | 43篇 |
2022年 | 89篇 |
2021年 | 156篇 |
2020年 | 120篇 |
2019年 | 164篇 |
2018年 | 160篇 |
2017年 | 157篇 |
2016年 | 254篇 |
2015年 | 480篇 |
2014年 | 470篇 |
2013年 | 524篇 |
2012年 | 764篇 |
2011年 | 738篇 |
2010年 | 400篇 |
2009年 | 391篇 |
2008年 | 571篇 |
2007年 | 518篇 |
2006年 | 528篇 |
2005年 | 504篇 |
2004年 | 490篇 |
2003年 | 422篇 |
2002年 | 373篇 |
2001年 | 113篇 |
2000年 | 70篇 |
1999年 | 88篇 |
1998年 | 93篇 |
1997年 | 72篇 |
1996年 | 60篇 |
1995年 | 58篇 |
1994年 | 62篇 |
1993年 | 31篇 |
1992年 | 41篇 |
1991年 | 37篇 |
1990年 | 31篇 |
1989年 | 22篇 |
1988年 | 26篇 |
1987年 | 20篇 |
1986年 | 24篇 |
1985年 | 18篇 |
1984年 | 30篇 |
1983年 | 17篇 |
1982年 | 17篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 8篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 6篇 |
1973年 | 17篇 |
排序方式: 共有9346条查询结果,搜索用时 15 毫秒
1.
Igor Loncaric Gabrielle L. Stalder Kemal Mehinagic Renate Rosengarten Franz Hoelzl Felix Knauer Chris Walzer 《PloS one》2013,8(12)
In order to test whether rooks (Corvus frugilegus) represent good indicators for the potential circulation of antibiotics in their native habitat, two populations with different migratory behavior were tested for the presence of beta-lactamase producing Enterobacteriaceae and methicillin-resistant Staphylococcus aureus (MRSA). In all, 54 and 102 samples of fresh feces of a migratory and a resident population were investigated. A total of 24 and 3 cefotaxime-resistant enterobacterial isolates were obtained from the migratory and resident population, respectively. In these isolates CTX-M-1 (n = 15), CTX-M-3 (n = 3), and CTX-M-15 (n = 3) genes were detected. TEM-1 and OXA-1 were associated with CTX-M in 3 and 2 isolates, respectively. In two E. coli isolates CMY-2 could be detected, where from one isolate displayed an overexpression of chromosomal AmpC as well. Among E. coli isolates the most common phylogenetic group was A (n = 11) and ST1683 (n = 5). In one E. coli of B2-ST131 the rfbO25b locus was detected. Three Enterobacter isolates were stably derepressed AmpC-producers. In five samples of the migratory population, PVL positive MRSA could be isolated. Two isolates were typed SCCmec IVa, spa type t127, and ST1. Three isolates carried a SCCmec type IVc, with spa type t852 and ST22. The highly significant difference of the occurrence of antibiotic resistance between the migratory population from eastern Europe compared to resident population in our study indicates that rooks may be good indicator species for the evaluation of environmental contamination with antibiotic resistant bacteria, especially due to their ecology, foraging behavior and differing migratory behavior. 相似文献
2.
3.
Eliot C. Bush Anne E. Clark Chris M. DeBoever Lillian E. Haynes Sidra Hussain Singer Ma Matthew M. McDermott Adam M. Novak John S. Wentworth 《PloS one》2012,7(11)
A significant proportion of enzymes display cooperativity in binding ligand molecules, and such effects have an important impact on metabolic regulation. This is easiest to understand in the case of positive cooperativity. Sharp responses to changes in metabolite concentrations can allow organisms to better respond to environmental changes and maintain metabolic homeostasis. However, despite the fact that negative cooperativity is almost as common as positive, it has been harder to imagine what advantages it provides. Here we use computational models to explore the utility of negative cooperativity in one particular context: that of an inhibitor binding to an enzyme. We identify several factors which may contribute, and show that acting together they can make negative cooperativity advantageous. 相似文献
4.
5.
6.
Lyndsey R. Buckner Angela M. Amedee Hannah L. Albritton Pamela A. Kozlowski Nedra Lacour Chris L. McGowin Danny J. Schust Alison J. Quayle 《PloS one》2016,11(1)
Chlamydia trachomatis causes a predominantly asymptomatic, but generally inflammatory, genital infection that is associated with an increased risk for HIV acquisition. Endocervical epithelial cells provide the major niche for this obligate intracellular bacterium in women, and the endocervix is also a tissue in which HIV transmission can occur. The mechanism by which CT infection enhances HIV susceptibility at this site, however, is not well understood. Utilizing the A2EN immortalized endocervical epithelial cell line grown on cell culture inserts, we evaluated the direct role that CT-infected epithelial cells play in facilitating HIV transmission events. We determined that CT infection significantly enhanced the apical-to-basolateral migration of cell-associated, but not cell-free, HIVBaL, a CCR5-tropic strain of virus, across the endocervical epithelial barrier. We also established that basolateral supernatants from CT-infected A2EN cells significantly enhanced HIV replication in peripheral mononuclear cells and a CCR5+ T cell line. These results suggest that CT infection of endocervical epithelial cells could facilitate both HIV crossing the mucosal barrier and subsequent infection or replication in underlying target cells. Our studies provide a mechanism by which this common STI could potentially promote the establishment of founder virus populations and the maintenance of local HIV reservoirs in the endocervix. Development of an HIV/STI co-infection model also provides a tool to further explore the role of other sexually transmitted infections in enhancing HIV acquisition. 相似文献
7.
Summary Equivalent-circuit impedance analysis experiments were performed on the urinary bladders of freshwater turtles in order to quantify membrane ionic conductances and areas, and to investigate how changes in these parameters are associated with changes in the rate of proton secretion in this tissue. In all experiments, sodium reabsorption was inhibited thereby unmasking the electrogenic proton secretion process. We report the following: (1) transepithelial impedance is represented exceptionally well by a simple equivalent-circuit model, which results in estimates of the apical and basolateral membrane ionic conductances and capacitances; (2) when sodium transport is inhibited with mucosal amiloride and serosal ouabain, the apical and basolateral membrane conductances and capacitances exhibit a continual decline with time; (3) this decline in the membrane parameters is most likely caused by subtle time-dependent changes in cell volume, resulting in changes in the areas of the apical and basolateral membranes; (4) stable membrane parameters are obtained if the tissue is not treated with ouabain, and if the oncotic pressure of the serosal solution is increased by the addition of 2% albumin; (5) inhibition of proton secretion using acetazolamide in CO2 and HCO
3
–
-free bathing solutions results in a decrease in the area of the apical membrane, with no significant change in its specific conductance; (6) stimulation of proton transport with CO2 and HCO
3
–
-containing serosal solution results in an increase in the apical membrane area and specific conductance. These results show that our methods can be used to measure changes in the membrane electrophysiological parameters that are related to changes in the rate of proton transport. Notably, they can be used to quantify in the live tissue, changes in membrane area resulting from changes in the net rates of endocytosis and exocytosis which are postulated to be intimately involved in the regulation of proton transport. 相似文献
8.
Jason H Mateika Chris Mendello Dany Obeid M Safwan Badr 《Journal of applied physiology》2004,96(3):1197-205; discussion 1196
We hypothesized that the acute ventilatory response to hypoxia is enhanced after exposure to episodic hypoxia in awake humans. Eleven subjects completed a series of rebreathing trials before and after exposure to eight 4-min episodes of hypoxia. During the rebreathing trials, subjects initially hyperventilated to reduce the partial pressure of carbon dioxide (Pet(CO(2))) below 25 Torr. Subjects then breathed from a bag containing normocapnic (42 Torr), low (50 Torr), or high oxygen (140 Torr) gas mixtures. During the trials, Pet(CO(2)) increased while a constant oxygen level was maintained. The point at which ventilation began to rise in a linear fashion as Pet(CO(2)) increased was considered to be the ventilatory recruitment threshold. The ventilatory response below and above the recruitment threshold was determined. Ventilation did not persist above baseline values immediately after exposure to episodic hypoxia; however, Pet(CO(2)) levels were reduced compared with baseline. In contrast, compared with baseline, the ventilatory response to progressive increases in carbon dioxide during rebreathing trials in the presence of low but not high oxygen levels was increased after exposure to episodic hypoxia. This increase occurred when carbon dioxide levels were above but not below the ventilatory recruitment threshold. We conclude that long-term facilitation of ventilation (i.e., increases in ventilation that persist when normoxia is restored after episodic hypoxia) is not expressed in awake humans in the presence of hypocapnia. Nevertheless, despite this lack of expression, the acute ventilatory response to hypoxia in the presence of hypercapnia is increased after exposure to episodic hypoxia. 相似文献
9.
10.
Fingerlings of brown trout ( Salmo trulta m. fario L.) were introduced to sections of different types of streams situated in natural catchments and those modified by Man's activity. At stations where environmental conditions were modified by such forms of impact as pollution, flow variability and impoundment, trout did not survive 5 months. In the natural river sections mortality rates increased downstream along the river continuum and were associated with increased predation. Growth rates in the upper reaches were primarily restricted by abiotic factors—temperature and trophic status: however, they were to a large extent modified by density-dependent regulation and intraspecific competition. The influence of the abiotic/biotic regulatory process, expressed as fish metabolic performance, is discussed as a framework for the determination of the carrying capacity of the riverine ecosystem. 相似文献