全文获取类型
收费全文 | 1176篇 |
免费 | 124篇 |
专业分类
1300篇 |
出版年
2021年 | 15篇 |
2019年 | 18篇 |
2018年 | 9篇 |
2017年 | 18篇 |
2016年 | 32篇 |
2015年 | 38篇 |
2014年 | 38篇 |
2013年 | 63篇 |
2012年 | 57篇 |
2011年 | 70篇 |
2010年 | 61篇 |
2009年 | 48篇 |
2008年 | 60篇 |
2007年 | 61篇 |
2006年 | 61篇 |
2005年 | 63篇 |
2004年 | 71篇 |
2003年 | 60篇 |
2002年 | 35篇 |
2001年 | 23篇 |
2000年 | 19篇 |
1999年 | 14篇 |
1998年 | 18篇 |
1997年 | 21篇 |
1996年 | 6篇 |
1995年 | 14篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 18篇 |
1991年 | 18篇 |
1990年 | 14篇 |
1989年 | 17篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 13篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1983年 | 13篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 8篇 |
1974年 | 12篇 |
1972年 | 10篇 |
1971年 | 9篇 |
1970年 | 6篇 |
1967年 | 6篇 |
排序方式: 共有1300条查询结果,搜索用时 0 毫秒
41.
Ecosystem tracer-level additions would benefit from a stable isotope-labeled source of complex organic molecules. We tested
a method to label tree C with 13C and create a stable isotope tracer for stream dissolved organic carbon (DOC) using tulip poplar (Liriodendron tulipifera L.) seedlings. In 2000, seedlings were grown with 0.82 moles of 13CO2 to assess the distribution and level of 13C enrichment in the tree tissues. In 2001, seedlings were grown with 25 times more 13CO2 to generate tissues with a 13C signal strong enough for a 13C-DOC stream tracer addition. 13C enrichment in the trees varied in each year and by tissue age and type. Tissues formed during labeling (new) were more enriched
in 13C than tissues established prior to the 13CO2 injection (old). Stems were most enriched in 13C in both new and old tissues. A higher percentage of 13CO2 was incorporated into seedlings in 2000 (59% ±1) than 2001 (43% ±0). Percent 13C incorporation among tree tissue types paralleled biomass distributions. Although tree C and 13C were equally soluble in both years, a greater percentage of tree C went into solution in 2001 (30%) than 2000 (20%). The
water-soluble tree C accounted for approximately 12% of the injected 13CO2 and had both humic and polysaccharide components. Results from a whole-stream 13C-DOC tracer addition demonstrated that tree C could be sufficiently labeled with 13CO2 to create a stream DOC isotope tracer with some polymeric constituents. 相似文献
42.
43.
Raina K. Plowright Peggy Eby Peter J. Hudson Ina L. Smith David Westcott Wayne L. Bryden Deborah Middleton Peter A. Reid Rosemary A. McFarlane Gerardo Martin Gary M. Tabor Lee F. Skerratt Dale L. Anderson Gary Crameri David Quammen David Jordan Paul Freeman Lin-Fa Wang Jonathan H. Epstein Glenn A. Marsh Nina Y. Kung Hamish McCallum 《Proceedings. Biological sciences / The Royal Society》2015,282(1798)
Viruses that originate in bats may be the most notorious emerging zoonoses that spill over from wildlife into domestic animals and humans. Understanding how these infections filter through ecological systems to cause disease in humans is of profound importance to public health. Transmission of viruses from bats to humans requires a hierarchy of enabling conditions that connect the distribution of reservoir hosts, viral infection within these hosts, and exposure and susceptibility of recipient hosts. For many emerging bat viruses, spillover also requires viral shedding from bats, and survival of the virus in the environment. Focusing on Hendra virus, but also addressing Nipah virus, Ebola virus, Marburg virus and coronaviruses, we delineate this cross-species spillover dynamic from the within-host processes that drive virus excretion to land-use changes that increase interaction among species. We describe how land-use changes may affect co-occurrence and contact between bats and recipient hosts. Two hypotheses may explain temporal and spatial pulses of virus shedding in bat populations: episodic shedding from persistently infected bats or transient epidemics that occur as virus is transmitted among bat populations. Management of livestock also may affect the probability of exposure and disease. Interventions to decrease the probability of virus spillover can be implemented at multiple levels from targeting the reservoir host to managing recipient host exposure and susceptibility. 相似文献
44.
It is now apparent that the signaling molecule 3',5'-cyclic diguanylic acid (c-di-GMP) is a central regulator of the prokaryote biofilm lifestyle and recent evidence also links this molecule to virulence. Environmentally responsive signal transduction systems that control expression and/or activity of the enzymes (GGDEF and EAL domain containing proteins) that are responsible for synthesis and degradation of c-di-GMP have recently been identified. Members of the phosphorelay family feature prominently amongst these systems, which include several with hybrid polydomain sensors and one that is similar to well-characterized chemotaxis-controlling pathways. These findings support the hypothesis that c-di-GMP levels are tightly controlled in response to a broad range, in terms of both diversity and intensity, of extracellular signals. Insight into how c-di-GMP affects changes in gene expression and/or protein activity has come from the demonstration that proteins containing the PilZ domain can bind c-di-GMP and control phenotypes involved in biofilm formation and virulence. These recent developments should pave the way for researchers to answer the important question of how a vast array of extracellular signals that are sensed by multiple sensory transduction pathways which all lead to the production or destruction of c-di-GMP are coordinated such that the appropriate phenotypic response is produced. 相似文献
45.
Tilman D. Rachner Peggy Benad Martina Rauner Claudia Goettsch Shiv K. Singh Michael Schoppet Lorenz C. Hofbauer 《Journal of cellular biochemistry》2009,108(1):106-116
Osteoprotegerin (OPG) is a decoy receptor for receptor activator of NF‐κB ligand (RANKL) and TNF‐related apoptosis‐inducing ligand (TRAIL). While RANKL is essential for osteoclastogenesis and facilitates breast cancer migration into bone, TRAIL promotes breast cancer apoptosis. We analyzed the expression of OPG and TRAIL and its modulation in estrogen receptor‐positive MCF‐7 cells and receptor‐negative MDA‐MB‐231 cells. In both cells, OPG mRNA levels and protein secretion were dose‐ and time‐dependently enhanced by interleukin (IL)‐1β and suppressed by dexamethasone. In contrast to MCF‐7 cells, MDA‐MB‐231 abundantly expressed TRAIL mRNA, which was enhanced by IL‐1β and inhibited by dexamethasone. TRAIL activated pro‐apoptotic caspase‐3, ‐7, and poly‐ADP‐ribose polymerase and decreased cell numbers of MDA‐MB‐231, but had no effect on MCF‐7 cells. Gene silencing siRNA directed against OPG resulted in a 31% higher apoptotic rate compared to non‐target siRNA‐treated MDA‐MB‐231 cells. Furthermore, TRAIL induced significantly less apoptosis in cells cultured in conditioned media (containing OPG) compared to cells exposed to TRAIL in fresh medium lacking OPG (P < 0.01) and these protective effects were reversed by blocking OPG with its specific ligand RANKL (P < 0.05). The association between cancer cell survival and OPG production by MDA‐MB‐231 cells was further supported by the finding, that modulation of OPG secretion using IL‐1β or dexamethasone prior to TRAIL exposure resulted in decreased and increased rate of apoptosis, respectively (P < 0.05). Thus, OPG secretion by breast cancer cells is modulated by cytokines and dexamethasone, and may represent a critical resistance mechanism that protects against TRAIL‐induced apoptosis. J. Cell. Biochem. 108: 106–116, 2009. © 2009 Wiley‐Liss, Inc. 相似文献
46.
This report describes the responses of an experienced gorilla mother to inappropriate maternal behavior displayed by her young adult daughter toward a newborn baby and repeated acts of baby-transfer between these two females in a captive social group of lowland gorillas (Gorilla g. gorilla). The quality of infant care by the young adult daughter clearly improved during the first 4 days after birth, and this improvement was at least partly based on her mothers encouragement. Thus, the mothers activities can be considered scaffolding for her daughter with regard to maternal infant care. 相似文献
47.
48.
49.
50.
Berlin YA 《Current issues in molecular biology》1999,1(1-2):21-30
Splicing by directed ligation (SDL) is a method of in-phase joining of PCR-generated DNA fragments that is based on a pre-designed combination of class IIS restriction endonuclease recognition and cleavage sites. Since these enzymes cleave outside of their recognition sites, the resulting sticky end can have any desired sequence, and the site itself can be removed and does not appear in the final spliced DNA product. SDL is based on the addition of class IIS recognition sites onto primers used to amplify DNA sequences. Cleavage of the PCR products results in elimination of the recognition site-containing flanking sequences and leaves the DNA fragments crowned with protruding ends. With careful design of the sticky ends, several segments can be ligated together in a predetermined order in a single reaction. SDL requires fewer rounds of amplification than overlap extension methods, and is particularly useful for creating a series of recombinants that differ in one segment. 相似文献