全文获取类型
收费全文 | 5931篇 |
免费 | 519篇 |
国内免费 | 3篇 |
专业分类
6453篇 |
出版年
2023年 | 42篇 |
2022年 | 116篇 |
2021年 | 222篇 |
2020年 | 93篇 |
2019年 | 127篇 |
2018年 | 145篇 |
2017年 | 104篇 |
2016年 | 210篇 |
2015年 | 364篇 |
2014年 | 330篇 |
2013年 | 407篇 |
2012年 | 569篇 |
2011年 | 498篇 |
2010年 | 317篇 |
2009年 | 241篇 |
2008年 | 351篇 |
2007年 | 358篇 |
2006年 | 317篇 |
2005年 | 283篇 |
2004年 | 285篇 |
2003年 | 234篇 |
2002年 | 215篇 |
2001年 | 45篇 |
2000年 | 28篇 |
1999年 | 38篇 |
1998年 | 57篇 |
1997年 | 28篇 |
1996年 | 26篇 |
1995年 | 19篇 |
1994年 | 31篇 |
1993年 | 29篇 |
1992年 | 19篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 14篇 |
1984年 | 18篇 |
1983年 | 15篇 |
1982年 | 14篇 |
1981年 | 8篇 |
1980年 | 13篇 |
1979年 | 7篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1971年 | 11篇 |
排序方式: 共有6453条查询结果,搜索用时 9 毫秒
971.
972.
Spatial management tools, such as marine spatial planning and marine protected areas, are playing an increasingly important role in attempts to improve marine management and accommodate conflicting needs. Robust data are needed to inform decisions among different planning options, and early inclusion of stakeholder involvement is widely regarded as vital for success. One of the biggest stakeholder groups, and the most likely to be adversely impacted by spatial restrictions, is the fishing community. In order to take their priorities into account, planners need to understand spatial variation in their perceived value of the sea. Here a readily accessible, novel method for quantitatively mapping fishers’ spatial access priorities is presented. Spatial access priority mapping, or SAPM, uses only basic functions of standard spreadsheet and GIS software. Unlike the use of remote-sensing data, SAPM actively engages fishers in participatory mapping, documenting rather than inferring their priorities. By so doing, SAPM also facilitates the gathering of other useful data, such as local ecological knowledge. The method was tested and validated in Northern Ireland, where over 100 fishers participated in a semi-structured questionnaire and mapping exercise. The response rate was excellent, 97%, demonstrating fishers’ willingness to be involved. The resultant maps are easily accessible and instantly informative, providing a very clear visual indication of which areas are most important for the fishers. The maps also provide quantitative data, which can be used to analyse the relative impact of different management options on the fishing industry and can be incorporated into planning software, such as MARXAN, to ensure that conservation goals can be met at minimum negative impact to the industry. This research shows how spatial access priority mapping can facilitate the early engagement of fishers and the ready incorporation of their priorities into the decision-making process in a transparent, quantitative way. 相似文献
973.
Isobel J. Whitehouse J. Scott Miners Elizabeth B. C. Glennon Patrick G. Kehoe Seth Love Katherine A. B. Kellett Nigel M. Hooper 《PloS one》2013,8(4)
The cellular prion protein (PrPC) has been implicated in the development of Alzheimer''s disease (AD). PrPC decreases amyloid-β (Aβ) production, which is involved in AD pathogenesis, by inhibiting β-secretase (BACE1) activity. Contactin 5 (CNTN5) has also been implicated in the development of AD by a genome-wide association study. Here we measured PrPC and CNTN5 in frontal cortex samples from 24 sporadic AD and 24 age-matched control brains and correlated the expression of these proteins with markers of AD. PrPC was decreased in sporadic AD compared to controls (by 49%, p = 0.014) but there was no difference in CNTN5 between sporadic AD and controls (p = 0.217). PrPC significantly inversely correlated with BACE1 activity (rs = −0.358, p = 0.006), Aβ load (rs = −0.456, p = 0.001), soluble Aβ (rs = −0.283, p = 0.026) and insoluble Aβ (rs = −0.353, p = 0.007) and PrPC also significantly inversely correlated with the stage of disease, as indicated by Braak tangle stage (rs = −0.377, p = 0.007). CNTN5 did not correlate with Aβ load (rs = 0.040, p = 0.393), soluble Aβ (rs = 0.113, p = 0.223) or insoluble Aβ (rs = 0.169, p = 0.125). PrPC was also measured in frontal cortex samples from 9 Down''s syndrome (DS) and 8 age-matched control brains. In contrast to sporadic AD, there was no difference in PrPC in the DS brains compared to controls (p = 0.625). These data are consistent with a role for PrPC in regulating Aβ production and indicate that brain PrPC level may be important in influencing the onset and progression of sporadic AD. 相似文献
974.
Edith K. MacRae Katherine B. Pryzwansky Marjorie H. Cooney John K. Spitznagel 《Cell and tissue research》1980,209(1):65-70
Summary Changes in surface morphology, as observed by scanning electron microscopy, appear rapidly when human polymorphonuclear neutrophils (PMN) are challenged with bacteria. Monolayers of PMN adhering to glass were incubated with opsonized E. coli from 5 sec to 10 min, and then fixed and prepared for SEM. As early as 5 sec after phagocytic challenge, E. coli are found in contact with PMN and in the process of engulfment into open cavities formed by lamellipodia. The shape of the mouth of the forming phagocytic vacuole is related to the orientation of bacteria during entry. Bacteria engulfed into early forming phagosomes are surrounded by a large open space between the bacteria and the phagosome wall. As phagocytosis proceeds, the space is reduced and the loose fit around the entering bacteria becomes tight. By 30 sec, bacteria may be completely internalized and by 1 min phagocytized E. coli are packed into bulging PMN. The observations reveal the variability and rapidity of the phagocytic response and confirm the presence of sensitive mechanisms for host defense by PMN.This work was supported by research grants from the University of North Carolina Research Council and the National Institutes of Health (A1 02430) 相似文献
975.
Trapido-Rosenthal Henry G.; Sharp Katherine H.; Galloway Tamara S.; Morrall Clare E. 《Integrative and comparative biology》2001,41(2):247-257
The presence of nitric oxide synthase (NOS) activity is demonstratedin the tropical marine cnidarian Aiptasia pallida and in itssymbiotic dinoflagellate algae, Symbiodinium bermudense. Enzymeactivity was assayed by measuring the conversion of arginineto citrulline. Biochemical characterization of NOS from Aiptasiawas characterized with respect to cellular localization, substrateand cofactor requirements, inhibitors, and kinetics. In responseto acute temperature shock, anemones retracted their tentacles.Animals subjected to such stress had lower NOS activities thandid controls. Treatment with NOS inhibitors caused tentacularretraction, while treatment with the NOS substrate L-arginineinhibited this response to stress, as did treatment with NOdonors. These results provide a preliminary biochemical characterizationof, and suggest a functional significance for, NOS activityin anthozoan-algal symbiotic assemblages. 相似文献
976.
977.
Katherine Hall 《Bioethics》1997,11(3&4):241-245
The ethics of treating the seriously and critically ill have not been static throughout the ages. Twentieth century medicine has inherited from the nineteenth century a science which places an inappropriate weight on diagnosis over prognosis and management, combined with a seventeenth century duty to prolong life. However other earlier ethical traditions, both Hippocratic and Christian, respected both the limitations of medicine and emphasised the importance of prognosis. This paper outlines some of the historical precedents for the treatment of the critically ill, and also how the current paradigm limits clinical practice and causes ethical tensions. An understanding that other paradigms have been ethically acceptable in the past allows wider consideration and acceptance of alternatives for the future. However future alternatives will also have to address the role of technology, given its importance in this area of medicine. 相似文献
978.
Joost Willemse Beata Ruban-O?mialowska David Widdick Katherine Celler Matthew I. Hutchings Gilles P. van Wezel Tracy Palmer 《Journal of bacteriology》2012,194(22):6272-6281
The Tat pathway transports folded proteins across the bacterial cytoplasmic membrane and is a major route of protein export in the Streptomyces genus of bacteria. In this study, we have examined the localization of Tat components in the model organism Streptomyces coelicolor by constructing enhanced green fluorescent protein (eGFP) and mCherry fusions with the TatA, TatB, and TatC proteins. All three components colocalized dynamically in the vegetative hyphae, with foci of each tagged protein being prominent at the tips of emerging germ tubes and of the vegetative hyphae, suggesting that this may be a primary site of Tat secretion. Time-lapse imaging revealed that localization of the Tat components was highly dynamic during tip growth and again demonstrated a strong preference for apical sites in growing hyphae. During aerial hypha formation, TatA-eGFP and TatB-eGFP fusions relocalized to prespore compartments, indicating repositioning of Tat components during the Streptomyces life cycle. 相似文献
979.
Richard G. James Kathryn C. Davidson Katherine A. Bosch Travis L. Biechele Nicholas C. Robin Russell J. Taylor Michael B. Major Nathan D. Camp Kerry Fowler Timothy J. Martins Randall T. Moon 《PloS one》2012,7(12)
The Wnt/ß-catenin signaling pathway controls important cellular events during development and often contributes to disease when dysregulated. Using high throughput screening we have identified a new small molecule inhibitor of Wnt/ß-catenin signaling, WIKI4. WIKI4 inhibits expression of ß-catenin target genes and cellular responses to Wnt/ß-catenin signaling in cancer cell lines as well as in human embryonic stem cells. Furthermore, we demonstrate that WIKI4 mediates its effects on Wnt/ß-catenin signaling by inhibiting the enzymatic activity of TNKS2, a regulator of AXIN ubiquitylation and degradation. While TNKS has previously been shown to be the target of small molecule inhibitors of Wnt/ß-catenin signaling, WIKI4 is structurally distinct from previously identified TNKS inhibitors. 相似文献
980.