全文获取类型
收费全文 | 1814篇 |
免费 | 230篇 |
出版年
2022年 | 10篇 |
2021年 | 18篇 |
2020年 | 20篇 |
2019年 | 20篇 |
2018年 | 23篇 |
2017年 | 20篇 |
2016年 | 32篇 |
2015年 | 55篇 |
2014年 | 75篇 |
2013年 | 65篇 |
2012年 | 95篇 |
2011年 | 95篇 |
2010年 | 66篇 |
2009年 | 64篇 |
2008年 | 85篇 |
2007年 | 105篇 |
2006年 | 91篇 |
2005年 | 69篇 |
2004年 | 71篇 |
2003年 | 70篇 |
2002年 | 69篇 |
2001年 | 68篇 |
2000年 | 59篇 |
1999年 | 48篇 |
1998年 | 30篇 |
1997年 | 26篇 |
1996年 | 25篇 |
1995年 | 11篇 |
1994年 | 12篇 |
1993年 | 15篇 |
1992年 | 29篇 |
1991年 | 36篇 |
1990年 | 32篇 |
1989年 | 39篇 |
1988年 | 32篇 |
1987年 | 34篇 |
1986年 | 21篇 |
1985年 | 36篇 |
1984年 | 26篇 |
1983年 | 28篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 16篇 |
1979年 | 13篇 |
1978年 | 15篇 |
1977年 | 18篇 |
1975年 | 19篇 |
1974年 | 12篇 |
1973年 | 8篇 |
1972年 | 9篇 |
排序方式: 共有2044条查询结果,搜索用时 15 毫秒
961.
Jesse F. Gregory III Youngja Park Yvonne Lamers Nirmalya Bandyopadhyay Yueh-Yun Chi Kichen Lee Steven Kim Vanessa da Silva Nikolas Hove Sanjay Ranka Tamer Kahveci Keith E. Muller Robert D. Stevens Christopher B. Newgard Peter W. Stacpoole Dean P. Jones 《PloS one》2013,8(6)
Marginal deficiency of vitamin B-6 is common among segments of the population worldwide. Because pyridoxal 5′-phosphate (PLP) serves as a coenzyme in the metabolism of amino acids, carbohydrates, organic acids, and neurotransmitters, as well as in aspects of one-carbon metabolism, vitamin B-6 deficiency could have many effects. Healthy men and women (age: 20-40 y; n = 23) were fed a 2-day controlled, nutritionally adequate diet followed by a 28-day low-vitamin B-6 diet (<0.5 mg/d) to induce marginal deficiency, as reflected by a decline of plasma PLP from 52.6±14.1 (mean ± SD) to 21.5±4.6 nmol/L (P<0.0001) and increased cystathionine from 131±65 to 199±56 nmol/L (P<0.001). Fasting plasma samples obtained before and after vitamin B6 restriction were analyzed by 1H-NMR with and without filtration and by targeted quantitative analysis by mass spectrometry (MS). Multilevel partial least squares-discriminant analysis and S-plots of NMR spectra showed that NMR is effective in classifying samples according to vitamin B-6 status and identified discriminating features. NMR spectral features of selected metabolites indicated that vitamin B-6 restriction significantly increased the ratios of glutamine/glutamate and 2-oxoglutarate/glutamate (P<0.001) and tended to increase concentrations of acetate, pyruvate, and trimethylamine-N-oxide (adjusted P<0.05). Tandem MS showed significantly greater plasma proline after vitamin B-6 restriction (adjusted P<0.05), but there were no effects on the profile of 14 other amino acids and 45 acylcarnitines. These findings demonstrate that marginal vitamin B-6 deficiency has widespread metabolic perturbations and illustrate the utility of metabolomics in evaluating complex effects of altered vitamin B-6 intake. 相似文献
962.
Agricultural landscapes can be characterized as a mosaic of habitat patches interspersed with hostile matrix, or as a gradient of patches ranging from suitable to unsuitable for different species. Arthropods moving through these landscapes encounter a range of edges, with different permeability. Patches of native vegetation in these landscapes may support natural enemies of crop pests by providing alternate hosts for parasitic wasps and/or acting as a source for predatory insects. We test this by quantifying species interactions and measuring movement across different edge-types. A high diversity of parasitoid species used hosts in the native vegetation patches, however we recorded few instances of the same parasitoid species using hosts in both the native vegetation and the crop (canola). However, we did find overall greater densities of parasitoids moving from native vegetation into the crop. Of the parasitoid groups examined, parasitoids of aphids (Braconidae: Aphidiinae) frequently moved from native vegetation into canola. In contrast, parasitoids of caterpillars (Braconidae: Microgastrinae) moved commonly from cereal fields into canola. Late season samples showed both aphids and parasitoids moving frequently out of native vegetation, in contrast predators moved less commonly from native vegetation (across the whole season). The season-long net advantage or disadvantage of native vegetation for pest control services is therefore difficult to evaluate. It appears that the different edge-types alter movement patterns of natural enemies more so than herbivorous pest species, and this may impact pest control services. 相似文献
963.
Pietro Mesirca Laurine Marger Futoshi Toyoda Riccardo Rizzetto Matthieu Audoubert Stefan Dubel Angelo G. Torrente Mattia L. DiFrancesco Jana Christina Muller Anne-Laure Leoni Brigitte Couette Jo?l Nargeot David E. Clapham Kevin Wickman Matteo E. Mangoni 《The Journal of general physiology》2013,142(2):113-126
Parasympathetic regulation of sinoatrial node (SAN) pacemaker activity modulates multiple ion channels to temper heart rate. The functional role of the G-protein–activated K+ current (IKACh) in the control of SAN pacemaking and heart rate is not completely understood. We have investigated the functional consequences of loss of IKACh in cholinergic regulation of pacemaker activity of SAN cells and in heart rate control under physiological situations mimicking the fight or flight response. We used knockout mice with loss of function of the Girk4 (Kir3.4) gene (Girk4−/− mice), which codes for an integral subunit of the cardiac IKACh channel. SAN pacemaker cells from Girk4−/− mice completely lacked IKACh. Loss of IKACh strongly reduced cholinergic regulation of pacemaker activity of SAN cells and isolated intact hearts. Telemetric recordings of electrocardiograms of freely moving mice showed that heart rate measured over a 24-h recording period was moderately increased (10%) in Girk4−/− animals. Although the relative extent of heart rate regulation of Girk4−/− mice was similar to that of wild-type animals, recovery of resting heart rate after stress, physical exercise, or pharmacological β-adrenergic stimulation of SAN pacemaking was significantly delayed in Girk4−/− animals. We conclude that IKACh plays a critical role in the kinetics of heart rate recovery to resting levels after sympathetic stimulation or after direct β-adrenergic stimulation of pacemaker activity. Our study thus uncovers a novel role for IKACh in SAN physiology and heart rate regulation. 相似文献
964.
May photoinhibition be a consequence, rather than a cause, of limited plant productivity? 总被引:1,自引:0,他引:1
William W. Adams III Onno Muller Christopher M. Cohu Barbara Demmig-Adams 《Photosynthesis research》2013,117(1-3):31-44
Photoinhibition in leaves in response to high and/or excess light, consisting of a decrease in photosynthesis and/or photosynthetic efficiency, is frequently equated to photodamage and often invoked as being responsible for decreased plant growth and productivity. However, a review of the literature reveals that photoinhibited leaves characterized for foliar carbohydrate levels were invariably found to possess high levels of sugars and starch. We propose that photoinhibition should be placed in the context of whole-plant source–sink regulation of photosynthesis. Photoinhibition may represent downregulation of the photosynthetic apparatus in response to excess light when (1) more sugar is produced in leaves than can be utilized by the rest of the plant and/or (2) more light energy is harvested than can be utilized by the chloroplast for the fixation of carbon dioxide into sugars. 相似文献
965.
966.
Gunter P. Eckert Uta Lipka Walter E. Muller 《Prostaglandins, leukotrienes, and essential fatty acids》2013,88(1):105-114
Mitochondrial dysfunction represents a common early pathological event in brain aging and in neurodegenerative diseases, e.g., in Alzheimer’s (AD), Parkinson’s (PD), and Huntington’s disease (HD), as well as in ischemic stroke. In vivo and ex vivo experiments using animal models of aging and AD, PD, and HD mainly showed improvement of mitochondrial function after treatment with polyunsaturated fatty acids (PUFA) such as docosahexaenoic acid (DHA). Thereby, PUFA are particular beneficial in animals treated with mitochondria targeting toxins. However, DHA showed adverse effects in a transgenic PD mouse model and it is not clear if a diet high or low in PUFA might provide neuroprotective effects in PD. Post-treatment with PUFA revealed conflicting results in ischemic animal models, but intravenous administered DHA provided neuroprotective efficacy after acute occlusion of the middle cerebral artery. In summary, the majority of preclinical data indicate beneficial effects of n-3 PUFA in neurodegenerative diseases, whereas most controlled clinical trials did not meet the expectations. Because of the high half-life of DHA in the human brain clinical studies may have to be initiated much earlier and have to last much longer to be more efficacious. 相似文献
967.
Florent Pantin Jeanne Renaud François Barbier Alain Vavasseur Didier Le Thiec Christophe Rose Thierry Bariac Stuart Casson Deirdre H. McLachlan Alistair M. Hetherington Bertrand Muller Thierry Simonneau 《Current biology : CB》2013,23(18):1805-1811
- Download : Download high-res image (162KB)
- Download : Download full-size image
968.
969.
Vuilleumier S Nadalig T Ul Haque MF Magdelenat G Lajus A Roselli S Muller EE Gruffaz C Barbe V Médigue C Bringel F 《Journal of bacteriology》2011,193(18):5035-5036
Hyphomicrobium sp. strain MC1 is an aerobic methylotroph originally isolated from industrial sewage. This prosthecate bacterium was the first strain reported to grow with chloromethane as the sole carbon and energy source. Its genome, consisting of a single 4.76-Mb chromosome, is the first for a chloromethane-degrading bacterium to be formally reported. 相似文献
970.
Chevalier MF Jülg B Pyo A Flanders M Ranasinghe S Soghoian DZ Kwon DS Rychert J Lian J Muller MI Cutler S McAndrew E Jessen H Pereyra F Rosenberg ES Altfeld M Walker BD Streeck H 《Journal of virology》2011,85(2):733-741
Functional defects in cytotoxic CD8(+) T cell responses arise in chronic human viral infections, but the mechanisms involved are not well understood. In mice, CD4 cell-mediated interleukin-21 (IL-21) production is necessary for the maintenance of CD8(+) T cell function and control of persistent viral infections. To investigate the potential role of IL-21 in a chronic human viral infection, we studied the rare subset of HIV-1 controllers, who are able to spontaneously control HIV-1 replication without treatment. HIV-specific triggering of IL-21 by CD4(+) T cells was significantly enriched in these persons (P = 0.0007), while isolated loss of IL-21-secreting CD4(+) T cells was characteristic for subjects with persistent viremia and progressive disease. IL-21 responses were mediated by recognition of discrete epitopes largely in the Gag protein, and expansion of IL-21(+) CD4(+) T cells in acute infection resulted in lower viral set points (P = 0.002). Moreover, IL-21 production by CD4(+) T cells of HIV controllers enhanced perforin production by HIV-1-specific CD8(+) T cells from chronic progressors even in late stages of disease, and HIV-1-specific effector CD8(+) T cells showed an enhanced ability to efficiently inhibit viral replication in vitro after IL-21 binding. These data suggest that HIV-1-specific IL-21(+) CD4(+) T cell responses might contribute to the control of viral replication in humans and are likely to be of great importance for vaccine design. 相似文献