全文获取类型
收费全文 | 422篇 |
免费 | 36篇 |
国内免费 | 62篇 |
出版年
2023年 | 4篇 |
2022年 | 5篇 |
2021年 | 10篇 |
2020年 | 7篇 |
2019年 | 16篇 |
2017年 | 7篇 |
2016年 | 5篇 |
2015年 | 15篇 |
2014年 | 9篇 |
2013年 | 19篇 |
2012年 | 30篇 |
2011年 | 14篇 |
2010年 | 32篇 |
2009年 | 22篇 |
2008年 | 19篇 |
2007年 | 12篇 |
2006年 | 16篇 |
2005年 | 17篇 |
2004年 | 9篇 |
2003年 | 11篇 |
2002年 | 12篇 |
2001年 | 8篇 |
2000年 | 10篇 |
1999年 | 11篇 |
1998年 | 10篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 8篇 |
1992年 | 7篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 14篇 |
1988年 | 15篇 |
1987年 | 7篇 |
1986年 | 6篇 |
1985年 | 11篇 |
1984年 | 8篇 |
1983年 | 6篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1977年 | 9篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1972年 | 5篇 |
1971年 | 4篇 |
1969年 | 3篇 |
1954年 | 3篇 |
排序方式: 共有520条查询结果,搜索用时 187 毫秒
31.
BRAD M. BEBOUT TORI M. HOEHLER BO THAMDRUP DAN ALBERT STEVEN P. CARPENTER MARY HOGAN KENDRA TURK DAVID J. DES MARAIS 《Geobiology》2004,2(2):87-96
Cyanobacterial mats collected in hypersaline salterns were incubated in a greenhouse under low sulphate concentrations ([]) and examined for their primary productivity and emissions of methane and other major carbon species. Atmospheric greenhouse warming by gases such as carbon dioxide and methane must have been greater during the Archean than today in order to account for a record of moderate to warm palaeoclimates, despite a less luminous early sun. It has been suggested that decreased levels of oxygen and sulphate in Archean oceans could have significantly stimulated microbial methanogenesis relative to present marine rates, with a resultant increase in the relative importance of methane in maintaining the early greenhouse. We maintained modern microbial mats, models of ancient coastal marine communities, in artificial brine mixtures containing both modern [] (c. 70 mm ) and ‘Archean’[] (<0.2 mm ). At low [], primary production in the mats was essentially unaffected, while rates of sulphate reduction decreased by a factor of three, and methane fluxes increased by up to 10‐fold. However, remineralization by methanogenesis still amounted to less than 0.4% of the total carbon released by the mats. The relatively low efficiency of conversion of photosynthate to methane is suggested to reflect the particular geometry and chemical microenvironment of hypersaline cyanobacterial mats. Therefore, such mats were probably relatively weak net sources of methane throughout their 3.5 Ga history, even during periods of low environmental levels oxygen and sulphate. 相似文献
32.
Molecular mechanics of mouse cardiac myosin isoforms 总被引:2,自引:0,他引:2
Alpert NR Brosseau C Federico A Krenz M Robbins J Warshaw DM 《American journal of physiology. Heart and circulatory physiology》2002,283(4):H1446-H1454
Two myosin isoforms are expressed in myocardium, alphaalpha-homodimers (V(1)) and betabeta-homodimers (V(3)). V(1) exhibits higher velocities and myofibrillar ATPase activities compared with V(3). We also observed this for cardiac myosin from normal (V(1)) and propylthiouracil-treated (V(3)) mice. Actin velocity in a motility assay (V(actin)) over V(1) myosin was twice that of V(3) as was the myofibrillar ATPase. Myosin's average force (F(avg)) was similar for V(1) and V(3). Comparing V(actin) and F(avg) across species for both V(1) and V(3), our laboratory showed previously (VanBuren P, Harris DE, Alpert NR, and Warshaw DM. Circ Res 77: 439-444, 1995) that mouse V(1) has greater V(actin) and F(avg) compared with rabbit V(1). Mouse V(3) V(actin) was twice that of rabbit V(actin). To understand myosin's molecular structure and function, we compared alpha- and beta-cardiac myosin sequences from rodents and rabbits. The rabbit alpha- and beta-cardiac myosin differed by eight and four amino acids, respectively, compared with rodents. These residues are localized to both the motor domain and the rod. These differences in sequence and mechanical performance may be an evolutionary attempt to match a myosin's mechanical behavior to the heart's power requirements. 相似文献
33.
Identification of a histamine H4 receptor on human eosinophils--role in eosinophil chemotaxis 总被引:5,自引:0,他引:5
O'Reilly M Alpert R Jenkinson S Gladue RP Foo S Trim S Peter B Trevethick M Fidock M 《Journal of receptor and signal transduction research》2002,22(1-4):431-448
Eosinophils are recruited to sites of inflammation via the action of a number of chemical mediators, including PAF, leukotrienes, eotaxins, ECF-A and histamine. Although many of the cell-surface receptors for these mediators have been identified, histamine-driven chemotaxis has not been conclusively attributed to any of the three known histamine receptor subtypes, suggesting the possibility of a 4th histamine-responsive receptor on eosinophils. We have identified and cloned a novel G protein-coupled receptor (GPCR), termed Pfi-013, from an IL-5 stimulated eosinophil cDNA library which is homologous to the human histamine H3 receptor, both at the sequence and gene structure level. Expression data indicates that Pfi-013 is predominantly expressed in peripheral blood leukocytes, with lower expression levels in spleen, testis and colon. Ligand-binding studies using Pfi-013 expressed in HEK-293Galpha15 cells, demonstrates specific binding to histamine with a Kd of 3.28 +/- 0.76 nM and possesses a unique rank order of potency against known histaminergic compounds in a competitive ligand-binding assay (histamine > clobenpropit > iodophenpropit > thioperamide > R-alpha-methylhistamine > cimetidine > pyrilamine). We have therefore termed this receptor human histamine H4. Chemotaxis studies on isolated human eosinophils have confirmed that histamine is chemotactic and that agonists of the known histamine receptors (H1, H2, and H3) do not induce such a response. Furthermore, studies employing histamine-receptor antagonists have shown an inhibition of chemotaxis only by the H3 antagonists clobenpropit and thioperamide. Since these compounds are also antagonists of hH4 we postulate that the receptor mediating histaminergic chemotaxis is this novel histamine H4 receptor. 相似文献
34.
35.
Ahmed Haouz Charles Twist Christian Zentz Patrick Tauc B. Alpert 《European biophysics journal : EBJ》1998,27(1):19-25
The catalytic oxidation of β-D-glucose by the enzyme glucose oxidase involves a redox change of the flavin coenzyme. The structure and the dynamics of
the two extreme glucose oxidase forms were studied by using infrared absorption spectroscopy of the amide I′ band, tryptophan
fluorescence quenching and hydrogen isotopic exchange. The conversion of FAD to FADH2 does not change the amount of α-helix present in the protein outer shell, but reorganises a fraction of random coil to β-sheet structure. The dynamics of the protein interior vary with the redox states of the flavin without affecting the motions
of the structural elements near the protein surface. From the structure of glucose oxidase given by X-ray crystallography,
these results suggest that the dynamics of the interface between the two monomers are involved in the catalytic mechanism.
Received: 27 December 1996 / Accepted: 18 July 1997 相似文献
36.
37.
Patrick L Cable J Potts D Ignace D Barron-Gafford G Griffith A Alpert H Van Gestel N Robertson T Huxman TE Zak J Loik ME Tissue D 《Oecologia》2007,151(4):704-718
Global climate models predict that in the next century precipitation in desert regions of the USA will increase, which is
anticipated to affect biosphere/atmosphere exchanges of both CO2 and H2O. In a sotol grassland ecosystem in the Chihuahuan Desert at Big Bend National Park, we measured the response of leaf-level
fluxes of CO2 and H2O 1 day before and up to 7 days after three supplemental precipitation pulses in the summer (June, July, and August 2004).
In addition, the responses of leaf, soil, and ecosystem fluxes of CO2 and H2O to these precipitation pulses were also evaluated in September, 1 month after the final seasonal supplemental watering event.
We found that plant carbon fixation responded positively to supplemental precipitation throughout the summer. Both shrubs
and grasses in watered plots had increased rates of photosynthesis following pulses in June and July. In September, only grasses
in watered plots had higher rates of photosynthesis than plants in the control plots. Soil respiration decreased in supplementally
watered plots at the end of the summer. Due to these increased rates of photosynthesis in grasses and decreased rates of daytime
soil respiration, watered ecosystems were a sink for carbon in September, assimilating on average 31 mmol CO2 m−2 s−1 ground area day−1. As a result of a 25% increase in summer precipitation, watered plots fixed eightfold more CO2 during a 24-h period than control plots. In June and July, there were greater rates of transpiration for both grasses and
shrubs in the watered plots. In September, similar rates of transpiration and soil water evaporation led to no observed treatment
differences in ecosystem evapotranspiration, even though grasses transpired significantly more than shrubs. In summary, greater
amounts of summer precipitation may lead to short-term increased carbon uptake by this sotol grassland ecosystem. 相似文献
38.
Cohen G Riahi Y Alpert E Gruzman A Sasson S 《Archives of physiology and biochemistry》2007,113(4-5):259-267
Vascular endothelial cell (VEC) dysfunction in diabetes has been associated with hyperglycaemia-induced intra- and extracellular glycation of proteins and to overproduction of glucose-derived free radicals. VEC protect their intracellular environment against an increased influx of glucose in face of hyperglycaemia by reducing the expression and plasma membrane abundance of their glucose transporter-1 (GLUT-1). We investigated the hypothesis that glucose-derived free radicals induce this down-regulatory mechanism in VEC, but proved the contrary. In fact, pro-oxidants significantly increased the expression and plasma membrane abundance of GLUT-1 and the rate of glucose transport in VEC while abolishing high-glucose-induced down-regulation of the hexose transport system. The resulting uncontrolled influx of glucose followed by overproduction of glucose-derived ROS further up-regulates the rate of glucose transport, and vice versa. This perpetuating glycoxidative stress finally leads to the collapse of the auto-regulatory protective mechanism and accelerates the development of dysfunctional endothelium in blood vessels. 相似文献
39.
40.