全文获取类型
收费全文 | 3120篇 |
免费 | 346篇 |
国内免费 | 6篇 |
专业分类
3472篇 |
出版年
2022年 | 21篇 |
2021年 | 48篇 |
2020年 | 28篇 |
2019年 | 39篇 |
2018年 | 42篇 |
2017年 | 52篇 |
2016年 | 70篇 |
2015年 | 86篇 |
2014年 | 88篇 |
2013年 | 145篇 |
2012年 | 142篇 |
2011年 | 123篇 |
2010年 | 90篇 |
2009年 | 81篇 |
2008年 | 118篇 |
2007年 | 102篇 |
2006年 | 121篇 |
2005年 | 117篇 |
2004年 | 89篇 |
2003年 | 105篇 |
2002年 | 98篇 |
2001年 | 76篇 |
2000年 | 97篇 |
1999年 | 70篇 |
1998年 | 41篇 |
1997年 | 34篇 |
1996年 | 28篇 |
1995年 | 43篇 |
1994年 | 46篇 |
1993年 | 34篇 |
1992年 | 64篇 |
1991年 | 51篇 |
1990年 | 74篇 |
1989年 | 71篇 |
1988年 | 79篇 |
1987年 | 60篇 |
1986年 | 62篇 |
1985年 | 45篇 |
1984年 | 38篇 |
1983年 | 34篇 |
1982年 | 27篇 |
1980年 | 36篇 |
1979年 | 46篇 |
1978年 | 36篇 |
1977年 | 32篇 |
1976年 | 29篇 |
1975年 | 45篇 |
1974年 | 34篇 |
1973年 | 40篇 |
1970年 | 21篇 |
排序方式: 共有3472条查询结果,搜索用时 15 毫秒
991.
992.
J C Chien L C Dickinson T L Mason 《Biochemical and biophysical research communications》1975,63(4):853-857
An improved synthesis for cobalt-cytochrome has been developed; its half reduction potential is ?140 ± 20mV. Reduced Cocyt-3 is oxidized by bovine heart cytochrome oxidase at a rate ~45% that of the native cytochrome . It is not reduced by mitochondrial NADH or succinate cytochrome reductase nor by microsomal NADH or NADPH cytochrome reductase. 相似文献
993.
M Bogoyevitch R Mitchell L C Ward S Mason 《The International journal of biochemistry》1987,19(2):173-177
The activities of leucine aminotransferase (BCAT) and 2-oxoisocaproate dehydrogenase (OADH) were measured in rat heart in vitro. The effect upon these enzyme activities of both ethanol and acetaldehyde, administered either acutely or chronically, was determined. Enzyme activities were not significantly altered by either acetaldehyde or ethanol when given chronically. Ethanol administered acutely to rats decreased OADH activity but BCAT was unaffected. Acetaldehyde administered acutely did not alter significantly BCAT activity but significantly increased OADH activity. 相似文献
994.
Consequential life cycle assessment: a review 总被引:1,自引:0,他引:1
J. Mason Earles Anthony Halog 《The International Journal of Life Cycle Assessment》2011,16(5):445-453
Purpose
Over the past two decades, consequential life cycle assessment (CLCA) has emerged as a modeling approach for capturing environmental impacts of product systems beyond physical relationships accounted for in attributional LCA (ALCA). Put simply, CLCA represents the convergence of LCA and economic modeling approaches. 相似文献995.
996.
Human 8-oxoguanine-DNA glycosylase 1 (hOGG1) repairs 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) which results from oxidation of guanine. Reactive oxygen species (ROS) formed in response to ultraviolet (UV) radiation cause this DNA damage, which is involved in pathological processes such as carcinogenesis and aging. The initiation of skin tumors probably requires penetration of UV to the actively dividing basal layer of the epidermis in order for acute damage to become fixed as mutations. Previously, the majority of UVB fingerprint mutations have been found in the upper layers of human skin tumors, while UVA mutations have been found mostly in the lower layer. Our aim was to determine whether this localization of UVA-induced DNA damage is related to stratification of the repair-enzyme hOGG1. Anti-hOGG1 immunohistochemical staining of frozen sections of human foreskin, adult buttock skin, and reconstructed human skin samples showed the highest expression of hOGG1 in the superficial epidermal layer (stratum granulosum). Study of the hOGG1 mRNA expression again showed the highest level in the upper region of the epidermis. This was not regulated by UV irradiation but by the differentiation state of keratinocytes as calcium-induced differentiation increased hOGG1 gene expression. UVA-induced 8-oxo-dG was repaired more rapidly in the upper layer of human skin compared to the lower layers. Our results indicate that weaker expression of the nuclear form of hOGG1 enzyme in the basal cells of the epidermis may lead to a lack of DNA repair in these cells and therefore accumulation of UVA-induced oxidative DNA mutations. 相似文献
997.
Yellow Fever Vaccine: Direct Challenge of Monkeys Given Graded Doses of 17D Vaccine 总被引:1,自引:0,他引:1 下载免费PDF全文
Richard A. Mason Nicola M. Tauraso Richard O. Spertzel Robert K. Ginn 《Applied microbiology》1973,25(4):539-544
In rhesus monkeys a wide dosage range of 17D yellow fever (YF) vaccine extending to a level even below that recommended for vaccination of man elicited an immune response providing solid protection to challenge with virulent YF virus. Forty-three of 45 monkeys vaccinated with 10(2.3) or greater weanling mouse mean lethal doses of 17D vaccine were resistant to challenge 20 weeks later with virulent Asibi strain YF virus. Monkeys given graded doses of lesser amounts of vaccine were progressively more susceptible to challenge. With a vaccine dose >/= 10(2.3) weanling mouse mean lethal doses, plaque neutralization (PN) seroconversion rates were 90% or greater, whereas hemagglutination-inhibiting (HI) and complement-fixing (CF) seroconversion rates were unrelated to vaccine dosage and were generally in the range of 20 to 80%. Ninety-six percent (51 of 54) of immune monkeys had PN titers >/=0.7 log(10) (fivefold) neutralization index as compared to approximately 55 to 65% who showed HI or CF titers >/=2 log(2) (fourfold) neutralization index. After challenge with Asibi strain YF virus, antibody titers of all three tests increaed equally. In rhesus monkeys PN antibody titers were well correlated with YF immunity, whereas HI and CF antibody titers were not. 相似文献
998.
John Buettner-Janusch Richard Reisman Dorian Coppenhaver George A. Mason Vina Buettner-Janusch 《American journal of physical anthropology》1973,38(3):661-669
Transferrin phenotypes of 403 Malagasy were determined by autoradiography after electrophoresis of plasma or serum. Three phenotypes, Tf C, Tf CD, and Tf D, were found. The frequency of TfD was 0.051 in the total sample, 0.041 in tribes of the Madagascar plateau, and 0.075 in tribes of the coastal and lowland regions. Haptoglobin phenotypes of 402 Malagasy were determined. The frequency of Hp1 was 0.41 in the total sample, 0.38 in tribes of the plateau, and 0.49 in coastal-lowland tribes. There is a significant difference in the frequency of Hp1 between the tribes of the plateau and those of the coastal and lowland regions. This confirms earlier observations of significant differences in frequencies of haptoglobins and hemoglobin S. Plasma or serum samples from 405 Malagasy were examined by electrophoresis to determine ceruloplasmin phenotypes. Seven distinct ceruloplasmin bands were observed. These were A, B, Galveston, Bridgeport, New Haven, Tananarive, and X, the last two described for the first time. Ten different phenotypes composed of one or two of the bands were observed. The frequency of the allele for ceruloplasmin B (CpB) was 0.78 in the total sample, 0.73 in tribes of the plateau, and 0.92 in coastal and lowland tribes. There is a significant difference in the frequency of this allele in the two major tribal groups, and this provides additional evidence of genetic differences between the peoples of the plateau and those of the coast and lowland regions of Madagascar. 相似文献
999.
The design of minimum-weight structures that retain their integrity under dynamic loading regimes has long challenged engineers. One solution to this problem found in both human and biological design is the optimization of weight and strength by hollowing a structure and replacing its inner core with supportive struts. In animals, this design is observed in sand dollar test, avian beak, and the cancellous bone of tetrapod limbs. Additionally, within the elasmobranch fishes, mineralized trabeculae (struts) have been reported in the jaws of durophagous myliobatid stingrays (Elasmobranchii: Batoidea), but were believed to be absent in basal members of the batoid clade. This study, however, presents an additional case of batoid trabeculation in the lesser electric ray, Narcine brasiliensis (Torpediniformes). The trabeculae in these species likely play different functional roles. Stingrays use their reinforced jaws to crush bivalves, yet N. brasiliensis feeds by ballistically protruding its jaws into the sediment to capture polychaetes. In N. brasiliensis, trabeculae are localized to areas likely to experience the highest load: the quadratomandibular jaw joints, hyomandibular-cranial joint, and the thinnest sections of the jaws immediately lateral to the symphyses. However, the supports perform different functions dependent on location. In regions where the jaws are loaded transversely (as in durophagous rays), "load leading" trabeculae distribute compressive forces from the cortex through the lumen of the jaws. In the parasymphyseal regions of the jaws, "truss" trabeculae form cross-braces perpendicular to the long axes of the jaws. At peak protrusion, the jaw arch is medially compressed and the jaw loaded axially such that these trabeculae are positioned to resist buckling associated with excavation forces. "Truss" trabeculae function to maintain the second moment of area in the thinnest regions of the jaws, illustrating a novel function for batoid trabeculation. Thus, this method of structural support appears to have arisen twice independently in batoids and performs strikingly different ecological functions associated with the distribution of extreme loading environments. 相似文献
1000.
Mercury methylation by dissimilatory iron-reducing bacteria 总被引:3,自引:0,他引:3
Kerin EJ Gilmour CC Roden E Suzuki MT Coates JD Mason RP 《Applied and environmental microbiology》2006,72(12):7919-7921
The Hg-methylating ability of dissimilatory iron-reducing bacteria in the genera Geobacter, Desulfuromonas, and Shewanella was examined. All of the Geobacter and Desulfuromonas strains tested methylated mercury while reducing Fe(III), nitrate, or fumarate. In contrast, none of the Shewanella strains produced methylmercury at higher levels than abiotic controls under similar culture conditions. Geobacter and Desulfuromonas are closely related to known Hg-methylating sulfate-reducing bacteria within the Deltaproteobacteria. 相似文献