全文获取类型
收费全文 | 3730篇 |
免费 | 390篇 |
国内免费 | 2篇 |
出版年
2021年 | 40篇 |
2020年 | 28篇 |
2019年 | 35篇 |
2018年 | 41篇 |
2017年 | 36篇 |
2016年 | 60篇 |
2015年 | 89篇 |
2014年 | 110篇 |
2013年 | 143篇 |
2012年 | 174篇 |
2011年 | 199篇 |
2010年 | 105篇 |
2009年 | 138篇 |
2008年 | 158篇 |
2007年 | 162篇 |
2006年 | 126篇 |
2005年 | 147篇 |
2004年 | 132篇 |
2003年 | 148篇 |
2002年 | 150篇 |
2001年 | 82篇 |
2000年 | 106篇 |
1999年 | 69篇 |
1998年 | 55篇 |
1997年 | 39篇 |
1996年 | 53篇 |
1995年 | 34篇 |
1994年 | 44篇 |
1993年 | 38篇 |
1992年 | 59篇 |
1991年 | 60篇 |
1990年 | 63篇 |
1989年 | 52篇 |
1988年 | 60篇 |
1987年 | 50篇 |
1986年 | 42篇 |
1985年 | 52篇 |
1984年 | 51篇 |
1983年 | 48篇 |
1982年 | 42篇 |
1981年 | 57篇 |
1980年 | 38篇 |
1979年 | 46篇 |
1978年 | 45篇 |
1977年 | 46篇 |
1976年 | 38篇 |
1975年 | 36篇 |
1974年 | 49篇 |
1973年 | 34篇 |
1972年 | 27篇 |
排序方式: 共有4122条查询结果,搜索用时 251 毫秒
61.
62.
Carl W. David Harvey B. Haukaas Juris G. Kalnins Robert Schor 《Biophysical journal》1967,7(5):505-510
In attempting to understand the yield region of the α β transformation in keratins (Astbury and Woods, 1933), we recently proposed a statistical-mechanical model (David and Schor, 1965) which generalized the work done by others on the helix random coil transformation (Zimm and Bragg, 1959; Gibbs and DiMarzio, 1959) (thermal denaturation) to the case of a polypeptide under external tension (Birstein, 1962). We wish now to report the comparison of the quantitative aspects of this model to the observed tension-length isotherms (in the yield region) of Cotswold wool. 相似文献
63.
64.
Studies on Chrysophyceae from some ponds and lakes in Alaska 总被引:2,自引:1,他引:1
Summary A new variety, Mallomonopsis elliptica var. salina, of M. elliptica var. elliptica Matwienko is described as seen in the living state in the light microscope, and in the electron microscope. The locality in which it was found is described.
Zusammenfassung Eine neue Varietät, Mallomonopsis elliptica var. salina, von M. elliptica var. elliptica Matwienko wird beschrieben. Sie wurde in lebendiger Gestalt in dem Lichtmikroskop and in dem Elektronenmikroskop beobachtet. Der Fundort wird beschrieben.相似文献
65.
J E Hagstrom S Harvey B Madden D McCormick E D Wieben 《Molecular endocrinology (Baltimore, Md.)》1989,3(11):1797-1806
The guinea pig seminal vesicle epithelium is an androgen-dependent tissue that synthesizes and secretes four major secretory proteins (SVP-1, SVP-2, SVP-3, and SVP-4). Sequencing of near full-length cDNA clones corresponding to the two most abundant mRNAs produced by the seminal vesicle reveals that all four secretory proteins are cleaved from two secretory protein precursors. Amino acid sequences from purified SVP-2 match the central region of the predicted amino acid sequences from the smaller cDNA clone, GP2 (581 nucleotides). Similar analysis demonstrates that the predicted amino acid sequence from the longer cDNA clone, GP1 (1368 nucleotides), codes for the related proteins SVP-3 and SVP-4 as well as SVP-1. The 43.2 kilodalton polyprotein precursor coded by GP1 contains two different sets of 24 amino acid tandemly repeated sequences. The two secretory protein precursors have extensive regions of peptide sequence homology, particularly in regions where protein processing must occur to produce the mature secretory proteins. Analysis of the predicted secondary structure of the two precursor polypeptides revealed a strong correlation between structural features and sites of protein processing. 相似文献
66.
In anoxic salt marsh sediments of Sapelo Island, GA, USA, the vertical distribution of CH4 production was measured in the upper 20 cm of surface sediments in ten locations. In one section of high marsh sediments,
the concentration and oxidation of acetate in sediment porewaters and the rate and amount of14C acetate and14CO2 incorporation into cellular lipids of the microbial population were investigated. CH4 production rates ranged from <1 to 493 nM CH4 gram sediment−1 day−1 from intact subcores incubated under nitrogen. Replacement with H2 stimulated the rate of methane release up to nine fold relative to N2 incubations. Rates of lipid synthesis from CO2 averaged 39.2 ×10−2nanomoles lipid carbon cm3 sediment−1 hr−1, suggesting that CO2 may be an important carbon precursor for microbial membrane synthesis in marsh sediments under anoxic conditions. Qualitative
measurements of lipid synthesis rates from acetate were found to average 8.7 × 10−2 nanomoles. Phospholipids were the dominant lipids synthesized by both substrates in sediment cores, accounting for an average
of 76.6% of all lipid radioactivity. Small amounts of ether lipids indicative of methanogenic bacteria were observed in cores
incubated for 7 days, with similar rates of synthesis for both CO2 and acetate. The low rate of ether lipid synthesis suggests that either methanogen lipid biosynthesis is very slow or that
methanogens represent a small component of total microbial lipid synthesis in anoxic sediments.
present address: The University of Maryland,, Chesapeake Biological Laboratory, Box 38, Solomons, MD 20688, USA 相似文献
67.
68.
Glutamine and Aspartate Loading of Synaptosomes: A Reevaluation of Effects on Calcium-Dependent Excitatory Amino Acid Release 总被引:5,自引:3,他引:2
Guinea-pig cerebral cortical synaptosomes were preincubated for 60 min with 100 microM D-aspartate, L-aspartate, or L-glutamate. The total D- plus L-aspartate content of the synaptosomal fraction increased to 235%, 195%, or 164%, respectively, of the control. Despite this no increase was seen in the very low KCl evoked, Ca2+-dependent release of aspartate. Preincubation with the three amino acids changed the synaptosomal glutamate content to 78% (D-aspartate), 149% (L-aspartate), or 168% (L-glutamate) of control. However there was no statistically significant effect of these preincubations on the extent of Ca2+-dependent glutamate release. Thus the Ca2+-dependent release of aspartate and glutamate is not determined by the total synaptosomal content of these amino acids. The addition of 0.1-0.5 mM glutamine to the incubation caused a massive appearance of glutamate in the extrasynaptosomal medium. Analysis of specific activities showed that glutamine was hydrolysed directly by an extrasynaptosomal glutaminase, and that intrasynaptosomal glutamate was predominantly labelled by uptake of this glutaminase-derived glutamate. No increase was seen in the extent of Ca2+-dependent release of glutamate (by fluorimetry) either after preincubation with glutamine or in the continued presence of glutamine. Thus we are unable to confirm reports that glutamine expands the transmitter pool of glutamate. The extrasynaptosomal glutaminase activity in the synaptosomal preparation was inhibited by Ca2+ and activated by phosphate. Identical kinetics were obtained with "free" brain mitochondria, confirming the origin of the glutamine-derived glutamate. 相似文献
69.
70.
Validation of signature polarlipid fatty acid biomarkers for alkane-utilizing bacteria in soils and subsurface aquifer materials 总被引:3,自引:0,他引:3
David B. Ringelberg John D. Davis Glen A. Smith Susan M. Pfiffner Peter D. Nichols Janet S. Nickels J.Michael Henson John T. Wilson Marylynn Yates Donald H. Kampbell Harvey W. Read Thomas T. Stocksdale David C. White 《FEMS microbiology letters》1989,62(1):39-50
Abstract Extractable cell membrane-derived polarlipid ester-linked fatty acids (PLFA) obtained from aerated soils gassed with methane or propane and from methane- and propane-oxidizing bacteria isolated from the soils were analyzed by capillary gas chromatography/mass spectrometry. Exposure of aerated soils to methane resulted in the formation of a high proportion of an unusual 18-carbon mono-unsaturated PLFA, 18:lw8c. High proportions of this fatty acid biomarker are found in monocultures from this soil grown in minimal media with methane. This PLFA has been previously established as associated with authentic type II methane-oxidizing bacteria. The microbiota in aerated soils exposed to hydrocarbons containing propane, formed a suite of PLFA characterized by high proportions of a 16-carbon mono-unsaturated acid, 16:lw6c, and an 18-carbon saturated fatty acid with an additional methyl branch at the 10 position, 10 Me 18:0. This PLFA pattern has been detected in several monocultures enriched from the soil with propane-amended minimal media. The correspondence of high proportions of these unusual mono-unsaturated PLFA in the isolated monocultures and in situ in the soils after stimulation with the appropriate hydrocarbon is a strong validation of the utility of these biomarkers in defining the community structure of the surface soil microbial community. 相似文献