全文获取类型
收费全文 | 776篇 |
免费 | 63篇 |
专业分类
839篇 |
出版年
2013年 | 13篇 |
2012年 | 17篇 |
2011年 | 14篇 |
2010年 | 22篇 |
2009年 | 23篇 |
2008年 | 17篇 |
2007年 | 21篇 |
2006年 | 15篇 |
2005年 | 13篇 |
2004年 | 11篇 |
2003年 | 7篇 |
2002年 | 7篇 |
2000年 | 7篇 |
1999年 | 8篇 |
1998年 | 9篇 |
1997年 | 17篇 |
1995年 | 13篇 |
1993年 | 14篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 13篇 |
1985年 | 10篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1976年 | 13篇 |
1975年 | 8篇 |
1974年 | 6篇 |
1972年 | 9篇 |
1971年 | 8篇 |
1959年 | 19篇 |
1958年 | 41篇 |
1957年 | 40篇 |
1956年 | 36篇 |
1955年 | 38篇 |
1954年 | 28篇 |
1953年 | 27篇 |
1952年 | 23篇 |
1951年 | 29篇 |
1950年 | 22篇 |
1949年 | 20篇 |
1948年 | 16篇 |
1946年 | 6篇 |
排序方式: 共有839条查询结果,搜索用时 0 毫秒
51.
52.
53.
IN VITRO studies have suggested that adenosine 3′,′-monophosphate (cyclic AMP) regulates cell morphology. During treatment with the dibutyryl analogue of cyclic AMP, N6,O2′-dibutyryl cyclic AMP, transformed fibroblasts acquire several morphological characteristics of untransformed fibroblasts1,3. Cell processes are extended, the cells occupy a greater surface area and in some cases there is a parallel alignment of cells. Chinese hamster ovary cells are affected in the same way. In neuroblastoma cells5, dibutyryl cyclic AMP induces neurite extension and increases the activity of acetylcholinesterase, an indicator of biochemical differentiation6. Cyclic AMP is known to control the dispersion of melanin7,8 and the differentiation of melanoblasts into melanocytes. We have now found that during treatment with dibutyryl cyclic AMP, melanoma cells spread out, appear larger and produce considerably more pigment than untreated cells. 相似文献
54.
Biosynthesis of lysosomal hydrolases: their synthesis in bound polysomes and the role of co- and post-translational processing in determining their subcellular distribution 总被引:17,自引:20,他引:17 下载免费PDF全文
By in vitro translation of mRNA’s isolated from free and membrane-bound polysomes, direct evidence was obtained for the synthesis of two lysosomal hydrolases, β-glucuronidase of the rat preputial gland and cathespin D of mouse spleen, on polysomes bound to rough endoplasmic reticulum (ER) membranes. When the mRNA’s for these two proteins were translated in the presence of microsomal membranes, the in vitro synthesized polypeptides were cotranslationally glycosylated and transferred into the microsomal lumen. Polypeptides synthesized in the absence of microsomal membranes were approximately 2,000 daltons larger than the respective unglycosylated microsomal polypeptides found after short times of labeling in cultured rat liver cells treated with tunicamycin. This strongly suggests that nascent chains of the lysosomal enzymes bear transient amino terminal signals which determine synthesis on bound polysomes and are removed during the cotranslational insertion of the polypeptides into the ER membranes. In the line of cultured rat liver cells used for this work, newly synthesized lysosomal hydrolases showed a dual destination; approximately 60 percent of the microsomal polypeptides detected after short times of labeling were subsequently processed proteolytically to lower molecular weight forms characteristic of the mature enzymes. The remainder was secreted from the cells without further proteolytic processing. As previously observed by other investigations in cultured fibroblasts (A. Gonzalez-Noriega, J.H. Grubbs, V. Talkad, and W.S. Sly, 1980, J Cell Biol. 85: 839-852; A. Hasilik and E.F. Neufeld, 1980, J. Biol. Chem., 255:4937-4945.) the lysosomotropic amine chloroquine prevented the proteolytic maturation of newly synthesized hydrolases and enhanced their section. In addition, unglycosylated hydrolases synthesized in cells treated with tunicamycin were exclusively exported from the cells without undergoing proteolytic processing. These results support the notions that modified sugar residues serve as sorting out signals which address the hydrolases to their lysosomal destination and that final proteolytic cleavage of hydrolase precursors take place within lysosome itself. Structural differences in the carbohydrate chains of intracellular and secreted precursors of cathespin D were detected from their differential sensitivity to digestion with endoglycosidases H and D. These observations suggest that the hydrolases exported into the medium follow the normal secretory route and that some of their oligosaccharides are subject to modifications known to affect many secretory glycoproteins during their passage through the Golgi apparatus. 相似文献
55.
Individually mark–mass release–resight study elucidates effects of patch characteristics and distance on host patch location by an insect herbivore 下载免费PDF全文
1. How organisms locate their hosts is of fundamental importance in a variety of basic and applied ecological fields, including population dynamics, invasive species management and biological control. However, tracking movement of small organisms, such as insects, poses significant logistical challenges. 2. Mass‐release and individual–mark–recapture techniques were combined in an individually mark–mass release–resight (IMMRR) approach to track the movement of over 2000 adult insects in an economically important plant–herbivore system. Despite its widespread use for the biological control of the invasive thistle Carduus nutans, the host‐finding behaviour of the thistle head weevil Rhinocyllus conicus has not previously been studied. Insects were released at different distances from a mosaic of artificially created host patches with different areas and number of plants to assess the ecological determinants of patch finding. 3. The study was able to characterize the within‐season dispersal abilities and between‐patch movement patterns of R. conicus. Weevils found host plant patches over 900 m away. Large patches, with tall plants, situated close to the nearest release point had the highest first R. conicus resights. Patch area and plant density had no effect on the number of weevils resighted per plant; however, R. conicus individuals were more likely to disperse out of small patches and into large patches. 4. By understanding how R. conicus locates host patches of C. nutans, management activities for the control of this invasive thistle can be better informed. A deeper mechanistic understanding of host location will also improve prediction of coupled plant–herbivore spatial dynamics in general. 相似文献
56.
SYNOPSIS. The carbohydrate of variant-specific surface antigen glycoproteins from bloodstream forms of 13 cloned variants of Trypanosoma brucei was analyzed by gas-liquid chromatography. The glycoproteins contained from 6 to 17% carbohydrate by weight, and all contained the same 4 sugars: mannose, galactose, glucose, and glucosamine (probably as N-acetyl-glucosamine). The glycoprotein from variant 048, strain 427 contained (±20%) 11 mannose, 4 galactose, 4 glucose, and 5 glucosamine residues/mole of glycoprotein (molecular weight 65,000). Glucose was an integral component of the glycoproteins, not dissociable by sodium dodecyl sulphate, 8 M urea, or 1 M acetic acid. Some of the glucose was dissociated by trichloroacetic acid. Most of the glycoproteins formed precipitin bands with concanavalin A in Ouchterlony double diffusion, but none formed such bands with wheat germ agglutinin or Ricinus communis lectin (molecular weight 120,000). 相似文献
57.
IAN J. JACKSON PETER BUDD JACQUELINE M. HORN RUTH JOHNSON SOPHIE RAYMOND KAREN STEEL 《Pigment cell & melanoma research》1994,7(2):73-80
The formation of mouse coat color is a relatively complex developmental process that is affected by a large number of mutations, both naturally occurring and induced. The cloning of the genes in which these mutations occur and the elucidation of the mechanisms by which these mutations disrupt the normal pigmentation pattern is leading to an understanding of the way interactions between gene products lead to a final phenotype. 相似文献
58.
Microbial dissimilatory iron reduction (DIR) is widespread in anaerobic sediments and is a key producer of aqueous Fe(II) in suboxic sediments that contain reactive ferric oxides. Previous studies have shown that DIR produces some of the largest natural fractionations of stable Fe isotopes, although the mechanism of this isotopic fractionation is not yet well understood. Here we compare Fe isotope fractionations produced by similar cultures of Geobacter sulfurreducens strain PCA and Shewanella putrefaciens strain CN32 during reduction of hematite and goethite. Both species produce aqueous Fe(II) that is depleted in the heavy Fe isotopes, as expressed by a decrease in 56Fe/54Fe ratios or δ56Fe values. The low δ56Fe values for aqueous Fe(II) produced by DIR reflect isotopic exchange among three Fe inventories: aqueous Fe(II) (Fe(II)aq), sorbed Fe(II) (Fe(II)sorb), and a reactive Fe(III) component on the ferric oxide surface (Fe(III)reac). The fractionation in 56Fe/54Fe ratios between Fe(II)aq and Fe(III)reac was –2.95‰, and this remained constant over the timescales of the experiments (280 d). The Fe(II)aq – Fe(III)reac fractionation was independent of the ferric Fe substrate (hematite or goethite) and bacterial species, indicating a common mechanism for Fe isotope fractionation during DIR. Moreover, the Fe(II)aq – Fe(III)reac fractionation in 56Fe/54Fe ratios during DIR is identical within error of the equilibrium Fe(II)aq – ferric oxide fractionation in abiological systems at room temperatures. This suggests that the role of bacteria in producing Fe isotope fractionations during DIR lies in catalyzing coupled atom and electron exchange between Fe(II)aq and Fe(III)reac so that equilibrium Fe isotope partitioning occurs. Although Fe isotope fractionation between Fe(II)aq and Fe(III)reac remained constant, the absolute δ56Fe values for Fe(II)aq varied as a function of the relative proportions of Fe(II)aq, Fe(II)sorb, and Fe(III)reac during reduction. The temporal variations in these proportions were unique to hematite or goethite but independent of bacterial species. In the case of hematite reduction, the small measured Fe(II)aq – Fe(II)sorb fractionation of −0.30‰ in 56Fe/54Fe ratios, combined with the small proportion of Fe(II)sorb, produced insignificant (<0.05‰) isotopic effects due to sorption of Fe(II). Sorption of Fe(II) produced small, but significant effects during reduction of goethite, reflecting the higher proportion of Fe(II)sorb and larger measured Fe(II)aq – Fe(II)sorb fractionation of –0.87‰ in 56Fe/54Fe ratios for goethite. The isotopic effects of sorption on the δ56Fe values for Fe(II)aq were largest during the initial stages of reduction when Fe(II)sorb was the major ferrous Fe species during goethite reduction, on the order of 0.3 to 0.4‰. With continued reduction, however, the isotopic effects of sorption decreased to <0.2‰. These results provide insight into the mechanisms that produce Fe isotope fractionation during DIR, and form the basis for interpretation of Fe isotope variations in modern and ancient natural systems where DIR may have driven Fe cycling. 相似文献
59.
60.
AUDREY J. MAGOUN JUSTINA C. RAY DEVIN S. JOHNSON PATRICK VALKENBURG F. NEIL DAWSON JEFF BOWMAN 《The Journal of wildlife management》2007,71(7):2221-2229
ABSTRACT We designed a novel approach to determining extent of distribution and area of occupancy for wolverines (Gulo gulo) by using aerial surveys of tracks in snow and hierarchical spatial modeling. In 2005 we used a small, fixed-wing aircraft with pilot and one observer to search 575 of 588 survey units for wolverine tracks in approximately 60,000 km2 of boreal forest in northwestern Ontario, Canada. We used sinuous flight paths to scan open areas in the forest in the 100-km2 survey units. We detected tracks in 138 (24%) of the 575 sampled units. There was strong evidence of occurrence (probability of occurrence >0.80) in 30% of the 588 survey units, weak evidence of occurrence (0.50–0.80) in 12%, weak evidence of absence (0.20–0.50) in 15%, and strong evidence of absence (< 0.20) in 43%. Wolverine range comprised 59% of the study area and area of occupancy was 33,400 km2. With information on probability of occurrence and core areas of occupation for wolverines in our study area, resource managers and others can examine factors that influence wolverine distribution patterns and use this information to formulate best management practices that will maintain wolverines on the landscape in the face of increasing resource development. Comparing future survey results with those of our 2005 survey will provide an objective way to assess the efficacy of management practices. 相似文献