全文获取类型
收费全文 | 35227篇 |
免费 | 83篇 |
国内免费 | 90篇 |
专业分类
系统科学 | 334篇 |
丛书文集 | 855篇 |
教育与普及 | 75篇 |
理论与方法论 | 107篇 |
现状及发展 | 15851篇 |
研究方法 | 1299篇 |
综合类 | 16384篇 |
自然研究 | 495篇 |
出版年
2013年 | 168篇 |
2012年 | 415篇 |
2011年 | 945篇 |
2010年 | 176篇 |
2008年 | 519篇 |
2007年 | 612篇 |
2006年 | 587篇 |
2005年 | 604篇 |
2004年 | 537篇 |
2003年 | 596篇 |
2002年 | 492篇 |
2001年 | 1091篇 |
2000年 | 1111篇 |
1999年 | 634篇 |
1992年 | 637篇 |
1991年 | 538篇 |
1990年 | 578篇 |
1989年 | 513篇 |
1988年 | 549篇 |
1987年 | 562篇 |
1986年 | 547篇 |
1985年 | 715篇 |
1984年 | 525篇 |
1983年 | 449篇 |
1982年 | 396篇 |
1981年 | 435篇 |
1980年 | 540篇 |
1979年 | 1131篇 |
1978年 | 929篇 |
1977年 | 923篇 |
1976年 | 726篇 |
1975年 | 813篇 |
1974年 | 1111篇 |
1973年 | 939篇 |
1972年 | 906篇 |
1971年 | 1093篇 |
1970年 | 1448篇 |
1969年 | 1185篇 |
1968年 | 1019篇 |
1967年 | 1073篇 |
1966年 | 946篇 |
1965年 | 694篇 |
1964年 | 181篇 |
1959年 | 425篇 |
1958年 | 663篇 |
1957年 | 469篇 |
1956年 | 409篇 |
1955年 | 371篇 |
1954年 | 405篇 |
1948年 | 257篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
In 1986, Brown and Clemmons (Proc. natl Acad. Sci. USA83 (1986) 3321) showed that platelets contain a substance, platelet-derived growth inhibitor (PDGI), that inhibits in vitro endothelial cell replication. Although platelets are rich in transforming grwoth factor (TGF-), PDGI was considered not to be related to TGF-, on the basis of its reported properties (extraction from platelets at neutral pH, binding to heparin-Sepharose). However, we purified PDGI to near homogeneity and showed that on the basis of HPLC retention behavior, in vitro growth inhibitory activities with several cell types, receptor binding, and immunoneutralization of growth inhibitory activity with specific anti-TGF- type 1 antibodies, PDGI is most probably identical with TGF- type 1. 相似文献
132.
V. Ralevic P. Milner K. A. Kirkpatrick G. Burnstock 《Cellular and molecular life sciences : CMLS》1992,48(1):31-34
Adenosine 5-triphosphate (ATP) was released into the perfusate of rat isolated mesenteric arterial beds during each of two consecutive increases in flow. There was no significant difference between the amounts of ATP released on each occasion. Substance P was also released into the perfusate by increased flow, although its release was more variable. Removal of the endothelium of the mesenteric vessels with sodium deoxycholate led to a significant reduction (74%) in the amount of ATP released compared with the release before the endothelium had been removed. This suggests that the ATP released into the mesenteric arterial perfusate during increased flow arises from endothelial cells. 相似文献
133.
Autophagic degradation of cytoplasm (including protein, RNA etc.) is a non-selective bulk process, as indicated by ultrastructural evidence and by the similarity in autophagic sequestration rates of various cytosolic enzymes with different half-lives. The initial autophagic sequestration step, performed by a poorly-characterized organelle called a phagophore, is subject tofeedback inhibition by purines and amino acids, the effect of the latter being potentiated by insulin and antagonized by glucagon. Epinephrine and other adrenergic agonists inhibit autophagic sequestration through a prazosin-sensitive 1-adrenergic mechanism. The sequestration is also inhibited by cAMP and by protein phosphorylation as indicated by the effects of cyclic nucleotide analogues, phosphodiesterase inhibitors and okadaic acid.Asparagine specifically inhibits autophagic-lysosomal fusion without having any significant effects on autophagic sequestration, on intralysosomal degradation or on the endocytic pathway. Autophaged material that accumulates in prelysosomal vacuoles in the presence of asparagine is accessible to endocytosed enzymes, revealing the existence of an amphifunctional organelle, the amphisome. Evidence from several cell types suggests that endocytosis may be coupled to autophagy to a variable extent, and that the amphisome may play a central role as a collecting station for material destined for lysosomal degradation.Protein degradation can also take place in a salvage compartment closely associated with the endoplasmic reticulum (ER). In this compartment unassembled protein chains are degraded by uncharacterized proteinases, while resident proteins roturn to the ER and assembled secretory and membrane proteins proceed through the Golgi apparatus. In thetrans-Golgi network some proteins are proteolytically processed by Ca2+-dependent proteinases; furthermore, this compartment sorts proteins to lysosomes, various membrane domains, endosomes or secretory vesicles/granules. Processing of both endogenous and exogenous proteins can occurr in endosomes, which may play a particularly important role in antigen processing and presentation. Proteins in endosomes or secretory compartments can either be exocytosed, or channeled to lysosomes for degradation. The switch mechanisms which decide between these options are subject to bioregulation by external agents (hormones and growth factors), and may play an important role in the control of protein uptake and secretion. 相似文献
134.
135.
R. Wiesendanger B. Martinoni T. Boller D. Arigoni 《Cellular and molecular life sciences : CMLS》1986,42(2):207-209
Summary Under the action of the appropriate synthase from ripe tomatoes a 11 mixture of (3S, 4R)-[3,4-2H2] and (3R, 4S)-[3,4-2H2]-(2S)-adenosylmethionine is transformed into a 11 mixture of the two meso forms of [2H2]-1-aminocyclopropanecarboxylic acid, a result which proves the operation of an inversion mechanism and which is consistent with direct nucleophilic displacement of the leaving group in the substrate. 相似文献
136.
Identification of a new predaceous stink bug pheromone and its attractiveness to the eastern yellowjacket 总被引:1,自引:0,他引:1
J. R. Aldrich W. R. Lusby J. P. Kochansky 《Cellular and molecular life sciences : CMLS》1986,42(5):583-585
Summary Males ofPodisus fretus (Hemiptera: Pentatomidae) release a long-range attractant pheromone containing linalool (49.0%), (E)-2-hexenal (34.5%), benzyl alcohol (12.0%), nerolidol (2.0%),-terpineol (1.1%), and traces of several other compounds. The eastern yellowjacket,Vespula maculifrons (Hymenoptera: Vespidae), is attracted to artificial pheromones forP. fretus and for the sympatric species,Podisus maculiventris.The authors thank Mr T.J. Henry of the USDA Systematic Entomology Laboratory and Dr J.E. McPherson, Southern Illinois University, for examining the pentatomid species and Dr A.S. Menke, USDA-SEL, for determining the yellowjacket species. We also thank S. Wilzer for technical assistance. Mention of a company name does not imply endorsement by the US Department of Agriculture. 相似文献
137.
G. L. Mendz C. R. Middlehurst P. W. Kuchel P. A. Crossie 《Cellular and molecular life sciences : CMLS》1986,42(9):1016-1018
Summary A technique to assay erythrocyte pyrimidine 5-nucleotidase activity in situ using31P nuclear magnetic resonance spectroscopy is presented. The assay is chemically specific, simple and applicable to untreated lysates. A comparison of enzyme levels in normal controls and in multiple sclerosis patients employing the assay yielded no significant differences between both groups. Difficulties encountered in the quantitative analysis of the assay using1H-NMR spectroscopy are briefly discussed. 相似文献
138.
The median procedure for n-trees 总被引:2,自引:2,他引:0
Let (X,d) be a metric space The functionM:X
k 2
x
defined by
is the minimum } is called themedian procedure and has been found useful in various applications involving the notion of consensus Here we present axioms that characterizeM whenX is a certain class of trees (hierarchical classifications), andd is the symmetric difference metricWe would like to thank the referees and Editor for helpful comments 相似文献
139.
R. M. Zinkernagel 《Cellular and molecular life sciences : CMLS》1986,42(9):970-972
Summary The proposal is made, illustrated and supported by experimental evidence that T cell-mediated immunopathology triggered initially by low- or non-cytopathic infectious agents may cause diseases, susceptibility to which is linked to the major histocompatibility gene complex.This summary is an updated version of the paper given on the occasion of the Paul Ehrlich Prize ceremonies in 1983; it was also presented at the meeting New Trends in Allergy II in München 1985, and is reproduced here with the permission of Springer Verlag, Heidelberg. 相似文献
140.
S. Suzuki S. Kurasawa H. Kitai M. Oba S. Komatsu K. Yoda R. Iizuka 《Cellular and molecular life sciences : CMLS》1986,42(7):795-798
Summary Porcine or human follicular fluid inhibited the spontaneous maturation of isolated hamster oocytes in vitro during the first 1.5 h of culture. Moreover, the presence of 50% follicular fluid combined with 100 M dbcAMP cooperatively reduced the incidence of germinal vesicle breakdown. The addition of FSH also inhibited the resumption of meiosis, and the presence of LH did not overcome the inhibitory effects of follicular fluid and tended to impede isolated hamster oocyte maturation in vitro. 相似文献