全文获取类型
收费全文 | 3547篇 |
免费 | 320篇 |
出版年
2022年 | 23篇 |
2021年 | 65篇 |
2020年 | 37篇 |
2019年 | 36篇 |
2018年 | 73篇 |
2017年 | 49篇 |
2016年 | 80篇 |
2015年 | 136篇 |
2014年 | 149篇 |
2013年 | 178篇 |
2012年 | 229篇 |
2011年 | 252篇 |
2010年 | 157篇 |
2009年 | 126篇 |
2008年 | 194篇 |
2007年 | 187篇 |
2006年 | 206篇 |
2005年 | 163篇 |
2004年 | 194篇 |
2003年 | 174篇 |
2002年 | 167篇 |
2001年 | 37篇 |
2000年 | 25篇 |
1999年 | 43篇 |
1998年 | 38篇 |
1997年 | 31篇 |
1996年 | 38篇 |
1995年 | 40篇 |
1994年 | 26篇 |
1993年 | 23篇 |
1992年 | 31篇 |
1991年 | 28篇 |
1990年 | 28篇 |
1989年 | 30篇 |
1988年 | 25篇 |
1987年 | 34篇 |
1986年 | 23篇 |
1985年 | 31篇 |
1984年 | 18篇 |
1983年 | 18篇 |
1982年 | 36篇 |
1981年 | 37篇 |
1980年 | 25篇 |
1979年 | 36篇 |
1978年 | 31篇 |
1977年 | 27篇 |
1976年 | 28篇 |
1975年 | 16篇 |
1974年 | 20篇 |
1973年 | 17篇 |
排序方式: 共有3867条查询结果,搜索用时 781 毫秒
41.
42.
Martha J. Powell 《Protoplasma》1994,181(1-4):123-141
Summary In development of the primitive fungi, chytridiomycetes, unwalled zoospores bearing single, posterior flagella are transformed into walled, round-cells which elaborate the thallus. Production, structural modification, or release of extracellular material are involved with each transition of developmental stage. This article reviews the variety and developmental changes of extracellular materials found at the cell surface of chytridiomycetes. A cell coat, produced from Golgi-derived vesicles during zoosporogenesis, is visible around free swimming zoospores of some chytridiomycetes. How the zoospore surface receives and transduces signals is not widely explored, but it is known that fenestrated cisternae and simple cisternae, which are integrated into the microbody-lipid globule complex, are spatially and structurally associated with the plasma membrane and flagellar apparatus. This spatial association, as well as the cytochemical localization of calcium in fenestrated cisternae, suggest a mechanism for signal transduction and for regulation of zoospore motility. Zoospores become encased in a new layer of extracellular material as the zoospore encysts. Among some chytrids the source of this material is preexisting vesicles which fuse with the plasma membrane. Among other zoospores, a readily identifiable population of encystment vesicles is not apparent, demonstrating that there is no single pattern or mechanism for zoospore encystment in chytridiomycetes. Encysted zoospores developing into thalli, typically produce cell walls with a microfibrillar substructure. Ultrastructural analysis of walls reveals distinctive architecture and remarkable sculpturing which have been used in systematics of some members of chytridiomycetes. Nothing is known as to underlying controls of cytoskeletal elements and plasma membrane enzyme complexes in wall biogenesis. Many changes in cell surface structures accompany thallus maturation. Septa, many traversed with plasmodesmata, are produced in most chytrid thallus types. As sporangia and resting spores prepare for the production and release of zoospores, additional extracellular layers of material are frequently produced. Polarized deposits of extracellular material become discharge plugs, discharge vesicles, or endoopercula. Interstitial material is also released into cleavage furrows. Circumscissile or localized digestion of walls produce operculate or inoperculate exit ports for zoospore release. Cryofixation preserves more extensive extracellular material than does conventional chemical fixation, and broader application of cryofixation may radically alter our current view of cell surface structure. Thus chytridiomycetes exhibit a range in patterns for the occurrence and subsequent modifications of extracellular materials, even for members within the same order. The most universally recognized role for these extracellular materials is protection. Although there is a reasonable view of the types of extracellular material involved in chytridiomycete development, we have only limited understandings of their biogenesis or roles in regulation and communication, areas awaiting more investigations.Abbreviations DIC
Nomarski-differential contrast optics
- TEM
transmission electron microscopy 相似文献
43.
Effects of modeling and lineage on fishing behavior in the small-eared bushbaby (otolemur garnettii)
Sheree L. Watson Martha Schiff Jeannette P. Ward 《International journal of primatology》1994,15(4):507-519
Thirty-eight bushbabies(Otolemur garnettii)were subjects in an observational learning study. We exposed them to one of three modeling conditions: (1) fishing model—one
that actually performed fishing behavior; (2) nonfishing model—one that performed as a model in every way except performance
of fishing behavior; and (3) no model. We assessed them with regard to latency to approach the fishbowl, latency to make an
initial fishing attempt, duration of time spent in the vicinity of the fishbowls, and number of actual fishing attempts. Results
indicate that subjects that were exposed to either fishing or nonfishing models were faster to approach the fishbowls and
spent more time in the vicinity of the fishbowls than animals in the no-model condition Lineage, i.e., whether or not the
animals’ parents fished, rather than modeling condition, was the best predictor of the latency to initial fishing attempt
and the number of attempts made. 相似文献
44.
The Regional Distribution of N-Acetylaspartylglutamate (NAAG) and Peptidase Activity Against NAAG in the Rat Nervous System 总被引:4,自引:4,他引:0
Stefanie Fuhrman Miklos Palkovits Martha Cassidy Joseph H. Neale 《Journal of neurochemistry》1994,62(1):275-281
Abstract: N -Acetylaspartylglutamate (NAAG), a prevalent peptide in the vertebrate nervous system, may be hydrolyzed by extracellular peptidase activity to produce glutamate and N -acetylaspartate. Hydrolysis can be viewed as both inactivating the peptide after synaptic release and increasing synaptic levels of ambient glutamate. To test the hypothesis that NAAG and the peptidase activity that hydrolyzes it coexist as a unique, two-stage system of chemical neurotransmission, 50 discrete regions of the rat CNS were microdissected for assay. In each microregion, the concentration of NAAG was determined by radioimmunoassay and the peptidase activity was assayed using tritiated peptide as substrate. The NAAG concentration ranged from 2.4 nmol/mg of soluble protein in median eminence to 64 in thoracic spinal cord. Peptidase activity against NAAG ranged from 54 pmol of glutamate produced per milligram of membrane protein per minute in median eminence to 148 in superior colliculus. A linear relationship was observed between NAAG peptidase and NAAG concentration in 46 of the 50 areas, with a slope of 2.26 and a correlation coefficient of 0.45. These data support the hypothesis that hydrolysis of NAAG to glutamate and N -acetylaspartate is a consistent aspect of the physiology and metabolism of this peptide after synaptic release. The ratio of peptide concentration to peptidase activity was >0.3 in the following four areas: ventrolateral medulla and reticular formation where the peptide is concentrated in axons of passage, thoracic spinal cord, where NAAG is concentrated in ascending sensory tracts as well as motoneuron cell bodies, and ventroposterior thalamic nucleus. 相似文献
45.
Summary In asexual reproduction of the water mold,Saprolegnia ferax, four distinct and sequentially produced spores are involved in dispersal, two of which are motile and two of which are nonmotile. Composition of cell surface glycoproteins may be important in dispersal strategies for each of these stages. Binding patterns of fluorescently labelled lectins were investigated to identify differences in glycoproteins of asexually produced dispersal stages. The pattern of lectin binding to zoospores was diverse. FITC-Con A bound to surfaces of zoospores and membranes of the water expulsion vacuole system, indicating the prescence of mannosyl and glucosyl residues. In zoospores incubated for more than 30 min in FITC-WGA and FITC-GS II. which bind N-acetyl glucosamine, fluorescence was sometimes localized in peripheral, intracellular patches. In shorter incubations, secondary zoospores bound these lectins along the groove region where K-bodies were located. Surfaces of cystospores typically bound FITC-WGA, but not FITC-GS II. FITC-GS II, however, bound to empty cystospore walls, probably because reactive sugars were available at the inner surface of the wall. Germ tubes emerging from cystospores bound labelled WGA and GS II, but not Con A. The same lectin binding pattern was found along discharge papilla of primary cystospores, indicating that modifications in cystospore walls associated with direct germination and zoospore discharge were similar. Thus, glycoproteins involved in early establishment of the hyphal system differ from those forming the cell surface of cystospores. Differences in the binding pattern of lectins to zoospores and cystospores highlight differences between cell surface carbohydrates of motile and nonmotile asexual stages.Abbreviations BPA
lectin fromBauhinia purpurea
- C1
primary cystospore
- C2
secondary cystospore
- Con A
concanavalin A, lectin fromCanavalia ensiformis
- DBA
lectin fromDolichos biflorus
- DIC
Nomarski differential interference contrast optics
- DS
dilute salts
- FITC
fluorescein isothiocyanate
- FUC
fucose
- Gal
galactose
- GalNAc
N-acetyl galactosamine
- Glc
glucose
- GlcNAc
N-acetyl glucosamine
- GS I
Griffonia simplicifolia lectin I
- GS II
G. simplicifolia lectin II
- Man
mannose
- MPA
lectin fromMaclura pomifera
- PC
phase contrast optics
- PNA
lectin fromArachis hypogaea
- SBA
soybean agglutinin, lectin fromGlycine max
- UEA-1
lectin fromUlex europaeus
- WGA
wheat germ agglutinin fromTriticum vulgare
- WV
water expulsion vacuole 相似文献
46.
Sarah M. Nour John R. Lawrence Hong Zhu George D. W. Swerhone Martha Welsh Tom W. Welacky Edward Topp 《Applied microbiology》2003,69(1):607-615
The soybean cyst nematode (SCN), Heterodera glycines, causes economically significant damage to soybeans (Glycine max) in many parts of the world. The cysts of this nematode can remain quiescent in soils for many years as a reservoir of infection for future crops. To investigate bacterial communities associated with SCN cysts, cysts were obtained from eight SCN-infested farms in southern Ontario, Canada, and analyzed by culture-dependent and -independent means. Confocal laser scanning microscopy observations of cyst contents revealed a microbial flora located on the cyst exterior, within a polymer plug region and within the cyst. Microscopic counts using 5-(4,6-dichlorotriazine-2-yl)aminofluorescein staining and in situ hybridization (EUB 338) indicated that the cysts contained (2.6 ± 0.5) × 105 bacteria (mean ± standard deviation) with various cellular morphologies. Filamentous fungi were also observed. Live-dead staining indicated that the majority of cyst bacteria were viable. The probe Nile red also bound to the interior polymer, indicating that it is lipid rich in nature. Bacterial community profiles determined by denaturing gradient gel electrophoresis analysis were simple in composition. Bands shared by all eight samples included the actinobacterium genera Actinomadura and Streptomyces. A collection of 290 bacteria were obtained by plating macerated surface-sterilized cysts onto nutrient broth yeast extract agar or on actinomycete medium. These were clustered into groups of siblings by repetitive extragenic palindromic PCR fingerprinting, and representative isolates were tentatively identified on the basis of 16S rRNA gene sequence. Thirty phylotypes were detected, with the collection dominated by Lysobacter and Variovorax spp. This study has revealed the cysts of this important plant pathogen to be rich in a variety of bacteria, some of which could presumably play a role in the ecology of SCN or have potential as biocontrol agents. 相似文献
47.
Inhibition of Salmonella typhimurium by the products of tartrate metabolism by a Veillonella species
The inhibition of the growth of Salmonella typhimurium by a Veillonella species grown on media supplemented with tartrate was examined. Growth of Salmonella typhimurium was not inhibited by the concentrations of products metabolized by Veillonella cultures on media supplemented with 0 or 50 mmol 1-1 of tartrate, but was inhibited on media supplemented with 100 or 150 mmol 1-1 of tartrate. Inhibition of Salm. typhimurium was correlated with the increased production of acetate and propionate from tartrate by the Veillonella species. 相似文献
48.
In 1976, Crump, Hoel, Langley, and Peto described how almost any dose‐response relationship for carcinogens becomes linear at low doses when background cancers are taken into account. This has been used, by the U.S. Environmental Protection Agency, USEPA, as partial justification for a regulatory posture that assumes low‐dose linearity, as is illustrated by a discussion of regulation of benzene as a carcinogen. The argument depends critically on the assumption that the pollutant and the background proceed by the same biological mechanism. In this paper we show that the same argument applies to noncancer end points also. We discuss the application to a number of situations: reduction in lung function and consequent increase in death rate due to (particulate) air pollution; reduction in IQ and hence (in extreme cases) mental deficiency due to radiation in utero; reduction of sperm count and hence increase in male infertility due to DBCP exposure. We conclude that, although the biological basis for the health effect response is different, in each case low‐dose linearity might arise from the same mathematical effect discussed by Crump et al. (1976). We then examine other situations and toxic end points where low‐dose linearity might apply by the same argument. We urge that biologists and chemists should concentrate efforts on comparing the biological and pharmacokinetic processes that apply to the pollutant and the background. Finally, we discuss some public policy implications of the possibility that low dose linearity may be the rule rather than the exception for environmental exposures. 相似文献
49.
Selenoprotein P associates with endothelial cells in rat tissues 总被引:11,自引:0,他引:11
R. F. Burk Kristina E. Hill Martha E. Boeglin Ford F. Ebner Harold S. Chittum 《Histochemistry and cell biology》1997,108(1):11-15
Selenoprotein P is an extracellular heparin-binding protein that has been implicated in protecting the liver against oxidant
injury. Its location in liver, kidney, and brain was determined by conventional immunohistochemistry and confocal microscopy
using a polyclonal antiserum. Selenoprotein P is associated with endothelial cells in the liver and is more abundant in central
regions than in portal regions. It is also present in kidney glomeruli associated with capillary endothelial cells. Staining
of selenoprotein P in the brain is also confined to vascular endothelial cells. The heparin-binding properties of selenoprotein
P could be the basis for its binding to tissue. Its localization to the vicinity of endothelial cells is potentially relevant
to its oxidant defense function.
Accepted: 6 March 1997 相似文献
50.
The purpose of this investigation was to determine whether Madurella mycetomatis, the most frequent agent of eumycotic mycetomas, produces siderophores and synthesizes new outer membrane proteins under iron-starvation conditions. Siderophore production, only of the hydroxamate type, was demonstrated in all nine strains tested. It was regulated by extracellular iron concentrations. Under iron-restricted conditions, M. mycetomatis expressed various outer membrane iron-regulated proteins, particularly of 24-kilodalton, that may participate in iron metabolism. 相似文献