全文获取类型
收费全文 | 6950篇 |
免费 | 604篇 |
国内免费 | 8篇 |
出版年
2023年 | 38篇 |
2022年 | 98篇 |
2021年 | 245篇 |
2020年 | 124篇 |
2019年 | 163篇 |
2018年 | 164篇 |
2017年 | 125篇 |
2016年 | 252篇 |
2015年 | 392篇 |
2014年 | 404篇 |
2013年 | 475篇 |
2012年 | 595篇 |
2011年 | 569篇 |
2010年 | 359篇 |
2009年 | 291篇 |
2008年 | 404篇 |
2007年 | 376篇 |
2006年 | 312篇 |
2005年 | 302篇 |
2004年 | 308篇 |
2003年 | 260篇 |
2002年 | 289篇 |
2001年 | 59篇 |
2000年 | 52篇 |
1999年 | 66篇 |
1998年 | 51篇 |
1997年 | 47篇 |
1996年 | 41篇 |
1995年 | 30篇 |
1994年 | 37篇 |
1993年 | 33篇 |
1992年 | 27篇 |
1991年 | 20篇 |
1990年 | 17篇 |
1989年 | 25篇 |
1988年 | 17篇 |
1987年 | 22篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 14篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 28篇 |
1980年 | 18篇 |
1978年 | 12篇 |
1976年 | 20篇 |
1975年 | 16篇 |
1974年 | 13篇 |
1973年 | 18篇 |
1966年 | 11篇 |
排序方式: 共有7562条查询结果,搜索用时 390 毫秒
41.
Isolation of an anaerobic bacterium which reductively dechlorinates tetrachloroethene and trichloroethene 总被引:5,自引:0,他引:5
Strain TEA, a strictly anaerobic, motile rod with one to four lateral flagella and a crystalline surface layer was isolated from a mixed culture that completely reduces chlorinated ethenes to ethene. The organism coupled reductive dehalogenation of tetrachloroethene or trichloroethene to cis-1,2-dichloroethene to growth, using molecular hydrogen as the electron donor. It was unable to grow fermentatively or in the presence of tri- or tetrachloroethene with glucose, pyruvate, lactate, acetate or formate. The 16S rDNA sequence of strain TEA was 99.7% identical to that of Dehalobacter restrictus. The two organisms thus are representatives of the same species or the same genus within the Bacillus/Clostridium subphylum of the gram-positive bacteria. 相似文献
42.
Penner serotyping of Campylobacter isolates from poultry, with absorbed pooled antisera 总被引:1,自引:1,他引:0
The Penner serotyping system, based on detection of heat-stable antigens with a passive haemagglutination technique, was used in studies on Campylobacter epidemiology in poultry. Preparation of specific antisera by absorption allowed the use of pooled antisera. Over 80% of the Campylobacter isolates were typable with this modified Penner serotyping system. Typability of strains was clearly affected by storage of the strains before actual typing. Extracted antigens appeared to be stable for at least 6 months at 4°C. Therefore, it is advisable to store extracted antigens from freshly isolated Campylobacter strains instead of reculturing frozen-stored strains, when actual typing cannot be performed directly after primary isolation. Untypability of isolates may partly be explained by the detection of Campylobacter serovars not yet represented in the serotyping system. Experiments on repeated serotyping of several Campylobacter strains did not suggest any serovar instability within the strains. 相似文献
43.
An analogue of human melanin-concentrating hormone (MCH) suitable for radioiodination was designed in which Tyr13 and Val19 of the natural peptide were replaced by phenylalanyl and tyrosyl residues: [Phe13, Tyr19] -MCH. The peptide was synthesized by the continuous-flow solid-phase methodology using Fmocstrategy and Polyhipe PA 500 and PEG-PS resins. The linear MCH peptides with either acetamidomethyl-protected or free cysteinyl residues were purified to homogeneity and cyclized by iodine oxidation, yielding the final product with the correct molecular weight of 2434.61. Radioiodination of the C-terminal tyrosine was carried out enzymatically using solid-phase bound glucose oxidase/lactoperoxidase, followed by purification on a reversed-phase mini-column and by high-pressure liquid chromatography. The resulting [125I]-[Phe13, Tyr19]-MCH tracer was the first radiolabelled MCH peptide suitable for radioreceptor assay: saturation binding analysis using mouse G4F-7 melanoma cells demonstrated the presence of 1090 MCH receptors per cell. The dissociation constant (KD ) was 1.18 × 10?10 M, indicating high-affinity MCH receptors on these cells. MCH receptors were also found in other cell lines such as mouse B16-F1 and G4F and human RE melanoma cells as well as in PC12 and COS-7 cells. Competition binding analyses with a number of other peptides such as α-MSH, neuropeptide Y, substance P and pituitary adenylate cyclase activating peptide, demonstrated that the binding to the MCH receptor is specific. Atrial natriuretic factor was found to be a weak competitor of MCH, indicating topological similarities between MCH and ANF when interacting with MCH receptors. 相似文献
44.
Mary C. Maas 《American journal of physical anthropology》1994,95(2):221-241
This study describes the molar enamel microstructure of seven lemurid primates: Hapalemur griseus, Varecia variegata, Lemur catta, Lemur macaco, Lemur fulvus rufus, Lemur fulvus fulvus, and Lemur fulvus albifrons. Contrary to earlier accounts, which reported little or no prism decussation in lemurid enamel, both Lemur and Varecia molars contain a prominent inner layer of decussating prisms (Hunter-Schreger bands), in addition to an outer radial prism layer, and a thin, nonprismatic enamel surface layer. In contrast, Hapalemur enamel consists entirely of radial and, near the surface, nonprismatic enamel. In addition, for all species, prism packing patterns differ according to depth from the tooth surface, and for all species but Varecia (which also has the thinnest enamel of any lemurid), average prism area increases from the enamel-dentine junction to the surface; this may be a developmental solution to the problem of accommodating a larger outer surface area with enamel deposited from a fixed number of cells. Finally, contradicting some previous reports, Pattern 1 prisms predominate only in the most superficial prismatic enamel. In the deeper enamel, prism cross-sections include both closed (Pattern 1) and arc-shaped (Pattern 2 or, most commonly, Pattern 3). This sequence of depth-related pattern change is repeated in all taxa. It should also be emphasized that all taxa can exhibit all three prism patterns in their mature enamel. The high degree of quantitative and qualitative variation in prism size, shape, and packing suggests that these features should be used cautiously in phylogenetic studies. Hapalemur is distinguished from the other lemurids by unique, medially constricted or rectangular prism cross-sections at an intermediate depth and the absence of prism decussation, but, without further assessment of character polarity, these differences do not clarify lemurid phylogenetic relations. Some characters of enamel microstructure may represent synapomorphies of Lemuridae, or of clades within Lemuridae, but homoplasy is likely to be common. Homoplasy of enamel characters may reflect functional constraints. © 1994 Wiley-Liss, Inc. 相似文献
45.
dos Santos Silva Edinaldo N. Kakkassery Francy K. Maas Sibylle Dumont Henri J. 《Hydrobiologia》1994,288(2):119-128
Calanoid copepods, belonging to the new genus and species Keraladiaptomus rangareddyi, were collected from temporary ponds in Kerala State, India. The new genus belongs to the family Diaptomidae, subfamily Diaptominae. It is described in detail and its affinity to the related genera, Arctodiaptomus Kiefer, 1932 and Eodiaptomus Kiefer, 1932, discussed. 相似文献
46.
Heleen M. Van der Maas E. R. de Jong A. C. Van Aelst H. A. Verhoeven F. A. Krens J. L. Van Went 《Protoplasma》1994,178(1-2):48-56
Summary The cytoplasmic content and the distribution of intramembrane particles (IMPs) of the plasma membrane of isolated sperm cells of perennial ryegrass (Lolium perenne L.) have been characterized using flow cytometry, transmission electron microscopy, confocal scanning laser microscopy and freeze-fracture studies. The isolated haploid sperm cells contain a variety of cell organelles with the exception of microtubules. Proplastids and plastids with starch were observed, although only rarely. Vacuoles containing remnants of organelles and stacked lamellae of endoplasmic reticulum with cytoplasmic inclusions were observed frequently, indicating that autophagy takes place. The number of mitochondria varies from 11 to 26 with an average of 17. Generally, the nucleus has a lobed shape and displays various interphasic stages of chromatin condensation. The analysis of the number of mitochondria and the nuclear state did not show evidence of sperm cell dimorphism. The cytological variability observed, could be explained by differences in developmental stages already present in vivo at the moment of isolation. No correlation between the number of mitochondria and the nuclear cross-sectioned area and/or the condensation state of the chromatin could be found. The density of intramembrane particles of the plasma membrane on the exoplasmic fracture face is more than twice that on the protoplasmic fracture face. That is the opposite of what was found for sporophytic cells of perennial ryegrass. These results are discussed in relation to the potential use of these cells for biotechnology and developmental studies. 相似文献
47.
Three different stages of the avian schistosome Trichobilharziaocellata appeared to contain a novel N-acetylgalac-tosaminyltransferaseactivity. To investigate its function in the biosynthesis ofschistosome glycoconjugates, the enzyme was partially purifiedfrom cercariae, a free-living stage of the parasite, by affinitychromatography on UDP-Sepharose. Acceptor specificity studiesshowed that the enzyme catalyses the transfer of N-acetylgalactosamine(GalNAc) from UDP-GalNAc to oligosaccharides, glyco-peptidesand glycoproteins carrying a terminally 相似文献
48.
Corinne Beaudouin Gisèle Haurat Jean Alex Laffitte Bernard Renaud 《Journal of neurochemistry》1993,61(3):928-935
Abstract: (+)-S-Adenosyl- l -methionine [(+)-SAM] was isolated from rat brain and was quantified by HPLC followed by UV spectrophotometric measurements and by 1 H-NMR. Its estimated ratio in brain is 3% of total SAM. Because of its commercial unavailability, (+)-SAM was also prepared from chemically synthesized SAM by separation of the two diastereoisomers on a preparative reverse-phase Nucleosil C8 column. The (+) diastereoisomer thus obtained was then assayed in vitro both as an inhibitor and a substrate of phenylethanolamine N -methyltransferase. Enzymatic activity was measured by HPLC analysis. It was shown that (+)-SAM has no effect on phenylethanolamine N -methyltransferase activity; therefore, it is unlikely that (+)-SAM plays any possible role in regulation of adrenaline synthesis in the brain. 相似文献
49.
Conclusion: Medical decisions concerning the end of life are a difficult matter and they evoke much emotional response. What is needed, however, is an open debate in order to improve the moral quality of decision making, not "dances with data". The central question in this debate should be, as Callahan aptly notes, whether medicine should involve itself only in that kind of "suffering which is brought on by illness and dying as biological phenomena" (emphasis added) or whether it should concern itself with the wellbeing of the patient. Apart from the fundamental question as to what types of suffering are to be considered as, at least in part biological phenomena, in The Netherlands most doctors, ourselves included, think the medical profession should do the latter. 相似文献
50.
Max R. Brown Hannes Becher Sebastian Williams Alex D. Twyford 《American journal of botany》2023,110(1):e16100