全文获取类型
收费全文 | 9125篇 |
免费 | 499篇 |
国内免费 | 5篇 |
出版年
2022年 | 58篇 |
2021年 | 92篇 |
2020年 | 66篇 |
2019年 | 72篇 |
2018年 | 97篇 |
2017年 | 84篇 |
2016年 | 192篇 |
2015年 | 286篇 |
2014年 | 309篇 |
2013年 | 625篇 |
2012年 | 550篇 |
2011年 | 563篇 |
2010年 | 359篇 |
2009年 | 333篇 |
2008年 | 556篇 |
2007年 | 595篇 |
2006年 | 533篇 |
2005年 | 540篇 |
2004年 | 598篇 |
2003年 | 486篇 |
2002年 | 497篇 |
2001年 | 151篇 |
2000年 | 157篇 |
1999年 | 145篇 |
1998年 | 122篇 |
1997年 | 95篇 |
1996年 | 87篇 |
1995年 | 83篇 |
1994年 | 73篇 |
1993年 | 77篇 |
1992年 | 115篇 |
1991年 | 109篇 |
1990年 | 85篇 |
1989年 | 78篇 |
1988年 | 79篇 |
1987年 | 62篇 |
1986年 | 55篇 |
1985年 | 54篇 |
1984年 | 54篇 |
1983年 | 42篇 |
1982年 | 34篇 |
1981年 | 43篇 |
1980年 | 37篇 |
1979年 | 33篇 |
1978年 | 33篇 |
1977年 | 27篇 |
1976年 | 24篇 |
1974年 | 19篇 |
1973年 | 21篇 |
1972年 | 20篇 |
排序方式: 共有9629条查询结果,搜索用时 15 毫秒
71.
Tomoshiro Takeda Osamu Ueno Muneaki Samejima Takeshi Ohtani 《Journal of plant research》1985,98(4):393-411
Two hundred and twenty species of 38 genera in the Cyperaceae from Australia were examined for the possible occurrence of the C4 photosynthesis and the anatomical features of leaves and culms. The Kranz type of anatomy and the carbon isotope ratios typical of C4 plants were found in 84 species in the following six genera of four tribes belonging to subfamily Cyperoideae:Bulbostylis, Crosslandia, andFimbristylis (Fimbristylideae);Lipocarpha (Lipocarpheae);Cyperus (Cypereae);Rhynchospora (Rhynchosporeae). The anatomical observation revealed that the C4 species possessed any one of the three Kranz anatomical types found by previous investigators. It was suggested that in the Cyperaceae the C4 syndrome evolved independently within several taxa of the subfamily. The relative distribution of C3 and C4 species of the Cyperaceae in Australia was investigated by use of floristic data. It was recognized that the C4 species dominated in the northern part of the continent which was characterized by tropical and subtropical savannas and hot dry areas with summer rainfall, and the C3 species in the southern part, which contained temperate areas and mediterranean climatic areas with winter rainfall. 相似文献
72.
In an attempt to elucidate the relationship between phosphatidylinositol breakdown and alpha-adrenergic responses, effects of phosphatidic acid and phosphatidylinositol related metabolites on Ca2+ mobilization and glucose output in cultured hepatocytes were examined. Norepinephrine induced the net 45Ca2+ efflux from preloaded cells and stimulated glucose output via alpha-adrenergic receptor stimulation, whereas phosphatidic acid caused 45Ca2+ uptake to cells and did not stimulate glucose output. Myo-inositol-monophosphate, diglyceride and arachidonic acid, which are released by phosphatidylinositol breakdown, had no effect on 45Ca2+ efflux and glucose output in cells. These results suggest that phosphatidic acid and phosphatidylinositol related metabolites can not mimic the alpha-adrenergic actions in cultured hepatocytes. 相似文献
73.
Spores of Nosema bombycis derived from diseased insects were highly purified by Urografin density gradient centrifugation. Antheraea eucalypti cells were inoculated with the purified spores primed with 0.1 n KOH solution to start a continuous propagation of N. bombycis in cell culture. The first increase in the number of infected A. eucalypti cells was observed at 48 hr postinoculation, and it was caused by the secondary infective forms of N. bombycis. The secondary infective forms were produced during the course of sporoblast differentiation. The parasites in cell cultures divided synchronously until 36 hr postinoculation. Mature spores were observed initially 6 days postinoculation at 27°C. The infected cultures were subcultured extensively for more than 1 year with the addition of healthy A. eucalypti cells. 相似文献
74.
Atsushi Ikai Masaaki Nishigai Toshiya Osada Hideo Arakawa Masako Kikuchi 《The protein journal》1987,6(1):81-93
The plasma α2-macroglobulin and its egg white homologue ovomacroglobulin were purified from several different species and their structure before and after the reaction with proteinases studied by electron microscopy. The negatively stained specimens showed either a ringlike structure or a flowerlike one before the reaction with proteinses, but their structures changed into open rectangular ones after the reaction. The translational frictional ratio f/f 0 of human α2-macroglobulin and crocodilian ovomacroglobulin given in the literature is between 1.5 and 1.6 before and after the reaction with proteinases. The value reflects asymmetry due not to a high axial ratio, but rather to an openness of the structure resulting in a partially free draining character of the molecules. The computational method developed by Bloomfield and his co-workers based on the formalism of Kirkwood is used to calculate the frictional ratio of several models constructed from small spheres. The overall shape of the models is derived from electron micrographs. Although the degree of hydration is an unknown parameter in the calculation, reasonable agreement is obtained between the experimental values of f/f 0 and the calculated ones. Combination of electron microscopic and hydrodynamic methods would be fruitful in the structural study of giant proteins such as α2-macroglobulin. 相似文献
75.
Subhash Padhye Takeshi Kambara David N. Hendrickson Govindjee 《Photosynthesis research》1986,9(1-2):103-112
The recent model of Kambara and Govindjee for water oxidation [Kambara T. and Govindjee (1985) Proc. Natl. Acad. Sci. U.S.A., 82:6119–6123] has been extended in this paper by examining all the data in order to identify the most likely candidate for the redox-active ligand (RAL), suggested to operate between the water oxidizing complex (WOC) and Z, the electron donor to the reaction center P680. We have concluded that a very suitable candidate for RAL is the imidazole moiety of a histidine residue. The electrochemical data available on imidazole derivatives play heavily in this identification of RAL. Thus, we suggest that histidine might play the role of an electron mediator between the WOC and Z. A model of S-states in terms of their plausible chemical identity is presented here.Abbreviations J
electronic spin of ion
- P680
reaction center chlorophyll
- RAL
Redox active ligand
- Sn
state of the oxygen-evolving system
- WOC
water oxidation complex
- Z
electron donor to P680
Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement 相似文献
76.
77.
K Nakahama K Yoshimura R Marumoto M Kikuchi I S Lee T Hase H Matsubara 《Nucleic acids research》1986,14(14):5843-5855
The gene encoding an extracellular metalloproteinase from Serratia sp. E-15 has been cloned, and its complete nucleotide sequence determined. The amino acid sequence deduced from the nucleotide sequence reveals that the mature protein of the Serratia protease consists of 470 amino acids with a molecular weight of 50,632. The G+C content of the coding region for the mature protein is 58%; this high G+C content is due to a marked preference for G+C bases at the third position of the codons. The gene codes for a short pro-peptide preceding the mature protein. The Serratia protease gene was expressed in Escherichia coli and Serratia marcescens; the former produced the Serratia protease in the cells and the latter in the culture medium. Three zinc ligands and an active site of the Serratia protease were predicted by comparing the structure of the enzyme with those of thermolysin and Bacillus subtilis neutral protease. 相似文献
78.
M Misago S Chiba M Kikuchi J Tsukada H Suzuki 《International journal of cell cloning》1986,4(5):320-330
The serial changes of peripheral reticulocytes and marrow erythroid progenitor cells (CFU-e) in mice were monitored under the conditions of absolute or relative changes in red cell mass to study the regulatory mechanism of erythropoiesis. A decreased number of marrow CFU-e and peripheral reticulocytes was observed in the mice with relative polycythemia induced by dehydration as well as in the mice with absolute polycythemia induced by hypertransfusion. On the other hand, a transient increase in the number of marrow CFU-e followed by a gradual increase in the number of peripheral reticulocytes was seen after a considerable amount of exsanguination. Similar stimulatory effects on marrow CFU-e were also observed either by rehydrating the dehydrated mice or by overhydrating the untreated mice to relatively decrease the level of hematocrit. The results suggested that in addition to factors relating to the balance between oxygen supply and requirement, which has been well known, erythropoiesis is greatly affected by hematocrit. 相似文献
79.
Flavonoids in the leaves of twenty-eight species belonging to the Polygonaceae were studied. Thirty-three kinds of flavonoids
were isolated, and eighteen kinds were obtained as crystals. Quercetin glycosides were commonly found in the family. In the
quercetin glycosides, 3-O-rhamnoside was most frequently found: 3-O-glucuronide is also distributed widely. Myricetin glycosides were rare. Methylated flavonols were found in some species of
the sectionsEchinocaulon andPersicaria. Eleven kinds ofC-glycosylflavones were found in the present survey, andC-glycosylflavones were distributed in all species of the genusRheum and in almost all species of the section Tiniaria.Rumex Acetosella andPolygonum suffultum are exceptional, the former contains flavone glycoside and the latterC-glycosylflavones only, as main components. 相似文献
80.
Sodium-dependence of glycylglycine (Gly-Gly) influx and stimulation of Na+ transport by Gly-Gly were studied in everted sacs, sheet preparations and brush-border membrane vesicles isolated from guinea-pig ileum. Gly-Gly influx was found to be independent of the presence of Na+, while Na+ transport was stimulated by Gly-Gly as evidenced by increases in transmural potential difference (PDt), short-circuit current (Isc) and Na+ influx. The change in PDt (ΔPDt) induced by Gly-Gly was a saturable function of Gly-Gly concentration, showing a Michaelis-Menten type relationship. The half-saturation concentration for Gly-Gly estimated from the electrical data was nearly identical with that estimated from influx data. At a constant Gly-Gly concentration the relationship between Isc and Na+ concentration was sigmoid, and the Hill coefficient was 1.5. Kinetic analysis according to Garay Garrahan indicates that each Gly-Gly carrier has two equivalent non-interacting binding sites for Na+, and that translocation of Na+ occurs when the two Na+ sites on the carrier loaded with Gly-Gly are occupied by Na+. However, our results indicate that the resultant Na+ flow is not capable of stimulating Gly-Gly translocation. 相似文献