全文获取类型
收费全文 | 2585篇 |
免费 | 264篇 |
国内免费 | 1篇 |
专业分类
2850篇 |
出版年
2021年 | 39篇 |
2020年 | 18篇 |
2019年 | 27篇 |
2018年 | 24篇 |
2017年 | 27篇 |
2016年 | 44篇 |
2015年 | 84篇 |
2014年 | 82篇 |
2013年 | 116篇 |
2012年 | 137篇 |
2011年 | 138篇 |
2010年 | 88篇 |
2009年 | 78篇 |
2008年 | 100篇 |
2007年 | 99篇 |
2006年 | 104篇 |
2005年 | 110篇 |
2004年 | 88篇 |
2003年 | 123篇 |
2002年 | 118篇 |
2001年 | 84篇 |
2000年 | 95篇 |
1999年 | 84篇 |
1998年 | 34篇 |
1997年 | 42篇 |
1996年 | 27篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 61篇 |
1991年 | 62篇 |
1990年 | 53篇 |
1989年 | 56篇 |
1988年 | 41篇 |
1987年 | 34篇 |
1986年 | 37篇 |
1985年 | 34篇 |
1984年 | 23篇 |
1983年 | 35篇 |
1982年 | 24篇 |
1981年 | 26篇 |
1980年 | 14篇 |
1979年 | 28篇 |
1978年 | 31篇 |
1977年 | 16篇 |
1976年 | 19篇 |
1975年 | 14篇 |
1974年 | 25篇 |
1973年 | 13篇 |
1969年 | 15篇 |
排序方式: 共有2850条查询结果,搜索用时 15 毫秒
61.
Histidine residues in elongation factor EF-tu from Escherichia coli protected by aminoacyl-tRNA against photo-oxidation 总被引:2,自引:0,他引:2
Complexes of Escherichia coli elongation factor EF-Tu with GTP or GTP and aminoacyl-tRNA were photo-oxidized by irradiation with visible light in the presence of rose bengal dye. EF-Tu was isolated, digested with trypsin, the resulting tryptic peptides were separated by high-performance liquid chromatography (HPLC), and the position of most of the peptides on the chromatogram was determined. Irradiation of complexes resulted in the inactivation of the factor (as tested by its capacity to interact with aminoacyl-tRNA) and was accompanied by the loss of its histidine residues (as revealed by amino acid analysis) and by the decrease in the amount of some tryptic peptides (as detected by HPLC). Aminoacyl-tRNA, bound to EF-Tu during the irradiation, protected the protein from inactivation, from the loss of histidine residues and some of its peptides from photo-oxidative degradation. Comparison of quantities of individual tryptic peptides recovered from the irradiated EF-Tu X GTP X aminoacyl-tRNA complex with those from the irradiated EF-Tu X GTP complex revealed that histidine-containing peptides T12 and T15 as well as methionine-containing peptide T14 were in the ternary complex markedly protected against the photo-oxidative degradation. This finding suggests that their histidines, i.e. His-66 and His-118 respectively and at least one of the methionines (Met-91, 98 or 112) present in peptide T14 are located near to or at the binding site of EF-Tu for aminoacyl-tRNA and could be involved in the interaction between aminoacyl-tRNA and the factor. 相似文献
62.
In carrot cells (Daucus carota L.), cultured in the presence of gibberellic acid, anthocyanin synthesis is blocked at the level of chalcone synthase. By feeding suitable precursors for anthocyanins (naringenin, eriodictyol, dihydroquercetin) biosynthesis of cyanidin glycosides can be restored. After addition of these substrates to the culture medium in the presence of gibberellic acid, the activity of chalcone synthase remained as low as in the control without precursors. The highest increase in anthocyanin content was achieved using dihydroquercetin as the added precursor. The time course of this supplementation showed a rapid response; within 4 h a substantial increase in anthocyanin could be observed. In contranst, the flavonol quercetin is not a precursor for cyanidin. The fact that naringenin was also accepted for cyanidin synthesis leads to the conclusion that hydroxylation in 3-position of ring B in Daucus carota takes place at the flavonoid stage.Abbreviations CHI
Chalcone isomerase
- CHS
chalcone synthase
- DMSO
dimethylsulfoxide
- GA3
gibberellic acid
- PAL
phenylalanine ammonia-lyase 相似文献
63.
Lars Chr. Petersen 《BBA》1979,548(3):636-641
The effect of pH on the oxygen kinetics of cytochrome c oxidase incorporated into phospholipid vesicles is studied. The pH profiles of the oxygen kinetics of energized and deenergized oxidase vesicles are similar. An effect of pH on the slope of the reciprocal plot of rate against oxygen concentration is observed, and this may indicate that protons are involved in the rate limiting step of the reaction between oxygen and reduced oxidase. In contrast to the pH dependence of the oxygen kinetics, the binding of CO to the oxidase is not pH dependent. 相似文献
64.
Significant polymorphism of serum complement components Bf, C2, C3, C6, and C8 in the chimpanzee has been demonstrated. The data are consistent with the hypothesis thatC2 andBf are closely linked toChLA and argue against close linkage ofChLA toC3 or toC8, as in man. In addition, a blank allele for C6 and C6 deficiency was detected in several chimps. 相似文献
65.
1. The stable complex formed between mammalian cytochrome c and phosvitin at low ionic strength was studied by partition in an aqueous two-phase system. Oxidized cytochrome c binds to phosvitin with a higher affinity than reduced cytochrome c. The difference was equivalent to a decrease of the redox potential by 22 mV on binding. 2. Complex-formation with phosvitin strongly inhibited the reaction of cytochrome c with reagents that react as negatively charged species, such as ascorbate, dithionite, ferricyanide and tetrachlorobenzoquinol. Reaction with uncharged reagents such as NNN'N'-tetramethylphenylenediamine and the reduced form of the N-methylphenazonium ion (present as the methylsulphate) was little affected by complex-formation, whereas oxidation of the reduced cytochrome by the positively charged tris-(phenanthroline)cobalt(III) ion was greatly stimulated. 3. A similar pattern of inhibition and stimulation of reaction rates was observed when phosvitin was replaced by other macromolecular polyanions such as dextran sulphate and heparin, indicating that the results were a general property of complex-formation with polyanions. A weaker but qualitatively similar effect was observed on addition of inositol hexaphosphate and ATP. 4. It is suggested that the effects of complex-formation with polyanions on the reactivity of cytochrome c with redox reagents are mainly the result of replacing the positive charge on the free cytochrome by a net negative charge. Any steric effects on polyanion binding are small in comparison with such electrostatic effects. 相似文献
66.
Lars Sottrup-Jensen Torben E. Petersen Staffan Magnusson 《Analytical biochemistry》1980,107(2):456-460
The use of gradient elution with acetate-buffered ethanol for high-performance liquid chromatographic analysis of phenylthiohydantoins (PTHs) is described. The system separates all commonly encountered PTH derivatives in a total analysis time of 20 min on columns of Spherisorb 5 S ODS (4.6 × 250 mm) packed in the laboratory at moderate expense. Experience with routine operation of the method, particularly with regard to column stability is discussed. Ethanol is considerably cheaper and less toxic than previously described solvent systems. 相似文献
67.
Konrad Heintze Karl-Uwe Petersen Peter Olles Sethna H. Saverymuttu John R. Wood 《The Journal of membrane biology》1979,45(1-2):43-59
Summary Fluid transport and net fluxes of Na, K, Cl and HCO3 by guinea pig gallbladder were investigatedin vitro. A perfused gallbladder preparation was devised to simultaneously study unidirectional fluxes of22Na and36Cl. The net Cl flux exceeded the net Na flux during fluid absorption in the presence of HCO3. This Cl excess was counter-balanced by a net HCO3 secretion: a HCO3–Cl exchange. PGE1 reversed the direction of fluid transport and abolished the net Cl flux. The magnitude of the HCO3 secretion remained unchanged, but shifted from a HCO3–Cl exchange to a net secretion of NaHCO3 and KHCO3. Furosemide inhibited both the HCO3–Cl exchange and HCO3 secretion after PGE1 without influencing fluid absorption. Ouabain inhibited the HCO3–Cl exchange as well as fluid absorption; only the effect on the HCO3 secretion was entirely reversible. Secreted HCO3 appeared not to be derived from metabolic sources since HCO3 secretion was abolished in a HCO3-free bathing medium. HCO3 secretion was also dependent on the Na concentration of the bathing fluid. Three lines of evidence are presented in favor of an active HCO3 secretion in guinea pig gallbladder. HCO3 is secreted against: (i) a chemical gradient, (ii) an electrical gradient and (iii) the direction of fluid movement under control conditions. 相似文献
68.
Ionic currents across pancreatic acinar cell membranes and their role in fluid secretion 总被引:1,自引:0,他引:1
O H Petersen Y Maruyama J Graf R Laugier A Nishiyama G T Pearson 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1981,296(1080):151-166
Fluid and enzyme secretion from a number of mammalian exocrine glands is controlled by the action of neurotransmitters and hormones on acinar cell membranes. Sustained stimulation evoking sustained fluid and enzyme secretion also evokes sustained membrane depolarization and increase in conductance. Mouse and rat pancreatic fluid and enzyme secretion, as well as membrane depolarization and conductance increase evoked by sustained stimulation with acetylcholine or cholecystokinin-gastrin peptides, are acutely dependent on extracellular calcium. However, the initial stimulant-evoked conductance increase and secretion appear to be triggered by calcium released from inside the cells. Direct measurement of membrane current during sustained stimulation in voltage-clamp experiments with resolution of the total current into its Na, Cl and K components has allowed calculations of stimulant-evoked Na and Cl uptake into the acinar cells. The NaCl uptake is quantitatively sufficient to account for the stimulant-evoked fluid secretion. The role of the stimulant-evoked transmembrane ionic current appears to be the supply of salt for the fluid secretion. Calcium derived from intracellular sources in the initial phase of secretion, and from the extracellular fluid in the sustained phase, couples fluid and enzyme secretion to hormone-receptor interaction. 相似文献
69.
The nematode Drilomermis leioderma n. gen., n. sp. (Merrnithidae) is described from larvae of Cybister fimbriolatus (Say) (Dytiscidae: Coleoptera) in Louisiana. Diagnostic characters of the genus Drilomermis are: medium-sized nematodes with the cuticle appearing smooth (lacking cross fibers) under the light microscope, six cephalic papillae, without mouth papillae, six hypodermal cords at midbody, 2 extremely long spicules (longer than 10 times body width at anus) which are separate and parallel (not twisted), an S-shaped vagina, medium-sized amphids located near head papillae, and postparasitic juvenile with a tail appendage. D. leioderma possesses a ventrally displaced mouth, very long vagina, and male genital papillae arranged in 3 double rows in the vicinity of the cloacal opening. Even when containing multiple parasites, about 40% of the hosts sulwived emergence of the memithids and lived several more days. In nature, some of these hosts may be able to continue their development, which is unusual since most mermithid-parasitized hosts die soon after the nematode emerges. 相似文献
70.