全文获取类型
收费全文 | 6849篇 |
免费 | 759篇 |
国内免费 | 2篇 |
专业分类
7610篇 |
出版年
2022年 | 51篇 |
2021年 | 82篇 |
2020年 | 58篇 |
2019年 | 54篇 |
2018年 | 92篇 |
2017年 | 83篇 |
2016年 | 129篇 |
2015年 | 220篇 |
2014年 | 227篇 |
2013年 | 300篇 |
2012年 | 386篇 |
2011年 | 342篇 |
2010年 | 237篇 |
2009年 | 234篇 |
2008年 | 290篇 |
2007年 | 256篇 |
2006年 | 249篇 |
2005年 | 243篇 |
2004年 | 236篇 |
2003年 | 225篇 |
2002年 | 240篇 |
2001年 | 187篇 |
2000年 | 216篇 |
1999年 | 164篇 |
1998年 | 98篇 |
1997年 | 93篇 |
1996年 | 76篇 |
1995年 | 76篇 |
1994年 | 56篇 |
1993年 | 69篇 |
1992年 | 140篇 |
1991年 | 139篇 |
1990年 | 148篇 |
1989年 | 134篇 |
1988年 | 109篇 |
1987年 | 128篇 |
1986年 | 114篇 |
1985年 | 102篇 |
1984年 | 105篇 |
1983年 | 73篇 |
1982年 | 57篇 |
1981年 | 68篇 |
1980年 | 46篇 |
1979年 | 75篇 |
1978年 | 45篇 |
1977年 | 66篇 |
1975年 | 53篇 |
1974年 | 49篇 |
1973年 | 63篇 |
1971年 | 48篇 |
排序方式: 共有7610条查询结果,搜索用时 0 毫秒
21.
We have previously reported that in vitro HCV infection of cells of hepatocyte origin attenuates complement system at multiple steps, and attenuation also occurs in chronically HCV infected liver, irrespective of the disease stage. However, none of these regulations alone completely impaired complement pathways. Modulation of the upstream proteins involved in proteolytic processing of the complement cascade prior to convertase formation is critical in promoting the function of the complement system in response to infection. Here, we examined the regulation of C2 complement expression in hepatoma cells infected in vitro with cell culture grown virus, and validated our observations using randomly selected chronically HCV infected patient liver biopsy specimens. C2 mRNA expression was significantly inhibited, and classical C3 convertase (C4b2a) decreased. In separate experiments for C3 convertase function, C3b deposition onto bacterial membrane was reduced using HCV infected patient sera as compared to uninfected control, suggesting impaired C3 convertase. Further, iC3b level, a proteolytically inactive form of C3b, was lower in HCV infected patient sera, reflecting impairment of both C3 convertase and Factor I activity. The expression level of Factor I was significantly reduced in HCV infected liver biopsy specimens, while Factor H level remained unchanged or enhanced. Together, these results suggested that inhibition of C3 convertase activity is an additional cumulative effect for attenuation of complement system adopted by HCV for weakening innate immune response. 相似文献
22.
Summary Video-densitometric DNA measurements of Feulgenstained tissues of 42 day old eggs of the corn snake,Elaphe g. guttata (Columbridae, Serpentes), revealed a basic DNA content of 2C=2.17 pg, with somatic polyploidy in the allantois, the chorioallontois, the yolk sac, and other extraembryonic membranes. The maximum value determined was 128C (in binucleate cells 2×128C) at the distal pole of the egg. This is the first report of somatic polyploidy in a snake, and one of the first in reptiles in general. 相似文献
23.
Host discrimination studies were conducted with different species of Asobara, which are larval endo-parasitoids of Drosophilidae. Results indicated variable host discrimination which depended on the relatedness of the species. The closely related sibling species Asobara tabida (Nees) and A. rufescens (Foerster) were not only capable of intraspecific discrimination, but also avoided multiparasitism by discriminating between unparasitized host larvae and larvae previously parasitized by females of the other species. This ability to discriminate interspecifically does not seem functional as each species occupies its own microhabitat. As it was shown to be absent in less closely related Asobara species we concluded that interspecific discrimination by A. tabida and A. rufescens was due to their close relationship. 相似文献
24.
Phosphoenolpyruvate carboxylase (EC 4.1.1.31) purified from leaves of the crassulacean acid metabolism plant (Crassula argentea) was chemically modified by the specific arginyl reagent 2,3-butanedione. Modification resulted in enzyme inactivation which followed pseudo first-order kinetics. Participation of arginyl residues involved in the binding of or response to both phosphoenolpyruvate and malate, respectively, was established. Inactivation and protection studies suggest the presence of three sites involved in the binding of the substrate, phosphoenolpyruvate, the activator, glucose 6-phosphate, and the inhibitor, malate. Studies using both fluorescence measurements of binding and steady-state kinetic methods indicate that phosphoenolpyruvate can bind both to the active site and to the activator site. Evidence for stimulation of the activity of phosphoenolpyruvate carboxylase upon the binding of substrate to the activation site was provided by kinetic studies using AMP, previously shown to be a specific ligand for the activation site. 相似文献
25.
External cranial vault molding using dynamic splinting is an adjunct to surgery in the treatment of craniosynostosis skull deformities. The skull molding cap not only maintains desired skull form, but also provides further active molding to normalize skull shape. Dynamic skull remodeling from these devices occurs primarily by translational movements of bone. Traction and compression result in bony repositioning which allows further reshaping as the osteoblasts and osteoclasts respond to these stresses. Three basic designs have been described. In practice, each one must be modified to meet individual needs, and adaptations are made according to established principles of dynamic splinting. 相似文献
26.
Light-regulated methylation of chloroplast proteins 总被引:2,自引:0,他引:2
Protein carboxyl methyltransferases, which catalyze transfer of methyl groups from S-adenosyl-L-methionine to the free carboxyl groups of acidic amino acids in proteins, can be divided into two classes based on several characteristics, such as the stoichiometry of substrate protein methylation, base stability of the incorporated methyl group, specificity for substrate, and participation in a regulatory system with which methylesterases are associated. The presence of such an enzyme in a photosynthetic system was demonstrated in the present work. The extent of methylation of chloroplast proteins was stimulated 30% by light and then decreased by the same amount in the presence of the electron transport inhibitor 3-(3',4'-dichlorophenyl)-1', 1'-dimethylurea or uncouplers of phosphorylation, indicating a dependence of the methyltransferase activity on photosynthetic electron transport and the trans-membrane delta pH. The light-independent, as well as the light-dependent, activity is probably of chloroplast origin since the extent of light stimulation in the purified thylakoid membranes and the stromal fraction was similar, and at low concentrations of S-adenosyl-L-methionine the small subunit of ribulose-1,5-bisphosphate carboxylase:oxygenase was found to be the predominant substrate. The labeling pattern of chloroplast proteins and labeling of an exogenous nonchloroplast protein indicated that the methyltransferase activity was not substrate-specific, although at low concentrations of the methyl donor, the small subunit of ribulose-1,5-bisphosphate carboxylase:oxygenase was labeled almost exclusively. Based on the low stoichiometry (less than 100 pmol/mg protein) of the methylation, its base lability, irreversibility, and the lack of substrate specificity except at very low concentrations of methyl donor, it was inferred that the chloroplast methyltransferase is best classified as a class II system that may function as part of a repair mechanism to replace racemized amino acids. 相似文献
27.
Dario Cremaschi Giuliano Meyer Guido Bottà Carlo Rossetti 《The Journal of membrane biology》1987,95(3):219-228
Summary In the epithelium of rabbit gallbladder, in the nominal absence of bicarbonate, intracellular Cl– activity is about 25mm, about 4 times higher than intracellular Cl– activity at the electrochemical equilibrium. It is essentially not affected by 10–4
m acetazolamide and 10–4
m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS) even during prolonged exposures; it falls to the equilibrium value by removal of Na+ from the lumen without significant changes of the apical membrane potential difference. Both intracellular Cl– and Na+ activities are decreased by luminal treatment with 25mm SCN–; the initial rates of change are not significantly different. In addition, the initial rates of change of intracellular Cl– activity are not significantly different upon Na+ or Cl– entry block by the appropriate reduction of the concentration of either ion in the luminal solution. Luminal K+ removal or 10–5
m bumetanide do not affect intracellular Cl– and Na+ activities or Cl– influx through the apical membrane. It is concluded that in the absence of bicarbonate NaCl entry is entirely due to a Na+–Cl– symport on a single carrier which, at least under the conditions tested, does not cotransport K+. 相似文献
28.
Dario Cremaschi Giuliano Meyer Carlo Rossetti Guido Bottà Paola Palestini 《The Journal of membrane biology》1987,95(3):209-218
Summary Cl– influx at the luminal border of the epithelium of rabbit gallbladder was measured by 45-sec exposures to36Cl– and3H-sucrose (as extracellular marker). Its paracellular component was evaluated by the use of 25mm SCN– which immediately and completely inhibits Cl– entry into the cell. Cellular influx was equal to 16.7eq cm–2 hr–1 and decreased to 8.5eq cm–2 hr–1 upon removal of HCO
3
–
from the bathing media and by bubbling 100% O2 for 45 min. When HCO
3
–
was present, cellular influx was again about halved by the action of 10–4
m acetazolamide, 10–5 to 10–4
m furosemide, 10–5 to 10–4
m 4-acetamido-4-isothiocyanostilbene-2,2-disulfonate (SITS), 10–3
m amiloride. The effects of furosemide and SITS were tested at different concentrations of the inhibitor and with different exposure times: they were maximal at the concentrations reported above and nonadditive. In turn, the effects of amiloride and SITS were not additive. Acetazolamide reached its maximal action after an exposure of about 2 min. When exogenous HCO
3
–
was absent, the residual cellular influx was insensitive to acetazolamide, furosemide and SITS. When exogenous HCO
3
–
was present in the salines, Na+ removal from the mucosal side caused a slow decline of cellular Cl– influx; conversely, it immediately abolished cellular Cl– influx in the absence of HCO
3
–
. In conclusion, about 50% of cellular influx is sensitive to HCO
3
–
, inhibitable by SCN–, acetazolamide, furosemide, SITS and amiloride and furthermore slowly dependent on Na+. The residual cellular influx is insensitive to bicarbonate, inhibitable by SCN–, resistant to acetazolamide, furosemide, SITS and amiloride, and immediately dependent on Na+. Thus, about 50% of apical membrane NaCl influx appears to result from a Na+/H+ and Cl–/HCO
3
–
exchange, whereas the residual influx seems to be due to Na+–Cl– contranport on a single carrier. Whether both components are simultaneously present or the latter represents a cellular homeostatic counterreaction to the inhibition of the former is not clear. 相似文献
29.
Isolation of apical plasma membrane in rabbit gallbladder epithelium by Percoll density gradient centrifugation 总被引:1,自引:0,他引:1
The apical membranes of rabbit gallbladder epithelial cells were isolated by treating the homogenate with Ca2+ or Mg2+ and centrifuging the suspension in Percoll gradient. In this way brush-border membranes were obtained with enrichment factors ranging between 10 and 20 and yields of 15-30%. A second method is described with which membranes were isolated, without any preliminary treatment, first by differential centrifugation, then with Percoll gradient; the final membrane enrichment was over 15, however the yield was very low (3%). Many possible enzymatic markers of the apical plasma membrane were investigated: L-gamma-glutamyltransferase, alkaline phosphatase, leucine aminopeptidase, sucrase. The first appears to be that of choice. Apical membrane fraction could be also evidenced by autofluorescence or by labeling with Lotus tetragonolobus lectin. Preliminary experiments showed that apical plasma membranes isolated in this way form vesicles. 相似文献
30.
Structural elements of bactopterin from Pseudomonas carboxydoflava carbon monoxide dehydrogenase 总被引:2,自引:0,他引:2
Bactopterin is a novel pterin occurring in bacterial molybdoenzymes as the organic portion of the molybdenum cofactor. Its structure is investigated here. The compound contains a single pterin ring and carries a side chain at carbon atom 6 of the pterin nucleus as indicated by the formation of pterin-6-carboxylic acid upon alkaline permanganate oxidation. Studies with phosphate-cleaving enzymes revealed the presence of two monophosphoric acid monoesters. The affinity of reduced bactopterin for thiol-Sepharose points to the presence of thiol(s) in active bactopterin. 相似文献