全文获取类型
收费全文 | 145164篇 |
免费 | 9253篇 |
国内免费 | 25篇 |
出版年
2023年 | 823篇 |
2022年 | 777篇 |
2021年 | 1693篇 |
2020年 | 1618篇 |
2019年 | 1588篇 |
2018年 | 3820篇 |
2017年 | 3424篇 |
2016年 | 4771篇 |
2015年 | 6449篇 |
2014年 | 6509篇 |
2013年 | 8743篇 |
2012年 | 10580篇 |
2011年 | 9587篇 |
2010年 | 5983篇 |
2009年 | 4567篇 |
2008年 | 7720篇 |
2007年 | 7360篇 |
2006年 | 7142篇 |
2005年 | 6209篇 |
2004年 | 6061篇 |
2003年 | 5560篇 |
2002年 | 5104篇 |
2001年 | 3516篇 |
2000年 | 3530篇 |
1999年 | 2560篇 |
1998年 | 1195篇 |
1997年 | 833篇 |
1996年 | 876篇 |
1995年 | 819篇 |
1994年 | 749篇 |
1993年 | 660篇 |
1992年 | 1368篇 |
1991年 | 1226篇 |
1990年 | 1180篇 |
1989年 | 1246篇 |
1988年 | 1058篇 |
1987年 | 1011篇 |
1986年 | 962篇 |
1985年 | 1012篇 |
1984年 | 935篇 |
1983年 | 756篇 |
1982年 | 673篇 |
1979年 | 840篇 |
1978年 | 654篇 |
1975年 | 730篇 |
1974年 | 792篇 |
1973年 | 761篇 |
1972年 | 674篇 |
1969年 | 694篇 |
1968年 | 668篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
941.
We have studied the structure of solid films obtained by x-ray diffraction, from several basic polypeptides with a defined sequence. The alterating polypeptides poly(Ala-Lys), poly(Leu-Lys), poly(Val-Lys), and poly(Arg-Leu) all show a cross-β-structure in which layers of hydrophilic side chains alternate with layers containing hydrophobic side chains. The other polypeptides studied are not in the β-conformation and appear to be in the α-helical conformation. The helices obtained from poly(Lys-Ala-Ala) and poly(Lys-Ala-Ala-Lys) appear to be packed in an unusual fashion, which favors interaction between alanine side chains. Such behavior is not found in poly(Lys-Leu-Ala). 相似文献
942.
Interrelationships between the phase diagrams of the two-component phospholipid bilayers 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Biophysical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Basic relationships between the phase diagrams, previously considered independent of each other, are described. Phase diagrams of two-component phosphatidylcholine/phosphatidylcholine (PC/PC), phosphatidylethanolamine/phosphatidylethanolamine (PE/PE), and PC/PE lipid membranes are systematically investigated by means of the Landau theory. While gradually changing the chain length of one of the components, a characteristic peritectic-miscible-azeotropic-semiazeotropic-eutectic (P-M-A-S-E) series of the phase diagram was found in the PC/PE system and a peritectic-miscible-one-component-miscible-peritectic (P-M-O-M-P) series was found in the PC/PC and PE/PE systems. These serial catastrophic changes in the phase diagrams could be explained by the fusion and birth of the mixed phase regions in the phase diagram. Finally when we constructed the superdiagrams, we obtained all of the possible series of the phase diagrams in a wide class of the two-component mixtures. Moreover, one can predict the type of the phase diagram when the components r and p contain equal-length saturated hydrocarbon chains.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
943.
A thyroid binding peripheral membrane protein(s) has been characterized in human red cell. Two classes of affinity sites for triiodothyronine have been demonstrated. The high affinity, low capacity site showed values for dissociation constant of 2 X 10(-10)M. The binding activity depended on the presence of free -SH group and showed a high stereospecificity for L-triiodothyronine, L-thyroxine was less potent (about 1,000-fold) than L-triiodothyronine in competing for this site. The results are discussed with respect to their cellular significance. 相似文献
944.
Curves relating net photosynthetic rate to irradiance [P(I) curve relation] were estimated and analysed inMercurialis perennis L. plants stemming from three forest (spruce, beech and ash) stands with different tree leaf canopy development and different
light regime.
The saturating irradiance (Is) reached the highest values in plants of all three stands in spring (spruce forest: 438 W m−2, beech forest: 440 W m−2 and ash forest: 367 W m−2), it declined sharply in the middle of the growing season (283, 285 and 297 W m-2, respectively) and this Is level persisted until autumn. A pronounced dynamics in plants from spruce and beech forests made itself manifest also in
the adaptation (Ia) and compensating (Ic) irradiances, respectively. After a sudden decline in summer, values in autumn were close to those of the vernal season.
The most pronounced parameter, which optimally expressed the adaptation ofMercurialis perennis to various light conditions, was the photosynthetic efficiency (α) calculated as the slope of the linear part of the curve
relating net photosynthetic rate to irradiance.
At the time of the highest PN sat. value in course of the growing season (August) (spruce forest: 100, beech forest: 98.7 and ash forest: 85.8 μg CO2 m−2 s−1), RD was in its minimum; in autumn PN sat. reached the lowest values which corresponded to the most intensive RD.
It was found thatMercurialis perennis plants stemming from forest stands with different light conditions did not make use equally of the altering light conditions
in the course of the growing season. By the underlying analysis of P(I) curves this rhizomatous perennial herb (geophyte)
may be characterized as a shade tolerant species. 相似文献
945.
Effects of 2,3,5-triiodobenzoic acid (TIBA) and age of etiolated pea epicotyl segments on the indol-3-ylacetic acid (IAA)
stimulated transport of14C-abscisic acid (ABA) was studied. In spite of a slight decrease of IAA transport after the application of TIBA, the IAA stimulation
of14C-ABA transport did not change. In segments excised from epicotyls of different age,3H-IAA transport was identical and the induction of prolongation growth by IAA in segments from the upper part of the epicotyl
was observed. The IAA ap{ie226-01}ation to the growing segments was connected with intensive attraction of14C-ABA to the site {ie226-02}AA application, while the application of IAA to the older segments was growth ineffective ana
no stimulation of14C-ABA transport by IAA was observed. 相似文献
946.
947.
In the present work we have been able to demonstrate that phenobarbital and lead exert an inducing effect on the biosynthesis of delta-aminolevulinic acid synthase, ferrochelatase and cytochrome P-450 hemoproteins in isolated rat hepatocytes of normal adult rats. Dibutyryl cyclic AMP enhances the induction effect produced by phenobarbital in this in vitro system. Glucose inhibits the induction of delta-aminolevulinic acid synthase and ferrochelatase. This repression effect can be reversed with increasing concentrations of dibutyryl cyclic AMP. No glucose effect was observed on the phenobarbital- and lead-mediated inductions of cytochrome P-450. he present results add more experimental evidence to support the concept that the last enzyme of the heme pathway is inducible, and as such may have a significant role in regulatory mechanisms of porphyrin and heme biosynthesis. 相似文献
948.
The ouabain-insensitive, active Na+ uptake of inside-out vesicles prepared with basolateral plasma membranes from rat kidney proximal tubular cells can be increased by the presence of micromolar concentrations of Ca2+ in the assay medium. The concomitant ATP hydrolysis associated with the Na+ uptake is also increased by the presence of Ca2+. The Na+ uptake and the concomitant ATP hydrolysis are inhibited by 2 mM furosemide. The effect of Ca2+ is not due to the activity of an Na+-Ca2+ exchanger. The present results are in accordance with our previous model (Proverbio, F., Proverbio, T. and Marín, R. (1982) Biochim. Biophys. Acta 688, 757-763) in which we proposed that Ca2+ seems to modulate the activity of the ouabain-insensitive Na+ pump, in two different ways: (1) in a strong association with the membranes in which Ca2+ (stable component) is essential for the pump activity and (2) in a weak association with the membranes in which Ca2+ (labile component) can be quickly and easily removed by reducing the free Ca2+ concentration of the assay medium to values lower than 1 microM. The Ka for Ca2+ (for the labile component) is around 5 microM. The Ca2+ modulation of the ouabain-insensitive Na+ pump is an indication that Ca2+ could regulate the magnitude of the Na+ extrusion accompanied by Cl- and water present in rat kidney proximal tubular cells. 相似文献
949.
The uptake of 86Rb+ was assayed in isolated rat pancreatic acinar cells to determine the effect of calcium and carbamoylcholine on the ouabain-sensitive and ouabain-insensitive components. The presence of calcium in the medium bathing the cells during the preincubation and the main incubation periods was needed to preserve in optimum conditions the uptake of 86Rb+, the stimulation by carbamoylcholine and the sensitivity to ouabain. In the presence of calcium, the ouabain-sensitive component of 86Rb+ uptake was higher than the ouabain-insensitive. The ouabain-sensitive component was 3-times lower in cells incubated in a medium lacking calcium and containing 1 mM EGTA, as compared to cells incubated in the presence of calcium. Carbamoylcholine, at 5 X 10(-4) M, stimulated the uptake of 86Rb+ and this effect depended on the presence of calcium in the bathing medium. Maximal stimulation by carbamoylcholine was reached at 0.2 mM calcium. The nett stimulation by carbamoylcholine was inhibited up to 85% by 1 mM ouabain. As judged by digitonin-disruption of plasma membrane, the above-indicated effects were limited to a cytoplasmic pool of 86Rb+ and a leaky plasma membrane could be ruled out. The results suggest that in rat pancreatic acinar cells, carbamoylcholine stimulated the ouabain-sensitive uptake of 86Rb+ and required the presence of calcium in the bathing medium. 相似文献
950.
We studied the processes of interference vs language automatisms in a group of Slovak-Hungarian bilinguists (N = 19, age 13 years) and compared their performance in the Stroop test with two control groups - Slovak (N = 24) and Hungarian (N = 31) monolinguists of the same age. - The Stroop effect, whose essence is an interfering effect of the meaning of the word on its physical characteristic - colour, proved to be a suitable method in the study of information processing in the semantic memory of bilinguists as well as in determining dominance of one language over the other. Slovak-Hungarian bilinguists showed, as a rule, shorter times in Hungarian and approximated in their performance Hungarian monolinguists. It can be assumed that reaction times in the Stroop test are conditioned not only by interference of the other language but also by language automatisms, whereby interference and speech automatisms are in an indirect correlation. - The results thus indicate that processes of interference and language automatisms cannot be separated. Dominance of one language structure over the other (Hungarian over Slovak) was confirmed not only in our experimental method but also in the subjective rating. 相似文献