全文获取类型
收费全文 | 16493篇 |
免费 | 829篇 |
国内免费 | 24篇 |
专业分类
17346篇 |
出版年
2023年 | 114篇 |
2022年 | 282篇 |
2021年 | 447篇 |
2020年 | 266篇 |
2019年 | 262篇 |
2018年 | 408篇 |
2017年 | 436篇 |
2016年 | 511篇 |
2015年 | 697篇 |
2014年 | 773篇 |
2013年 | 1077篇 |
2012年 | 1192篇 |
2011年 | 1094篇 |
2010年 | 654篇 |
2009年 | 590篇 |
2008年 | 706篇 |
2007年 | 690篇 |
2006年 | 593篇 |
2005年 | 499篇 |
2004年 | 489篇 |
2003年 | 395篇 |
2002年 | 362篇 |
2001年 | 338篇 |
2000年 | 307篇 |
1999年 | 246篇 |
1998年 | 129篇 |
1997年 | 109篇 |
1996年 | 102篇 |
1995年 | 115篇 |
1994年 | 103篇 |
1993年 | 97篇 |
1992年 | 225篇 |
1991年 | 210篇 |
1990年 | 201篇 |
1989年 | 193篇 |
1988年 | 170篇 |
1987年 | 165篇 |
1986年 | 149篇 |
1985年 | 174篇 |
1984年 | 167篇 |
1983年 | 115篇 |
1982年 | 112篇 |
1981年 | 99篇 |
1980年 | 92篇 |
1979年 | 148篇 |
1978年 | 109篇 |
1977年 | 95篇 |
1974年 | 118篇 |
1973年 | 91篇 |
1972年 | 100篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
Removal of the embryonic axis prevents the normal decline of carboxypeptidase (Cpase) I in mung bean seedling cotyledons. Cpase I activity and protein, the latter manifested on western blots, almost completely disappear about 24 h before the cotyledon abscises. Of the 3 proteolytic enzyme patterns, only that of Cpase I can be restored by an exogenous supply of 10 m M CaCl2 in the agar growth medium. The calcium effect is dependent on [CaCl2 ] and is not manifested in the presence of chelators and calcium channel blockers. For detached cotyledons to show the normal low level of Cpase I by the eighth day of growth, calcium had to be supplied during seed imbibition and throughout the entire time from removal of the axis. The difference between detached cotyledons in the absence and presence of calcium was greatest when the cotyledons were detached 4–6 days after seed imbibition. Loss of Cpase I activity and protein can be demonstrated in vitro, with the maximum level of Cpase I-degrading activity measured 4 days after seed imbibition under the same growth conditions used to study the calcium effect. It is sensitive to pepstatin and has a pH optimum of 3, suggesting that this Cpase I-degrading activity is due to an aspartic protease. 相似文献
922.
A bacterial strain capable of biotransformation oflantadene A(22-angeloyloxy-3-oxo-olean-12-en-28-oic acid),the pentacyclic hepatotoxin of lantana (Lantanacamara var. aculeata) has been isolated fromsoil using lantadene A as the sole carbon source. Theorganism is Gram negative, rod shaped, motile,catalase positive and has been identified as Alcaligenes faecalis. The isolate has been found tobe specific for lantadene A and did not utilizelantadene B. In studies using sucrose as an additionalcarbon source, A. faecalis elicitedbiotransformation of lantadene A to its trans isomer22-tigloyloxy-3-oxoolean-12-en-28-oic acid,designated as lantadene X and two other minormetabolites which could not be isolated in pure state. 相似文献
923.
Nielsen CB Singh SK Wengel J Jacobsen JP 《Journal of biomolecular structure & dynamics》1999,17(2):175-191
LNA (Locked Nucleic Acids) is a novel oligonucleotide analogue containing a conformationally restricted nucleotide with a 2'-O, 4'-C-methylene bridge that induces unprecedented thermal affinities when mixed with complementary single stranded DNA and RNA. We have used two-dimensional 1H NMR spectroscopy obtained at 750 and 500 MHz to determine a high resolution solution structure of an LNA oligonucleotide hybridized to the complementary DNA strand. The determination of the structure was based on a complete relaxation matrix analysis of the NOESY cross peaks followed by restrained molecular dynamics calculations. Forty final structures were generated for the duplex from A-type and B-type dsDNA starting structures. The root-mean-square deviation (RMSD) of the coordinates for the forty structures of the complex was 0.32A. The structures were analysed by use of calculated helix parameters. This showed that the values for rise and buckle in the LNA duplex is markedly different from canonical B-DNA at the modification site. A value of twist similar to A-DNA is also observed at the modification site. The overall length of the helix which is 27.3 A. The average twist over the sequence are 35.9 degrees +/- 0.3 degrees. Consequently, the modification does not cause the helix to unwind. The bis-intercalation of the thiazole orange dye TOTO to the LNA duplex was also investigated by 1H NMR spectroscopy to sense the structural change from the unmodified oligonucleotide. We observed that the bis-intercalation of TOTO is much less favourable in the 5'-CT(L)AG-3' site than in the unmodified 5'-CTAG-3' site. This was related to the change in the base stacking of the LNA duplex compared to the unmodified duplex. 相似文献
924.
Singh Anjana Pandey K.D. Dubey R.S. 《World journal of microbiology & biotechnology》1999,15(2):277-282
Reverse micelles serve as a novel tool to entrap enzymes and microbial whole cells within aqueous pockets and can be of great use in enhancing the efficiency and sustainability of the biological system. Photosynthetic bacterium Rhodopseudomonas sphaeroides entrapped inside the aqueous pool of reverse micelles prepared from benzene-sodium lauryl sulphate exhibited 25-fold enhancement of H2 photoproduction rate (1.67 ml H2 [mg protein]–1 h–1) compared to cells suspended in normal aqueous medium. Hydrogen photoproduction by the bacterium was catalysed by the nitrogenase enzyme system which was supported at a low light intensity of 12 Em–2 sec–1 photon flux energy at a wavelength of 520 nm. The optimum temperature for the process was 40 °C. 相似文献
925.
Nature of the chromophore binding site of bacteriorhodopsin: the potential role of Arg82 as a principal counterion
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)
The nature of the chromophore binding site of light-adapted bacteriorhodopsin is analyzed by using modified neglect of differential overlap with partial single and double configuration interaction (MNDO-PSDCI) molecular orbital theory to interpret previously reported linear and nonlinear optical spectroscopic measurements. We conclude that in the absence of divalent metal cations in close interaction with Asp85 and Asp212, a positively charged amino acid must be present in the same vicinity. We find that models in which Arg82 is pointed upward into the chromophore binding site and directly stabilizes Asp85 and Asp212 are successful in rationalizing the observed one-photon and two-photon properties. We conclude further that a water molecule is strongly hydrogen bonded to the chromophore imine proton. The chromophore "1Bu*+" and "1Ag*-" states, despite extensive mixing, exhibit significantly different configurational character. The lowest-lying "1Bu*+" state is dominated by single excitations, whereas the second-excited "1Ag*-" state is dominated by double excitations. We can rule out the possibility of a negatively charged binding site, because such a site would produce a lowest-lying "1Ag*-" state, which is contrary to experimental observation. The possibility that Arg82 migrates toward the extracellular surface during the photocycle is examined. 相似文献
926.
927.
Bicarbonate and chloride secretion in Calu-3 human airway epithelial cells 总被引:23,自引:0,他引:23
下载免费PDF全文
![点击此处可从《The Journal of general physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Devor DC Singh AK Lambert LC DeLuca A Frizzell RA Bridges RJ 《The Journal of general physiology》1999,113(5):743-760
Serous cells are the predominant site of cystic fibrosis transmembrane conductance regulator expression in the airways, and they make a significant contribution to the volume, composition, and consistency of the submucosal gland secretions. We have employed the human airway serous cell line Calu-3 as a model system to investigate the mechanisms of serous cell anion secretion. Forskolin-stimulated Calu-3 cells secrete HCO-3 by a Cl-offdependent, serosal Na+-dependent, serosal bumetanide-insensitive, and serosal 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS)-sensitive, electrogenic mechanism as judged by transepithelial currents, isotopic fluxes, and the results of ion substitution, pharmacology, and pH studies. Similar studies revealed that stimulation of Calu-3 cells with 1-ethyl-2-benzimidazolinone (1-EBIO), an activator of basolateral membrane Ca2+-activated K+ channels, reduced HCO-3 secretion and caused the secretion of Cl- by a bumetanide-sensitive, electrogenic mechanism. Nystatin permeabilization of Calu-3 monolayers demonstrated 1-EBIO activated a charybdotoxin- and clotrimazole- inhibited basolateral membrane K+ current. Patch-clamp studies confirmed the presence of an intermediate conductance inwardly rectified K+ channel with this pharmacological profile. We propose that hyperpolarization of the basolateral membrane voltage elicits a switch from HCO-3 secretion to Cl- secretion because the uptake of HCO-3 across the basolateral membrane is mediated by a 4,4 '-dinitrostilben-2,2'-disulfonic acid (DNDS)-sensitive Na+:HCO-3 cotransporter. Since the stoichiometry reported for Na+:HCO-3 cotransport is 1:2 or 1:3, hyperpolarization of the basolateral membrane potential by 1-EBIO would inhibit HCO-3 entry and favor the secretion of Cl-. Therefore, differential regulation of the basolateral membrane K+ conductance by secretory agonists could provide a means of stimulating HCO-3 and Cl- secretion. In this context, cystic fibrosis transmembrane conductance regulator could serve as both a HCO-3 and a Cl- channel, mediating the apical membrane exit of either anion depending on basolateral membrane anion entry mechanisms and the driving forces that prevail. If these results with Calu-3 cells accurately reflect the transport properties of native submucosal gland serous cells, then HCO-3 secretion in the human airways warrants greater attention. 相似文献
928.
In passerine songbirds, song learning often is restricted to an early sensitive period and requires the participation of several discrete regions within the anterior forebrain. Activation of N-methyl-D-aspartate (NMDA) receptors is implicated in song learning and in one forebrain song region, the lateral magnocellular nucleus of the anterior neostriatum (IMAN), NMDA receptors decrease in density, their affinity for the antagonist MK-801 increases, and their currents decay more quickly as young male zebra finches lose the ability to imitate new song elements. These developmental changes in NMDA receptor pharmacology and physiology suggest that the subunit composition of NMDA receptors changes developmentally. Here, we have used in situ hybridization and [3H]ifenprodil receptor autoradiography to study the developmental regulation of the NMDA receptor 2B subunit (NR2B) within the anterior forebrain of male zebra finches. NR2B mRNA expression within the IMAN was twice as great in 30-day-old males (early in the sensitive period for song learning) as in adult males, and this developmental decrease in NR2B mRNA expression was mirrored by a decrease in high-affinity (NR2B-associated) [3H]ifenprodil binding within this song region. In another anterior forebrain song region, Area X, NR2B mRNA also declined significantly after 30 days posthatch, but this decline was not accompanied by a significant decrease in [3H]ifenprodil binding. The results are consistent with the hypothesis that developmental changes in NMDA receptor function mediated by regulation of subunit composition contribute to the sensitive period for vocal learning in birds. 相似文献
929.
Adults Charles-Foster rats were prenatally treated to phenobarbitone (10 mg/kg, i.p.) from day 13 to 21 of gestation, this being the critical period of neural development. Pregnant control rats were similarly treated with equal volume of vehicle. Adult rat offsprings at 8-9 weeks of age were subjected to open-field exploratory behaviour, elevated plus-maze and elevated zero-maze tests. The rat offsprings displayed significantly increased ambulation and rearings in an open-field arena when compared to control offsprings whereas self-grooming and faecal droppings remain unchanged. On elevated plus-maze test these prenatally treated rat offsprings spent significantly less time on open arms and more time and more number of entries in enclosed arms as compared to controls. Prenatally exposed rats also showed significant less time on open arms, less number of head dips and stretched attend postures on elevated zero-maze test indicating increased anxiogenic behavioural pattern in these animals. The results suggest that prenatal exposure to phenobarbitone leaves a lasting effect on the anxiety state of the offsprings. 相似文献
930.
Kumar D Bansal A Thomas P Sairam M Sharma SK Mongia SS Singh R Selvamurthy W 《International journal of biometeorology》1999,42(4):201-204
High altitude stress leads to lipid peroxidation and free radical formation which results in cell membrane damage in organs
and tissues, and associated mountain diseases. This paper discusses the changes in biochemical parameters and antibody response
on feeding glutamate to male albino Sprague Dawley rats under hypoxic stress. Exposure of rats to simulated hypoxia at 7576
m, for 6 h daily for 5 consecutive days, in an animal decompression chamber at 32±2° C resulted in an increase in plasma malondialdehyde
level with a concomitant decrease in blood glutathione (reduced) level. Supplementation of glutamate orally at an optimal
dose (27 mg/kg body weight) in male albino rats under hypoxia enhanced glutathione level and decreased malondialdehyde concentration
significantly. Glutamate feeding improved total plasma protein and glucose levels under hypoxia. The activities of serum glutamate
oxaloacetate transaminase (SGOT) and serum glutamate pyruvate transaminase (SGPT) and the urea level remained elevated on
glutamate supplementation under hypoxia. Glutamate supplementation increased the humoral response against sheep red blood
cells (antibody titre). These results indicate a possible utility of glutamate in the amelioration of hypoxia-induced oxidative
stress.
Received: 23 March 1998 / Accepted: 19 October 1998 相似文献