全文获取类型
收费全文 | 73748篇 |
免费 | 4948篇 |
国内免费 | 7篇 |
专业分类
78703篇 |
出版年
2023年 | 556篇 |
2022年 | 510篇 |
2021年 | 1088篇 |
2020年 | 965篇 |
2019年 | 983篇 |
2018年 | 2368篇 |
2017年 | 2168篇 |
2016年 | 2784篇 |
2015年 | 3531篇 |
2014年 | 3537篇 |
2013年 | 4834篇 |
2012年 | 5859篇 |
2011年 | 5393篇 |
2010年 | 3268篇 |
2009年 | 2492篇 |
2008年 | 4173篇 |
2007年 | 3872篇 |
2006年 | 3668篇 |
2005年 | 3104篇 |
2004年 | 3061篇 |
2003年 | 2761篇 |
2002年 | 2529篇 |
2001年 | 1708篇 |
2000年 | 1768篇 |
1999年 | 1261篇 |
1998年 | 591篇 |
1997年 | 432篇 |
1996年 | 425篇 |
1995年 | 396篇 |
1994年 | 328篇 |
1993年 | 314篇 |
1992年 | 614篇 |
1991年 | 540篇 |
1990年 | 465篇 |
1989年 | 436篇 |
1988年 | 438篇 |
1987年 | 389篇 |
1986年 | 367篇 |
1985年 | 391篇 |
1984年 | 366篇 |
1983年 | 268篇 |
1982年 | 250篇 |
1981年 | 189篇 |
1980年 | 192篇 |
1979年 | 232篇 |
1978年 | 221篇 |
1975年 | 209篇 |
1974年 | 231篇 |
1973年 | 241篇 |
1972年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
The object of this study was to adapt the triple stain technique to diluted and incubated boar spermatozoa. Freshly ejaculated semen was resuspended in MR-A diluent to contain 3x10(7) cells/ml (diluted spermatozoa) and was subsequently capacitated (incubated spermatozoa). Experiments were conducted to show the conditions required for optimal staining quality and validation of triple stain technique. The most satisfactory staining solutions for diluted spermatozoa were 2% Trypan blue at 37 degrees C for 15 minutes, 0.8% Bismarck brown in 30% ethyl alcohol (pH 2.8) at 40 degrees C for 10 minutes and 0.8% rose Bengal in 0.1 M of Tris (pH 4.3) at 21 degrees C for 20 minutes. Satisfactory results were obtained for incubated spermatozoa with rose Bengal when the staining time was 10 minutes. Triple stain technique seemed to be a useful method for the simultaneous assessment of sperm vitality and acrosomal status; consequently, it should be valuable tool, for use in porcine in vitro fertilization systems. 相似文献
952.
953.
C Navarro-Ranninger P A Ochoa J M Pérez J H Rodríguez J R Masaguer C Alonso 《Journal of inorganic biochemistry》1992,48(3):163-171
Four platinum(II) aminobenzamidine complexes have been prepared and characterized by IR and 1H and 13C NMR spectroscopy, and tested for their ability to interact with the nicked and closed circular forms of the pUC8 plasmid DNA. The results show that the complexes of formula [Pt(LH)2Cl2]2X have a cis- geometry with an amino-Pt bonding, where L is either p- or m-aminobenzamidine and where 2X is 2Cl- or PtCl4(2-). It was observed that these complexes significantly alter the electrophoretic mobility of nicked and closed circular forms of DNA and that the alteration in electrophoretic mobility due to Pt(II)-p-aminobenzamidine binding is higher than that due to Pt(II)-m-aminobenzamidine. No difference in mobility was observed whether the DNA interacted with complexes having as counteranion Cl- or PtCl4(2-). The synthesized compounds were, in addition, assayed for antitumor activity in vitro against colon (CX-1), lung (LX-1), and mammary (MX-1) human tumor cells. The results show that these complexes inhibited the multiplication of the tumor cells and that they show higher specificity for lung cells. 相似文献
954.
Pablo Collazo Lluís Montoliu Pere Puigdomènech Joan Rigau 《Plant molecular biology》1992,20(5):857-867
The isolation and characterization of cDNA and homologous genomic clones encoding the lignin O-methyltransferase (OMT) from maize is reported. The cDNA clone has been isolated by differential screening of maize root cDNA library. Southern analysis indicates that a single gene codes for this protein. The genomic sequence contains a single 916 bp intron. The deduced protein sequence from DNA shares significant homology with the recently reported lignin-bispecific caffeic acid/5-hydroxyferulic OMTs from alfalfa and aspen. It also shares homology with OMTs from bovine pineal glands and a purple non-sulfur photosynthetic bacterium. The mRNA of this gene is present at different levels in distinct organs of the plant with the highest accumulation detected in the elongation zone of roots. Bacterial extracts from clones containing the maize OMT cDNA show an activity in methylation of caffeic acid to ferulic acid comparable to that existing in the plant extracts. These results indicate that the described gene encodes the caffeic acid 3-O-methyltransferase (COMT) involved in the lignin biosynthesis of maize. 相似文献
955.
956.
957.
Mario Díaz Antonio Lorenzo 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(2):189-196
Summary Bioelectrical parameters and unidirectional sodium and chloride fluxes were measured under voltageclamp conditions in groups of lizards submitted to single or chronic aldosterone treatment. Both acute (AT) and chronic (CT) treatment induced significant increases in the short-circuit current (I
sc), as well as in the mucosa-to-serosa (J
m-s
Na
) and net sodium flux (J
net
Na
). In AT tissues, aldosterone did not change net chloride flux (J
net
Cl
) but did so in CT tissues. Amiloride reduced the aldosterone-increased I
sc in AT and CT tissues, inhibited J
net
Na
in AT tissues and abolished it in CT colons. J
net
Cl
was also reduced by the diuretic in the group of AT colons, whereas no changes were observed in the CT tissues. Addition of luminal DIDS reduced Na+ absorption and totally inhibited Cl- absorption in the AT tissues, but did not change I
sc. However, in CT tissues neither Na+ nor Cl- transport were affected by DIDS. A good relationship between I
sc and J
m-s
Na
was apparent after DIDS treatment in AT tissues. In this group, simultaneous addition of DIDS and amiloride totally abolished J
net
Na
and reduced I
sc to untreated control values. Addition of serosal ouabain abolished I
sc and Na+ absorption in AT and CT colons, but Cl- absorption was only altered in AT tissues. These results support the hypothesis that aldosterone induces an electrogenic, amiloride-sensitive sodium absorption, and in a dose-dependent fashion suppresses electroneutral NaCl absorption in the lizard colon.Abbreviations AT
acutely treated
- CT
chronically treated animals
- DIDS
4-4-diisothiocyanatostibene-2-2-disulfonic acid
- DMSO
dimethylsulphoxide
-
G
t
tissue conductance
-
I
sc
short circuit current
- PD
transepithelial potential difference
- SITS
4-acetamido-4-isothiocyanatostilbene-2-2-disulfonic acid
- UC
untreated controls
Preliminary results of this paper were presented at the X
th meeting of the European Intestinal Transport Group (EITG), Askov Hojskole, Denmark, 16–19 September 1990 相似文献
958.
Summary The white-rot fungus Ganoderma australis selectively degrades lignin in the ecosystem palo podrido. Using conditions that simulate those of palo podrido in the laboratory, it was found that low nitrogen content and low O2 tension stimulate the productio of manganese peroxidase and lignin degradation, and depress cellulose degradation and cellulase production. The inverse is found at high nitrogen concentration and high O2 tension. This agrees with previous results indicating that low O2 tension and low nitrogen stimulate selective lignin degradation by this fungus.
Correspondence to: J. Eyzaguirre 相似文献
959.
Ammonium ingestion prevents depletion of hepatic energy metabolites induced by acute ammonium intoxication 总被引:1,自引:0,他引:1
E Kosenko V Felipo M D Mi?ana E Grau S Grisolía 《Archives of biochemistry and biophysics》1991,290(2):484-488
Ingestion of an ammonium containing diet produces hyperammonemia and protects rats against acute ammonium intoxication. Acute ammonium toxicity has been attributed to the depletion of energy metabolite intermediates. We show here that hyperammonemia affords considerable protection against depletion of hepatic energy metabolites evoked by ammonium acetate injection. In control rats there were marked decreases in the content of acetoacetate, beta-hydroxybutyrate, ATP, 2-oxoglutarate, lactate, and pyruvate while phosphoenolpyruvate increased markedly. In hyperammonemic rats beta-hydroxybutyrate, ATP, 2-oxoglutarate, and lactate were not significantly affected while pyruvate increased markedly and phosphoenolpyruvate slightly. These results suggest that in controls the activity of pyruvate kinase is inhibited after ammonium injection while in hyperammonemic rats it is not inhibited. The content of alanine (an inhibitor of pyruvate kinase) reached 2.8 mumol/g in controls and 1.6 mumol/g in hyperammonemic rats, 15 min after ammonium injection. This could explain the different effects of ammonium injection on control and hyperammonemic rats. 相似文献
960.
E del Río J M González-Ma?as J I Gurtubay F M Go?i 《Archives of biochemistry and biophysics》1991,291(2):300-306
Purple membrane bacteriorhodopsin can be easily solubilized by Triton X-100 and other detergents, but not by deoxycholate. In order to understand this behavior, we have examined the effects of a variety of surfactants. We show that detergents containing the cholane ring (cholate, taurocholate, 3[(3-cholamidopropyl)diethyl-ammonio]propanesulfonic acid...) are virtually unable to solubilize native bacteriorhodopsin. However, when the protein is reconstituted in dimyristoyl phosphatidylcholine and solubilization is assayed at a temperature such that bacteriorhodopsin is in the form of monomers, solubilization by cholane detergents does occur. We propose that steric factors prevent access of the rigid planar surfactant molecules to the hydrophobic protein regions. These are perhaps located in the monomer-monomer interface, whose solvation by surfactants is essential for solubilization to occur. We note that the capacity of some detergents to solubilize bacteriorhodopsin is always associated within the same range of surfactant concentrations with bleaching (partial or total) of the protein chromophore. The detergent-induced bleaching is at least partially reversible, suggesting that free retinal remains associated to some membrane components. While some surfactant molecules remain tightly bound to the membrane protein, cholane detergents can be completely removed from bacteriorhodopsin. Our results indicate that a structure-function relationship exists for detergents applied to the solubilization of bacteriorhodopsin. 相似文献