全文获取类型
收费全文 | 1390939篇 |
免费 | 124350篇 |
国内免费 | 1938篇 |
专业分类
1517227篇 |
出版年
2021年 | 17956篇 |
2019年 | 16210篇 |
2018年 | 17998篇 |
2017年 | 16717篇 |
2016年 | 28281篇 |
2015年 | 42748篇 |
2014年 | 50820篇 |
2013年 | 77119篇 |
2012年 | 39325篇 |
2011年 | 28597篇 |
2010年 | 44442篇 |
2009年 | 45575篇 |
2008年 | 26781篇 |
2007年 | 24820篇 |
2006年 | 29750篇 |
2005年 | 30693篇 |
2004年 | 29959篇 |
2003年 | 27279篇 |
2002年 | 25261篇 |
2001年 | 34973篇 |
2000年 | 32319篇 |
1999年 | 32167篇 |
1998年 | 25617篇 |
1997年 | 25535篇 |
1996年 | 24981篇 |
1995年 | 23162篇 |
1994年 | 22904篇 |
1993年 | 22014篇 |
1992年 | 28099篇 |
1991年 | 26664篇 |
1990年 | 25355篇 |
1989年 | 25995篇 |
1988年 | 23881篇 |
1987年 | 22532篇 |
1986年 | 21637篇 |
1985年 | 23343篇 |
1984年 | 22995篇 |
1983年 | 20350篇 |
1982年 | 20588篇 |
1981年 | 19779篇 |
1980年 | 18518篇 |
1979年 | 19000篇 |
1978年 | 18038篇 |
1977年 | 17159篇 |
1976年 | 16322篇 |
1975年 | 16132篇 |
1974年 | 16650篇 |
1973年 | 16960篇 |
1972年 | 14406篇 |
1971年 | 13289篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
171.
172.
A. Knob C. R. F. Terrasan E. C. Carmona 《World journal of microbiology & biotechnology》2010,26(3):389-407
β-Xylosidases are hydrolytic enzymes which play an important role in xylan degradation, hydrolyzing xylobiose and xylooligosaccharides to xylose from the non-reducing end. Filamentous fungi are particularly interesting producers of this enzyme from an industrial point of view, due to the fact that they secrete β-xylosidases into the medium. Besides, fungal β-xylosidases are highly advantageous for their elevated activity levels and specificity. Interest in xylanolytic enzymes has been increasing, for their possible application in many biotechnological processes. This fact has driven the isolation, purification and characterization of several β-xylosidases. In this review, the mechanisms of action, substrate specificities, physicochemical characteristics, regulation at molecular level, molecular cloning and classification of filamentous fungal β-xylosidases are described. The potential industrial applications of fungal β-xylosidases will also be presented. 相似文献
173.
The linkage of the Phi, Pgd, Po2, S, H and halothane sensitivity loci was followed in a Belgian Landrace family, heterozygous for these systems over 6 generations. Recombination next to the S locus occurred mainly in pigs belonging to this particular family. From this investigation the position of the S locus is proved to be outwith the Phi-Pgd region, next to Phi . Therefore the gene sequence S - Phi - Hal -H- Po2 -Pgd is proposed. Higher recombination rates were observed in the female parental line of the multiheterozygous family when compared to the male parental line. Additional data from animals, unrelated to this strain, confirm the evidence of close linkage of the S system to the nearest marker loci. 相似文献
174.
Content of red blood-cell sialic acid in BW 35 blood donors. Relation to magnesium concentration and pyruvate kinase activity 总被引:1,自引:0,他引:1
Previous studies have shown that the concentration of red blood cell (RBC) magnesium is significantly lower in subjects carrying an HLA-BW 35 antigen (p less than 0.001) than in non-carriers. As this finding might be related to modifications of the RBC membrane sialoglycoconjugates, RBC sialic acid was comparatively determined in BW 35+ and BW 35- subjects. Pyruvate-kinase activity mean RBC volume, and reticulocyte count have also been determined in order to estimate whether some significant variations in the level of these age markers could be detected between the HLA BW 35+ and BW 35- subjects. A significant negative correlation between sialic acid and RBC magnesium concentrations was observed for the whole population tested (n 57, p less than 0.005), 61% of the BW 35+ and only 25% of the BW 35- individuals having sialic acid values above, and magnesium values below the overall mean (p less than 0.01). The variance of mean RBC volume was also larger for the BW 35+ group. Other determinations did not show any significant variations, suggesting that the results are not related to RBC age. 相似文献
175.
Fumaria capreolata was taken into cell suspension culture. The culture yielded a biomass of about 12 g dry weight per liter of medium; the dried cells contained ca. 0.1% of alkaloids. Besides choline, the following ten known isoquinoline alkaloids were isolated from the cell extract in crystalline form: coptisine, dehydrocheilanthifoline; (+)-isoboldine; magnoflorine; N-methylcoclaurine; (+)-reticuline; (–)-pallidine; protopine; sanguinarine; (–)-scoulerine. This is the most diverse isoquinoline alkaloid spectrum thus far published for a cell suspension culture. 相似文献
176.
177.
EEG activity was recorded in rats submitted to osmotic opening of the BBB by intracarotid mannitol infusion.This procedure produced an immediate short-lasting depression of the EEG and a tardive paroxysmal EEG activity. Both these phenomena were more relevant on the ipsilateral hemisphere. In some instances a tonico-clonic seizure was recorded.Pre-treatment with diazepam abolished the occurrence of the tardive EEG and behavioral modifications.In accord with previous findings, focal seizure activity is likely to be responsible for the metabolic abnormalities associated with osmotic opening of the BBB. This preparation therefore produces in the brain unphysiological states in respect to local metabolism and electrical function. 相似文献
178.
179.
180.
The five cysteines, at positions 82, 191, 192, 270, and 401, of Escherichia coli aspartate aminotransferase (AATase) were, individually and in some combinations, converted to alanine by site-directed mutagenesis (C82A, C191A, C192A, C270A, C401A). Cys-191, which is conserved in all AATase isozymes, was mutated to serine as well (C191S). A quintuple mutant, with all cysteines converted to alanines (Quint), was also constructed. The effects of these single and multiple mutations were examined by steady-state kinetics and urea denaturation. The thermal stabilities of Quint and of the wild-type enzyme (WT) were determined by differential scanning calorimetry. The mutants had kcat values up to 50% greater than that of WT and KMAsp and KM alpha-KG values up to 1.5- and 3.3-fold higher than that of WT. The mutants C82A and C191A exhibit nearly the same CM in urea denaturation experiments as WT, while the other single mutants and Quint are less stable, with CM differences of up to 0.7 M urea. Quint is also less thermostable than WT, with a delta TM of 3.3-4.4 degrees C. Thus the five cysteine replacements yield small, but significant, changes in catalytic and denaturation parameters, but none of the cysteines was found to be essential. The changes manifested in the mutation of the conserved Cys-191 to alanine are no greater than those observed with the four nonconserved cysteines. We consider the evolutionary implications of these findings. 相似文献