全文获取类型
收费全文 | 44748篇 |
免费 | 3477篇 |
国内免费 | 123篇 |
出版年
2022年 | 414篇 |
2021年 | 863篇 |
2020年 | 599篇 |
2019年 | 765篇 |
2018年 | 1019篇 |
2017年 | 857篇 |
2016年 | 1236篇 |
2015年 | 1762篇 |
2014年 | 1947篇 |
2013年 | 2610篇 |
2012年 | 2837篇 |
2011年 | 2720篇 |
2010年 | 1857篇 |
2009年 | 1521篇 |
2008年 | 2123篇 |
2007年 | 2040篇 |
2006年 | 1936篇 |
2005年 | 1637篇 |
2004年 | 1642篇 |
2003年 | 1482篇 |
2002年 | 1505篇 |
2001年 | 1235篇 |
2000年 | 1142篇 |
1999年 | 987篇 |
1998年 | 486篇 |
1997年 | 461篇 |
1996年 | 376篇 |
1995年 | 374篇 |
1994年 | 294篇 |
1993年 | 331篇 |
1992年 | 635篇 |
1991年 | 604篇 |
1990年 | 560篇 |
1989年 | 500篇 |
1988年 | 414篇 |
1987年 | 402篇 |
1986年 | 385篇 |
1985年 | 421篇 |
1984年 | 396篇 |
1983年 | 348篇 |
1982年 | 254篇 |
1981年 | 265篇 |
1980年 | 238篇 |
1979年 | 312篇 |
1978年 | 275篇 |
1977年 | 284篇 |
1976年 | 276篇 |
1975年 | 282篇 |
1974年 | 263篇 |
1973年 | 245篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Lower levels of monogalactosyl diglyceride (MGDG) and digalactosyl diglyceride (DGDG) have been found in tetraploid wheats as compared with those in hexaploid wheats. The same difference has been found between hexaploid cultivars and tetraploid lines derived from them by D genome extraction. A lower level of MGDG and DGDG is also present in Triticum carthlicum (AABB) as compared with Aegilops squarrosa (DD) or with the synthetic T. spelta (AABBDD) obtained from them. Analysis of the appropriate nullitetrasomic and ditelosomic lines indicates that a gene or genes located in the short arm of chromosome 5D are responsible for the observed difference and that group 5 chromosomes can be ranked as to their influence on the MGDG and DGDG levels in the order 5B > 5D > 5A and 5D > 5B > 5A, respectively. These results further support our previous identification of DGDG as the lipid factor responsible for petroleum ether solubility of lipopurothionins. Since DGDG contributes to baking quality by improving the retention of fermentation gases, the present observations imply that the difference in bread-making quality between the two types of wheat is not due only to proteins contributed by the D genome. 相似文献
952.
Phage Mu-1 and a thermoinducible derivative, Mu-1 cts 62 were inserted into the broad host range R factor RP4. These hybrid plasmids were transferred by conjugation to a phytopathogenic bacterium Pseudomonas solanacearum GMI 1000 and a legume-root nodule bacterium Rhizobium meliloti 2011. The Mu genome is transcribed and tranlated in these new hosts: P. solanacearum (RP4:Mu cts) cultures have a spontaneous production of about 5 X 10(5) plaque-forming units ml-1 which is similar to the frequency of spontaneous Mu production in E. coli; the Mu production of R. meliloti is lower (about 10(2) plaque-forming units ml-1). 相似文献
953.
Summary Preparations of biomembranes, consisting of membrane vesicles, were analyzed with the analytical ultracentrifuge. Under certain conditions depending on the speed of rotation and the temperature, a sedimentation profile was observed that was highly characteristic for membranous material. From the sedimentation coefficients obtained, we calculated particle weights for the various well-defined membrane components. In certain types of preparations the particle weights of two adjacent components differed on average by a factor of 2. When vesicles obtained by fragmentation of biomembranes were compared with the granular vesicles present in intact cells, the accordance in diameters was striking. This may indicate that the size of vesicles is determined by purely physical factors. 相似文献
954.
Abstract— —The site of origin of transmitter amino acids released by depolarizing agents from nerve endings was studied. The model used was the incubated and depolarized synaptosome preparation from which the component soluble, synaptic vesicle, membrane and mitochondrial sub-fractions were obtained. Synaptosomal amino acids were radioactively labelled from D-[U-14 C]glucose in vivo by intraventricular injection and in vitro during subsequent incubation. The specific radioactivities of amino acids released in response to K+ (56 mM) or veratrine (75 μM) were found to closely resemble those of the soluble cytoplasmic fraction, in most cases differing significantly from those of the other fractions. The specific radioactivity of the GABA and aspartate released by K+ stimulation and the GABA and glutamate released by veratrine were significantly different from that of the vesicles in each case. The specific radioactivities of glutamate released by both agents, and also GABA with K+ stimulation, were approximately double that of the amino acid released in control conditions. Depletion of the soluble cytoplasmic pools of glutamate, GABA and aspartate occurred following stimulation, corresponding to the induced-release of these compounds. Turnover of the amino acids in the other subfractions was too low to account for their participation in the release process in addition to the soluble cytoplasmic pool. A cytoplasmic origin of release of neurotransmitter amino acids from nerve endings is proposed. 相似文献
955.
ASSAY AND PROPERTIES OF 4-AMINOBUTYRIC-2-OXOGLUTARIC ACID TRANSAMINASE AND SUCCINIC SEMIALDEHYDE DEHYDROGENASE IN RAT BRAIN TISSUE 总被引:5,自引:4,他引:1
Abstract— The activity of 4-aminobutyric-2-oxoglutaric acid transaminase (GABA transaminase) and succinic semialdehyde dehydrogenase was determined in total rat brain homogenate. GABA transaminase activity was measured using a coupled enzyme method which utilizes endogenous succinic semialdehyde dehydrogenase to convert the formed succinic semialdehyde into succinate. The concurrently produced NADH was used as an estimate of GABA transaminase activity. This method could be used since it was shown that the dehydrogenase was about twice as active as the transaminase and because no significant accumulation of the intermediate succinic semialdehyde could be detected. GABA transaminase was inhibited by high ionic strength. In contrast NaCl decreased the apparent K m and increased V max for succinic semialdehyde dehydrogenase at high but not al low tissue concentrations. Increasing tissue concentration also resulted in a decrease of the apparent K m , but did not change the Vmax of succinic semialdehyde dehydrogenase and it is suggested that this enzyme can exist in two distinct states of aggregation, one with a high and one with a low affinity for succinic semialdehyde. The high affinity form of the enzyme is thought to prevent succinic semialdehyde from accumulation in the GABA transaminase assay. It is concluded that within certain limits the coupled enzyme method described here can be used for the assay of GABA transaminase activity. 相似文献
956.
Use of an axenic medium for differentiation between pathogenic and nonpathogenic Naegleria fowleri isolates. 总被引:7,自引:4,他引:3
下载免费PDF全文
![点击此处可从《Applied microbiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
J De Jonckheere 《Applied microbiology》1977,33(4):751-757
Growth in an axenic medium composed by Chang (3rd Int. Congr. Parasitol. Munich Abstr. ICPIII 1:187-188, 1974) allowed separation of pathogenic from nonpathogenic Naegleria fowleri strains, since only the former show luxuriant growth in this medium. On the basis of these results, this medium was used in early screening for virulent Naegleria isolates. During an extensive ecological study, data were obtained on 102 Naegleria strains. Twenty of these strains grew luxuriantly in this liquid medium. Seventeen of them were tested by intranasal instillation in mice, and all proved to be highly pathogenic. Strains showing only moderate growth or no growth at all in this axenic medium were found to be nonpathogenic for mice. Moreover, it was found that using this medium in the early stage of Naegleria sampling favors isolation of pathogenic strains in mixtures of Naegleria. During these experiments, further evidence was obtained that thermal polluted waters are the main origin of N. fowleri in the environment. 相似文献
957.
B M Schoot A F Schoots J J De Pont F M Schuurmans Stekhoven S L Bonting 《Biochimica et biophysica acta》1977,483(1):181-192
1. Preincubation with N-ethylmaleimide inhibits the overall activity of highly purified (Na+ +K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) preparations of rabbit kidney outer medulla. 2. This inhibition is decreased by addition of ATP or 4-nitrophenylphosphate under non-phosphorylating conditions, and also by addition of ADP or adenylylimidodiphosphate. 3. N-ethylmaleimide treatment leads to inhibition of K+-stimulated 4-nitrophenylphosphatase activity, Na+-stimulated ATPase activity, and phosphorylation by ATP as well as by inorganic phosphate. These inhibitions strictly parallel that of the overal (Na+ +K+)-ATPase reaction. 4. N-ethylmaleimide lowers the number of sites which are phosphorylated by inorganic phosphate, without affecting the dissociation constant of the enzyme-phosphate complex. 5. N-ethylmaleimide does not affect the relative stimulation by ATP of the K+-stimulated 4-nitrophenylphosphatase activity. 6. These effects of N-ethylmaleimide can be explained as a complete loss of active enzyme, either by reaction of N-ethylmaleimide inside the active center, or by alterations in the quaternary structure through reactions outside the active center. 相似文献
958.
J M Van Amelsvoort J J De Pont A L Stols S L Bonting 《Biochimica et biophysica acta》1977,471(1):78-91
A study has been made to determine whether renal plasma membranes contain an HCO3 stimulated, ouabain insensitive Mg ATPase. Purified mitochondrial, microsomal and brush border membrane fractions have been isolated from rabbit kidney. The microsomal anion-sensitive ATPase activity appears to be entirely of mitochondrial origin on the basis of the effects of inhibitors of mitochondrial Mg ATPase. The brush border membrane fraction is contaminated with mitochondrial fragments and contains an Mg ATPase activity with low anion-sensitivity. Further purification of this fraction causes parallel decreases in anion-sensitivity of the Mg ATPase activity and in cytochrome c oxidase activity. These results indicate that conclusions previously reached by other investigators for a role of anion-sensitive Mg ATPase in the bicarbonate reabsorption of the proximal tubule may no longer be tenable. 相似文献
959.
Secretion and reabsorption of uterine luminal fluid in rats 总被引:3,自引:0,他引:3
Treatment of ovariectomized rats with oestradiol-17beta and progesterone demonstrated that oestradiol-17beta causes secretion of sodium, potassium and water into the lumen of the uterine horn and that progesterone causes reabsorption of these substances. 相似文献
960.
Silvia M.C. Dias João B. Fernandes José G.S. Maia Otto R. Gottlieb Hugo E. Gottlieb 《Phytochemistry》1982,21(7):1737-1740
The trunk wood of an Amazonian Aniba (Lauraceae) species contains, besides dillapiol and the benzodioxane-type neolignan eusiderin, four bicyclo(3.2.1)octanoid neolignans. These comprise representatives of the canellin-type: the known methoxycanellin-A and the novel compounds characterized as (1R, 3S, 4S, 5S, 6S, 7R)-1-allyl-4-hydroxy-3, 5-dimethoxy-7-methyl-6-(3′-methoxy-4′, 5′-methylenedioxyphenyl)-8-oxo-bicyclo(3.2.1)octane; (1R, 3S, 4S, 5S, 6S, 7R)-1-allyl-4-hydroxy-3, 5-dimethoxy-7-methyl-6-(3′, 4′, 5′-trimethoxyphenyl)-8-oxobicyclo(3.2.1)octane and (1R, 4R, 5R, 6S, 7R, 8S)-1-allyl-4, 8-dihydroxy-5-methoxy-7-methyl-6-(3′-methoxy-4′,5′-methylenedioxyphenyl)-3-oxobicyclo(3.2.1)octane. 相似文献