全文获取类型
收费全文 | 104928篇 |
免费 | 6498篇 |
国内免费 | 15篇 |
出版年
2022年 | 613篇 |
2021年 | 1526篇 |
2020年 | 1261篇 |
2019年 | 1409篇 |
2018年 | 2642篇 |
2017年 | 2381篇 |
2016年 | 3220篇 |
2015年 | 4235篇 |
2014年 | 4524篇 |
2013年 | 5873篇 |
2012年 | 6822篇 |
2011年 | 6140篇 |
2010年 | 4125篇 |
2009年 | 3405篇 |
2008年 | 5013篇 |
2007年 | 4793篇 |
2006年 | 4630篇 |
2005年 | 3919篇 |
2004年 | 3976篇 |
2003年 | 3505篇 |
2002年 | 3281篇 |
2001年 | 2709篇 |
2000年 | 2507篇 |
1999年 | 2053篇 |
1998年 | 1010篇 |
1997年 | 754篇 |
1996年 | 768篇 |
1995年 | 713篇 |
1994年 | 653篇 |
1992年 | 1233篇 |
1991年 | 1158篇 |
1990年 | 1105篇 |
1989年 | 1169篇 |
1988年 | 968篇 |
1987年 | 990篇 |
1986年 | 905篇 |
1985年 | 953篇 |
1984年 | 781篇 |
1983年 | 701篇 |
1982年 | 586篇 |
1979年 | 794篇 |
1978年 | 597篇 |
1977年 | 583篇 |
1975年 | 693篇 |
1974年 | 752篇 |
1973年 | 714篇 |
1972年 | 654篇 |
1970年 | 621篇 |
1969年 | 686篇 |
1968年 | 646篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
951.
Managing wild crops of Gracilaria in central Chile 总被引:3,自引:3,他引:0
952.
Differential effect of pertussis toxin on the affinity state for agonists of renal alpha 1- and alpha 2- adrenoceptors 总被引:4,自引:0,他引:4
J L Boyer A García C Posadas J A García-Sáinz 《The Journal of biological chemistry》1984,259(13):8076-8079
The effect of pertussis toxin treatment on the guanine nucleotide-induced modulation of the affinity of renal alpha 1- and alpha 2-adrenergic receptors was investigated. Pretreatment of rats with pertussis toxin did not induce any change in the number of or affinity for antagonists of alpha 1- or alpha 2-receptors studied using [3H]prazosin and [3H]yohimbine, respectively. Guanyl-5'-yl imidodiphosphate induced an "up-shift" in the number of alpha 2-adrenergic receptors; this up-shift was not observed for alpha 1-adrenergic receptors. Pertussis toxin treatment decreased the affinity of epinephrine for the [3H]yohimbine-binding sites and reduced the ability of guanine nucleotides to modulate alpha 2-adrenoceptor agonist affinity. The regulation by guanine nucleotides of alpha 1-adrenoceptor affinity for agonists was not altered. These results suggest that the modulation of alpha 1- and alpha 2-adrenoceptors by guanine nucleotides is probably exerted through different molecular entities. 相似文献
953.
Invertase immobilization via its carbohydrate moiety 总被引:1,自引:0,他引:1
After periodate oxidation of its glycosidic component, invertase was covalently bound onto three types of modified solid supports: glycidyl methacrylate, styrene-divinylbenzene copolymers, and bead cellulose. Direct reaction of the invertase aldehyde groups that were formed with amino groups of the support and use of the modified Ugi reaction have been employed as immobilization procedures. Apart from binding methods, the important effects of the buffer, support, conditions of periodate oxidation, and the length of the spacer on the activity of the enzyme conjugate have been investigated. Superior conjugate activity was obtained, via modified Ugi reaction, by the immobilization of a suitably oxidized invertase to a styrene-divinylbenzene copolymer having free amino groups. 相似文献
954.
Dion tomasellii sp. nov. occurs with an interrupted distribution from Guerrero to central Sonora. It is characterized by falcate to subfalcate leaflets. The populations from Sonora and northern Sinaloa are segregated asD. tomasellii var.sonorense on the basis of their narrower and glaucous leaflets. 相似文献
955.
Jan Dahmén Torbjörn Frejd Göran Magnusson Ghazi Noori Anne-Sofie Carlström 《Carbohydrate research》1984,125(2):237-245
The applicability of 2-bromoethyl glycosides in carbohydrate synthesis is demonstrated by the synthesis of glycosides of alpha-L-Fuc-(1----2)-D-Gal and beta-D-Gal-(1----4)-D-GlcNAc. The bromoethyl aglycon was transformed into the methoxycarbonylethylthioethyl spacer, which allowed coupling of the sugars to proteins (BSA and KLH). 相似文献
956.
Luis M. Vázquez de Miguel José A. Serrano Blázquez Fernando J. García Barros Manuel Gómez Guillén 《Carbohydrate research》1984,126(1):81-90
Ethyl 2-amino-4,6-O-benzylidene-2-deoxy-d-gluconate adds to acetylenic esters to give sugar enaminones. The following acetylene derivatives have been employed: methyl propiolate, ethyl phenylpropiolate, and dimethyl acetylenedicarboxylate (6). With compound 6, the reaction leads to a mixture of the expected enaminone and the isomeric oxazolidine derivative. The structures and configurations of the new compounds were studied by spectroscopic and chemical methods. 相似文献
957.
Modulatory effect of zinc on the proliferative response of murine spleen cells to polyclonal T cell mitogens 总被引:1,自引:0,他引:1
To study the effect of zinc on the proliferative response to polyclonal T cell mitogens, spleen cells from C57BL/6 mice were cultured with or without ZnCl2 and stimulated with graded doses of concanavalin A or phytohemagglutinin. Addition of 10(-4) M ZnCl2 inhibited proliferation whereas 10(-5) to 10(-6) M ZnCl2 did not modify the response to suboptimal doses of mitogen but increased DNA synthesis in cultures stimulated with high doses of mitogen (10 or 20 micrograms/ml of concanavalin A and 10 or 25 microliters/ml of phytohemagglutinin) which are supraoptimal for C57BL/6 mice, and inhibited proliferation in cultures of spleen cells from animals of this strain, low responder to T cell mitogens. In contrast, supplementation with ZnCl2 did not enhance the response to mitogen of spleen cells from high responder BALB/c mice. The enhancing effects of ZnCl2 on the proliferative response of C57BL/6 cells were not observed following depletion of adherent cells or in cultures supplemented with 5 X 10(-5) M 2-mercaptoethanol, both conditions capable of abrogating the inhibitory effect of high mitogen doses on the response of C57BL/6 cells. 相似文献
958.
Ramón F. Pinilla Amparo Cano Luis Boto Angel Pestan˜a 《Biochemical and biophysical research communications》1984,119(3):1141-1146
A transitory increase in ornithine decarboxylase activity has been observed soon after food removal from Dictyostelium discoideum amoeba. This increase can be prevented by supplementation of the differentiation buffer with the 11 amino acids known for their ability to retard the development of this slime mold. Lysine can replace the amino acid mixture with an apparent inhibition constant of 50 micromolar. This inhibition by lysine, which was only observed in vivo, took place within 5 min and was readily reversed upon lysine removal. 相似文献
959.
We have studied the structural properties of nucleosomal particles deficient in histones H2A and H2B produced by modification of histone amino groups with dimethylmaleic anhydride [Jordano, J., Montero, F., & Palacián, E. (1984) Biochemistry (preceding paper in this issue)]. Digestion with DNase I of residual particles containing only 15% of the original H2A . H2B complement produces only discrete DNA fragments no longer than 70 nucleotides. As compared with the original nucleosomes, thermal denaturation of the residual particles shows a decrease from 140 to about 90 in the number of nucleotide base pairs per particle that melt at the highest temperature transition as well as a drop in the temperature of this transition. Circular dichroism spectra of the residual particles give ellipticity values around 275 nm, much higher than those corresponding to the control nucleosomes, which appears to indicate a loss in the compact DNA tertiary structure. When regeneration of the modified amino groups of the residual particles takes place in the presence of the complementary fraction containing histones H2A and H2B, but not in its absence, nucleosomal particles with the structural properties of the original nucleosomes are reconstituted. Therefore, the structural change observed in the residual particles can be assigned to the lack of histones H2A and H2B and not to the modified amino groups of the histones present in the residual particles. The results are consistent with the stabilization by histones H2A and H2B of a DNA length of 50-70 base pairs per nucleosome. 相似文献
960.
In an open gas exchange system with a thermocouple psychrometer the transpiration rate of the first leaf in 8-day plants of
spring barley was measured in dependence on the water saturation deficit (ΔW
sat). The plants were cultivated in Richter’s nutrient solution, either complete, or deficient in nitrogen or phosphorus. The
cuticular transpiration (as measured in the dark) was unaffected by N and P deficiency. The N deficiency reduced the transpiration
rate by increasing stomatal resistance since full water saturation of the leaf (67% rate of the control variant) up to stomatal
closing at Δ Wsat = 14%. The P deficiency does not affect the transpiration rate at initial phases of wilting, but the stomata close only at
a higher Δ Wsat (25%) than those in the control. 相似文献