全文获取类型
收费全文 | 42882篇 |
免费 | 3762篇 |
国内免费 | 208篇 |
专业分类
46852篇 |
出版年
2021年 | 408篇 |
2018年 | 546篇 |
2017年 | 511篇 |
2016年 | 676篇 |
2015年 | 750篇 |
2014年 | 965篇 |
2013年 | 1157篇 |
2012年 | 1289篇 |
2011年 | 1347篇 |
2010年 | 889篇 |
2009年 | 897篇 |
2008年 | 1221篇 |
2007年 | 1213篇 |
2006年 | 1201篇 |
2005年 | 1033篇 |
2004年 | 985篇 |
2003年 | 934篇 |
2002年 | 902篇 |
2001年 | 2783篇 |
2000年 | 2471篇 |
1999年 | 1809篇 |
1998年 | 617篇 |
1997年 | 530篇 |
1996年 | 474篇 |
1995年 | 404篇 |
1994年 | 390篇 |
1992年 | 1283篇 |
1991年 | 1166篇 |
1990年 | 1100篇 |
1989年 | 1053篇 |
1988年 | 934篇 |
1987年 | 926篇 |
1986年 | 804篇 |
1985年 | 800篇 |
1984年 | 566篇 |
1983年 | 519篇 |
1979年 | 632篇 |
1978年 | 455篇 |
1977年 | 416篇 |
1976年 | 372篇 |
1975年 | 499篇 |
1974年 | 579篇 |
1973年 | 557篇 |
1972年 | 572篇 |
1971年 | 549篇 |
1970年 | 523篇 |
1969年 | 528篇 |
1968年 | 427篇 |
1967年 | 374篇 |
1966年 | 411篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
131.
Z Naor 《Biochemical and biophysical research communications》1990,167(3):982-992
Addition of the stable and permeable analog 8-bromo cyclic GMP (8-BR-cGMP) to myo-[2-3H]inositol prelabeled cultured rat pituitary cells results in enhanced formation of [3H]-myo-inositol monophosphate (IP1). The stimulatory effect of the cyclic nucleotide analog is additive to the effect of Li+, which accumulates IP1 via inhibition of inositol 1-monophosphatase, and also to the effect of gonadotropin releasing hormone (GnRH) which stimulates the formation of IP1, as well as that of inositol 1,4-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3) via enhanced hydrolysis of polyphosphoinositides. Many Ca2(+)-mobilizing hormones acting via phosphoinosite turnover also stimulate cGMP formation. The cyclic nucleotide might then serve as a modulator by further hydrolysis of phosphoinositides needed for protein kinase C activation. 相似文献
132.
Neomorph and leaf differentiation as alternative morphogenetic pathways in soybean tissue culture. 总被引:1,自引:0,他引:1
Somatic embryogenesis and organogenesis were induced on immature cotyledons of different soybean cultivars. The anatomical investigation of morphogenesis proved neomorph differentiation instead of somatic embryos, and leaf formation instead of shoot development. While normal embryos were induced in 0-3.1% of the explants, neomorphs developed at a much higher rate i.e. in 10.5-78.9% depending on the genotype. Likewise organogenesis preferably followed the pathway of leaflet development (3.1-26.3%) than that of shoot tip formation (0-2.6%). Low plant regeneration frequency of soybean can partly be explained with these two alternative abortive pathways of morphogenesis probably induced with higher frequency than the normal pathways by the generally used in vitro methods. 相似文献
133.
Studies concerning metabolism of catecholamines in mice differing with respect to the degree of pigmentation were based on determination of daily excretion of vanillylmandelic and homovanillic acid and tissue content of epinephrine, norepinephrine, dopamine and their methoxy derivatives. It was found that pigmented mice excrete more homovanillic acid, the metabolite of dopamine, than do albinotic mice. Tissue studies have shown that the brain of albinotic mice contains more dopamine and kidneys more epinephrine, norepinephrine and their methoxy derivatives than the respective organs of the pigmented mice. The probable reasons of differences in the rate of inactivation of catecholamines in albinotic and pigmented mice have been discussed. 相似文献
134.
Restriction fragment length polymorphism (RFLP) was used as a molecular genotyping approach to characterize differences in major histocompatibility complex class IV genes in meat-type chickens. A high level of polymorphism was observed following digestion with each of the two restriction endonucleases PvuII and BglII. Examination of DNA from 54 chickens revealed 23 polymorphic fragments. Application of RFLP techniques in the analysis of family groups should make possible the determination of B-G genotypes in the meat type chickens. 相似文献
135.
An analysis is made of the general expression for the density of a nonlinear charge induced in a magnetized plasma in the interaction between two arbitrary waves. Asymptotic expressions for the nonlinear induced charge density are derived for the first time in the case where both of the interacting waves are short-scale. 相似文献
136.
137.
138.
Y S Ho P J Sheffield J Masuyama H Arai J Li J Aoki K Inoue U Derewenda Z S Derewenda 《Protein engineering》1999,12(8):693-700
Platelet-activating factor acetylhydrolases (PAF-AHs) are unique PLA2s which hydrolyze the sn-2 ester linkage in PAF-like phospholipids with a marked preference for very short acyl chains, typically acetyl. The recent solution of the crystal structure of the alpha(1) catalytic subunit of isoform Ib of bovine brain intracellular PAF-AH at 1.7 A resolution paved the way for a detailed examination of the molecular basis of substrate specificity in this enzyme. The crystal structure suggests that the side chains of Thr103, Leu48 and Leu194 are involved in substrate recognition. Three single site mutants (L48A, T103S and L194A) were overexpressed and their structures were solved to 2.3 A resolution or better by X-ray diffraction methods. Enzyme kinetics showed that, compared with wild-type protein, all three mutants have higher relative activity against phospholipids with sn-2 acyl chains longer than an acetyl. However, for each of the mutants we observed an unexpected and substantial reduction in the V(max) of the reaction. These results are consistent with the model in which residues Leu48, Thr103 and Leu194 indeed contribute to substrate specificity and in addition suggest that the integrity of the specificity pocket is critical for the expression of full catalytic function, thus conferring very high substrate selectivity on the enzyme. 相似文献
139.
Iu V Karetny? Z N Vasil'eva M S Balaian V A Zenkov V A Sysoliatin G V Sobina G V Lysenko G V Tsvetova S Ch Tesheva L A Klimenko 《Zhurnal mikrobiologii, epidemiologii, i immunobiologii》1990,(10):71-76
Additional observation on the formation of humoral immunity to hepatitis A virus has been carried out in 2,375 adolescents aged 15-17 years and belonging to 12 groups at 3 areas of the USSR. This observation has shown that in the presence of a high level of the immune stratum the spread of infection occurs, as a rule, outside the group under observation. Besides, as revealed in this study, the risk of seroconversion and the loss of specific antibodies are, respectively, directly and inversely related to the level of the immune stratum in the group. The level of the immune stratum in a given age group supposedly reflects the intensity of the development of the epidemic process at a definite area. 相似文献
140.
Structural variants of mercury reductase containing the N-terminal domain, which is easily cleaved by trypsin, have been found in Gram-positive bacteria with a low genomic G + C content (Bacillus, Staphylococcus and, possibly, some other genera). Mercury reductases without the N-terminal domain and relatively resistant to limited proteolysis are typical for Gram-positive bacteria with a high genomic G + C content (Arthrobacter, Citreobacterium, Micrococcus, Mycobacterium, Rhodococcus). Both types of mercury reductase genes may be located on plasmids. 相似文献