全文获取类型
收费全文 | 39207篇 |
免费 | 113篇 |
国内免费 | 169篇 |
专业分类
系统科学 | 238篇 |
丛书文集 | 688篇 |
教育与普及 | 75篇 |
理论与方法论 | 145篇 |
现状及发展 | 17451篇 |
研究方法 | 1641篇 |
综合类 | 18577篇 |
自然研究 | 674篇 |
出版年
2013年 | 292篇 |
2012年 | 589篇 |
2011年 | 1234篇 |
2010年 | 245篇 |
2008年 | 681篇 |
2007年 | 812篇 |
2006年 | 764篇 |
2005年 | 810篇 |
2004年 | 859篇 |
2003年 | 702篇 |
2002年 | 733篇 |
2001年 | 1175篇 |
2000年 | 1107篇 |
1999年 | 777篇 |
1992年 | 734篇 |
1991年 | 558篇 |
1990年 | 610篇 |
1989年 | 566篇 |
1988年 | 540篇 |
1987年 | 562篇 |
1986年 | 604篇 |
1985年 | 768篇 |
1984年 | 578篇 |
1983年 | 494篇 |
1982年 | 415篇 |
1981年 | 444篇 |
1980年 | 543篇 |
1979年 | 1266篇 |
1978年 | 985篇 |
1977年 | 978篇 |
1976年 | 743篇 |
1975年 | 773篇 |
1974年 | 1123篇 |
1973年 | 951篇 |
1972年 | 1022篇 |
1971年 | 1151篇 |
1970年 | 1582篇 |
1969年 | 1185篇 |
1968年 | 1043篇 |
1967年 | 1128篇 |
1966年 | 1008篇 |
1965年 | 742篇 |
1964年 | 223篇 |
1959年 | 394篇 |
1958年 | 742篇 |
1957年 | 525篇 |
1956年 | 429篇 |
1955年 | 357篇 |
1954年 | 401篇 |
1948年 | 292篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
113.
114.
115.
Neurotransmission from mossy fibre terminals onto cerebellar granule cells is almost certainly mediated by L-glutamate. By taking advantage of the small soma size, limited number of processes and short dendrite length of granule cells, we have obtained high-resolution recordings of spontaneous miniature excitatory postsynaptic currents (m.e.p.s.cs) and evoked currents in thin cerebellar slices. Miniature currents have a similar time-course and pharmacology to evoked currents and consist of an exceptionally fast non-NMDA (N-methyl-D-aspartate) component (measured rise-time, 200 microseconds; estimated pre-filtered rise-time less than 100 microseconds; decay time constant, tau = 1.0 ms), followed by 50 pS NMDA channel openings that are directly resolvable. We could find no evidence for the recent proposal that miniature currents in granule cells are mediated solely by NMDA channels with a novel time course. The non-NMDA receptor component of m.e.p.s.cs has a skewed amplitude distribution, which suggests potential complications for quantal analysis. The difference in time course between the m.e.p.s.cs reported here and other synaptic currents in the brain could reflect differences in synaptic function or electrotonic filtering; the relative contribution of these possibilities has yet to be established. 相似文献
116.
We have studied the chemistry of aminoacyl AMP to model reactions at the 3' terminus of aminoacyl tRNA for the purpose of understanding the origin of protein synthesis. The present studies relate to the D, L preference in the esterification of 5'-AMP. All N-acetyl amino acids we studied showed faster reaction of the D-isomer, with a generally decreasing preference for D-isomer as the hydrophobicity of the amino acid decreased. The beta-branched amino acids, Ile and Val, showed an extreme preference for D-isomer. Ac-Leu, the gamma-branched amino acid, showed a slightly low D/L ratio relative to its hydrophobicity. The molecular basis for these preferences for D-isomer is understandable in the light of our previous studies and seems to be due to preferential hydrophobic interaction of the D-isomer with adenine. The preference for hydrophobic D-amino acids can be decreased by addition of an organic solvent to the reaction medium. Conversely, peptidylation with Ac-PhePhe shows a preference for the LL isomer over the DD isomer. 相似文献
117.
J A Martignetti A A Aqeel W A Sewairi C E Boumah M Kambouris S A Mayouf K V Sheth W A Eid O Dowling J Harris M J Glucksman S Bahabri B F Meyer R J Desnick 《Nature genetics》2001,28(3):261-265
The inherited osteolyses or 'vanishing bone' syndromes are a group of rare disorders of unknown etiology characterized by destruction and resorption of affected bones. The multicentric osteolyses are notable for interphalangeal joint erosions that mimic severe juvenile rheumatoid arthritis (OMIMs 166300, 259600, 259610 and 277950). We recently described an autosomal recessive form of multicentric osteolysis with carpal and tarsal resorption, crippling arthritic changes, marked osteoporosis, palmar and plantar subcutaneous nodules and distinctive facies in a number of consanguineous Saudi Arabian families. We localized the disease gene to 16q12-21 by using members of these families for a genome-wide search for homozygous-by-descent microsatellite markers. Haplotype analysis narrowed the critical region to a 1.2-cM region that spans the gene encoding MMP-2 (gelatinase A, collagenase type IV; (ref. 3). We detected no MMP2 enzymatic activity in the serum or fibroblasts of affected family members. We identified two family-specific homoallelic MMP2 mutations: R101H and Y244X. The nonsense mutation effects a deletion of the substrate-binding and catalytic sites and the fibronectin type II-like and hemopexin/TIMP2 binding domains. Based on molecular modeling, the missense mutation disrupts hydrogen bond formation within the highly conserved prodomain adjacent to the catalytic zinc ion. 相似文献
118.
量子力学是当前物理学及其相关科学的重要科学分支,也是当前许多新科学技术中的重要应用的基础。本书包含了现代新科学技术工程师所需要的量子力学方面的新材料,如研究和了解原子核、原子、分子和固体材料,以及激光器和其他量子光学器件所需要的科学资料等。 相似文献
119.
The fecundity of the blood-feeding insect,Rhodnius prolixus, was observed to increase in successive periods of egg production, each period being triggered by a single large blood meal. As previously published, the fecundity of mated animals was significantly higher than that of unmated animals for the first period of egg production. For a second period of egg production, fecundity increased significantly in both mated and unmated animals. By the fourth period, fecundity had returned to first-feed values for mated animals, but remained high for unmated animals, and the fecundity of mated and unmated animals was not significantly different. Thus, during successive periods of egg production, the processes which maintain fecundity of unmated animals below that of mated animals are overcome. 相似文献
120.
P. Levy G. Cherqui A. Robert D. Wicek J. Picard 《Cellular and molecular life sciences : CMLS》1989,45(6):588-591
Summary During the spontaneous differentiation (day 5 to day 15 of the culture) of Caco-2 cells, the sulfation of cell layer glycosaminoglycans increased, whereas protein kinase C activity was concomitantly redistributed from the membrane to the cytosol. The protein kinase C activators, 4-phorbol 12-myristate, 13-acetate and 1,2-dioctanoyl-glycerol inhibited glycosaminoglycan sulfation. By contrast, 4-phorbol 12, 13 didecanoate was ineffective.These results suggest that membrane-bound PKC may exert a modulatory effect on glycosaminoglycan sulfation, and this effect is gradually attenuated as Caco-2 cell differentiation progresses. 相似文献