全文获取类型
收费全文 | 16806篇 |
免费 | 1722篇 |
国内免费 | 3篇 |
专业分类
18531篇 |
出版年
2023年 | 102篇 |
2022年 | 222篇 |
2021年 | 394篇 |
2020年 | 241篇 |
2019年 | 280篇 |
2018年 | 341篇 |
2017年 | 309篇 |
2016年 | 529篇 |
2015年 | 996篇 |
2014年 | 963篇 |
2013年 | 1172篇 |
2012年 | 1534篇 |
2011年 | 1468篇 |
2010年 | 930篇 |
2009年 | 799篇 |
2008年 | 1040篇 |
2007年 | 1052篇 |
2006年 | 989篇 |
2005年 | 960篇 |
2004年 | 906篇 |
2003年 | 789篇 |
2002年 | 712篇 |
2001年 | 143篇 |
2000年 | 105篇 |
1999年 | 137篇 |
1998年 | 143篇 |
1997年 | 86篇 |
1996年 | 80篇 |
1995年 | 59篇 |
1994年 | 62篇 |
1993年 | 57篇 |
1992年 | 70篇 |
1991年 | 70篇 |
1990年 | 59篇 |
1989年 | 37篇 |
1988年 | 53篇 |
1987年 | 30篇 |
1986年 | 25篇 |
1985年 | 37篇 |
1984年 | 33篇 |
1983年 | 32篇 |
1982年 | 29篇 |
1981年 | 22篇 |
1980年 | 22篇 |
1978年 | 23篇 |
1977年 | 22篇 |
1976年 | 18篇 |
1974年 | 27篇 |
1972年 | 24篇 |
1971年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
2.
Jennifer S. Lund 《Brain Cell Biology》2002,31(3-5):203-209
It is clear from reviewing the findings of our own studies and those of others that the cerebral cortex has combined two very different strategies of organisation. Firstly it has a strictly defined genetically determined substrate of specific neurons classes, specific rules for which kinds of cells interconnect, a laminar architecture where efferent and afferent relays and interlaminar links are predetermined. But, as well, a second strategy allows great developmental lability in the precise spatial patterns of intralaminar circuits of the excitatory neurons and in the actual weights of excitatory and inhibitory synapses that are contributed to each neuron. This second strategy presumably allows the cortex to be tailor-made to the early experience of each individual and, as well, allow for lability of responses to different conditions of stimulation and adjustment of the system to compensate to some degree for injuries affecting afferents and circuitry in the adult system. 相似文献
3.
Inhibitory effect of the flavonoid silymarin on the erythrocyte hemolysis induced by phenylhydrazine
A Valenzuela T Barría R Guerra A Garrido 《Biochemical and biophysical research communications》1985,126(2):712-718
The flavonoid silymarin, which is used as a therapeutical agent in the treatment of liver diseases, can inhibit the hemolysis and lipid peroxidation induced by phenylhydrazine on erythrocytes obtained from rats treated with the flavonoid. This effect is ascribed to the antioxidant properties as a free radical scavenger exhibited by the flavonoid. Silymarin failed to inhibit the glutathione depletion induced by phenylhydrazine on erythrocytes. It is proposed that the flavonoid acts at the membrane level of the cell avoiding the lipid peroxidative and fluidizing effect of phenylhydrazine. 相似文献
4.
5.
Daily ingestion of iodide alone is not adequate to sustain production of the thyroid hormones, tri- and tetraiodothyronine. Proper maintenance of iodide in vivo also requires its active transport into the thyroid and its salvage from mono- and diiodotyrosine that are formed in excess during hormone biosynthesis. The enzyme iodotyrosine deiodinase responsible for this salvage is unusual in its ability to catalyze a reductive dehalogenation reaction dependent on a flavin cofactor, FMN. Initial characterization of this enzyme was limited by its membrane association, difficult purification and poor stability. The deiodinase became amenable to detailed analysis only after identification and heterologous expression of its gene. Site-directed mutagenesis recently demonstrated that cysteine residues are not necessary for enzymatic activity in contrast to precedence set by other reductive dehalogenases. Truncation of the N-terminal membrane anchor of the deiodinase has provided a soluble and stable source of enzyme sufficient for crystallographic studies. The structure of an enzyme·substrate co-crystal has become invaluable for understanding the origins of substrate selectivity and the mutations causing thyroid disease in humans. 相似文献
6.
We previously reported that aged mice lacking complement factor H (CFH) exhibit visual defects and structural changes in the retina. However, it is not known whether this phenotype is age-related or is the consequence of disturbed development. To address this question we investigated the effect of Cfh gene deletion on the retinal phenotype of young and mid-age mice. Cfh
−/− mouse eyes exhibited thickening of the retina and reduced nuclear density, but relatively normal scotopic and photopic electroretinograms. At 12 months there was evidence of subtle astroglial activation in the Cfh
−/− eyes, and significant elevation of the complement regulator, decay-accelerating factor (DAF) in Müller cells. In the retinal pigment epithelium (RPE) of young control and Cfh
−/− animals mitochondria and melanosomes were oriented basally and apically respectively, whereas the apical positioning of melanosomes was significantly perturbed in the mid-age Cfh
−/− RPE. We conclude that deletion of Cfh in the mouse leads to defects in the retina that precede any marked loss of visual function, but which become progressively more marked as the animals age. These observations are consistent with a lifelong role for CFH in retinal homeostasis. 相似文献
7.
8.
9.
Impacts of a native parasitic plant on an introduced and a native host species: implications for the control of an invasive weed 总被引:1,自引:0,他引:1
Background and Aims: While invasive species may escape from natural enemies in thenew range, the establishment of novel biotic interactions withspecies native to the invaded range can determine their success.Biological control of plant populations can be achieved by manipulationof a species' enemies in the invaded range. Interactions weretherefore investigated between a native parasitic plant andan invasive legume in Mediterranean-type woodlands of SouthAustralia. Methods: The effects of the native stem parasite, Cassytha pubescens,on the introduced host, Cytisus scoparius, and a co-occurringnative host, Leptospermum myrsinoides, were compared. The hypothesisthat the parasitic plant would have a greater impact on theintroduced host than the native host was tested. In a fieldstudy, photosynthesis, growth and survival of hosts and parasitewere examined. Key Results: As predicted, Cassytha had greater impacts on the introducedhost than the native host. Dead Cytisus were associated withdense Cassytha infections but mortality of Leptospermum wasnot correlated with parasite infection. Cassytha infection reducedthe photosynthetic rates of both hosts. Infected Cytisus showedslower recovery of photosystem II efficiency, lower transpirationrates and reduced photosynthetic biomass in comparison withuninfected plants. Parasite photosynthetic rates and growthrates were higher when growing on the introduced host Cytisus,than on Leptospermum. Conclusions: Infection by a native parasitic plant had strong negative effectson the physiology and above-ground biomass allocation of anintroduced species and was correlated with increased plant mortality.The greater impact of the parasite on the introduced host maybe due to either the greater resources that this host providesor increased resistance to infection by the native host. Thisdisparity of effects between introduced host and native hostindicates the potential for Cassytha to be exploited as a controltool. 相似文献
10.