全文获取类型
收费全文 | 7762篇 |
免费 | 796篇 |
国内免费 | 872篇 |
出版年
2024年 | 21篇 |
2023年 | 118篇 |
2022年 | 196篇 |
2021年 | 428篇 |
2020年 | 350篇 |
2019年 | 393篇 |
2018年 | 367篇 |
2017年 | 256篇 |
2016年 | 390篇 |
2015年 | 539篇 |
2014年 | 599篇 |
2013年 | 601篇 |
2012年 | 706篇 |
2011年 | 654篇 |
2010年 | 418篇 |
2009年 | 351篇 |
2008年 | 423篇 |
2007年 | 434篇 |
2006年 | 366篇 |
2005年 | 284篇 |
2004年 | 240篇 |
2003年 | 241篇 |
2002年 | 216篇 |
2001年 | 125篇 |
2000年 | 84篇 |
1999年 | 82篇 |
1998年 | 82篇 |
1997年 | 48篇 |
1996年 | 41篇 |
1995年 | 27篇 |
1994年 | 30篇 |
1993年 | 33篇 |
1992年 | 34篇 |
1991年 | 35篇 |
1990年 | 17篇 |
1989年 | 31篇 |
1988年 | 22篇 |
1987年 | 21篇 |
1986年 | 15篇 |
1985年 | 13篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 12篇 |
1978年 | 5篇 |
1976年 | 7篇 |
1974年 | 8篇 |
1968年 | 4篇 |
排序方式: 共有9430条查询结果,搜索用时 15 毫秒
21.
The effects of a single does of LiCl (2.5 or 10 mEq/kg) on brain inositol and inositol-1-phosphate (Ins1P), intermediates of brain phosphoinositude (PI) turnover, were determinated in male Han: Wistar rats. There was a remarkable, 36–58 fold elevation of brain Li+ as the single does of LiCl was increased 4-fold. Moreover, the accumulation of brain lithium was slow during repeated administration of LiCl. Brain lithium did not correlate with changes in brain PI turnover either after a single or repeated doses. Thus, after a single does of LiCl the increases in brain Ins1P were much less than the decreases in brain inositol. Also, brain inositol was significantly decreased only with the high dose of LiCl whereas brain Ins1P accumulation was more prominent with the lower dose. Moreover, repeated daily doses of LiCl only transiently increased brain Ins1P at 1 and 7 d whereas inositol remained at control levels throughout the 14 d observation period. Lithium probably caused the transient decrease in brain inositol by inhibiting several enzymes, in addition to the inhibition of myo-inositol mono-phosphates, in the PI cycle. Moreover, a slow dampening down of PI turnover by lithium, possible via an inhibitory action on G-protein-coupling, may also explain the present findings. 相似文献
22.
David D. McPherson Chun-Tao Che Geoffrey A. Cordell D. Doel Soejarto John M. Pezzuto Harry H.S. Fong 《Phytochemistry》1985,25(1):167-170
Three new furanoditerpenoids of the caesalpin-type have been isolated from the roots of Caesalpinia pulcherrima. The structures of these compounds, vouacapen-5α-ol, 6β-cinnamoyl-7β-hydroxy-vouacapen-5α-ol and 8,9,11,14-didehydrovouacapen-5α-ol, were elucidated through interpretation of their spectral data. Sitosterol was also obtained. 相似文献
23.
24.
S L Hauser A K Bahn M Che F Gilles H L Weiner 《Journal of immunology (Baltimore, Md. : 1950)》1984,133(6):3037-3042
Experimental allergic encephalomyelitis (EAE) was induced in SJL/J mice by using two injections of spinal cord homogenate in incomplete Freund's adjuvant supplemented with mycobacteria. Analysis of circulating Lyt-bearing subsets by indirect immunofluorescence during the course of acute EAE revealed the following: 1) during the pre-clinical phase of EAE (1 to 2 days before the onset of paralysis), there was a decrease in the percentage of Lyt-1- but not of Lyt-2-bearing cells in peripheral blood, and of both Lyt-1- and Lyt-2-bearing cells in spleen; 2) with the onset of clinically evident EAE, there was a decrease in both Lyt-1 and Lyt-2 cells in peripheral blood and an increase in the percentage of Lyt-1-bearing cells in pooled inguinal and axillary lymph node; and 3) after these early changes, there was a rapid reconstitution of the percentages of total Lyt-bearing cells and of both Lyt-1- and Lyt-2-bearing cells in peripheral blood. Immunohistochemical analysis of the central nervous system infiltrate revealed that the earliest lesions consisted predominantly of Lyt-1 T lymphocytes, with few Lyt-2 cells present. These results demonstrate that the influx of cells of the Lyt-1 inducer subset to the central nervous system in acute EAE is accompanied by a transient decrease in Lyt-1 cells in peripheral blood. 相似文献
25.
P Goliński S Wnuk J Che?kowski A Visconti M Schollenberger 《Applied and environmental microbiology》1986,51(4):743-745
A compound very similar to the mycotoxin citrinin was observed on thin-layer chromatographic plates during the screening analysis of grain extracts. This compound was produced by 22 of the tested Fusarium avenaceum (Corda ex Fries) Sacc. strains isolated from wheat, triticale, barley, corn, and potatoes. A chemical test confirmed the presence of an unknown compound, which was given the preliminary name of antibiotic Y (indicating yellow fluorescence). The following properties of the new metabolite are described: spectroscopic (UV, infrared, proton nuclear magnetic resonance, fluorescence, and mass spectrometry), phytotoxic, antibiotic (inhibitory effect of bacterial growth), and toxic (toxicity to Artemia salina, chicken embryos, and mouse fibroblasts). Elemental analysis of the compound showed that it had the general formula C15H10O8, in agreement with the mass spectrometric finding that the molecular ion had a molecular weight of 318. The structure of the compound is presently under study. 相似文献
26.
Molecular cloning of a human gene that regulates chromosome condensation and is essential for cell proliferation. 总被引:19,自引:3,他引:16 下载免费PDF全文
The tsBN2 cell line, a temperature-sensitive (ts) mutant of baby hamster kidney cell line BHK21/13, seems to possess a mutation in the gene that controls initiation of chromosome condensation. At the nonpermissive temperature (39.5 degrees C), the chromatin of tsBN2 cells is prematurely condensed, and the cells die. Using tsBN2 cells as a recipient of DNA-mediated gene transfer, we investigated a human gene that is responsible for regulation of chromosome condensation and cell proliferation. We found that the human gene complementing the tsBN2 mutation resides in the area of the 40- to 50-kilobase HindIII fragment, derived from HeLa cells. Based on this finding, we initiated cloning of a human gene complementing the tsBN2 mutation. From lambda and cosmid libraries carrying partial digests of DNA from the secondary transformants, the 41.8-kilobase HindIII fragment containing the human DNA was isolated. The cloned human DNA was conserved in ts+ transformants through primary and secondary transfections. Two cosmid clones convert the ts- phenotype of tsBN2 cells to ts+ with more than 100 times a higher efficiency, compared with cases of transfection with total human DNA. Thus, the cloned DNA fragments contain an active human gene that complements the tsBN2 mutation. 相似文献
27.
Cytosine residues of poly(C) and heat-denatured calf thymus DNA were transformed into 5,6-dihydrouracil-6-sulfonate (U(SO−3)) residues by treatment with bisulfite. The poly(U(SO−3)2, C3) and poly(U(SO−3)9, C1) prepared did not form inter-base binding with either poly(A) or poly(I) as judged by the absence of hypochromicity in ultraviolet absorbance. U(SO−3) residues in the DNA inactivated it to serve as template for E.coli DNA polymerase I, while the template activity was restored by conversion of the U(SO−3) residues into U. 相似文献
28.
J. G. Salway J. L. Harwood M. Kai G. L. White J. N. Hawthorne 《Journal of neurochemistry》1968,15(3):221-226
—The activities of four enzymes concerned with inositol lipid metabolism have been determined in homogenates of rat brains of different ages. The enzymes are CDP-diglyceride inositol phosphatidate transferase, phosphatidylinositol kinase, diphosphoinositide kinase and triphosphoinositide phosphomonoesterase. The activities of all the enzymes increased with age. Phosphatidylinositol kinase activity rose most sharply well before myelination, reaching a maximum at about 6 days of age. Diphosphoinositide kinase and triphosphoinositide phosphomonoesterase activities increased most rapidly during myelination. The increase in CDP-diglyceride inositol phosphatidate transferase showed no definite association with any period of development. It is concluded that triphosphoinositide metabolism is associated with myelin or a closely related structure. 相似文献
29.
Consequences of herbivory in the mountain birch (Betula pubescens ssp tortuosa): importance of the functional organization of the tree 总被引:3,自引:0,他引:3
Summary Three types of experiments indicate that the functional organization of the mountain birch may influence the ways in which the tree responds to simulated or natural herbivory. The first experiment showed that herbivory to both short and long shoot leaves affects plant development but, because growth largely proceeds by resources of the previous year, is manifested only in the year following the damage. The second experiment showed that even partial damage to a single long shoot leaf caused the axillary bud of that leaf to produce a shorter shoot the next year. Therefore, the value of a leaf depends also on the organ which it is subtending. In the third experiment we manipulated the apical dominance of shoots in ramets and caused improvement to leaf quality in extant shoots. Ramets within a tree responded individually, probably mediated by disturbance of the hormonal control because removal of apical buds elicited the response although removal of the same number of basal buds did not. Induced amelioration is a different response to induced resistance. The two responses are triggered by different cues and may occur in the same plant. By altering hormonal balance of shoots it is potentially possible for herbivores to induce amelioration of food quality. The ways in which herbivory is simulated may explain variability of results obtained when herbivory-induced responses in plants have been studied. 相似文献
30.
Che Fang-Sik; Cho Chol; Hyeon Sung-Be; Isogai Akira; Suzuki Akinori 《Plant & cell physiology》1990,31(1):45-50
The incorporation rate of choline chloride and allylcholinebromide into wheat protoplasts were rapid compared with theincorporation rate of benzylcholine bromide. Choline chloridewas metabolized via two pathways: choline betaine and choline phosphorylcholine phos-phatidylcholine. Allylcholine bromidewas metabolized via only one pathway: allylcholine phosphorylallylcholine phosphatidylallylcholine, and benzylcholine bromide was notmetabolized at all. These results suggest that the stimulationof photosynthesis (Hyeon et al. 1988) by these compounds iscaused directly by these choline analogs and not by their metabolites. (Received June 29, 1989; Accepted October 20, 1989) 相似文献