全文获取类型
收费全文 | 15638篇 |
免费 | 855篇 |
国内免费 | 891篇 |
专业分类
17384篇 |
出版年
2023年 | 152篇 |
2022年 | 248篇 |
2021年 | 286篇 |
2020年 | 311篇 |
2019年 | 390篇 |
2018年 | 414篇 |
2017年 | 341篇 |
2016年 | 355篇 |
2015年 | 398篇 |
2014年 | 872篇 |
2013年 | 1055篇 |
2012年 | 772篇 |
2011年 | 917篇 |
2010年 | 678篇 |
2009年 | 749篇 |
2008年 | 856篇 |
2007年 | 870篇 |
2006年 | 713篇 |
2005年 | 672篇 |
2004年 | 590篇 |
2003年 | 600篇 |
2002年 | 499篇 |
2001年 | 337篇 |
2000年 | 331篇 |
1999年 | 353篇 |
1998年 | 358篇 |
1997年 | 294篇 |
1996年 | 259篇 |
1995年 | 262篇 |
1994年 | 248篇 |
1993年 | 221篇 |
1992年 | 193篇 |
1991年 | 167篇 |
1990年 | 134篇 |
1989年 | 149篇 |
1988年 | 96篇 |
1987年 | 109篇 |
1986年 | 81篇 |
1985年 | 115篇 |
1984年 | 161篇 |
1983年 | 108篇 |
1982年 | 107篇 |
1981年 | 114篇 |
1980年 | 80篇 |
1979年 | 78篇 |
1978年 | 65篇 |
1977年 | 36篇 |
1976年 | 46篇 |
1975年 | 31篇 |
1974年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Neurotransmitter receptors and ion channels play a critical role in the transduction of signals at chemical synapses. The modulation of neurotransmitter receptor and ion channel function by protein phosphorylation is one of the major regulatory mechanisms in the control of synaptic transmission. The nicotinic acetylcholine receptor (nAcChR) has provided an excellent model system in which to study the modulation of neurotransmitter receptors and ion channels by protein phosphorylation since the structure and function of this receptor have been so extensively characterized. In this article, the structure of the nAcChR from the electric organ of electric fish, skeletal muscle, and the central and peripheral nervous system will be briefly reviewed. Emphasis will be placed on the regulation of the phosphorylation of nAcChR by second messengers and by neurotransmitters and hormones. In addition, recent studies on the functional modulation of nicotinic receptors by protein phosphorylation will be reviewed. 相似文献
22.
Crepis dinarica andC. froelichiana are two closely related species of theC. praemorsa complex. Even though they exhibit the same chromosome number (2n = 8) and similar idiogram shape, they differ widely in quantity and distribution of heterochromatin bands. The hybrids between these two species comprise three morphological types. Parental genomes were distinguished in hybrids by Giemsa differential staining (C-banding). Although meiosis presents only a few abnormalities (about 2.4%), the percentage of aborted pollen grains is very high (90%). 相似文献
23.
Summary Non-symbiotic heterotrophic N2 fixation in coniferous bark litter was investigated with the acetylene reduction assay under aerobic and anaerobic conditions. The litter studied was composed essentially of bark, of pH 5 and a C/N ratio of 101; the ratio of available C to available N, which governs N2 fixation, was considerably higher. The rate of N2 fixation was estimated as 2.5–4.4 g N. g–1 dry wt. day–1. Nitrogenase activity was still evident after seven months of incubation under aerobic conditions. The N2-ase activity was O2 dependent: under anaerobic conditions no N2-ase activity was found unless a fermentable C source was added. The importance of N2 fixation in N-poor litter for the maintenance of soil fertility is emphasized. 相似文献
24.
Root turnover and production by14C dilution: implications of carbon partitioning in plants 总被引:1,自引:1,他引:0
D. G. Milchunas W. K. Lauenroth J. S. Singh C. V. Cole H. W. Hunt 《Plant and Soil》1985,88(3):353-365
Summary Estimates of belowground net primary production (BNP) obtained by using traditional soil core harvest data are subject to a variety of potentially serious errors. In a controlled growth chamber experiment, we examined the aboveground-belowground, labile to structural tissue, and plant to soil dynamics of carbon to formulate a14C dilution technique for potential successful application in the field and to quantify sources of error in production estimates.Despite the fact that the majority of net14C movement between above- and belowground plant parts occurred between the initial labeling and day 5, significant quantities of14C were incorporated into cell-wall tissue throughout the growing period. The rate of this increase at late sampling dates was greater for roots than for shoots. Total loss of assimilated14C was 47% in wheat and 28% in blue grama. Exudation and sloughing in wheat and blue grama, respectively, was 15 and 6% of total uptake and 22 and 8% of total plant production.When root production estimates by14C dilution were corrected for the quantities of labile14C incorporated into structural carbon between two sampling dates, good agreement with actual production was found. The error associated with these estimates was ±2% compared with a range of –119 to –57% for the uncorrected estimates. Our results suggest that this technique has potential field application if sampling is performed the year after labelling.Sources of errors in harvest versus14C dilution estimates of BNP are discussed. 相似文献
25.
26.
Bernhard Knig Patricia A. DiNitto Peter M. Blumberg 《Journal of cellular biochemistry》1985,29(1):37-44
The major phorbol ester receptor is the Ca++-activated, phospholipid-dependent protein kinase C. Diacylglycerol stimulates protein kinase C in a fashion similar to the phorbol esters. Likewise, it inhibits phorbol ester binding competitively. Both results suggest that diacylglycerol is the/an endogenous phorbol ester analogue. Alternatively, the diacylglycerol might simply be acting to modify the phospholipid environment of the protein. If diacylglycerol were indeed functioning as an analogue, it should interact with the receptor stoichiometrically. This interaction can be quantitated by measuring the perturbation in apparent diacylglycerol binding affinity as a function of the ratio of diacylglycerol to receptor. We report here that 1,2-dioleoylglycerol interacts with the receptor with the predicted stoichiometry. 相似文献
27.
We have investigated the breakdown of membrane-bound phosphatidylinositol (PI) in homogenates of soybean (Glycine max) callus. The breakdown of PI was stimulated by the detergent deoxycholate. At pH 7.0 and 1·gl-1 of deoxycholate the loss of PI was rapid and extensive: more than 80% was broken down within 10 min. The breakdown of PI was also stimulated by millimolar concentrations of Ca2+. The products of breakdown of added PI (purified from soybean callus) in this system were identified from their chromatographic mobilities as 1,2-diacylglycerol, myo-inositol 1-phosphate and myo-inositol 1:2-cyclic monophosphate.Abbreviations DOC
deoxycholate
- EDTA
ethylenedi-aminetetraacetic acid,-acetate
- Pi
Inorganic phosphate
- PI
phosphatidylinositol
- PS
phosphatidylserine
- TLC
thinlayer chromatography 相似文献
28.
G. Stauder H. Riesemann W.M. Joester K.E. Joester 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,741(3)
A third DNA polymerase ‘C’ with low molecular weight was isolated and purified 3700-fold from ground hyphae of Neurospora crassa WT 74 A, which shows similarities to β- and γ-polymerases from higher eukaryotes: preference for poly(rA)(dT) as a template/primer, inhibition by p-chloromercuribenzoate, resistance against N-ethylmaleimide up to 10 mmol/l, and molecular weight of about 40 000. This polymerase elutes as a distinct peak from DEAE-cellulose at 0.60 mol/l KCl and has an optimum for K+ at 2–20 mmol/l, for Mn2+ at 0.8 mmol/l, for Mg2+ at 4.0 mmol/l, the pH optimum is 8.0. Its Km is 1.5 μmol/l using dTTP as substrate. The enzyme activity described here is free of endonuclease but contains detectable amounts of exonuclease. 相似文献
29.
Pierre Broquet Hélèna Baubichon-Cortay Pascal George Marie-Jo Peschard Pierre Louisot 《Journal of neurochemistry》1990,54(2):388-394
The tricyclic antidepressant desipramine, when added to culture medium, gave rise in C6 rat glioma cells to a decrease of the activity of the enzyme asialofetuin sialyltransferase. The inhibition was dose and time dependent and was observed in both multiplying cells and cells blocked with 2 mM thymidine or depletion of amino acids. This inhibition was rather specific to the sialyltransferase, as under the conditions where this enzyme was inhibited up to 70%, other enzymes such as dolichol phosphate mannose synthetase, glutamine synthetase, and glycerol phosphate dehydrogenase remained unaffected. This inhibition was not reversed after removal of desipramine from the medium and was not observed by direct addition of desipramine to the sialyltransferase incubation assay. Under the same conditions, W-7 [N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide], which is known to be a potent calmodulin antagonist and an inhibitor of calmodulin-dependent kinases, gave the same concentration-dependent inhibition profile of sialyltransferase as desipramine, whereas H-7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine], which is an inhibitor of protein kinase C and cyclic nucleotide-dependent kinases, had no effect. So, it is suggested that desipramine inhibits the sialyltransferase activity in C6 glioma cells through a calmodulin-dependent system. 相似文献
30.
A. C. Passaquin G. Coupin W. A. Schreier P. Poindron R. A. Cole J. de Vellis 《Neurochemical research》1989,14(10):987-993
We investigated the effect of rat interferon-/ (IFN) on the expression of glycerol phosphate dehydrogenase (E.C.1.1.1.8; GPDH), in both C6 cells and pure cultures of oligodendrocytes. IFNs are naturally produced inhibitors of cell growth that can also affect differentiated cell functions. GPDH is a biochemical marker for oligodendrocytes and is known to be developmentally regulated and steroid inducible. GPDH activity is induced by hydrocortisone (HC) 3.5 fold in C6 cells and 5 fold in oligodendrocytes compared to untreated cultures. A pretreatment of these cells with 75 U/ml of rat IFN-/ resulted in an inhibition of the HC induction of GPDH enzymatic activity by 50% and 40% in C6 cells and oligodendrocytes respectively. We also found that IFN impaired the accumulation of GPDH mRNA in both cell types. These results demonstrate that IFNs are capable of modifying the cellular response to hormones in cells of neuroepithelial origin, and suggest the possibility that IFNs may be able to influence the development and function of the brain.Special issue dedicated to Dr. Paola S. Timiras 相似文献