全文获取类型
收费全文 | 22204篇 |
免费 | 1133篇 |
国内免费 | 995篇 |
专业分类
24332篇 |
出版年
2023年 | 355篇 |
2022年 | 503篇 |
2021年 | 590篇 |
2020年 | 682篇 |
2019年 | 876篇 |
2018年 | 706篇 |
2017年 | 491篇 |
2016年 | 490篇 |
2015年 | 569篇 |
2014年 | 1158篇 |
2013年 | 1544篇 |
2012年 | 832篇 |
2011年 | 1228篇 |
2010年 | 832篇 |
2009年 | 999篇 |
2008年 | 1044篇 |
2007年 | 1084篇 |
2006年 | 922篇 |
2005年 | 844篇 |
2004年 | 733篇 |
2003年 | 616篇 |
2002年 | 536篇 |
2001年 | 372篇 |
2000年 | 273篇 |
1999年 | 319篇 |
1998年 | 331篇 |
1997年 | 280篇 |
1996年 | 297篇 |
1995年 | 239篇 |
1994年 | 238篇 |
1993年 | 223篇 |
1992年 | 217篇 |
1991年 | 192篇 |
1990年 | 195篇 |
1989年 | 160篇 |
1988年 | 185篇 |
1987年 | 161篇 |
1986年 | 174篇 |
1985年 | 248篇 |
1984年 | 365篇 |
1983年 | 265篇 |
1982年 | 297篇 |
1981年 | 239篇 |
1980年 | 259篇 |
1979年 | 239篇 |
1978年 | 196篇 |
1977年 | 152篇 |
1976年 | 139篇 |
1975年 | 116篇 |
1974年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Summary DNA sequencing has revealed an internal, tandemly repetitive structure in the family of giant polypeptides encoded by three types of Balbiani ring (BR) genes, in three different species ofChironomus. Each major BR repeat can be subdivided into two halves: a region consisting of short subrepeats and a more constant region that lacks obvious subrepeats. Comparative predictions of secondary structure indicate that an -helical segment is consistently present in the amino-terminal half of the constant region in all known BR proteins. Comparative predictions, coupled with consideration of the known phosphorylation of serine and threonine residues in BR proteins, suggest that the -helical structure may also extend into the carboxy-terminal half of the constant region, possibly interrupted by -turn(s). However, it is also possible that the structure is variable, and that a -strand is present in that half in some cases. All of the constant regions conserve one methionine and one phenylalanine residue, as well as all four cysteines; these residues presumably play roles in the packing or cross-linking of aligned constant regions. The structure of the subrepeat region is not clear, but the prevalence of a tripeptide pattern (basic-proline-acidic) suggests some type of structural regularity, possibly an extended helix. The possible significance of these conserved molecular features is discussed in the context of how they may serve the elasticity, insolubility, and hydrophilicity of the fibrils and threads formed by the BR polypeptides. 相似文献
32.
Triphosphoinositide breakdown and dense body release as the earliest events in thrombin-induced activation of human platelets 总被引:2,自引:0,他引:2
F Rendu P Marche J Maclouf A Girard S Levy-Toledano 《Biochemical and biophysical research communications》1983,116(2):513-519
The activation by thrombin of human platelets prelabelled with 32P induced a 30-40% decrease in 32P-triphosphoinositides (TPI) in the first 10 sec; the decrease in the other 32P-labelled phosphoinositides occurred by 20-30 sec. At 10 sec., the intensity of these effects was maximum with 0.2-0.4 U/ml thrombin. Under these conditions, 53, 20 and 15% of the dense granule, alpha-granule and lysosome constituents, respectively were released and thromboxane B2 synthesis reached only 10% of its maximum. Together with experiments carried out with chlorpromazine - or PGE1 - treated platelets, our results suggest the existence of a close relationship between TPI-breakdown and dense body release which appear to be the earliest events resulting from the activation of human platelets by thrombin. 相似文献
33.
The nature of the stable noncovalent dimers of band 3 protein from erythrocyte membranes in solutions of Triton X-100 总被引:2,自引:0,他引:2
Stable noncovalent dimers of band 3 protein from human erythrocyte membranes, in which state the protein is thought to exist after solubilization by the nonionic detergent Triton X-100, do not occur when purified batches of the detergent are used. Instead, the protein is in a monomer/dimer/tetramer association equilibrium. The stable dimers do appear, however, when the detergent has been 'aged'. They thus seem to be artifacts. 相似文献
34.
Antonio G. Gonzalez Jaime Bermejo Barrera Francisco J. Tolfdo Marante 《Phytochemistry》1983,22(4):1049-1050
The fruit bodies of the Basidiomycete Scleroderma polyrhizum have been shown to contain the steroids ergosta-4,6,8(14) 22-tetraen-3-one and 5α,8α-epidoxyergosta-6,22-dien-3β-ol and also palmitic and oleic acids. 相似文献
35.
Mutations induced by repeated EMS treatments were investigated by using mouse L5178Y cells. The frequency of TGr mutations increased linearly with the number of EMS treatments whereas the yield of BrdUrdr mutations showed a curvilinear dose-response curve. The BrdUrdr frequency was roughly proportional to the square of the TGr frequency and the results were compatible with the hypothesis that BrdUrdr cells were induced by two mutational events within a cell. Most of the BrdUrdr colonies isolated after 6 EMS treatments, however, were unstable. When BrdUrdr colonies that had arisen in BrdUrd medium after 2 weeks' incubation were isolated in normal medium, the descendant cells showed a nearly normal level of thymidine incorporation and low plating efficiencies of about 1% in BrdUrd medium. In contrast, after isolation of the same colonies in BrdUrd medium, a low level of thymidine incorporation and high plating efficiencies in BrdUrd medium were observed in the descendant cells.
Reverse selection from BrdUrdr to HATr was accomplished with frequencies of 10−6−10−3 for the descendants grown in BrdUrd medium, and AzaCyd treatment drastically increased the reversion frequency to nearly 10−1. Further re-revertants from HATr to BrdUrdr were also found with frequencies of 10−3−10−2 without treatment. These results indicate that the initial BrdUrdr cells did not result from inactivation of the thymidine-kinase gene but that the mode of gene expression was altered in some way. 相似文献
36.
The 1H nuclear magnetic resonance (nmr) spectra of complexes of soybean ferric leghemoglobin with 3-substituted pyridines and 5-substituted nicotinic acids have been recorded in order to determine the influence of axial ligands on heme electronic structure. The hyperfine shifted resonances of the heme group were assigned by analogy to previous assignments for the pyridine and nicotinic acid complexes of leghemoglobin. The spectra are characteristic of predominantly low-spin ferric heme complexes. For the pyridine complexes, the rate of ligand exchange was found to increase with decreasing ligand pKA. For many of the complexes, optical and nmr spectra reveal the presence of an equilibrium mixture of high- and low-spin states of the iron atom. The percentage of high-spin component increases with decreasing ligand pKA Smaller hyperfine shifts are noted for leghemoglobin complexes with ligands capable of weak ligand → metal π bonding. The pattern of hyperfine shifted resonances is similar for all complexes studied and indicates that the overall heme electronic structure is dominated by the bonding to the proximal histidine. 相似文献
37.
Cristina Rinaldi-Garaci Enrico Garaci Vera Del Gobbo Cartesio Favalli Teresa Jezzi Allan L. Goldstein 《Cellular immunology》1983,80(1):57-65
The effects of thymosin-α1 on the stimulation of specific release of prostaglandin E2 (PGE2) from splenic lymphocytes and thymocytes were studied. Experiments were also performed to study in parallel the absolute levels of thymosin-α1 in the blood and the induction of serum FTS activity and of azathioprine sensitivity of spleen cells from adult thymectomized (ATx) mice. A significant difference in the release of PGE2 between normal splenocytes and splenocytes from ATx mice was observed. Thymosin-α1 at certain concentrations was able to stimulate PGE2 release from lymphocytes of ATx mice while inhibiting release in lymphocytes of normal mice. Also, thymocytes were stimulated to release PGE2 after incubation with α1 in a manner similar to that seen in spleen cells of ATx mice. Approximately the same concentration of α1 was found to also correct the low azathioprine sensitivity of splenocytes from ATx mice. Determinations of FTS-like activity in the blood and the pharmacokinetics of α1 after administration of this synthetic molecule show a clear dissociation. A maximum peak of α1 activity was obtained after 1 hr, while maximal FTS-like activity was observed after 24 hr. The inhibition of the induction by α1 of FTS-like activity and of Thy 1.2 antigen by indomethacin suggests that the action of α1 requires prostaglandin biosynthesis. 相似文献
38.
The antioxidant capacity of 5-hydroxy-tryptophan. 5-hydroxy-indole. and DOPA (3,4-dihydroxy-phenyI-alanine) was tested in the Fe-induced lipid peroxidation of liver microsomes of normal- and vitamin E-deficient rats, using ascorbate as a reductant. Lipid peroxidation was monitored as low-level chemilu-minescence, indicative of generation of electronically-excited states arising from the recombination of secondary lipid peroxyl radicals. 相似文献
39.
In view of the likely production of monohydroxyeicosatetraenoic acid (HETE's) in bronchial asthma, the role of these lipoxygenase products in the development of a classical clinical element of airway disease, namely airway hyperreactivity, has been investigated. Tracheas removed from guinea-pigs actively sensitized to ovalbumin produced, upon antigenic challenge (0.01 μg/ml), a 17-fold increase (0.97 ± 0.34 ng/ml to 16.73 ± 1.58 ng/ml) in the amount of 5-hydroxyeicosatetraenoic acid (5-HETE) as measured by radioimmunoassay of the tissue-bath fluid, indicating that this tissue is capable of producing 5-HETE. While 5-HETE alone, at concentrations equal to or greater than those found during the above antigenic response (0.001 to 1.0 μM), failed to produce intrinsic contractions of normal, nonsensitized guinea-pig trachea, a 30 min pretreatment with 5-HETE (1.0 μM) enhanced subsequent LTD4-induced contractions. Pretreatment with either 12- or 15-HETE, at similar concentrations and conditions, failed to potentiate LTD4 concentration-response curves. The effect of 5-HETE was time-dependent, since pretreatment for either 15 or 60 min had little or no effect on subsequent LTD4 responses. Also, the 5-HETE-induced enhancement seemed specific fot LTD4, since contractions to LTC4 (in the presence of l-serine borate), acetylcholine, histamine, PGD2 or U-46619 were unaffected by 5-HETE. Therefore, 5-HETE may have a role in the development of airway hyperreactivity by interacting with released LTD4 to exacerbate airway smooth muscle contraction in asthma. 相似文献
40.
James M. May 《The Journal of membrane biology》1989,108(3):227-233
Summary The sulfhydryl reagent 5, 5-dithiobis (2-nitrobenzoic acid) (DTNB) was used to study the functional role of an exofacial sulfhydryl group on the human erythrocyte hexose carrier. Above 1mm DTNB rapidly inhibited erythrocyte 3-O-methylglucose influx, but only to about half of control rates. Efflux was also inhibited, but to a lesser extent. Uptake inhibition was completely reversed by incubation and washing with 10mm cysteine, whereas it was only partially reduced by washing in buffer alone, suggesting both covalent and noncovalent interactions. The covalent thiol-reversible reaction of DTNB occurred on the exofacial carrier, since (i) penetration of DTNB into cells was minimal, (ii) blockade of potential uptake via the anion transporter did not affect DTNB-induced hexose transport inhibition, and (iii) DTNB protected from transport inhibition by the impermeant sulfhydryl reagent glutathione-maleimide-I. Maltose at 120mm accelerated the covalent transport inhibition induced by DTNB, whereas 6.5 m cytochalasin B had the opposite effect, indicating under the one-site carrier model that the reactive sulfhydryl is on the outward-facing carrier but not in the substrate-binding site. In contrast to glutathione-maleimide-I, however, DTNB did not restrict the ability of the carrier to reorient inwardly, since it did not affect equilibrium cytochalasin B binding. Thus, carrier conformation determines exposure of the exofacial carrier sulfydryl, but reaction of this group may not always lock the carrier in an outward-facing conformation. 相似文献