全文获取类型
收费全文 | 18377篇 |
免费 | 3047篇 |
国内免费 | 8506篇 |
出版年
2024年 | 338篇 |
2023年 | 995篇 |
2022年 | 1155篇 |
2021年 | 1149篇 |
2020年 | 1041篇 |
2019年 | 1070篇 |
2018年 | 851篇 |
2017年 | 994篇 |
2016年 | 947篇 |
2015年 | 1016篇 |
2014年 | 1346篇 |
2013年 | 1170篇 |
2012年 | 1457篇 |
2011年 | 1605篇 |
2010年 | 1407篇 |
2009年 | 1377篇 |
2008年 | 1544篇 |
2007年 | 1281篇 |
2006年 | 1171篇 |
2005年 | 1075篇 |
2004年 | 1023篇 |
2003年 | 812篇 |
2002年 | 704篇 |
2001年 | 670篇 |
2000年 | 505篇 |
1999年 | 488篇 |
1998年 | 369篇 |
1997年 | 345篇 |
1996年 | 300篇 |
1995年 | 262篇 |
1994年 | 198篇 |
1993年 | 176篇 |
1992年 | 206篇 |
1991年 | 197篇 |
1990年 | 164篇 |
1989年 | 131篇 |
1988年 | 95篇 |
1987年 | 78篇 |
1986年 | 43篇 |
1985年 | 78篇 |
1984年 | 26篇 |
1983年 | 38篇 |
1982年 | 5篇 |
1981年 | 12篇 |
1980年 | 2篇 |
1963年 | 4篇 |
1958年 | 2篇 |
1957年 | 1篇 |
1955年 | 1篇 |
1950年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
以线性表位肽P14(a3127-148)作为抗原建立适合免疫治疗的抗肾小球基底膜(anti-glomerular basement membrane,GBM)病大鼠模型。采用大鼠后脚垫三点注射P14(a3127-148)与弗氏佐剂乳化物的方法进行单次免疫,免疫前后每周采集24 h尿样和血样,所有大鼠在免疫后7 w处死。大鼠免疫后,肾炎模型组在各时间点的24 h尿蛋白、尿蛋白肌酐比值(albumin/urine creatinine ratio,ACR)、血肌酐及血尿素氮均明显高于对照组(P<0.05);循环抗P14(a3127-148)IgG抗体水平明显高于对照组(P<0.001);PAS染色可见节段性纤维素样坏死,富于细胞型新月体;免疫组化染色可见肾小球有明显的中性粒细胞和巨噬细胞浸润;免疫荧光检测可见IgG沿GBM呈强线性沉积;电镜观察到GBM的断裂和收缩;而对照组均未见改变。HE染色在所有大鼠中均未观察到肺部病变。使用P14(a3127-148)线性肽免疫WKY大鼠成功建立了大鼠抗GBM病模型,有助于开发更为特异的免疫疗法。 相似文献
852.
光是影响植物分枝的重要外在环境因素,但光信号因子HY5(ELONGATED HYPOCOTYL5)是否调控植物分枝目前尚不清楚。创制了HY5转基因超表达植株,并获得商业化T-DNA插入突变体纯合植株。通过比较野生型(WT)、超表达植株(HY5-OE)、突变体(hy5-215)的分枝数目发现,与野生型相比,超表达植株分枝数目显著增加,而突变体分枝数目则显著减少。进一步比较这些遗传材料的拟南芥植株分枝的负调控关键因子BRC1(BRANCHED1)转录本水平差异,发现与野生型相比,超表达植株中BRC1转录本显著下调、突变体中显著上调。研究结果表明,HY5通过抑制拟南芥分枝关键负调控因子BRC1的转录水平,进而促进拟南芥的分枝。研究结果为阐明HY5调控分枝的生物学功能提供了一定的理论依据。 相似文献
853.
巴洛沙星的合成研究 总被引:4,自引:0,他引:4
合成新型抗菌药——巴洛沙星。以3-溴吡啶为原料,在催化剂作用下经氨解、加氢还原制备关键中间体3-甲胺基哌啶,并考察加氢过程中催化剂类型和反应温度的影响,再以甲氧环合酯(1-环丙基-6,7-二氟-8-甲氧基-1,4二氢-4-氧代喹啉-3-羧酸乙酯)为起始物,在乙酐中与硼酸螯合,并与3-甲胺基哌啶缩合,最后水解制备巴洛沙星。合成巴洛沙星的总质量收率58.7%,合成3-甲胺基哌啶的质量收率为74.1%。使用乙腈作为缩合的溶剂和后期水解反应的碱性物质,改进合成巴洛沙星的路线,提高了收率;制备3-甲胺基哌啶的加氢反应,在选择Rh/C为催化剂、反应温度为80℃时,质量收率为78%。 相似文献
854.
Xlao-Ling Wang Suo-Lang Gesang Wei Jiao Xun Liao Li-Sheng Ding 《植物学报(英文版)》2007,49(5):609-614
Two new sesqulterpenoid glucosides, namely α-hydroxycostic acid 6-β-D-glucopyranoside (compound 1) and 11 βH-11,13-dlhydrodehydrocostuslactone 8α-O-(6'-acetyl)-β-D-glucopyranoside (compound 2), along with 11 known sesqulterpenoids (compounds 3-13) were isolated from the aerial parts of Saussurea involucrate (Kar. et Kir.) Sch.- BIp. The structures of the new sesquiterpenoid glucosides were established by one- and two-dimensional nuclear magnetic resonance and mass spectrometry analysis. 相似文献
855.
Libo Shan Ping He Jen Sheen 《植物学报(英文版)》2007,49(1):105-111
Plants possess innate Immune systems to prevent most potential Infections. The ancient and conserved innate immune responses are triggered by microbe-associated molecular patterns (MAMPs) and play important roles in broad-spectrum defenses. However, successful bacterial pathogens evolved type Ⅲ virulence effectors to suppress MAMP-medlated immunity. To survive, plants further developed highly specific resistance (R) genes to trigger gene-for-gene-mediated immunity and turn the virulent pathogens into avirulent ones. We summarize here the very recent advances in this dynamic coevolution of plantbacterium interaction. 相似文献
856.
857.
Purification and characterization of cytosolic glyceraldehyde-3-phosphate dehydrogenase from the dromedary camel 总被引:1,自引:1,他引:0
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (EC 1.2.1.12),a key enzyme ofcarbon metabolism,was purified and characterized to homogeneity from skeletal muscle of Camelusdromedarius.The protein was purified approximately 26.8 folds by conventional ammonium sulphatefractionation followed by Blue Sepharose CL-6B chromatography,and its physical and kinetic propertieswere investigated.The native protein is a homotetramer with an apparent molecular weight of approximately146 kDa.Isoelectric focusing analysis showed the presence of only one GAPDH isoform with an isoelectricpoint of 7.2.The optimum pH of the purified enzyme was 7.8.Studies on the effect of temperature onenzyme activity revealed an optimal value of approximately 28-32 ℃ with activation energy of 4.9 kcal/mol.The apparent K_m values for NAD~ and DL-glyceraldehyde-3-phophate were estimated to be 0.025±0.040mM and 0.21±0.08 mM, respectively. The V_(max) of the purified protein was estimated to be 52.7±5.9 U/mg.These kinetic parameter values were different from those described previously, reflecting protein differencesbetween species. 相似文献
858.
859.
β-amyloid peptide (Aβ) is considered to be responsible for the formation of senile plaques,which is the hallmark of Alzheimer's disease (AD).Oxidative stress,manifested by protein oxidation andlipid peroxidation,among other alterations,is a characteristic of AD brain.A growing body of evidence hasbeen presented in support of Aβ_(1-40) forming an oligomeric complex that binds copper at a CuZn superoxidedismutase-like binding site. Aβ_(1-40)Cu(Ⅱ) complexes generate neurotoxic hydrogen peroxide (H_2O_2) from O_2via Cue reduction,though the precise reaction mechanism is unclear.The toxicity of Aβ_(1-40) or the Aβ_(1-40)Cu(Ⅱ)complexes to cultured primary cortical neurons was partially attenuated when ( )-α-tocopherol (vitamin E)as free radical antioxidant was added at a concentration of 100 μM.The data derived from lactate dehydro-genase (LDH) release and the formation of H_2O_2 confirmed the results from the MTT assay.These findingsindicate that copper binding to Aβ_(1-40) can give rise to greater production of H_2O_2, which leads to a break-down in the integrity of the plasma membrane and subsequent neuronal death.Groups treated with vitaminE exhibited much slighter damage,suggesting that vitamin E plays a key role in protecting neuronal cellsfrom dysfunction or death. 相似文献
860.