全文获取类型
收费全文 | 10473篇 |
免费 | 282篇 |
国内免费 | 475篇 |
专业分类
系统科学 | 382篇 |
丛书文集 | 327篇 |
教育与普及 | 449篇 |
理论与方法论 | 29篇 |
现状及发展 | 121篇 |
研究方法 | 69篇 |
综合类 | 9850篇 |
自然研究 | 3篇 |
出版年
2024年 | 48篇 |
2023年 | 127篇 |
2022年 | 178篇 |
2021年 | 218篇 |
2020年 | 136篇 |
2019年 | 45篇 |
2018年 | 104篇 |
2017年 | 105篇 |
2016年 | 107篇 |
2015年 | 232篇 |
2014年 | 336篇 |
2013年 | 282篇 |
2012年 | 355篇 |
2011年 | 382篇 |
2010年 | 374篇 |
2009年 | 460篇 |
2008年 | 460篇 |
2007年 | 462篇 |
2006年 | 384篇 |
2005年 | 329篇 |
2004年 | 323篇 |
2003年 | 441篇 |
2002年 | 628篇 |
2001年 | 600篇 |
2000年 | 425篇 |
1999年 | 537篇 |
1998年 | 447篇 |
1997年 | 414篇 |
1996年 | 351篇 |
1995年 | 345篇 |
1994年 | 297篇 |
1993年 | 248篇 |
1992年 | 204篇 |
1991年 | 183篇 |
1990年 | 175篇 |
1989年 | 160篇 |
1988年 | 137篇 |
1987年 | 83篇 |
1986年 | 45篇 |
1985年 | 20篇 |
1984年 | 8篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1976年 | 3篇 |
1974年 | 3篇 |
1970年 | 3篇 |
1969年 | 2篇 |
1958年 | 2篇 |
1957年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Iron in surface seawater has been demonstrated to be the limiting nutrient factor for primary productivity in certain oceanic regions where other major nutrients are abundant[1—5]. The available Fe to the phytoplankton in seawater is related to the uptake of carbon dioxide through the ocean and, in turn, to the global greenhouse effect[6—11]. Recent reports showed that the available Fe in the seawater is closely correlative to nitrogen fixation in the ocean[12—17]. Understanding which Fe s… 相似文献
992.
<Emphasis Type="Italic">In situ</Emphasis> generation and detection of methyl radical by voltammetry
The chemistry of free radicals is one of the advanced subjects of fundamental sciences such as biomedicine, environment protection, new energy sources and new material. The detection technique of free radicals is an essential tool for studying free radicals. Methyl radical is one of the representatives of alkyl radicals and the studies on it attract more attention. The current approaches to obtaining methyl radical are based on pulse radiolysis of organic substances that contain the methyl gro… 相似文献
993.
TANG Zhiwen WANG Kemin TAN Weihong LI Jun LIU Lingfeng GUO Qiuping MENG Xiangxian HUANG Shasheng LI Du & LUO Weifang State Key Laboratory of Chemo/Biosensing Chemometrics Institute of Biological Technology College of Chemistry Chemical Engi-neering Hunan University Changsha China Correspondence should be addressed to Wang Kemin 《科学通报(英文版)》2003,48(12):1215-1218
The replication, ligation and repair of nucleic acids are essential life processes that based the survival, pro-creation and evolution of creatures. These interactions involving proteins (enzymes) and nucleic acids have fas-cinated great interest of scientists[1]. The nucleic acids ligation has been revealed that it is catalyzed by ligase and co-enzyme (ATP or NAD)[2—4]. Along with in-depth re-search, the nucleic acids ligation becomes a significant and common tool employed in bioengineerin… 相似文献
994.
The effect of sodium nitroprusside (SNP), a donor of nitric oxide, on meiotic maturation of mouse oocytes was studied by injecting Nw-nitro-L-arginine methyl ester (L-NAME) intra-peritoneal (ip), a nitric oxide synthase inhibitor, or culturing oocytes in the medium supplemented with L-NAME or hypoxanthine (HX) to arrest the spontaneous oocyte maturation in vitro. The results showed that the inhibitory effect of L-NAME by injecting 10 mg/kg ip on extrusion of the first polar body only could be reversed by injecting 2.5 mg/kg SNP with L-NAME simultaneously (P < 0.05). Half an hour later ten mice died when given 10 mg/kg SNP ip. The treatment of some concentrations of SNP (10–7, 10–6, 10–5mol/L) significantly stimulated meiotic maturation to metaphase Ⅱ stages in cumulus enclosed oocytes in the presence of HX. However, other concentrations of SNP (10–8, 10–4, 10–3 mol/L) had no effect on HX-arrested oocyte meiotic maturation. The optimal concentration of SNP on CEOs had no effect on DOs. The dose of 10–3 mol/LL-NAME demonstrated a significant suppression in formation of PB1, but not in GVBD. This inhibition was reversed by the addition of SNP. These results indicated that the physiological levels of NO produced by cumulus cells could stimulate meiotic maturation of mouse oocytes both in vivo and in vitro. 相似文献
995.
Nicastrin modulates presenilin-mediated notch/glp-1 signal transduction and betaAPP processing 总被引:26,自引:0,他引:26
Yu G Nishimura M Arawaka S Levitan D Zhang L Tandon A Song YQ Rogaeva E Chen F Kawarai T Supala A Levesque L Yu H Yang DS Holmes E Milman P Liang Y Zhang DM Xu DH Sato C Rogaev E Smith M Janus C Zhang Y Aebersold R Farrer LS Sorbi S Bruni A Fraser P St George-Hyslop P 《Nature》2000,407(6800):48-54
Nicastrin, a transmembrane glycoprotein, forms high molecular weight complexes with presenilin 1 and presenilin 2. Suppression of nicastrin expression in Caenorhabditis elegans embryos induces a subset of notch/glp-1 phenotypes similar to those induced by simultaneous null mutations in both presenilin homologues of C. elegans (sel-12 and hop-1). Nicastrin also binds carboxy-terminal derivatives of beta-amyloid precursor protein (betaAPP), and modulates the production of the amyloid beta-peptide (A beta) from these derivatives. Missense mutations in a conserved hydrophilic domain of nicastrin increase A beta42 and A beta40 peptide secretion. Deletions in this domain inhibit A beta production. Nicastrin and presenilins are therefore likely to be functional components of a multimeric complex necessary for the intramembranous proteolysis of proteins such as Notch/GLP-1 and betaAPP. 相似文献
996.
997.
998.
Detection of preinvasive cancer cells 总被引:12,自引:0,他引:12
999.
The tumour-suppressor genes lgl and dlg regulate basal protein targeting in Drosophila neuroblasts 总被引:7,自引:0,他引:7
Drosophila neuroblasts are a model system for studying asymmetric cell division: they divide unequally to produce an apical neuroblast and a basal ganglion mother cell that differ in size, mitotic activity and developmental potential. During neuroblast mitosis, an apical protein complex orients the mitotic spindle and targets determinants of cell fate to the basal cortex, but the mechanism of each process is unknown. Here we show that the tumour-suppressor genes lethal giant larvae (lgl) and discs large (dlg) regulate basal protein targeting, but not apical complex formation or spindle orientation, in both embryonic and larval neuroblasts. Dlg protein is apically enriched and is required for maintaining cortical localization of Lgl protein. Basal protein targeting requires microfilament and myosin function, yet the lgl phenotype is strongly suppressed by reducing levels of myosin II. We conclude that Dlg and Lgl promote, and myosin II inhibits, actomyosin-dependent basal protein targeting in neuroblasts. 相似文献
1000.