全文获取类型
收费全文 | 6078篇 |
免费 | 1016篇 |
国内免费 | 3746篇 |
出版年
2024年 | 105篇 |
2023年 | 258篇 |
2022年 | 364篇 |
2021年 | 397篇 |
2020年 | 405篇 |
2019年 | 438篇 |
2018年 | 270篇 |
2017年 | 283篇 |
2016年 | 278篇 |
2015年 | 398篇 |
2014年 | 593篇 |
2013年 | 493篇 |
2012年 | 695篇 |
2011年 | 636篇 |
2010年 | 552篇 |
2009年 | 559篇 |
2008年 | 593篇 |
2007年 | 584篇 |
2006年 | 512篇 |
2005年 | 447篇 |
2004年 | 332篇 |
2003年 | 283篇 |
2002年 | 221篇 |
2001年 | 228篇 |
2000年 | 225篇 |
1999年 | 148篇 |
1998年 | 82篇 |
1997年 | 69篇 |
1996年 | 37篇 |
1995年 | 41篇 |
1994年 | 20篇 |
1993年 | 20篇 |
1992年 | 18篇 |
1991年 | 21篇 |
1990年 | 22篇 |
1989年 | 32篇 |
1988年 | 14篇 |
1987年 | 12篇 |
1986年 | 20篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 22篇 |
1981年 | 8篇 |
1977年 | 6篇 |
1965年 | 6篇 |
1964年 | 5篇 |
1959年 | 3篇 |
1953年 | 5篇 |
1950年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
E Estébanez-Perpi?á A Bayés J Vendrell M A Jongsma D P Bown J A Gatehouse R Huber W Bode F X Avilés D Reverter 《Journal of molecular biology》2001,313(3):629-638
The cotton bollworm Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) is one of the most serious insect pests in Australia, India and China. The larva causes substantial economical losses to legume, fibre, cereal oilseed and vegetable crops. This pest has proven to be difficult to control by conventional means, mainly due to the development of pesticide resistance. We present here the 2.5 A crystal structure from the novel procarboxypeptidase (PCPAHa) found in the gut extracts from H. armigera larvae, the first one reported for an insect. This metalloprotease is synthesized as a zymogen of 46.6 kDa which, upon in vitro activation with Lys-C endoproteinase, yields a pro-segment of 91 residues and an active carboxypeptidase moiety of 318 residues. Both regions show a three-dimensional structure quite similar to the corresponding structures in mammalian digestive carboxypeptidases, the most relevant structural differences being located in the loops between conserved secondary structure elements, including the primary activation site. This activation site contains the motif (Ala)(5)Lys at the C terminus of the helix connecting the pro- and the carboxypeptidase domains. A remarkable feature of PCPAHa is the occurrence of the same (Ala)(6)Lys near the C terminus of the active enzyme. The presence of Ser255 in PCPAHa instead of Ile and Asp found in the pancreatic A and B forms, respectively, enlarges the S1' specificity pocket and influences the substrate preferences of the enzyme. The C-terminal tail of the leech carboxypeptidase inhibitor has been modelled into the PCPAHa active site to explore the substrate preferences and the enzymatic mechanism of this enzyme. 相似文献
2.
3.
急性脑梗死约占全部脑卒中的70%,病死率和致残率高,且极易复发。但目前针对急性脑梗死在时间窗内溶栓、抗凝等治疗手段不能从根本上切实有效地修复受损脑组织,且伴有出血等风险。寻找脑梗死形成发展的原因并予以治疗迫在眉睫。酸中毒是引起缺血性脑损伤的重要机制。大量实验研究表明,酸中毒能加重神经元的缺血性损伤,且其梗死面积与酸中毒的程度直接相关。但缺血产生的酸中毒如何引起神经元损伤的确切机制尚不明确。最近研究发现酸中毒能激活一种在中枢及周围神经中广泛存在的膜通道,即酸敏感离子通道,它对Ca2+通透,能引起细胞内Ca2+超载,同时能激活胞内酶引起细胞内蛋白质、脂类及核酸的降解,加重缺血后脑损伤。本文就酸敏感离子通道1a与脑梗死做一综述。 相似文献
4.
Maarten Jongsma Maarten Koornneef Pim Zabel Jacques Hille 《Plant molecular biology》1987,8(5):383-394
Tomato protoplasts have been transformed with plasmid DNA's, containing a chimeric kanamycin resistance gene and putative tomato origins of replication. A calcium phosphate-DNA mediated transformation procedure was employed in combination with either polyethylene glycol or polyvinyl alcohol. There were no indications that the tomato DNA inserts conferred autonomous replication on the plasmids. Instead, Southern blot hybridization analysis of seven kanamycin resistant calli revealed the presence of at least one kanamycin resistance locus per transformant integrated in the tomato nuclear DNA. Generally one to three truncated plasmid copies were found integrated into the tomato nuclear DNA, often physically linked to each other. For one transformant we have been able to use the bacterial ampicillin resistance marker of the vector plasmid pUC9 to rescue a recombinant plasmid from the tomato genome. Analysis of the foreign sequences included in the rescued plasmid showed that integration had occurred in a non-repetitive DNA region. Calf-thymus DNA, used as a carrier in transformation procedure, was found to be covalently linked to plasmid DNA sequences in the genomic DNA of one transformant. A model is presented describing the fate of exogenously added DNA during the transformation of a plant cell. The results are discussed in reference to the possibility of isolating DNA sequences responsible for autonomous replication in tomato. 相似文献
5.
黑龙江省三江平原丹顶鹤的数量分布 总被引:2,自引:0,他引:2
在地面调查的基础上,我们使用Y-11轻型飞机对黑龙江省三江平原地区的丹顶鹤的数量分布近行了调查,调查时飞行高度80米,航速140公里/小时,续航里程共3748公里。调查结果表明,丹顶鹤在三江平原主要分布在8个地区,其中嘟噜河下游、洪河自然保护区、七星河流域和兴凯湖低地是主要繁殖地,总数量共309只。 相似文献
6.
在整理青海省蚤类标本中,发现长刺双蚤 Amphipsylla longispina Scalon, 1950一新亚种,记述如下。 长刺双蚤共和亚种 Amphipsylla longis pina gongheensis新亚种 鉴别特征 新亚种可动突前缘较直,不动突较细长,阳茎钩突端部似鸟头状; 相似文献
7.
Structural relatedness of lysis proteins from colicinogenic plasmids and icosahedral coliphages 总被引:4,自引:0,他引:4
The host-lysis-inducing functions of phi X174 protein E and MS2 protein L
were recently shown to reside on the N-terminal and C-terminal halves of
the two respective lysis proteins. In the present study it is shown that
the small lysis proteins encoded in various colicinogenic plasmids share
local sequence similarities and certain structural characteristics with the
essential peptides of their coliphage-coded counterparts. Despite their
dissimilar sizes and origins, it is suggested that the colicinogenic lysis
proteins are functionally analogous and evolutionarily related to those of
icosahedral single- stranded DNA and RNA phages.
相似文献
8.
本实验在14只麻醉开胸狗心脏上观察了氟碳乳剂与右旋糖酐稀释血液对心肌耗氧量与供应缺血心肌氧量关系的影响。以左室压力-时间指数(SPTI)作为心肌耗氧量的指标,根据冠脉有效侧支血流量(ECF)、PaO_2和 Hb 浓度计算供应缺血心肌的氧量。实验结果表明,低分子右旋糖酐稀释血液后,SPTI 暂时性轻度增加(稀释后30min 时较对照增加7.1±2.7%,P<0.05,稀释后60min 时增加2.8±1.2%,P>0.05),ECF 明显增多(稀释后30min 时较对照增加58.5±6.1%,P<0.01),缺血区边缘心肌氧供需关系未发生明显变化。氟碳乳剂稀释血液后,SPTI 的变化规律与右旋糖酐稀释后相同(稀释后30min 和60min 时分别较对照增加2.5±0.7%和1.9±0.8%)ECF 和 PaO_2升高(稀释后30min 时分别较对照增加53.9±6.7%和93±8.9%),供应缺血心肌的氧量显著增加,缺血区边缘心肌氧供需矛盾明显改善。 相似文献
9.
10.
Alloxan-induced diabetes mellitus in pregnant sheep and chronic fetal catheterization 总被引:1,自引:0,他引:1
25 female sheep of the Texel breed were made hyperglycaemic by administration of alloxan monohydrate (ALX) in early pregnancy and 15 ewes served as controls. Average venous glucose levels (mean +/- standard deviation) increased from 3.5 +/- 0.2 to 14.0 +/- 1.8 mmol/l. All hyperglycaemic sheep were treated with long-acting insulin in doses adjusted individually (0.2-1.0 U/kg per day) to keep glucose levels above 8 mmol/l. After a temporary significant increase, maternal venous concentrations of urea and creatinine returned to normal levels. One sheep died on day 6 after administration of ALX. Another hyperglycaemic sheep died at induction of anaesthesia. Eight hyperglycaemic ewes aborted between days 90 and 128 of gestation. Between days 103 and 135 of gestation the remaining hyperglycaemic (n = 15) and control (n = 15) ewes were operated upon and the fetuses were provided with EEG, nuchal EMG and ECG electrodes and catheters in the trachea, amniotic fluid, jugular vein and carotid artery. Use of the chronic sheep preparation for the study of diabetes mellitus and fetal reactions was successful in 10 out of 25 cases, as in the diabetic group postoperative intra-uterine fetal survival varied between 2 and 19 days and in 10 cases was at least 5 days. Postoperative intrauterine fetal survival in the controls was significantly longer and varied between 4 and 28 days, and in 13 cases was at least 5 days. A highly significant correlation (P less than 10-6) between maternal and fetal blood glucose levels was seen.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献