全文获取类型
收费全文 | 20343篇 |
免费 | 1639篇 |
国内免费 | 1968篇 |
出版年
2024年 | 35篇 |
2023年 | 239篇 |
2022年 | 726篇 |
2021年 | 1205篇 |
2020年 | 821篇 |
2019年 | 958篇 |
2018年 | 907篇 |
2017年 | 640篇 |
2016年 | 889篇 |
2015年 | 1331篇 |
2014年 | 1562篇 |
2013年 | 1610篇 |
2012年 | 1917篇 |
2011年 | 1681篇 |
2010年 | 1019篇 |
2009年 | 920篇 |
2008年 | 1023篇 |
2007年 | 885篇 |
2006年 | 792篇 |
2005年 | 733篇 |
2004年 | 550篇 |
2003年 | 514篇 |
2002年 | 407篇 |
2001年 | 321篇 |
2000年 | 300篇 |
1999年 | 293篇 |
1998年 | 186篇 |
1997年 | 179篇 |
1996年 | 203篇 |
1995年 | 144篇 |
1994年 | 165篇 |
1993年 | 101篇 |
1992年 | 123篇 |
1991年 | 123篇 |
1990年 | 91篇 |
1989年 | 82篇 |
1988年 | 57篇 |
1987年 | 58篇 |
1986年 | 45篇 |
1985年 | 31篇 |
1984年 | 38篇 |
1983年 | 20篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1979年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
眼斑螳属Creobroter Serville,1839隶属于花螳螂亚科Hymenopodinae。迄今已知19种,主要分布于印度、锡金、泰国、菲律宾、斯里兰卡,印度尼西亚和爪哇,我国已记载2种,分布于四川和广东(梅县)。我们在整理本属标本时,发现二新种及一新记录。新种、新记录记述如后,并列出种检索表。 模式标本均保存在中国科学院上海昆虫研究所。 相似文献
32.
MacELISA、RPHI和IFAT用于流行性出血热早期诊断的比较 总被引:2,自引:0,他引:2
比较了IgM捕获ELISA(MacELISA)、反向间接血凝抑制试验(RPHI)和间接免疫荧光抗体试验(IFAT)检测流行性出血热(EHF)病人血清特异性抗体的结果。MacELISA对急性期血清IgM抗体的阳性检出率与RPHI对总抗体的阳性检出率相近,两法都具有较高的敏感性。而IFAT检测IgG抗体的阳性率则较低。总抗体滴度(RPHI)与IgG抗体滴度(IFAT)相关(r=0.542,P<0.01),而与IgM抗体滴度(MacELISA)无明显相关(r<0.1)。但进一步研究发现,3日内血清IgM抗体滴度与总抗体滴度(RPHI)存在相关关系(r=0.701,P<0.01),表明IgM抗体可能也与发病初期RPHI的较高的阳性检出率有关。本工作表明,MacELISA作为一种早期诊断方法具有高度的特异性和敏感性,而RPHI操作简便、快速、敏感性高,但存在一定的非特异性。研究还发现,流行区临床诊断为EHF的病人,IFAT(IgG)和RPHI检测均阳性,而MacELISA(IgM)阴性,提示用RPHI进行血清学诊断时,检查双份血清是必要的。 相似文献
33.
血管内皮舒张因子在氧自由基所致慢性缺氧大鼠肺内动... 总被引:1,自引:1,他引:0
The role of endothelium-derived relaxing factor (EDRF) on the effect of oxygen-derived free radicals (generated by xanthine-xanthine oxidase system) on intrapulmonary arterial in chronic hypoxic rats was studied by a microbioassay method. Intrapulmonary artery rings with intact or denuded endothelium of hypoxic (5,000 m, 10 days) and normoxic rats were prepared for observation of oxygen-derived free radicals induced contraction. It was shown that oxygen-derived free radicals induced contractions of intrapulmonary arterial rings with intact endothelium were obviously augmented in hypoxic rats than in normoxic controls. The augmented responses could be further potentiated by the addition of EDRF inactivator reduced hemoglobin (RHb), but diminished or even abolished by applying superoxide dismutase (Cu-Zn SOD). However, no effect on denuded rings was observed when RHb or SOD was added. It is concluded that chronic hypoxia may attenuate the action of EDRF in the enhancement of the reactivity of intrapulmonary artery to oxygen-derived free radicals. 相似文献
34.
电损毁海马CA3区及连合前穹窿对大鼠血浆胰岛素水平... 总被引:1,自引:0,他引:1
Bilateral electrical lesioning of the hippocampal CA3 region (HCA3-EL) or anterior commissura hippocampi (ACHF-EL) caused marked elevations in plasma basal levels of insulin. 2 weeks later, fasting blood glucose levels were also augmented with decreased glucose tolerance. In contrast, the secretory response of pancreatic B cells to glucose stimulation was markedly enhanced. Following intravenous glucose tolerance test (IVGTT), the relative amounts of glucagon-like and insulin-like immunoreactants were reduced in the pancreatic islets of both HCA3-EL and ACHF-EL rats in comparison with the controls. In the HCA3-EL group, the relative amounts of somatostatin-like immunoreactants and gross numbers of such immunostained cells in islets were also decreased as compared with the control. No difference was seen in pancreatic-polypeptide-like immunoreactivities as assessed by immunohistochemistry plus microphotometry method. The above results suggest strongly that HCA3 and ACHF exert a tonic inhibitory action on the insulin secretion in the rat. 相似文献
35.
Colocalization of the genes for the alpha 3(IV) and alpha 4(IV) chains of type IV collagen to chromosome 2 bands q35-q37. 总被引:5,自引:0,他引:5
Each type of basement membrane in man contains between two and five genetically distinct type IV collagens: alpha 1(IV)-alpha 5(IV). Genes for alpha 1(IV), alpha 2(IV), alpha 3(IV), and alpha 5(IV) have been isolated. We have recently isolated partial cDNAs for the fifth member of the family, designated alpha 4(IV). On the basis of comparison of the deduced peptide sequences of all five chains, the type IV collagens can be divided into two families: alpha 1-like, comprising alpha 1(IV), alpha 3(IV), and alpha 5(IV); and alpha 2-like, comprising alpha 2(IV) and alpha 4(IV). Genes encoding the alpha 1(IV) and alpha 2(IV) chains (COL4A1 and COL4A2) both map to human chromosome 13q34 and have been shown to be transcribed from opposite DNA strands using a common bidirectional promoter that allows coordinate regulation of the two chains. Indeed, these two chains are commonly found together in basement membrane and form [alpha 1]2.[alpha 2] heterotrimers. Whereas alpha 1(IV) and alpha 2(IV) have been found in all basement membranes studied hitherto, it has been shown that alpha 3(IV) and alpha 4(IV) are found in only a subset of basement membranes. In basement membranes where either of these molecules is present, however, they are found together. In view of this relationship and the structural similarities between alpha 1(IV) and alpha 3(IV) and between alpha 2(IV) and alpha 4(IV), we hypothesized that COL4A3 and COL4A4, the genes encoding alpha 3(IV) and alpha 4(IV), respectively, have a genomic organization similar to that of COL4A1 and COL4A2.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
36.
In vivo regulatory phosphorylation site in c(4)-leaf phosphoenolpyruvate carboxylase from maize and sorghum 总被引:4,自引:3,他引:1
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Reversible seryl-phosphorylation contributes to the light/dark regulation of C4-leaf phosphoenolpyruvate carboxylase (PEPC) activity in vivo. The specific regulatory residue that, upon in vitro phosphorylation by a maize-leaf protein-serine kinase(s), leads to an increase in catalytic activity and a decrease in malate-sensitivity of the target enzyme has been recently identified as Ser-15 in 32P-phosphorylated/activated dark-form maize PEPC (J-A Jiao, R Chollet [1990] Arch Biochem Biophys 283: 300-305). In order to ascertain whether this N-terminal seryl residue is, indeed, the in vivo regulatory phosphorylation site, [32P]phosphopeptides were isolated and purified from in vivo 32P-labeled maize and sorghum leaf PEPC and subjected to automated Edman degradation analysis. The results show that purified light-form maize PEPC contains 14-fold more 32P-radioactivity than the corresponding dark-form enzyme on an equal protein basis and, more notably, only a single N-terminal serine residue (Ser-15 in maize PEPC and its structural homolog, Ser-8, in the sorghum enzyme) was found to be 32P-phosphorylated in the light or dark. These in vivo observations, combined with the results from our previous in vitro phosphorylation studies (J-A Jiao, R Chollet [1989] Arch Biochem Biophys 269: 526-535; [1990] Arch Biochem Biophys 283: 300-305), demonstrate that an N-terminal seryl residue in C4 PEPC is, indeed, the regulatory site that undergoes light/dark changes in phosphorylation-status and, thus, plays a major, if not cardinal role in the light-induced changes in catalytic and regulatory properties of this cytoplasmic C4-photosynthesis enzyme in vivo. 相似文献
37.
A comparative study on tyrosine oxidation was made with a pure cationic and anionic peroxidase from peanut cell culture medium. The results showed that both isozymes possessed almost identical capacity to oxidize tyrosine to dityrosine, isodityrosine and polytyrosine with the main difference being the pH optimum (pH 4 for the anionic and pH 7 for the cationic isozyme). Variation of reaction time after 1.5 h incubation had little effect on the quantity and quality of the oxidation products. On the other hand, increase of enzyme units correspondingly increased tyrosine-oxidation. The removal of heme and carbohydrate moieties from the holoenzyme arrested the reaction thereby suggesting the role played by these moieties in stabilizing the active site of peroxidase isoenzymes. Isolated cell wall extracts catalyzed the tyrosine-oxidation equally well as the purified peroxidase. Even though polyclonal antibodies against anionic peroxidase inhibited the in vitro tyrosine reaction they did not affect the tyrosine oxidation by the cell walls, while the cationic antibodies did.Abbreviations A.PRX
anionic peanut peroxidase
- C.PRX
cationic peanut peroxidase
- PcAb
polyclonal antibodies
- ELISA
enzyme-linked-immuno-sorbent-assay
- TFMS
trifluoromethane sulfonic acid 相似文献
38.
39.
The purpose of this study is to investigate the role of paraventricular nucleus of the hypothalamus (PVN) and alpha 1 adrenergic receptor of PVN in the pressor responses to stimulation of renal afferent nerve in alpha 1-chloralose-anesthetized cats with carotid sinoaortic denervation and vagotomy. The pressor response to stimulation of renal afferent nerve consisted of a primary and a second components. The primary component response was completely blocked while the second component was not blocked by autonomic blocking agents (hexomethonium and atropine). Bilateral lesions of PVN greatly attenuated the pressor response before and after autonomic blockade. Intracerebroventricular and PVN injection alpha 1, adrenergic antagonist (prazosin) significantly decreased in the pressor response to stimulation of renal afferent nerve. These results indicate that paraventricular nucleus of the hypothalamus and alpha 1 adrenergic receptors in central nervous system, especially in PVN, play an important role in the pressor responses to stimulation of renal afferent nerve. 相似文献
40.