全文获取类型
收费全文 | 3126篇 |
免费 | 59篇 |
国内免费 | 126篇 |
专业分类
3311篇 |
出版年
2023年 | 15篇 |
2022年 | 33篇 |
2021年 | 36篇 |
2020年 | 39篇 |
2019年 | 40篇 |
2018年 | 59篇 |
2017年 | 25篇 |
2016年 | 52篇 |
2015年 | 51篇 |
2014年 | 127篇 |
2013年 | 262篇 |
2012年 | 96篇 |
2011年 | 164篇 |
2010年 | 84篇 |
2009年 | 139篇 |
2008年 | 139篇 |
2007年 | 145篇 |
2006年 | 153篇 |
2005年 | 156篇 |
2004年 | 138篇 |
2003年 | 141篇 |
2002年 | 111篇 |
2001年 | 86篇 |
2000年 | 69篇 |
1999年 | 66篇 |
1998年 | 80篇 |
1997年 | 63篇 |
1996年 | 65篇 |
1995年 | 72篇 |
1994年 | 60篇 |
1993年 | 48篇 |
1992年 | 39篇 |
1991年 | 36篇 |
1990年 | 40篇 |
1989年 | 45篇 |
1988年 | 27篇 |
1987年 | 23篇 |
1986年 | 20篇 |
1985年 | 37篇 |
1984年 | 43篇 |
1983年 | 18篇 |
1982年 | 32篇 |
1981年 | 24篇 |
1980年 | 14篇 |
1979年 | 19篇 |
1978年 | 19篇 |
1977年 | 14篇 |
1976年 | 12篇 |
1974年 | 9篇 |
1973年 | 11篇 |
排序方式: 共有3311条查询结果,搜索用时 15 毫秒
41.
42.
Anionic peroxidase isoenzymes, separated on acrylamide gels, were examined in two flax genotrophs and in their reciprocal F2 hybrids. Isoenzyme 1 exhibited a significant difference in Rm between stem base and apex and there was a gradient of decreasing Rm and activity between base and apex. Isoenzyme 2 displayed only the activity gradient. The parents differed significantly in the Rm's and activities of isoenzymes 1 and 2, and the F2's showed complete dominance of the L parent for Rm, with activities being approximately intermediate. 相似文献
43.
【目的】大豆过氧化物酶(SBP)作用底物广泛、比活高、热稳定性好,使其在免疫检测、工业污染废水处理领域有着广泛的应用潜力。现有的生产方法主要是从大豆壳中提取,这种方法产量低,成本高,远不能满足于工业应用要求,本研究希望实现在毕赤酵母中高效表达有功能活性的大豆过氧化物酶。【方法】将大豆过氧化物酶基因以及C末端截短20个氨基酸的基因克隆pPIC-9K载体中,并在毕赤酵母X-33中诱导表达。同时还将糖基化位点的天冬酰胺突变成为谷氨酰胺,研究糖基化位点对表达的影响。【结果】全长SBP在毕赤酵母中表达是无活性的,只有截短的SBP△20在试管发酵的表达活力达23.5 U/mL,经过糖基化位点的突变表明130、144、185、197对酶活非常重要,不能突变;211和216位点去糖基化突变对酶活有所提高。【结论】经过发酵条件的优化,在5 L的发酵罐中发酵液上清最高酶活力达510 U/mL,是目前报道的最高水平。 相似文献
44.
45.
Physiological and biochemical events leading to vitrification of plants cultured in vitro 总被引:12,自引:0,他引:12
Water content, peroxidase activity and isoperoxidases, phenylalanine ammonia-lyase activity and phenolic content were comparatively analyzed in tissues of normal and vitreous plants cultured in vitro. The release of ethylene in flask atmospheres by normal and vitrifying plants was also measured. On the basis of the results, it is hypothesized that vitrification results from a burst of ethylene controlled by the peroxidase-IAA-oxidase system. An initiating stress (e.g. excess of cytokinins or of NH4 + ions) would mediate the enhancement of the activity of soluble and membrane-bound peroxidases through a rapid modification of the phenolic level. The excess of ethylene in the atmosphere of stressed plants would retroinhibit its own biosynthesis and as a consequence decrease the activities of PAL and acidic peroxidases, thus hindering lignification processes. A parallel decrease in cellulose synthesis due to a diverted conversion of sugars to amino acids is expected (from data in the literature). Deficiency of both cellulose and lignin would allow more water uptake due to reduced wall pressure and bring about the hyperhydric malformations. 相似文献
46.
Cadmium (Cd) is a widespread heavy metal pollutant and environmental and human health hazard, which may be partially resolved using green and cost-effective phytoremediation techniques. However, the efficiency of phytoremediation is often limited by the small biomass of Cd-hyperaccumulator plants. Although cattail (Typha angustifolia L.) is tolerant of heavy metals and has a high biomass, there is little information available on its detoxification mechanisms for heavy metals, especially Cd. In the present study we investigated the tolerance of cattail to Cd and mechanisms involved in its Cd detoxification. Our results show that: (a) cattail is tolerant of Cd; (b) the root Casparian band, cell wall, vacuole, glutathione (GSH), and glutathione peroxidase (GPX) play important roles in Cd detoxification; and (c) mechanisms of Cd detoxification differ in leaf cell cytoplasm (mainly a GSH-related antioxidant defense system) and root cell cytoplasm (mainly a GSH-related chelation system). In summary, cattail possesses multiple detoxification mechanisms for Cd and is a promising species for phytoremediation of Cd-polluted environments. 相似文献
47.
Ahsan Mushir Shemsi Firdous Ahmad Khanday Ahsanulhaq Qurashi Amjad Khalil Gea Guerriero Khawar Sohail Siddiqui 《Biotechnology advances》2019,37(3):357-381
Numerous enzymes of biotechnological importance have been immobilized on magnetic nanoparticles (MNP) via random multipoint attachment, resulting in a heterogeneous protein population with potential reduction in activity due to restriction of substrate access to the active site. Several chemistries are now available, where the modifier can be linked to a single specific amino acid in a protein molecule away from the active-site, thus enabling free access of the substrate. However, rarely these site-selective approaches have been applied to immobilize enzymes on nanoparticles. In this review, for the first time, we illustrate how to adapt site-directed chemical modification (SDCM) methods for immobilizing enzymes on iron-based MNP. These strategies are mainly chemical but may additionally require genetic and enzymatic methods. We critically examine each method and evaluate their scope for simple, quick, efficient, mild and economical immobilization of enzymes on MNP. The improvements in the catalytic properties of few available examples of immobilized enzymes are also discussed. We conclude the review with the applications and future prospects of site-selectively modified magnetic enzymes and potential benefits of this technology in improving enzymes, including cold-adapted homologues, modular enzymes, and CO2-sequestering, as well as non-iron based nanomaterials. 相似文献
48.
荞麦属种质资源发芽种子过氧化物酶同工酶研究 总被引:2,自引:1,他引:2
用聚丙烯酰胺凝胶电泳对荞麦属9个种(含大粒组8个和小粒组1个)32个收集系栽培及野生荞麦种子的过氧化物酶同工酶进行了研究。结果表明:过氧化物酶同工酶酶带23条,不同物种的酶带数4到8条。其中,甜荞有7条带,而苦荞为4条。酶带分析及聚类分析表明:大粒组荞麦种的谱带与F.gracilipes等小粒组荞麦种间差异极大,甜荞和苦荞酶带分别与F.megaspartanium和F.pilus相似,并分别与F.megas-partanium和F.pilus聚类最近,支持F.megaspartanium和F.pilus可能分别是甜荞和苦荞祖先种的假说。 相似文献
49.
Nafion膜固定的亚甲基蓝为介体的生物传感器 总被引:3,自引:0,他引:3
制成了以亚甲基蓝为介体的电流型过氧化氢生物传感器,通过离子交换牢固地固定在Nafion膜中的亚甲基蓝,能有效地在辣根过氧化物酶和玻碳电极之间传递电子.探讨了pH值、温度、工作电位和抗坏血酸等物质对此传感器生物电催化还原H2O2的影响.此生物传感器选择性好、灵敏度高,对H2O2线性响应范围为5.0×10-7~2×10-4 mol/L,响应时间少于30 s. 相似文献
50.
Salt and oxidative stress: similar and specific responses and their relation to salt tolerance in Citrus 总被引:45,自引:0,他引:45
Salt damage to plants has been attributed to a combination of several factors including mainly osmotic stress and the accumulation
of toxic ions. Recent findings in our laboratory showed that phospholipid hydroperoxide glutathione peroxidase (PHGPX), an
enzyme active in the cellular antioxidant system, was induced by salt in citrus cells and mainly in roots of plants. Following
this observation we studied the two most important enzymes active in elimination of reactive oxygen species, namely, superoxide
dismutase (SOD) and ascorbate peroxidase (APX), to determine whether a general oxidative stress is induced by salt. While
Cu/Zn-SOD activity and cytosolic APX protein level were similarly induced by salt and methyl viologen, the response of PHGPX
and other APX isozymes was either specific to salt or methyl viologen, respectively. Unlike PHGPX, cytosolic APX and Cu/Zn-SOD
were not induced by exogenously added abscisic acid. Salt induced a significant increase in SOD activity which was not matched
by the subsequent enzyme APX. We suggest that the excess of H2O2 interacts with lipids to form hydroperoxides which in turn induce and are removed by PHGPX. Ascorbate peroxidase seems to
be a key enzyme in determining salt tolerance in citrus as its constitutive activity in salt-sensitive callus is far below
the activity observed in salt-tolerant callus, while the activities of other enzymes involved in the defence against oxidative
stress, namely SOD, glutathione reductase and PHGPX, are essentially similar.
Received: 10 January 1997 / Accepted: 28 May 1997 相似文献