首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4405篇
  免费   224篇
  国内免费   27篇
  2023年   86篇
  2022年   108篇
  2021年   336篇
  2020年   173篇
  2019年   201篇
  2018年   271篇
  2017年   178篇
  2016年   244篇
  2015年   315篇
  2014年   329篇
  2013年   339篇
  2012年   347篇
  2011年   298篇
  2010年   195篇
  2009年   168篇
  2008年   178篇
  2007年   159篇
  2006年   119篇
  2005年   113篇
  2004年   76篇
  2003年   63篇
  2002年   46篇
  2001年   16篇
  2000年   20篇
  1999年   20篇
  1998年   15篇
  1997年   10篇
  1996年   12篇
  1995年   13篇
  1994年   10篇
  1993年   11篇
  1992年   11篇
  1991年   12篇
  1989年   13篇
  1988年   9篇
  1987年   11篇
  1986年   10篇
  1985年   11篇
  1983年   7篇
  1982年   7篇
  1979年   7篇
  1978年   6篇
  1977年   7篇
  1975年   10篇
  1974年   9篇
  1973年   5篇
  1972年   6篇
  1971年   6篇
  1970年   6篇
  1969年   6篇
排序方式: 共有4656条查询结果,搜索用时 15 毫秒
1.
2.
Perspective texture synthesis has great significance in many fields like video editing, scene capturing etc., due to its ability to read and control global feature information. In this paper, we present a novel example-based, specifically energy optimization-based algorithm, to synthesize perspective textures. Energy optimization technique is a pixel-based approach, so it’s time-consuming. We improve it from two aspects with the purpose of achieving faster synthesis and high quality. Firstly, we change this pixel-based technique by replacing the pixel computation with a little patch. Secondly, we present a novel technique to accelerate searching nearest neighborhoods in energy optimization. Using k- means clustering technique to build a search tree to accelerate the search. Hence, we make use of principal component analysis (PCA) technique to reduce dimensions of input vectors. The high quality results prove that our approach is feasible. Besides, our proposed algorithm needs shorter time relative to other similar methods.  相似文献   
3.
Cellulases are the key enzymes used in the biofuel industry. A typical cellulase contains a catalytic domain connected to a carbohydrate-binding module (CBM) through a flexible linker. Here we report the structure of an atypical trimodular cellulase which harbors a catalytic domain, a CBM46 domain and a rigid CBM_X domain between them. The catalytic domain shows the features of GH5 family, while the CBM46 domain has a sandwich-like structure. The catalytic domain and the CBM46 domain form an extended substrate binding cleft, within which several tryptophan residues are well exposed. Mutagenesis assays indicate that these residues are essential for the enzymatic activities. Gel affinity electrophoresis shows that these tryptophan residues are involved in the polysaccharide substrate binding. Also, electrostatic potential analysis indicates that almost the entire solvent accessible surface of CelB is negatively charged, which is consistent with the halophilic nature of this enzyme.  相似文献   
4.
Risk assessment is part of the risk analysis process as it is used in veterinary medicine to estimate risks related to international trade and food safety. Data from monitoring and surveillance systems (MO&SS) are used throughout the risk assessment process for hazard identification, release assessment, exposure assessment and consequence assessment. As the quality of risk assessments depends to a large extent on the availability and quality of input data, there is a close relationship between MO&SS and risk assessment. In order to improve the quality of risk assessments, MO&SS should be designed according to minimum quality standards. Second, recent scientific developments on state-of-the-art design and analysis of surveys need to be translated into field applications and legislation. Finally, knowledge about the risk assessment process among MO&SS planners and managers should be promoted in order to assure high-quality data.  相似文献   
5.
The differentiation of spermatids in the marmoset (Callithrix jacchus, n = 9) testis is described here at the light-microscopic level employing serial semithin sections. The definition of 8 different phases of spermiogenesis, i.e. the formation of spermatids, is based upon the changes in the development of nucleus, acrosome and flagellum.  相似文献   
6.
We present evidence for a dimorphic life cycle in the vacuolate sulfide-oxidizing bacteria that appears to involve the attachment of a spherical Thiomargarita-like cell to the exteriors of invertebrate integuments and other benthic substrates at methane seeps. The attached cell elongates to produce a stalk-like form before budding off spherical daughter cells resembling free-living Thiomargarita that are abundant in surrounding sulfidic seep sediments. The relationship between the attached parent cell and free-living daughter cell is reminiscent of the dimorphic life modes of the prosthecate Alphaproteobacteria, but on a grand scale, with individual elongate cells reaching nearly a millimeter in length. Abundant growth of attached Thiomargarita-like bacteria on the integuments of gastropods and other seep fauna provides not only a novel ecological niche for these giant bacteria, but also for animals that may benefit from epibiont colonization.  相似文献   
7.
Catalytically defective rare variants of Sialic acid Acetyl Esterase (SIAE) have previously been linked to autoimmunity. Studies presented here confirm that the M89V SIAE protein and all other products of common variant alleles of SIAE are catalytically normal. Although overexpressing transfected non-lymphoid cells secrete small amounts of SIAE that can associate with the cell surface, normal human lymphocytes do not exhibit cell surface SIAE, supporting genetic evidence in mice that indicates that this protein functions in a lymphocyte intrinsic manner. Analyses of the plasma proteome also indicate that SIAE is not secreted in vivo. A re-analysis exclusively of catalytically defective rare variant alleles of SIAE in subjects in which this gene was completely sequenced confirmed an association of SIAE with autoimmunity. A subset of catalytically defective rare variant SIAE alleles has previously been typed in a large genotyping study comparing a diverse group of disease subjects and controls; our re-analysis of this data shows that catalytically defective alleles are enriched in disease subjects. These data suggest that SIAE may be associated with autoimmunity and that further study of catalytically defective rare variant SIAE alleles in terms of autoimmune disease susceptibility is strongly warranted.  相似文献   
8.
Some P-450 systems, notably aromatase and 14-demethylase catalyse not only the hydroxylate reaction but also the oxidation of an alcohol into a carbonyl compound as well as a C---C bond cleavage process. All these reactions occur at the same active site. A somewhat analogous situation is noted with 17-hydroxylase-17,20-lyase that participates in hydroxylation as well as C---C bond cleavage process. The C---C bond cleavage reactions catalysed by the above enzymes conform to the general equation:

It is argued that all three types of reaction catalyzed by these enzymes may be viewed as variations on a common theme. In P-450 dependent hydroxylation the initially formed FeIII---O---O. species is converted into FeIII---O---OH and the heterolysis of the oxygen—oxygen bond of the latter then gives the oxo-derivative for which a number of canonical structures are possible; for example FeV = O ↔ (+.)FeIV = O ↔ FeIV---O.. One of these, FeIV---O. behaves like an alkoxyl radical and participates in hydrogen abstraction from C---H bond to produce FeIV---OH and carbon radical. The latter is then quenched by the delivery of hydroxyl radical from FeIV---OH. The latter species may thus be regarded as a carrier of hydroxyl radical. We have proposed that the C---C bond cleavage reaction occurs through the participation of the FeIII---O---OH species that is trapped by the electrophilic property of the carbonyl compound giving a peroxide adduct that fragments to produce an acyl—carbon cleavage. Scientific developments leading up to this conclusion are considered. In the first author's views,

“The study of mechanisms is not a scientific but a cultural activity. Mechanisms do not aim at an absolute truth but are intended to be a “running” commentary on the status of knowledge in a field. As the structural knowledge in a field advances Mechanisms evolve to take note of the new findings. Just as a constructive “running” commentary provides the stimulus for higher standards of performance, so Mechanisms call for better and firmer structural information from their practitioners”.  相似文献   

9.
The present research was conducted in district Jhang, Pakistan, to evaluate the concentration of metals/metalloids in soil and pumpkin (Cucurbita maxima) irrigated with domestic wastewater. Data revealed that the levels of metals and metalloids in soil samples from two different sites were below the safe limits except Cd, whereas, in the vegetable, the concentrations of As, Se, Ni, Mo, Pb, Mn, and Cu were above the safe limits. The levels of 12 metals and metalloids in the soil were ranged between 0.14 to 22.76 mg/kg at site-I and 0.16 to 22.13 mg/kg at site-II. The levels of these metals in the vegetable were found 0.35 to 61.13 mg/kg at site-I and 0.31 to 53.63 mg/kg at site-II. The transfer factor at both sites was highest for As and Co. The pollution load index recorded for Se, Cu, Cd, Mo, Pb, and Co was greater than 1. The daily intake of As, Mn, and Mo was above the oral reference dose, which reflects that the intake of pumpkin is not safe for the inhabitants of the selected sites. The control measures should be taken to phytoextract heavy metals and metalloids from polluted sites so as to reduce the health risks.  相似文献   
10.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号