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   5365篇
  免费   298篇
  国内免费   30篇
  5693篇
  2024年   11篇
  2023年   88篇
  2022年   210篇
  2021年   354篇
  2020年   176篇
  2019年   209篇
  2018年   283篇
  2017年   192篇
  2016年   259篇
  2015年   354篇
  2014年   367篇
  2013年   379篇
  2012年   423篇
  2011年   363篇
  2010年   228篇
  2009年   195篇
  2008年   212篇
  2007年   182篇
  2006年   144篇
  2005年   131篇
  2004年   88篇
  2003年   99篇
  2002年   75篇
  2001年   42篇
  2000年   42篇
  1999年   40篇
  1998年   26篇
  1997年   27篇
  1996年   24篇
  1995年   23篇
  1994年   17篇
  1993年   21篇
  1992年   44篇
  1991年   27篇
  1990年   15篇
  1989年   26篇
  1988年   26篇
  1987年   41篇
  1986年   27篇
  1985年   17篇
  1984年   24篇
  1983年   28篇
  1982年   14篇
  1981年   18篇
  1980年   12篇
  1979年   16篇
  1978年   11篇
  1976年   8篇
  1973年   8篇
  1972年   12篇
排序方式: 共有5693条查询结果,搜索用时 15 毫秒
1.
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   
2.
Seven barley(Hordeum vulgäre L.) cultivars tested varied greatly in their responses to root medium salinity (electrical conductivity of 3, 5, 10, 15 and 20 dS nr-1)-lant growth was relatively more adversely affected than seed germination. Dry/fresh mass ratio increased at higher salinity levels in all barley cultivars indicating reduced water uptake. Higher K/Na ratio in plant shoots compared to that in the root medium solution indicated selective uptake of K that seems to be among processes involved in tolerance of cultivars to salinity stress.  相似文献   
3.
Cadmium (200 ppm) applied through the rooting medium to 30-day-old wheat plants decreased chlorophyll content, net CO2 exchanges and PSII activity by 34, 54 and 43% respectively. Thylakoid total lipids, total glycolipids, total phospholipids and total neutral lipids decreased by 22, 23, 12 and 25%, respectively, under cadmium treatment. Thylakoid membrane glycolipids had three major constituents, viz. monogalactosyl diacylglycerol, digalactosyl diacylglycerol and sulphoquinovosyl diacylglycerol. Monogalactosyl diacylglycerol and digalactosyl diacylglycerol contents decreased by 32 and 27%, respectively, under cadmium. Cadmium application also decreased the concentration of phosphatidyl glycerol and phosphatidyl choline to the extent of about 57 and 31%, respectively. On the other hand, phosphatidic acid and free fatty acids content showed an increase. These compositional changes in thylakoid membranes might be responsible for reduced PSII activity and rate of photosynthesis as observed under cadmium treatment.  相似文献   
4.
5.
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   

6.
Phenol, p-cresol, and volatile fatty acids (VFA; acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids) were used as odor indicators of swine waste. Aeration of the waste allowed the indigenous microorganisms to grow and degrade these malodorous substances. The time required for degradation of these substances varied according to the waste used, and it was not necessarily related to their concentrations. Using a minimal medium which contained one of the malodorous compounds as sole carbon source, we have selected from swine waste microorganisms that can grow in the medium. The majority of these microorganisms were able to degrade the same substrate when inoculated in sterilized swine waste but with an efficiency varying from one strain to the other. None of these strains was able to degrade all malodorous substances studied. Within 6 days of incubation these selected strains degraded the following: Acinetobacter calcoaceticus, phenol and all VFA; Alcaligenes faecalis, p-cresol and all VFA; Corynebacterium glutamicum and Micrococcus sp., phenol, p-cresol, and acetic and propionic acids; Arthrobacter flavescens, all VFA. On a laboratory scale, the massive inoculation of swine waste with C. glutamicum or Micrococcus sp. accelerated degradation of the malodorous substances. However, this effect was not observed with all of the various swine wastes tested. These results suggest that an efficient deodorization process of various swine wastes could be developed at the farm level based on the aerobic indigenous microflora of each waste.  相似文献   
7.
H C Cheung  C K Wang  N A Malik 《Biochemistry》1987,26(18):5904-5907
We have determined the free energy of formation of the binary complexes formed between skeletal troponin C and troponin T (TnC.TnT) and between troponin T and troponin I (TnT.TnI). This was accomplished by using TnC fluorescently modified at Cys-98 with N-(iodoacetyl)-N'-(5-sulfo-1-naphthyl)ethylenediamine for the first complex and TnI labeled at Cys-133 with the same probe for the other complex. The free energy of the ternary complex formed between troponin C and the binary complex TnT.TnI [TnC.(TnT.TnI)] was also measured by monitoring the emission of 5-(iodoacetamido)eosin attached to Cys-133 of the troponin I in TnT.TnI. The free energies were -9.0 kcal.mol-1 for TnC.TnT, -9.2 kcal.mol-1 for TnT.TnI, and -8.7 kcal.mol-1 for TnC.(TnT.TnI). In the presence of Mg2+ the free energies of TnC.TnT and TnC.(TnT.TnI) were -10.3 and -10.9 kcal.mol-1, respectively; in the presence of Ca2+ the corresponding free energies were -10.6 and -13.5 kcal.mol-1. Mg2+ and Ca2+ had negligible effect on the free energy of TnT.TnI. From these results the free energies of the formation of troponin from the three subunits were found to be -16.8 kcal.mol-1, -18.9 kcal.mol-1, and -21.6 kcal.mol-1 in the presence of EGTA, Mg2+, and Ca2+, respectively. Most of the free energy decrease caused by Ca2+ binding to the Ca2+-specific sites is derived from stabilization of the TnI-TnC linkage.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
8.
A comparison of different habitats, biogas plant, rumen fluid and sewage sludge, for cellulolytic organisms indicated sewage studge was the best source. Enrichment cultura gave a mixed culture which exhibited CMCase activity as well as extracellular Avicelase, xylanase, -glucosidase, -xylosidase activities and cell-bound -glucosidase, and -xylosidase production in a synthetic medium with eleven different cellulosic and lignocellulosic substrates. The activity of extracellular -glucosidase and -xylosidase production was significantly higher than endogenous activities. Hemicellulases were induced better than cellulases. The anzyme system was stable under aerobic conditions. Of the different lignocellulosic substrates, kallar grass was the best inducer of extracellular enzymes.
Résumé La comparaison de différents habitats: digesteur méthanique, fluide du rumen ou boue de station d'épuration, pour leur contenu en organismes cellulolytiques, indiquent que la boue de station d'épuration est la meilleure source. Une culture par enrichissement a produit une culture mixte qui a exhibé aussi bien une activité CMCase que des activitiés extracellulaires avicelasique, xylanasique, -glucosidasique et -xylosidasique et qu'une production de -glucosidase et de -xylosidase liées à la cellule, dans un milieu synthétique et pour onze substrats cellulosiques et lignocellulosiques différents. L'activité de la -glucosidase extracellulaire et la production de -xylosidase sont significativement plus élevées que les activitiés endogènes. Les hemicellulases sont mieux induites que les cellulases. Le système enzymatique est stable dans des conditions aérobies. Parmi les divers substrats lignocellulosiques, l'herbe Kallar est le meilleur inducteur d'enzymes extracellufaires.
  相似文献   
9.
A rapid, one-step procedure has been developed for inducing direct organogenesis and somatic embryogenesis in cultures of Phaseolus coccineus L., P. acutifolius A., P. aureus L. [Vigna radiata L. Wilczek] and P. wrightii L. Development of somatic embryos and shoot buds occurred within 6–8 weeks of culture from intact seedlings raised on MS (Murashige and Skoog 1962) medium supplemented with N6-benzylaminopurine (BAP). Shoot buds or embryoids originated from subepidermal tissue of the regions adjacent to the shoot apex, hypocotyl and cotyledonary axils. While P. acutifolius and P. aureus were regenerated via shoot formation and P. wrightii by somatic embryogenesis, both embryogenesis and shoot regeneration were observed in P. coccineus. Relatively higher levels of BAP, 50–80 M, were found to be optimal for inducing regeneration while lower concentrations were ineffective. About 40–70 shoots and 70–250 somatic embryos were produced per responding seedling. Regenerated shoots and somatic embryos developed into whole plants on a basal medium or the one supplemented with 1 M naphthaleneacetic acid.  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