全文获取类型
收费全文 | 4644篇 |
免费 | 338篇 |
国内免费 | 1篇 |
出版年
2022年 | 29篇 |
2021年 | 58篇 |
2020年 | 37篇 |
2019年 | 43篇 |
2018年 | 54篇 |
2017年 | 56篇 |
2016年 | 109篇 |
2015年 | 164篇 |
2014年 | 212篇 |
2013年 | 242篇 |
2012年 | 318篇 |
2011年 | 324篇 |
2010年 | 216篇 |
2009年 | 164篇 |
2008年 | 268篇 |
2007年 | 240篇 |
2006年 | 245篇 |
2005年 | 214篇 |
2004年 | 183篇 |
2003年 | 186篇 |
2002年 | 171篇 |
2001年 | 68篇 |
2000年 | 66篇 |
1999年 | 65篇 |
1998年 | 70篇 |
1997年 | 64篇 |
1996年 | 34篇 |
1995年 | 39篇 |
1994年 | 43篇 |
1993年 | 38篇 |
1992年 | 40篇 |
1991年 | 40篇 |
1990年 | 29篇 |
1989年 | 30篇 |
1988年 | 42篇 |
1987年 | 30篇 |
1986年 | 31篇 |
1985年 | 29篇 |
1984年 | 28篇 |
1983年 | 26篇 |
1982年 | 28篇 |
1981年 | 30篇 |
1980年 | 21篇 |
1979年 | 24篇 |
1977年 | 25篇 |
1976年 | 20篇 |
1974年 | 33篇 |
1973年 | 16篇 |
1971年 | 16篇 |
1969年 | 22篇 |
排序方式: 共有4983条查询结果,搜索用时 14 毫秒
71.
We have purified for the first time from green leaves a multifunctional protein (MFP) involved in fatty acid -oxidation. The protein, designated MFP IV, was extracted from green leaves of three-week-old cucumber (Cucumis sativus L.) plants. Chromatography on cation exchanger, separation on hydroxylapatite, and fast-protein liquid chromatography on Phenylsuperose led to a more than 7000-fold purification and to the isolation of an apparently homogeneous 80-kDa monomeric protein. This protein is immunologically related to the glyoxysomal MFP II, as evidenced by immunodecoration with antiserum raised against MFP II. Comparison of molecular masses of all MFPs presently known revealed that the MFP prepared from green leaves (MFP IV) is distinct from MFP II (76.5 kDa) and MFP I (74 kDa) from dark-grown cotyledons. By including other properties in this comparison, we demonstrated that MFP IV can also be distinguished from the glyoxysomal MFP III (81 kDa) and the bacterially expressed MFP-a (80 kDa). Moreover, MFP IV is a constituent of leaf peroxisomes and contains the activities of 2-enoyl-CoA hydratase (EC 4.2.1.17),l-3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) and 3-hydroxyacyl-CoA epimerase.Abbreviation MFP
multifunctional protein
This work was supported by grants from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. 相似文献
72.
73.
Water channels in Chara corallina 总被引:4,自引:0,他引:4
Water relations parameters ofChara corallina inter-nodes weremeasured using the single cell pressure probe. The effect ofmercurials, which are recognized as non-specific water channelinhibitors, was examined. HgCl2 concentrations greater than5 mmol m3 were found to inhibit hydraulic conductivity{Lp) close to 90%, whereas pCMPS was found to have no effecton Lp. The activation energy of water flow was increased significantlyfrom 21.0 kJ mol1 to 45.6 kJ mol1, following theapplication of HgCl2. These results are in accordance with evidencefor Hg2+sensitive water channels in the plasma membrane of charophytes(Henzler and Steudle, 1995; Tazawa et al., 1996). The metaboliceffects must, however, be considered in view of the rapid inhibitionof respiration and the depolarization of the membrane potentialwith HgCl2 concentrations lower than those found to affect Lp.It was possible to measure simultaneously water relations andmembrane PD, in order to examine the contribution of potassiumchannels to Lp. Cells were induced into a K+ permeable state.The K+ channels, assumed to be open, were subsequently blockedby various blockers. No significant difference in Lp was foundfor any of these treatments. Finally, the permeability of C.corallina membranes to ethanol was examined. HgCl2 was foundto cause a decrease in reflection coefficient, coinciding witha decrease in Lp, but there was no change in the ethanol permeabilitycoefficient. This has been interpreted in terms of both thefrictional model and composite model of non-electrolyte membranetransport. Key words: Water channels, Chara, hydraulic, conductivity, membrane transport models, reflection coefficient 相似文献
74.
The distribution and morphology of crustacean cardioactive peptide-immunoreactive neurons in the brain of the locust Locusta migratoria has been determined. Of more than 500 immunoreactive neurons in total, about 380 are interneurons in the optic lobes. These neurons invade several layers of the medulla and distal parts of the lobula. In addition, a small group of neurons projects into the accessory medulla, the lamina, and to several areas in the median protocerebrum. In the midbrain, 12 groups or individual neurons have been reconstructed. Four groups innervate areas of the superior lateral and ventral lateral protocerebrum and the lateral horn. Two cell groups have bilateral arborizations anterior and posterior to the central body or in the superior median protocerebrum. Ramifications in subunits of the central body and in the lateral and the median accessory lobes arise from four additional cell groups. Two local interneurons innervate the antennal lobe. A tritocerebral cell projects contralaterally into the frontal ganglion and appears to give rise to fibers in the recurrent nerve, and in the hypocerebral and ingluvial ganglia. Varicose fibers in the nervi corporis cardiaci III and the corpora cardiaca, and terminals on pharyngeal dilator muscles arise from two subesophageal neurons. Some of the locust neurons closely resemble immunopositive neurons in a beetle and a moth. Our results suggest that the peptide may be (1) a modulatory substance produced by many brain interneurons, and (2) a neurohormone released from subesophageal neurosecretory cells. 相似文献
75.
Many theories of human stereovision are based on feature matching and
the related correspondence problem. In this paper, we present
psychophysical experiments indicating that localized image features
such as Laplacian zerocrossings, intensity extrema, or centroids are
not necessary for binocular depth perception. Smooth one-dimensional
intensity profiles were combined into stereograms with
mirror-symmetric half-images such that these localized image features
were either absent or did not carry stereo information. In a
discrimination task, subjects were asked to distinguish between
stereograms differing only by an exchange of these half-images (ortho-
vs. pseudoscopic stereograms). In a depth ordering task, subjects had
to judge which of the two versions appeared in front. Subjects are
able to solve both tasks even in the absence of the mentioned image
features. The performance is compared to various possible stereo
mechanisms. We conclude that localized image features and the
correspondences between them are not necessary to perceive
stereoscopic depth. One mechanism accounting for our data is
correlation or mean square difference.
Received: 8 February 1994 / Accepted in revised form: 15
September
1994 相似文献
76.
A direct chiral chromatographic reversed phase method for the determination of the enantiomers of felodipine is described. The influence of charged and uncharged modifiers as well as the effect of the mobile phase pH on the enantiomeric resolution is discussed. A high mobile phase pH and the addition of 2-propanol as organic modifier gave the highest separation factor (α = 1.3). The high mobile phase pH (pH = 7.6) is outside the recommended pH limit of silica based columns but was necessary to achieve baseline resolution of (R)- and (S)-felodipine. Improvement of column efficiency by increasing column temperature was utilized for optimization of the enantiomeric resolution (Rs = 1.7). The enantiomers of felodipine and three related compounds were separated within 15 min. The enantiomeric purity of (R)- and (S)-felodipine in injections and (R)-felodipine in bulk substance was higher than 99.5% and no racemization was observed after storage at accelerated conditions. A poor Chiral-AGP® column used for a long period was restored using a simple wash step together with repacking the top of the chromatographic column. © 1995 Wiley-Liss, Inc. 相似文献
77.
Christoph Winterhalter Peter Heinrich Anton Candussio Günther Wich Wolfgang Liebl 《Molecular microbiology》1995,15(3):431-444
A segment of Thermotoga maritima strain MSB8 chromosomal DNA was isolated which encodes an endo-1,4-β-D-xylanase, and the nucleotide sequence of the xylanase gene, designated xynA, was determined. With a half-life of about 40 min at 90°C at the optimal pH of 6.2, purified recombinant XynA is one of the most thermostable xylanases known. XynA is a 1059-amino-acid (?120 kDa) modular enzyme composed of an N-terminal signal peptide and five domains, in the order A1-A2-B-C1-C2. By comparison with other xylanases of family 10 of glycosyl hydrolases, the central ?340-amino-acid part (domain B) of XynA represents the catalytic domain. The N terminal ?150-amino-acid repeated domains (A1-A2) have no significant similarity to the C-terminal ?170-amino-acid repeated domains (C1-C2). Cellulose-binding studies with truncated XynA derivatives and hybrid proteins indicated that the C-terminal repeated domains mediate the binding of XynA to microcrystalline cellulose and that C2 alone can also promote cellulose binding. C1 and C2 did not share amino acid sequence similarity with any other known cellulose-binding domain (CBD) and thus are CBDS of a novel type. Structurally related protein segments which are probably also CBDs were found in other multi-domain xylanolytic enzymes. Deletion of the N-terminal repeated domains or of all the non-catalytic domains resulted In substantially reduced tbermostability while a truncated xylanase derivative lacking the C-terminal tandem repeat was as thermostable as the full-length enzyme. It is argued that the multidomain organization of some enzymes may be one of the strategies adopted by thermophiles to protect their proteins against thermal denaturation. 相似文献
78.
Chlorophyll fluorescence as a tool in plant physiology 总被引:43,自引:0,他引:43
79.
Summary Three cultivation systems were compared. In one system the alders were grown hydroponically. In the two other systems the alders were planted in gravel and either given water and nutrients at intervals or the nutrient solution was continuously supplied. Alders continuously supplied with nutrients and water showed a significantly more rapid growth, higher biomass production and higher nitrogen content than did alders given nutrients and water at intervals or alders hydroponically grown. Alders continuously supplied with water and nutrients had a constant RE (relative efficiency of nitrogenase) of about 0.80 throughout the experimental period while alders supplied with water and nutrients at intervals showed a slight decrease in RE at the end of the experimental period. No strict relationship was found between RE and nitrogen content or between RE and plant productivity. 相似文献
80.