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   2331篇
  免费   120篇
  国内免费   1篇
  2022年   11篇
  2021年   32篇
  2020年   19篇
  2019年   20篇
  2018年   20篇
  2017年   24篇
  2016年   44篇
  2015年   59篇
  2014年   79篇
  2013年   194篇
  2012年   132篇
  2011年   147篇
  2010年   89篇
  2009年   67篇
  2008年   130篇
  2007年   139篇
  2006年   127篇
  2005年   143篇
  2004年   112篇
  2003年   124篇
  2002年   143篇
  2001年   23篇
  2000年   18篇
  1999年   31篇
  1998年   41篇
  1997年   21篇
  1996年   25篇
  1995年   20篇
  1994年   30篇
  1993年   22篇
  1992年   17篇
  1991年   28篇
  1990年   21篇
  1989年   28篇
  1988年   22篇
  1987年   17篇
  1986年   21篇
  1985年   20篇
  1984年   20篇
  1983年   17篇
  1982年   14篇
  1981年   22篇
  1980年   19篇
  1979年   11篇
  1978年   8篇
  1976年   12篇
  1975年   11篇
  1974年   13篇
  1973年   7篇
  1971年   8篇
排序方式: 共有2452条查询结果,搜索用时 15 毫秒
71.
Summary A remarkable increase in number of endothelial specific granules was observed in the rabbit umbilical veins between 2 and 5 days after birth. Electron microscopy indicated that the granules were segregated in the Golgi complex of the endothelial cells and released into the vascular lumen during the postnatal obliteration stage of this vessel.Incubation of the postnatal vessels in Ringer solution containing a histamine releasing compound induced remarkable morphological alterations of these cytoplasmic components; a reduction of their osmiophilia, swelling with a widened space separating the granular matrix from the limiting membrane, fusion to each other and expulsion of their contents into the vascular lumen, as in mast cell degranulation by this drug, were noted.High-performance liquid chromatography of the homogenized vessels demonstrated appreciable concentrations of histamine in the postnatal samples. There was a correlation between the histamine concentration and the quantity of granules in the respective postnatal samples.The present study strongly suggests that the granules are reservoirs of histamine and have an important role in the obliteration of this vessel.This work was supported in part by Grant in Aid for Scientific Research (# 448087) to S. Fujimoto from the Ministry of Education of Japan  相似文献   
72.
Summary Disomic and trisomic cells of a patient with Down syndrome mosaic were used to study the effect of the additional chromosome 21 against an identical genetic background. The frequency of Ag staining and the participation in satellite associations were determined for each pair of acrocentric chromosomes. The additional chromosome 21 of the trisomic cells and its homologues proved to be regularly Ag positive. Therefore the trisomic cells showed more Ag positive chromosomes and more satellite associations per cell than the diploid cells. Thus, no compensation for the additional rRNA-gene dose could be found in the cells of the trisomic line.  相似文献   
73.
An enzyme hydrolyzing the water-insoluble glucans produced from sucrose by Streptococcus mutans was purified from the culture concentrate of Streptomyces chartreusis strain F2 by ion-exchange chromatography on diethylaminoethyl cellulose and carboxymethyl cellulose columns and gel filtration on Bio-Gel A-1.5m. The purification achieved was 6.4-fold, with an overall yield of 27.3%. Electrophoresis of the purified enzyme protein gave a single band on a sodium dodecyl sulfate-polyacrylamide gel slab. Its molecular weight was estimated to be approximately 68,000, but there is a possibility that the native enzyme exists in an aggregated form or is an oligomer of the peptide subunits, have a molecular weight larger than 300,000. The pH optimum of the enzyme was 5.5 to 6.0, and its temperature optimum was 55 degrees C. The enzyme lost activity on heating at 65 degrees C for 10 min. The enzyme activity was completely inhibited by the presence of 1 mM Mn2+, Hg2+, Cu2+, Ag2+, or Merthiolate. The Km value for the water-insoluble glucan of S. mutans OMZ176 was an amount of glucan equivalent to 1.54 mM glucose, i.e., 0.89 mM in terms of the alpha-1,3-linked glucose residue. The purified enzyme was specific for glucans containing an alpha-1,3-glucosidic linkage as the major bond. The enzyme hydrolyzed the S. mutans water-insoluble glucans endolytically, and the products were oligosaccharides. These results indicate that the enzyme elaborated by S. chartreusis strain F2 is an endo-alpha-1,3-glucanase (EC 3.2.1.59).  相似文献   
74.
Summary To develop an efficient method for continuous production of L-malic acid from fumaric acid using immobilized microbial cells, screening of microorganisms having high fumarase activity was carried out and cultural conditions of selected microorganisms were investigated. As a result of screening microorganisms belonging to the genera Brevibacterium, Proteus, Pseudomonas, and Sarcina were found to produce fumarase in high levels. Among these microorganisms Brevibacterium ammoniagenes, B. flavum, Proteus vulgaris, and Pseudomonas fluorescens were further selected for their high fumarase levels in the cultivation on several media. These 4 microorganisms were entrapped into a k-carrageenan gel lattice, and the resultant immobilized B. flavum showed the highest fumarase activity and operational stability.Cultural conditions for the fumarase formation and the operational stability of fumarase activity of immobilized B. flavum are detailed. Productivity for L-malic acid using immobilized B. flavum with k-carrageenan was 2.3 fold of that using immobilized B. ammoniagenes with polyacrylamide.Presented at the Annual Meeting of the Agricultural Chemical Society of Japan, Nagoya, April 3, 1978  相似文献   
75.
Summary The ventral prostatic secretory epithelial cells in older rats were studied by light and electron microscopy. The cells vary in height in different parts of the same organ, and ultrastructurally they show the presence of a developed secretory apparatus such as well-developed Golgi body and abundant rough endoplasmic reticulum. They also show signs of a depressed secretory activity, involving occasional emiocytosis of apical secretory vacuoles and a paucity of condensing vacuoles in the Golgi region and above it. Further, they are characterized by the frequent occurrence of supra and paranuclear pleomorphic lysosomes.  相似文献   
76.
77.
A novel component which specifically binds butyrate was found in rat and mouse liver. This component, termed butyrate binding protein (BBP), is localized in the cytosolic fraction and exhibits protein characteristics, such as heat- and protease-sensitivity. The size of BBP was found to be 7.6S, while it was converted to subunits of 45,000--48,000 dalton by treatment with sodium dodecyl sulfate. The dissociation constant of the binding of BBP with butyrate was 2.22 X 10(-6) M in the standard assay. 30-Fold purification of BBP was achieved by batch-wise adsorption and elution from CM-cellulose and hydroxylapatite column chromatography. BBP is clearly distinguishable from the fatty acid-binding protein found previously on the basis of its size and binding specificity.  相似文献   
78.
Summary To localize the binding sites of aminoglycosides in the cochlea, immunocytochemistry was used with the antibody to gentamicin and the protein-A/gold complex. We found that the main binding sites were the stereocilia, the cuticular plates of hair cells, the head plates of Deiters' cells, cell filaments and the cones of pillar cells, tectorial membranes, basilar membranes, the matrix of the spiral limbus, plasma membranes, mitochondria, and the chromatin of various kinds of cells. Triphosphoinositide and acidic glycosaminoglycans are the two most likely candidates for the cause of binding activity.  相似文献   
79.
Pandanus boninensis, endemic to the Ogasawara Islands, Japan, is distributed on both the older Bonin and younger Volcano Islands. In this study, we conducted population genetic analyses of P. boninensis on these islands to examine the population diversity and structure across old and young islands, to assess potential differences in population demography with island age, and to collect any evidence of migration between old and young islands. We found that the genetic diversity of expressed sequence tag (EST)–based microsatellite (SSR) markers, the nucleotide diversity of nuclear DNA sequences, and the haplotype diversity of chloroplast DNA on young islands were lower than those on old islands. Clustering analyses of EST‐SSR indicated that populations on old islands were strongly diverged from those on young islands. Approximate Bayesian computation analysis of EST‐SSR suggested that population expansion occurred on old islands while population reduction occurred on young islands. We also found evidence of migration among old islands (mostly from south to north), while it appears that there have been very few migration events between old and young islands. These differences could be due to the fact that young islands tend to be geographically isolated and support smaller populations that began a shorter time ago from limited founders. The P. boninensis populations on the Volcano Islands are interesting from an evolutionary perspective as they constitute a classic example of the early stages of progressive colonization on oceanic islands with small effective population sizes and low genetic diversity.  相似文献   
80.
Molecular Biology Reports - Pyridoxine (PN), one of the vitamers of vitamin B6, plays an important role in the maintenance of epidermal function and is used to treat acne and rough skin. Clinical...  相似文献   
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