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   33839篇
  免费   3068篇
  国内免费   22篇
  2023年   149篇
  2022年   369篇
  2021年   800篇
  2020年   431篇
  2019年   559篇
  2018年   665篇
  2017年   550篇
  2016年   998篇
  2015年   1679篇
  2014年   1763篇
  2013年   2104篇
  2012年   2728篇
  2011年   2788篇
  2010年   1708篇
  2009年   1411篇
  2008年   2074篇
  2007年   2066篇
  2006年   2009篇
  2005年   1749篇
  2004年   1757篇
  2003年   1569篇
  2002年   1517篇
  2001年   354篇
  2000年   286篇
  1999年   322篇
  1998年   326篇
  1997年   241篇
  1996年   222篇
  1995年   195篇
  1994年   172篇
  1993年   165篇
  1992年   164篇
  1991年   188篇
  1990年   167篇
  1989年   152篇
  1988年   134篇
  1987年   123篇
  1986年   116篇
  1985年   134篇
  1984年   158篇
  1983年   140篇
  1982年   139篇
  1981年   135篇
  1980年   113篇
  1979年   99篇
  1978年   74篇
  1977年   86篇
  1976年   80篇
  1975年   80篇
  1973年   78篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
To determine whether mesenchyme plays a part in the differentiation of gut endocrine cells, proventricular endoderm from 4- to 5-day chick or quail embryos was associated with mesenchyme from the dorsal pancreatic bud of chick embryos of the same age. The combinations were grown on the chorioallantoic membranes of host chick embryos until they reached a total incubation age of 21 days. Proventricular or pancreatic endoderm of the appropriate age and species reassociated with its own mesenchyme provided the controls. Morphogenesis in the experimental grafts corresponded closely to that in proventricular controls, i.e. the pancreatic mesenchyme supported the development of proventricular glands from proventricular endoderm. Insulin, glucagon and somatostatin cells and cells with pancreatic polypeptide-like immunoreactivity differentiated in the pancreatic controls. The latter three endocrine cell types, together with neurotensin and bombesin/gastrin-releasing polypeptide (GRP) cells, developed in proventricular controls and experimental grafts. The proportions of the major types common to proventriculus and pancreas (somatostatin and glucagon cells) were in general similar when experimental grafts were compared with proventricular controls but different when experimental and pancreatic control grafts were compared. Hence pancreatic mesenchyme did not materially affect the proportions of these three cell types in experimental grafts, induced no specific pancreatic (insulin) cell type and allowed the differentiation of the characteristic proventricular endocrine cell types, neurotensin and bombesin/GRP cells. However, an important finding was a significant reduction in the proportion of bombesin/GRP cells, attributable in part to a decrease in their number and in part to an increase in the numbers of endocrine cells of the other types. This indicates that mesenchyme may well play a part in determining the regional specificity of populations of gut endocrine cells.  相似文献   
102.
Andrew  Kitchener 《Journal of Zoology》1987,213(4):621-639
The keratinous horns of bovids are used in intraspecific combat to gain access to females in oestrus. Horn sheath keratin is a composite material consisting of stiff protein fibres and a pliant protein matrix. Unlike antlers, horns are permanent structures which are likely to accumulate damage during fighting. Therefore, horn sheath keratin should be resistant to fracture (tough) and insensitive to surface defects (scratches and cracks) which may weaken horns by acting as stress concentrators.
The effect of water on the toughness and notch-sensitivity of horn sheath keratin was investigated in three-point bending and tensile tests. Several measures of toughness were made on dry (0% water content), fresh (20%) and wet (40%) horn keratin, including total work of fracture, Gurney & Hunt work of fracture, critical strain energy release rate and critical stress intensity factor.
The mean total work of fracture of fresh horn is about 40 kJ/m2 which is relatively much greater than most biological and synthetic materials. Most of the work of fracture is due to plastic yielding of the matrix (50–75%); the rest is due to crack-tip specific fracture mechanisms such as fibre pull-out and Cook Gordon crack-stopping. Dehydration reduces the total work of fracture of horn keratin by preventing the yielding of the matrix.
The strength of fresh and wet horn is insensitive to notches, but dry horn is very notch-sensitive. Therefore, bovids must avoid dehydration of their horns due to the desiccating effect of the environment. The 'horning' behaviour of bovids may be a maintenance activity which ensures that the horn sheath is adequately hydrated to remain tough and notch-insensitive.  相似文献   
103.
104.
Members of the Chenopodiaceae can accumulate high levels (>100 mol·(g DW)-1) of glycine betaine (betaine) in leaves when salinized. Chenopodiaceae synthesize betaine by a two-step oxidation of choline (cholinebetaine aldehyde betaine), with the second step catalyzed by betaine aldehyde dehydrogenase (BADH, EC 1.2.1.8). High betaine levels have also been reported in leaves of species from several distantly-related families of dicotyledons, raising the question of whether the same betaine-synthesis pathway is used in all cases.Fast atom bombardment mass spectrometry showed that betaine levels of >100 mol·(g DW)-1 are present in Lycium ferocissimum Miers (Solanaceae), Helianthus annuus L. (Asteraceae), Convolvulus arvensis L. (Convolvulaceae), and Amaranthus caudatus L. (Amaranthaceae), that salinization promotes betaine accumulation in these plants, and that they can convert supplied choline to betaine aldehyde and betaine. Nicotiana tabacum L. and Lycopersicon lycopersicum (L.) Karst. ex Farw. (Solanaceae), Lactuca sativa L. (Asteraceae) and Ipomoea purpurea L. (Convolvulaceae) also contained betaine, but at a low level (0.1–0.5 mol·(g DW)-1. Betaine aldehyde dehydrogenase activity assays, immunotitration and immunoblotting demonstrated that the betaine-accumulating species have a BADH enzyme recognized by antibodies raised against BADH from Spinacia oleracea L. (Chenopodiaceae), and that the Mr of the BADH monomer is in all cases close to 63 000. These data indicate that the cholinebetaine aldehydebetaine pathway may have evolved by vertical descent from an early angiosperm ancestor, and might be widespread (albeit not always strongly expressed) among flowering plants. Consistent with these suggestions, Magnolia x soulangiana was found to have a low level of betaine, and to express a protein of Mr 63 000 which cross-reacted with antibodies to BADH from Spinacia oleracea.Abbreviations BADH Betaine aldehyde dehydrogenase - DCIMS desorption chemical ionization mass spectrometry - FABMS fast atom bombardment mass spectrometry - Mr relative molecular mass - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate - TLC thin-layer chromatography  相似文献   
105.
Simian acquired immune deficiency syndrome (SAIDS) caused by the type D retrovirus SRV-1 results in opportunistic infections and a spectrum of oral lesions similar to those seen in humans with AIDS. To better understand the pathogenesis of these oral lesions we have retrospectively examined the oral mucosa from ten rhesus monkeys that died with SAIDS and prospectively examined the oral mucosa of ten additional animals inoculated with SRV-1 to determine at what time, and in what cells SRV-1 infection of the oral mucosa occurs. Using single and double label immunohistologic techniques, and electron microscopy we detected SRV-1 in clusters of oral epithelial cells and rare Langerhans cells as early as 1 month postinoculation.  相似文献   
106.
An overhanging 3'' terminus is a conserved feature of telomeres.   总被引:32,自引:10,他引:22       下载免费PDF全文
The reactivity of single-stranded thymidines with osmium tetraoxide was used to demonstrate the existence of a terminal overhang of the G-rich strand of telomeres from two distantly related eucaryotes, the ciliated protozoan Tetrahymena spp. and the acellular slime mold Didymium spp. Conservation of a G-strand overhang at the molecular terminus of telomeres is consistent with our suggestion that an unusual DNA structure formed by the G-strand overhang is important for telomere function (E. Henderson, C. C. Hardin, S. K. Wolk, I. Tinoco Jr., and E. H. Blackburn, Cell 51:899-908, 1987).  相似文献   
107.
Interest in the biological behavior of a growing number of elements, along with increasing recognition of the importance of interactions among them, demands a versatile and reliable technique for multielement analysis of biological samples. Significant improvements over the sensitivity achieved with conventional inductively coupled plasma (ICP) optical emission spectrometries have been realized with the introduction of quadrupole mass spectrometry (MS) for detection of ions in the plasma. The hybrid technique of ICP-MS promises to be a method of rapid multielement analysis, at detection limits that approach or surpass those of other technologies. However, the application of ICP-MS to analyses of biological interest is truly in its infancy. Here we report the use of ICP-MS for the determination of more than 30 elements of biological interest in a tissue and a biological fluid (rat liver and serum, respectively). Experimental values of the elements serve as a basis for discussion of analytical protocols, performance criteria, and certain problems peculiar to ICP-MS.  相似文献   
108.
Summary Bacillus brevis 47 was cultivated in 2-1 fermentors to study the effect of medium supplementation on extracellular protein production. Additional polypeptone, when supplied initially or at 12 h (late exponential phase), had little stimulatory effect on extracellular protein levels, which reached 6–7 g/l after 48h. A large increase in protein production was observed, however, when polypeptone was added at 21 h (stationary phase). This addition resulted in the accumulation in the medium of 14 g/l protein after 48 h, and a total of 16 g/l when cell-bound protein was included. In all cases, glucose was consumed only very slowly.  相似文献   
109.
Methane monooxygenase (MMO) is the enzyme responsible for the conversion of methane to methanol in methanotrophic bacteria. In addition, this enzyme complex oxidizes a wide range of aliphatic and aromatic compounds in a number of potentially useful biotransformations. In this study, we have used biochemical data obtained from purification and characterization of the soluble MMO from Methylococcus capsulatus (Bath), to identify structural genes encoding this enzyme by oligonucleotide probing. The genes encoding the and subunits of MMO were found to be chromosomally located and were linked in this organism. We report here on the analysis of a recombinant plasmid containing 12 kilobases of Methylococcus DNA and provide the first evidence for the localization and linkage of genes encoding the methane monooxygenase enzyme complex. DNA sequence analysis suggests that the primary structures of the and subunit of MMO are completely novel and the complete sequence of these genes is presented.  相似文献   
110.
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