全文获取类型
收费全文 | 48733篇 |
免费 | 3509篇 |
国内免费 | 19篇 |
出版年
2023年 | 149篇 |
2022年 | 557篇 |
2021年 | 961篇 |
2020年 | 595篇 |
2019年 | 705篇 |
2018年 | 1057篇 |
2017年 | 923篇 |
2016年 | 1510篇 |
2015年 | 2401篇 |
2014年 | 2732篇 |
2013年 | 3053篇 |
2012年 | 4050篇 |
2011年 | 3882篇 |
2010年 | 2447篇 |
2009年 | 2233篇 |
2008年 | 3095篇 |
2007年 | 3005篇 |
2006年 | 2593篇 |
2005年 | 2408篇 |
2004年 | 2216篇 |
2003年 | 1915篇 |
2002年 | 1676篇 |
2001年 | 1320篇 |
2000年 | 1252篇 |
1999年 | 1014篇 |
1998年 | 409篇 |
1997年 | 349篇 |
1996年 | 252篇 |
1995年 | 218篇 |
1994年 | 214篇 |
1993年 | 177篇 |
1992年 | 338篇 |
1991年 | 312篇 |
1990年 | 285篇 |
1989年 | 242篇 |
1988年 | 189篇 |
1987年 | 177篇 |
1986年 | 143篇 |
1985年 | 117篇 |
1984年 | 94篇 |
1983年 | 97篇 |
1982年 | 72篇 |
1981年 | 62篇 |
1980年 | 69篇 |
1979年 | 78篇 |
1978年 | 61篇 |
1977年 | 55篇 |
1975年 | 50篇 |
1974年 | 71篇 |
1973年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Localization and Pattern of Graviresponse across the Pulvinus of Barley Hordeum vulgare 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Pulvini of excised stem segments from barley (Hordeum vulgare cv `Larker') were pretreated with 1 millimolar coumarin before gravistimulation to reduce longitudinal cell expansion and exaggerate radial cell enlargement. The cellular localization and pattern of graviresponse across individual pulvini were then evaluated by cutting the organ in cross-section, photographing the cross-section, and then measuring pulvinus thickness and the radial width of cortical and epidermal cells in enlargements of the photomicrographs. With respect to orientation during gravistimulation, we designated the uppermost point of the cross-section 0° and the lowermost point 180°. A gravity-induced increase in pulvinus thickness was observable within 40° of the vertical in coumarin-treated pulvini. In upper halves of coumarin-treated gravistimulated pulvini, cells in the inner cortex and inner epidermis had increased radial widths, relative to untreated gravistimulated pulvini. In lower halves of coumarin-treated pulvini, cells in the central and outer cortex and in the outer epidermis showed the greatest increase in radial width. Cells comprising the vascular bundles also increased in radial width, with this pattern following that of the central cortex. These results indicate (a) that all cell types are capable of showing a graviresponse, (b) that the graviresponse occurs in both the top and the bottom of the responding organ, and (c) that the magnitude of the response increases approximately linearly from the uppermost point to the lowermost. These results are also consistent with models of gravitropism that link the pattern and magnitude of the graviresponse to graviperception via statolith sedimentation. 相似文献
92.
The AIDS epidemic has transformed the importance of cytomegalovirus (CMV) as a pathogen for the adult human central nervous system (CNS). At autopsy, about 25 percent of AIDS cases have cytopathologic evidence of CNS infection by CMV. Since almost nothing is known of the host CNS-viral interactions, we have developed a laboratory model of CMV infection of the brain in the guinea pig. In the present paper, we review the syndromes of CMV infection of the human CNS and compare the neuropathological findings of the opportunistic CMV brain infection in AIDS with the model. Destructive meningoencephalitis, perivascular infiltrates, and subependymal inflammation are found in both, but the glial nodule is the most characteristic feature of each. Thus, we demonstrate that the model faithfully reflects the histopathology of the human disease. Furthermore, since we have found that CNS infection is achieved following systemic infection in the guinea pig, the model recapitulates the sequence of infection in humans. 相似文献
93.
94.
95.
Summary In enzymatic synthesis of cephalexin (CEX) from 7-amino-3-deacetoxycephalosporanic acid (7-ADCA) and D--phenylglycine methyl ester (PGM) using an acylase fromXanthomonas citri, it was found that the synthetic activity and conversion yield were enhanced markedly by depressing the water activity (a
w
) of reaction system. This enhancement was probably resulted from the change of thermodynamic equilibrium and maximized at a range ofa
w
from 0.96 to 0.97. 相似文献
96.
A stable shuttle vector which replicates in Escherichia coli and Clostridium perfringens was constructed by ligating a 3.6-kilobase (kb) fragment of plasmid pBR322 with C. perfringens plasmid pHB101 (3.1 kb). The marker for this shuttle plasmid originated from the 1.3-kb chloramphenicol resistance gene of plasmid pHR106. The resulting shuttle vector, designated pAK201, is 8 kb in size and codes for resistance to 20 micrograms of chloramphenicol per ml in both E. coli and C. perfringens. Following shuttle vector construction in E. coli, plasmid pAK201 was transformed into E. coli HB101 and C. perfringens ATCC 3624A, using intact cell electroporation. The transformation frequencies were 10(6) and 10(4) transformants per microgram of DNA in E. coli and C. perfringens, respectively. Restriction enzyme analysis of the chimera isolated from transformants of both microorganisms suggested that the plasmids were identical. Reciprocal transformation experiments in E. coli and C. perfringens indicated no difference in transformation frequency. Plasmid pAK201 was stable in C. perfringens following repeated transfer in the absence of chloramphenicol pressure. The restriction map of plasmid pAK201 shows six unique cut sites which should be useful for future genetic analysis and C. perfringens gene library construction. 相似文献
97.
Two subunits of mannose permease, II-PMan and II-MMan, of Escherichia coli mediate coliphage N4 infection 总被引:1,自引:0,他引:1
Mutant strains of Escherichia coli that lack one or all of the intact components of mannose permease do not support the growth of phage N4. Complementation experiments using three recombinant plasmids containing DNA fragments coding for the subunits of mannose permease revealed that among the three component subunits, II-PMan and II-MMan alone are sufficient to confer N4 sensitivity. 相似文献
98.
R Morita S Morimoto E Koh K Fukuo S Kim K Itoh K Taniguchi T Onishi T Ogihara 《Biochemistry international》1989,18(3):647-653
Low density lipoprotein (LDL), a major cholesterol-carrying lipoprotein in the plasma, binds to its receptor through apoprotein B (Apo-B). The addition of LDL and Apo-B induced rapid (5 s), but transient increase in the inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) level with K0.5 values of 1.1 and 0.07 microgram/ml, accompanied by increases of cytosolic free Ca2+ concentration [( Ca2+]i), in vascular smooth muscle cells (VSMC). The increases by LDL and Apo-B were both reduced by pretreatment of the VSMC with pertussis toxin. The early change in Ins-1,4,5-P3 involving a GTP-binding protein may function as an initial signal for the action of LDL in VSMC. 相似文献
99.
The cDNA cloning of a pea (Pisum sativum) seed lipoxygenase. Sequence comparisons of the two major pea seed lipoxygenase isoforms. 总被引:6,自引:0,他引:6
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Cloning and sequencing of two cDNAs from mRNA of maturing pea (Pisum sativum) seeds allowed the deduction of the complete amino acid sequence of a lipoxygenase polypeptide which is most similar to that of soya-bean lipoxygenase 2. The predicted Mr of this polypeptide is 97134, and its sequence permits comparisons between the lox2-type and the lox3-type lipoxygenase isoforms from pea and soya bean (Glycine max). 相似文献
100.
DNA can be removed or separated by the selective adsorption/desorption on positively charged submicronsized polymeric particles (SSPP). The selective adsorption of DNA, in the presence of protein, on positively charged SSPP was accomplished by increasing the concentration of potassium phosphate or sodium phosphate. The adsorption of DNA was not affected by the concentration of potassium phosphate or sodium phosphate up to 1.2M. On the other hand, the adsoprtion of a protein (bovine serum albumin) was completely impeded by 170mM potassium phosphate. DNA adsorbed on SSPP could be desorbed by increasing the concentration of NaCl or KCl, thus it can be recovered. DNA desorbed from SSPP when the concentration of NaCl or KC was higher than 0.6M. A complete desorption of DNA was achieved at the concentration of NaCl or KCl above 1.2M. 相似文献