全文获取类型
收费全文 | 8739篇 |
免费 | 504篇 |
国内免费 | 6篇 |
出版年
2023年 | 71篇 |
2022年 | 111篇 |
2021年 | 256篇 |
2020年 | 152篇 |
2019年 | 169篇 |
2018年 | 238篇 |
2017年 | 216篇 |
2016年 | 275篇 |
2015年 | 355篇 |
2014年 | 409篇 |
2013年 | 568篇 |
2012年 | 585篇 |
2011年 | 556篇 |
2010年 | 338篇 |
2009年 | 281篇 |
2008年 | 395篇 |
2007年 | 356篇 |
2006年 | 339篇 |
2005年 | 336篇 |
2004年 | 273篇 |
2003年 | 228篇 |
2002年 | 233篇 |
2001年 | 205篇 |
2000年 | 178篇 |
1999年 | 166篇 |
1998年 | 60篇 |
1997年 | 58篇 |
1996年 | 65篇 |
1995年 | 52篇 |
1994年 | 59篇 |
1993年 | 37篇 |
1992年 | 118篇 |
1991年 | 120篇 |
1990年 | 88篇 |
1989年 | 95篇 |
1988年 | 89篇 |
1987年 | 92篇 |
1986年 | 89篇 |
1985年 | 94篇 |
1984年 | 83篇 |
1983年 | 52篇 |
1982年 | 49篇 |
1981年 | 43篇 |
1980年 | 50篇 |
1979年 | 82篇 |
1978年 | 59篇 |
1977年 | 51篇 |
1976年 | 42篇 |
1974年 | 41篇 |
1973年 | 44篇 |
排序方式: 共有9249条查询结果,搜索用时 187 毫秒
81.
Possible implications of reciprocity between ethylene and aflatoxin biogenesis in Aspergillus flavus and Aspergillus parasiticus. 总被引:2,自引:1,他引:1 下载免费PDF全文
Aspergillus flavus and Aspergillus parasiticus produced ethylene during early growth. However, the onset of toxin biosynthesis was marked by the absence of ethylene evolution. 2-Chloroethyl phosphonic acid, an ethylene-generating compound, inhibited aflatoxin biosynthesis in vivo. The reciprocal relationship between the production of aflatoxin and ethylene by the organism may indicate the involvement of the latter in the regulation of aflatoxin biogenesis. 相似文献
82.
Cell surface tubulin in leukemic cells: molecular structure, surface binding, turnover, cell cycle expression, and origin 下载免费PDF全文
We report here new characteristics of cell surface tubulin from a human leukemia cell line. These cells (CEM cells) possess tubulin that is readily iodinated on the surface of living cells, turns over at a rate identical to that of other surface proteins, and is present throughout the cell cycle. When removed with trypsin, it rapidly returns to the surface. Peptide mapping of iodinated surface tubulin indicates that it possesses a similar, but not identical, primary structure to total CEM and rat brain tubulin. Living CEM cells are able to bind specifically a subfraction of CEM tubulin from metabolically labeled high speed supernatants of lysed CEM cells. Surface tubulin is more basic than the total tubulin pool. The binding, which is saturable, is inhibited by unlabeled CEM high speed supernatants but not by excess thrice-cycled rat or bovine brain tubulin. Surface tubulin is also shown to bind to living nontransformed normal rat kidney cells but not to normal, circulating, mononuclear white cells. Activated lymphocytes produce a tubulin that binds to CEM cells. Since CEM tubulin was detected in the media of 6-h cultures of CEM cells, we must conclude that at least some of the surface tubulin comes from the media. We further conclude that these leukemic cells produce an unusual tubulin that may bind specifically to any membrane. The presence of iodinatable surface tubulin, however, appears to require both the production of a unique tubulin and the presence of a "receptor-like" surface binding component. 相似文献
83.
In vivo recombinant interleukin 2 administration enhances survival against a lethal challenge with Toxoplasma gondii 总被引:22,自引:0,他引:22
S D Sharma J M Hofflin J S Remington 《Journal of immunology (Baltimore, Md. : 1950)》1985,135(6):4160-4163
Administration of recombinant interleukin 2 (rIL 2) resulted in a significant (p less than 0.01) decrease in mortality in mice infected with a dose of Toxoplasma gondii that killed 100% of untreated mice. Mice treated with rIL 2 had a significantly (less than 0.005) lower numbers of cysts in the brains. The protection afforded by rIL 2 could not be correlated with increased antibody synthesis or be explained by increased macrophage killing in the treated mice. Mice treated with rIL 2 after Toxoplasma infection demonstrated increased natural killer (NK) cell activity compared with either Toxoplasma-infected or rIL 2-treated mice. rIL 2 failed to reverse the suppressed proliferative response of lymphocytes to concanavalin A and lipopolysaccharide in mice acutely infected with a virulent strain of T. gondii. These results reveal that rIL 2 may have a remarkably protective effect against intracellular parasites. 相似文献
84.
Ultraviolet-irradiated Escherichia coli K12 uvrA(B,C) cells show higher survival if plated on minimal growth medium (MM) rather than on rich growth medium (RM). This phenomenon has been referred to as 'minimal medium recovery' (MMR). UV-irradiated (4 J/m2) uvrA cells showed a similar rate of protein synthesis, whether incubated in MM or RM, however, they showed a severe depression in DNA synthesis when incubated in MM that lasted for about 30 min, and the normal rate of DNA synthesis was not reestablished until about 60 min after irradiation. When a sample of these same cells was switched to RM immediately after UV-irradiation, there was only a slight slowing of DNA synthesis, and the normal rate of synthesis was reestablished by 60 min. An additional mmrA mutation or growth retardation by valine blocked both this extra DNA synthesis in RM, and the inhibitory effect of RM on survival. These findings suggest that the absence of a marked delay in DNA synthesis observed in RM may be responsible for the inhibitory effect of RM on the survival of UV-irradiated excision-deficient cells. Two hypotheses, which are not mutually exclusive, are proposed and supported by data to explain why a fast rate of DNA synthesis after UV-irradiation partially inhibits postreplication repair and enhances cell lethality. 相似文献
85.
The lipid level (fresh weight basis) of Cuscuta reflexa Roxb. was related to the lipid content of the host plants Meilicago saliva L., Helianthus annuus L., Pisum sativum L. and Lantana camara L. Parasitizing by the dodder significantly increased the total lipid level of the hosts. The increase was mainly due to enhancement in the neutral lipid fraction.
The level of phospholipid in the parasite was always higher than in its hosts. Phospholidyl choline and phosphatidyl ethanolamine constituted about 65% of the total phospholipid of Cuscuta. This was followed by phosphatidyl inositol (ca 20%) and phosphatidyl glycerol (ca 12%). Phosphatidic acid constituted only ca 3% of the phospholipids of Cuscuta. Although the total phospholipid levels of various host plants were not affected as a result of the infection by Cuscuta, a significant decrease occurred in the levels of phosphatidyl eholine and phosphatidyl ethanolamine as well as marked increases in phosphatidyl inositol and phosphatidic acid. The infected tissue showed an increase in phospholipase D activity as compared with the controls. The results have been discussed in relation to changes in permeability of the infected tissue. 相似文献
The level of phospholipid in the parasite was always higher than in its hosts. Phospholidyl choline and phosphatidyl ethanolamine constituted about 65% of the total phospholipid of Cuscuta. This was followed by phosphatidyl inositol (ca 20%) and phosphatidyl glycerol (ca 12%). Phosphatidic acid constituted only ca 3% of the phospholipids of Cuscuta. Although the total phospholipid levels of various host plants were not affected as a result of the infection by Cuscuta, a significant decrease occurred in the levels of phosphatidyl eholine and phosphatidyl ethanolamine as well as marked increases in phosphatidyl inositol and phosphatidic acid. The infected tissue showed an increase in phospholipase D activity as compared with the controls. The results have been discussed in relation to changes in permeability of the infected tissue. 相似文献
86.
The tissue distributions of sinapic acid esters (1-sinapoylglucose, sinapolyl-l-malate, 6,3-disinapoylsucrose), kaempferol glycosides, free malic acid and of the enzyme involved in the synthesis of sinapoyl-l-malate, 1-sinapoylglucose: l-malate sinapoyltransferase (SMT), have been investigated in cotyledons of Raphanus sativus L. seedlings. The kaempferol glycosides were mainly localized in the upper epidermis. The sinapoyl esters were found in all tissues, but differed markedly in their concentrations. While disinapoylsucrose was localized predominantly in the mesophyll, most sinapoylmalate was found in the epidermal layers, as was most SMT activity. Ultraviolet microscopy and microfluorospectrophotometry of isolated epidermal peels indicated that the epidermal sinapoyl esters were restricted to guard cells, guard mother cells and adjacent epidermal cells. Upon excitation by UV light (365 nm) these exhibited strong blue fluorescence with an emission maximum at about 480 nm. Our results indicate a highly tissue-and cell-specific secondary metabolism in Raphanus cotyledons and indicate that the biosynthesis of sinapoylmalate is intimately related to the malic-acid metabolism of the guard cells.Abbreviations HPLC
high-performance liquid chromatography
- SMT
1-sinapoylglucose: l-malate sinapoyltransferase 相似文献
87.
The movement of radioactively labelled polypeptides into the microtubule-associated transport channels in the ovaries of a hemipteran insect has been analysed using SDS-polyacrylamide slab gel electrophoresis and fluorography. The patterns of label suggest that the microtubules which pack the transport channels form a relatively static cytoskeleton while other components move independently from them along the channels. As well as illustrating the functional organisation of microtubule-associated transport in this system our studies of labelled proteins have also provided clues as to the mechanism of transport itself. 相似文献
88.
89.
90.
The geminate recombination of CO with Hb following dissociation by a 10-ns laser pulse has been studied as a function of pH (9.2 and 7.0 without inositol hexaphosphate and 6.0 with inositol hexaphosphate) and temperature (5-35 degrees C). The hemoglobins studied included adult, Rothschild, rabbit, opossum, and carp. Despite significant differences in their structural and functional properties, the first four of these hemoglobins show similar trends in the yields, rates, and activation energies of the geminate recombination. The nature of the "cage recombination" in hemoglobin is discussed in the light of such findings. Neither a slow diffusion model nor a model based upon a specific non-heme binding site accounts for the observations. 相似文献