全文获取类型
收费全文 | 4729篇 |
免费 | 368篇 |
出版年
2023年 | 26篇 |
2022年 | 43篇 |
2021年 | 80篇 |
2020年 | 53篇 |
2019年 | 63篇 |
2018年 | 73篇 |
2017年 | 80篇 |
2016年 | 118篇 |
2015年 | 221篇 |
2014年 | 246篇 |
2013年 | 255篇 |
2012年 | 360篇 |
2011年 | 340篇 |
2010年 | 252篇 |
2009年 | 196篇 |
2008年 | 278篇 |
2007年 | 258篇 |
2006年 | 253篇 |
2005年 | 240篇 |
2004年 | 241篇 |
2003年 | 222篇 |
2002年 | 234篇 |
2001年 | 50篇 |
2000年 | 41篇 |
1999年 | 58篇 |
1998年 | 64篇 |
1997年 | 45篇 |
1996年 | 47篇 |
1995年 | 35篇 |
1994年 | 36篇 |
1993年 | 44篇 |
1992年 | 36篇 |
1991年 | 37篇 |
1990年 | 22篇 |
1989年 | 45篇 |
1988年 | 33篇 |
1987年 | 20篇 |
1986年 | 25篇 |
1985年 | 16篇 |
1984年 | 25篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 21篇 |
1980年 | 19篇 |
1979年 | 16篇 |
1978年 | 20篇 |
1977年 | 14篇 |
1976年 | 15篇 |
1974年 | 20篇 |
1973年 | 15篇 |
排序方式: 共有5097条查询结果,搜索用时 31 毫秒
91.
92.
Pamela J. Weathers 《Applied microbiology》1984,48(6):1251-1253
Evidence is presented here that axenic cultures of Chlorella, Scenedesmus, Coelastrum, and Chlorococcum spp. evolve N2O when grown on NO2−, showing that the Chlorophyceae are a source of N2O in aquatic systems. 相似文献
93.
Ammonia oxidation, as measured by nitrite production, was inhibited by 2-chloro-6-trichloromethyl-pyridine (nitrapyrin, N-serve) in the methane-oxidizing bacterium,Methylosinus trichosporium OB3b, and the autotrophic nitrifying organisms,Nitrosococcus oceanus andNitrosomonas marina. 6-Chloropicolinic acid, a hydrolysis product of nitrapyrin, was ineffective as an inhibitor of ammonia oxidation by either the methanotroph or the autotrophs. Picolinic acid (2-carboxy-pyridine), in contrast, inhibited nitrification by the methane-oxidizing bacterium but not by the autotrophic cultures. Picolinic acid may provide a means for differentiating ammonia oxidation attributable to methanotrophs from that resulting from autotrophs in environmental studies. 相似文献
94.
Aphanocapsa 6308 metabolizes both NaHCO3 and Na2CO3. The short term incorporation (5-s) metabolic pattern and the patterns of incorporation of bicarbonate for exponential versus stationary phase cultures differ, however. Cells were equilibrated for 10 min in air and distilled water prior to injection of either NaH14CO3 at pH 8.0, or Na2
14CO3 at pH 11.0. Hot ethanol extracts were analyzed via paper chromatography and autoradiography for products of CO2 fixation. At 5 s, malate (51.5%) predominates slightly as a primary bicarbonate fixation product over 3-phosphoglycerate (40.3%); 3-phosphoglycerate is the primary product of carbonate fixation. At 60 s, the carbonate and bicarbonate labelling patterns are similar. Cells in stationary phase fix in 5 s a greater proportion of bicarbonate into malate (36% vs. 14% for 3-phosphoglycerate) than do cells in exponential growth. Likewise, 60 s incorporations show a large amount of bicarbonate fixed into aspartate (30.9%) in stationary phase cells over that of exponential phase (11.6%). These data suggest an operative C4 pathway for purposes not related to carbohydrate synthesis but rather as compensation for the incomplete tricarboxylic acid cycle in cyanobacteria. The enhancement of both aspartate fixation and CO2 fixation into citrulline in stationary phase correlates with an increase in cyanophycin granule production which requires both aspartate and arginine.Nonstandard Abbreviations 3-PGA
3-phosphoglyceric acid
- TCA
tricarboxylic acid 相似文献
95.
From the unripe fruits of Sapium indicum, three aliphatic esters of the tigliane nucleus were isolated. These compounds were derivatives of 4-deoxyphorbol. Sapatoxin A was identified as 12-O-[n-deca-2,4,6-trienoyl]-4-deoxyphorbol-13-acetate, B as 12-O-[n-deca-2,4,6-trienoyl]-4-deoxy-5-hydroxyphorbol-13-acetate and C as 12-O-[n-deca-2,4,6-trienoyl]-4,20-dideoxy-5-hydroxyphorbol-13-acetate, by spectroscopic analysis and hydrolysis reactions. 相似文献
96.
C3H mice were infected subcutaneously with 105 promastigotes of Leishmania mexicana and subsequent lesions were examined at 3, 5, and 8 months. All animals developed persistent nonulcerating nodules of variable size which did not metastasize. The nodules contained amastigotes with a mononuclear infiltrate of histiocytes, lymphocytes, and plasma cells, but without formation of tuberculoid-type granulomas. Neutrophils and eosinophils were also encountered in some cases. Specific antileishmanial antibodies and delayed-type hypersensitivity to leishmanial antigen were present at 3, 5, and 8 months postinfection. L. mexicana infection in C3H mice differs from classic self-healing cutaneous leishmaniasis by the pesistence of nonhealing, nonulcerating, nonmetastasizing lesions, despite evidence of cellular and humoral immunity. 相似文献
97.
The rate of spontaneous mutation to resistance to the 12,13-epoxytrichothecene trichodermin has been determined under different growth limiting conditions in continuous cultures of the microbial eukaryote Schizosaccharomyces pombe. In agreement with data obtained in bacterial systems by previous workers the kinetics observed for the accumulation of mutations is found to be dependent upon the nutrient used to limit the growth of the population. Under conditions of glucose-limitation mutation accumulation is directly proportional to the rate of cell division, while under histidine-limitation it is proportional to chronological time. Various possible explanations for these observations are discussed. 相似文献
98.
G G Choudhury V L Sylvia A Y Sakaguchi 《The Journal of biological chemistry》1991,266(34):23147-23151
Receptor tyrosine kinases couple to multiple intracellular effector molecules that are crucial for normal cell growth and transformation. Stimulation of membrane phospholipid hydrolysis by receptor tyrosine kinases is one such pathway for generating intracellular second messengers that may be important for mitogenesis. Certain receptor tyrosine kinases tyrosine phosphorylate a phosphoinositide-specific phospholipase C that hydrolyses the membrane phospholipid phosphatidylinositol 4,5-bisphosphate. In contrast, the glycoprotein receptor for colony stimulating factor 1, a transmembrane tyrosine kinase, does not utilize this pathway, but rather stimulates the hydrolysis of phosphatidylcholine. Here we show that eluates of antiphosphotyrosine affinity purified lysates of colony-stimulating factor 1-stimulated cells contain elevated levels of phosphatidylcholine-specific phospholipase C activity. The affinity-purified activity is sensitive to tyrosine-specific T-cell phosphatase, and is detected in the membrane fraction of stimulated cells. Recovery of phospholipase C activity in the antiphosphotyrosine protein fraction is reduced by pertussis toxin pretreatment of cells. The phosphatidylcholine phospholipase C activity in isolated membranes of colony-stimulating factor 1-treated cells was also reduced by pertussis toxin treatment and stimulated by guanosine 5'-3-O-(thio)triphosphate. These results indicate that colony stimulating factor 1 receptor-mediated stimulation of phosphatidylcholine-specific phospholipase C requires tyrosine phosphorylation, and might be affected by a G-protein coupled pathway. 相似文献
99.
Interleukin-1 (IL-1) plays an important role in cartilage destruction associated with inflammatory and degenerative arthritis because of its ability to induce matrix degrading enzymes. Previously, we have shown that the IL-1-induced chondrocyte protease activity was inhibited by transforming growth factor-β (TGF-β). In this paper, we show that TGF-β inhibits the IL-1-induced synthesis of collagenase and stromelysin by reducing the steady-state mRNA levels in rabbit articular chondrocytes. We further demonstrate that TGF-β-treated chondrocytes show reduced 125I-IL-1 binding that returns to a normal level when TGF-β is removed from the culture medium. The inhibitory effect of TGF-β is observed for both naturally occurring as well as fibroblast growth factor (FGF)-inducible binding sites (receptors). Scatchard analysis of receptor—ligand interactions demonstrate that the reduced binding is due to a reduction in the number of receptors for IL-1 and is not due to changes in affinity. Affinity cross-linking studies suggest that control chondrocytes contain two major cross-linked bands of Mr =116 and 80 kDa and a minor band of Mr =100 kDa. FGF-treated cells show enhanced levels of all the bands, plus an additional 200-kDa band. TGF-β treatment of chondrocytes results in the reduction of all of these bands in both control as well as FGF-induced cells. These observations suggest that the ability of TGF-β to down-regulate the IL-1 receptor may be a mechanism by which it exerts its effects in antagonizing the IL-1 activity on chondrocytes. 相似文献
100.
Pamela Manzi Giovannella Bruscalupi Flavia Castellano Anna Trentalance 《Bioscience reports》1992,12(3):215-219
In female frogs (Rana Esculenta) during gametogenesis the cholesterol synthesized in the liver by 3-hydroxy-3-methylglutaryl coenzyme A reductase is mostly exported into the blood and taken up by the oocytes.In order to understand the fate of the neosynthesized cholesterol, female and male frogs and estrogenized male controls were injected with the labelled precursor14C mevalonate.In females and in estrogenized controls, mevalonate-derived radioactivity is found in a plasmatic lipoprotein that has been identified as vitellogenin by immunological detection.The increased 3-hydroxy-3-methylglutaryl coenzyme A reductase activity present in females in Fall is likely to be committed to provide cholesterol for the lipidation of this cholesterol-rich protein. 相似文献