全文获取类型
收费全文 | 15209篇 |
免费 | 1032篇 |
国内免费 | 1篇 |
出版年
2023年 | 116篇 |
2022年 | 225篇 |
2021年 | 443篇 |
2020年 | 292篇 |
2019年 | 358篇 |
2018年 | 445篇 |
2017年 | 406篇 |
2016年 | 632篇 |
2015年 | 953篇 |
2014年 | 1016篇 |
2013年 | 1258篇 |
2012年 | 1487篇 |
2011年 | 1332篇 |
2010年 | 832篇 |
2009年 | 714篇 |
2008年 | 871篇 |
2007年 | 857篇 |
2006年 | 804篇 |
2005年 | 663篇 |
2004年 | 628篇 |
2003年 | 556篇 |
2002年 | 445篇 |
2001年 | 103篇 |
2000年 | 74篇 |
1999年 | 102篇 |
1998年 | 85篇 |
1997年 | 78篇 |
1996年 | 64篇 |
1995年 | 48篇 |
1994年 | 54篇 |
1993年 | 37篇 |
1992年 | 35篇 |
1991年 | 22篇 |
1990年 | 25篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 18篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 17篇 |
1983年 | 14篇 |
1982年 | 5篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 9篇 |
1976年 | 4篇 |
1975年 | 6篇 |
1972年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Characterization by photoaffinity labeling of a steroid binding protein in rat liver plasma membrane
Iñaki Ibarrola Ana Alejandro Aida Marino María J. Sancho José M. Macarulla Miguel Trueba 《The Journal of membrane biology》1992,125(2):185-192
Summary The mechanism of steroid uptake by the cell remains controversial. [3H]R5020 was utilized to characterize by photoaffinity labeling the steroid binding site in plasma membrane. This binding was saturable, reversible and had one type of binding site (K
d
= 33 ± 4 nm, B
max
= 32 ± 2 pmol/mg). [3H]R5020 could be prevented from binding by a variety of steroids (cortisol, progesterone, deoxycorticosterone, and levonorgestrel); estradiol did not have affinity for this binding site. The kinetics of R5020 photoactivation was time dependent and saturable. SDS-PAGE showed a specific band which corresponded to a 53-kDa peptide. The sucrose density gradient analysis has revealed the existence of a protein with a sedimentation coefficient of 3.6 ± 0.2 S. This polypeptide shows different characteristics than cytosolic steroid receptor or serum steroid binding proteins. This binding protein could correspond to the steroid binding site previously found in the plasma membrane.This work was supported by grants PB85-0461 from the Comisión Asesora de Investigatión Científica y Técnica and PGV-8612 from the Departamento de Educatión, Universidades e Investigation del Gobierno Vasco. We thank Roussel-Uclaf (France) for the nonradioactive RU-steroids kindly provided. 相似文献
112.
Summary Growth reduction or cessation is an initial response of Atriplex nummularia L. cells to NaCl. However, A. nummularia L. cells that are adapted to 342 and 428 mM NaCl are capable of sustained growth in the presence of salt. Cells that are adapted to NaCl exhibit a reduced rate of division compared to unadapted cells. Unlike salt adapted cells of the glycophyte Nicotiana tabacum L., A. nummularia L. cells do not exhibit reduced rate of cell expansion after adaptation. However, the cell expansion rate of unadapted A. nummularia L. cells is considerably slower than that of unadapted glycophyte cells and this normally low rate of cell expansion may contribute to the enhanced capacity of the halophyte to tolerate salt. Turgor of NaCl adapted cells was equivalent to unadapted cells indicating that the cells of the halophyte do not respond to salt by osmotic over adjustment as reported for the glycophyte tobacco (Binzel et al. 1985, Plant Physiol. 79:118–125). 相似文献
113.
Effects of storage were tested on germination ofDioscorea composita (Dioscoreaceae) seeds. Freshly collected seeds and seeds stored at 25°C in paper bags from 1 to 11 mo or for 4 and 5 yr were used in most of the experiments. Seeds were tested for germination at 20, 25, 30, 35, 25–20, and 25–35°C in white light and in darkness. Initiation of germination was delayed in freshly harvested seeds, and dormancy was reduced in seeds stored for about 9 mo. Viability of the seeds decreased after 4 and 5 yr of storage. 相似文献
114.
The relative efficiency of a buffered medium containing a high salt concentration and EDTA as a means to solubilize collagen-tailed molecular forms of acetylcholinesterase has been examined in four brain areas of several species belonging to different vertebrate classes. This extraction procedure has proved successful in most cases, with the yield of tailed enzyme varying between less than 1 and 26% of the total tissue activity. The solubilization values are consistently higher in more primitive vertebrates than in mammals and, for a given species, are usually lower in the telencephalon than in other brain structures. Our results confirm that the vertebrate central nervous system contains collagen-tailed quaternary structural forms of acetylcholinesterase. 相似文献
115.
Dulce E. Oliveira Ana Lúcia C. Santos Neto Anita D. Panek 《Analytical biochemistry》1981,113(1):188-192
A quantitative in situ assay of yeast α-glucosidase involving permeabilization of the cells by freezing and thawing is described. The assay was applied to different strains in different physiological states and was shown to give results comparable to those obtained with total cell homogenates. The primary advantage of the in situ assay was the possibility of analyzing a large number of samples from the same culture during a growth curve using a very reduced cell mass. 相似文献
116.
M.T. Silva J.C.F. Sousa J.J. Polónia M.A.E. Macedo Ana M. Parente 《生物化学与生物物理学报:生物膜》1976,443(1):92-105
The ultrastructural study of membrane organization in gram-positive bacteria related to the OsO4 fixation conditions revealed that large, complex mesosomes are observed only when the bacteria are subjected to an initial fixation with 0.1% OsO4 in the culture broth, as in the prefixation step of the Ryter-Kellenberger procedure. Evidence was obtained suggesting that the large mesosomes are produced by this prefixation. The kinetic study of the membrane morphological alterations occurring during the prefixation of Bacillus cereus with 0.1% OsO4 in the culture broth showed that the amount of mesosome material increases linearly from zero to a maximum observed at 1.7 min of prefixation and that at about this time a maximum is reached for the number of mesosomes per unity of cell area and for the average individual mesosome area. The large mesosomes observed in gram-positives fixed by the complete Ryter-Kellenberger procedure would be the result of the membrane-damaging action of 0.1% OsO4. Such damaging action was deduced from the observation that 0.1% OsO4 quickly lyses protoplasts and induces a quick and extensive leakage of intracellular K+ from B. cereus and Streptococcus faeculis. In support of that interpretation is the observation that in bacteria subjected to several membrane-damaging treatments, mesosome-like structures are seen after three different fixation procedures. In bacteria initially fixed with 1% OsO4, 4% OsO4 or 2.5% glutaraldehyde, no large, complex mesosomes are observed, small and simple invaginations of the cytoplasmic membrane being present. The size of these minute mesosomes is inversely proportional that causes of fixation. Uranyl acetate was found among the studied fixatives the one to the rate the least damage to bacterial membranes. This fixative satisfactorily preserves protoplasts. In bacteria initially fixed with uranyl acetate no mesosomes were found. The results of the present work throw serious doubts on the existence of mesosomes, both large and small, as real structures of bacterial cells. It is proposed that a continuous cytoplasmic membrane without infoldings (mesosomes) would be the real pattern of membrane organization in gram-positives. 相似文献
117.
Photosynthetic fructose-1,6-diphosphatase (FDPase) fractions I and II, earlier purified from spinach leaves, show a similar amino acid composition, with the exception of a higher glutamic acid content in the latter. In both fractions glutamic and aspartic acids are the main amino acids. pH activity profiles of fractions I and II are similar, with optima at 8·65–8·70, both showing a high specificity for fructose- 1,6-diphosphate. These two fractions are Mg2+-dependent for activity, with an Optimum Mg2+ concentration of 10 mM in standard conditions, which shifts to 5 mM when the MG2+/EDTA ratio is increased to 10; Mn2+ and Co2+ are slightly active. EDTA enhances FDPase activity slightly, with an optimum at 0·4–0·8 mM. Cysteine has no activating effect, and acts as an inhibitor above 10 mM. Both I and II have an optimum substrate concentration of 4 mM, and the substrate inhibits at concns above this value. Kinetic velocity curves are sigmoidal, with the concave zone located in the range of physiological substrate concns. (Hill coefficient 1·75 for both). This suggests a strong regulatory role of fructose-1,6-diphosphate. Km values are 1·4 × 10−3 M (fraction I) and 1·1 × 10−3 M (fraction II). The highest activity rate occurs at 60°, in accordance with the high thermostability of both fractions; the activation energies are 14·3 kcal/mol (fraction I) and 13·0 kcal/mol (fraction II). 相似文献
118.
We studied the effect of high-fat diet on the expression and activation of the three caveolins in rat skeletal muscle and their association with the insulin signalling cascade. Initial response was characterized by increased signalling through Cav-1 and Cav-3 phosphorylation, suggesting that both participate in an initial acute response to the calorie surplus. Afterwards, Cav-1 signalling was slightly reduced, whereas Cav-3 remained active. Late chronic phase signalling through both proteins was impaired inducing a prediabetic state. Summarizing, caveolins seem to mediate a time-dependent regulation of insulin cascade in response to high-fat diet in muscle. 相似文献
119.
Catarina Macedo-Silva Vera Miranda-Gonalves Ana Lameirinhas Joana Lencart Alexandre Pereira Joo Lobo Rita Guimares Ana Teresa Martins Rui Henrique Isabel Bravo Carmen Jernimo 《Cell death & disease》2020,11(12)
Esophageal squamous cell carcinoma (ESCC), the most frequent esophageal cancer (EC) subtype, entails dismal prognosis. Hypoxia, a common feature of advanced ESCC, is involved in resistance to radiotherapy (RT). RT response in hypoxia might be modulated through epigenetic mechanisms, constituting novel targets to improve patient outcome. Post-translational methylation in histone can be partially modulated by histone lysine demethylases (KDMs), which specifically removes methyl groups in certain lysine residues. KDMs deregulation was associated with tumor aggressiveness and therapy failure. Thus, we sought to unveil the role of Jumonji C domain histone lysine demethylases (JmjC-KDMs) in ESCC radioresistance acquisition. The effectiveness of RT upon ESCC cells under hypoxic conditions was assessed by colony formation assay. KDM3A/KDM6B expression, and respective H3K9me2 and H3K27me3 target marks, were evaluated by RT-qPCR, Western blot, and immunofluorescence. Effect of JmjC-KDM inhibitor IOX1, as well as KDM3A knockdown, in in vitro functional cell behavior and RT response was assessed in ESCC under hypoxic conditions. In vivo effect of combined IOX1 and ionizing radiation treatment was evaluated in ESCC cells using CAM assay. KDM3A, KDM6B, HIF-1α, and CAIX immunoexpression was assessed in primary ESCC and normal esophagus. Herein, we found that hypoxia promoted ESCC radioresistance through increased KDM3A/KDM6B expression, enhancing cell survival and migration and decreasing DNA damage and apoptosis, in vitro. Exposure to IOX1 reverted these features, increasing ESCC radiosensitivity and decreasing ESCC microtumors size, in vivo. KDM3A was upregulated in ESCC tissues compared to the normal esophagus, associating and colocalizing with hypoxic markers (HIF-1α and CAIX). Therefore, KDM3A upregulation in ESCC cell lines and primary tumors associated with hypoxia, playing a critical role in EC aggressiveness and radioresistance. KDM3A targeting, concomitant with conventional RT, constitutes a promising strategy to improve ESCC patients’ survival.Subject terms: Predictive markers, Cancer 相似文献
120.
The outer membrane protein (omp40) component from the chemolithoautotrophic acidophilic Thiobacillus ferrooxidans is apparently regulated by the external pH and the concentration of phosphorus. Its amino-terminal sequence showed little identity with the Escherichia coli OmpC, OmpF or PhoE porins, but was 38.5% identical to the outer membrane channel-forming protein NosA from Pseudomonas stutzeri, whose expression is also regulated environmentally. In addition, the partial amino acid sequence of T. ferrooxidans omp40 showed between 34 and 38% identity with the amino-terminal end of the small outer membrane proteins Rck and PagC from Salmonella typhimurium and OmpX from Enterobacter cloacae. 相似文献