全文获取类型
收费全文 | 5221篇 |
免费 | 338篇 |
国内免费 | 91篇 |
出版年
2023年 | 33篇 |
2022年 | 40篇 |
2021年 | 64篇 |
2020年 | 114篇 |
2019年 | 158篇 |
2018年 | 184篇 |
2017年 | 107篇 |
2016年 | 97篇 |
2015年 | 96篇 |
2014年 | 244篇 |
2013年 | 268篇 |
2012年 | 187篇 |
2011年 | 195篇 |
2010年 | 114篇 |
2009年 | 138篇 |
2008年 | 185篇 |
2007年 | 233篇 |
2006年 | 175篇 |
2005年 | 176篇 |
2004年 | 106篇 |
2003年 | 105篇 |
2002年 | 90篇 |
2001年 | 63篇 |
2000年 | 51篇 |
1999年 | 53篇 |
1998年 | 63篇 |
1997年 | 47篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 44篇 |
1993年 | 47篇 |
1992年 | 40篇 |
1991年 | 36篇 |
1990年 | 38篇 |
1989年 | 41篇 |
1988年 | 28篇 |
1987年 | 34篇 |
1985年 | 131篇 |
1984年 | 260篇 |
1983年 | 208篇 |
1982年 | 259篇 |
1981年 | 247篇 |
1980年 | 166篇 |
1979年 | 159篇 |
1978年 | 132篇 |
1977年 | 108篇 |
1976年 | 64篇 |
1975年 | 43篇 |
1974年 | 35篇 |
1973年 | 29篇 |
排序方式: 共有5650条查询结果,搜索用时 15 毫秒
91.
A high recovery method for concentrating microgram quantities of protein from large volumes of solution 总被引:4,自引:0,他引:4
A method is presented for the recovery of 40-80% of the protein from a 1 microgram/ml solution. The final protein pellet is free of detergent and other ionic compounds and is thus compatible with any denaturing solution. The primary structure of the protein is unaffected by the procedure, making the final pellet an ideal sample for any analytical procedure to determine protein structure. 相似文献
92.
93.
Fluorography--limitations on its use for the quantitative detection of 3H- and 14C-labeled proteins in polyacrylamide gels 总被引:1,自引:0,他引:1
The suitability of fluorography for the detection of 3H- and 14C-labeled proteins on polyacrylamide gradient gels has been investigated. It was found that the absorbance of the fluorographic film image produced by a given level of radioactivity decreased as the acrylamide concentration in the gel increased. The use of Coomassie brilliant blue protein dyes to stain the gel prior to fluorography reduced the absorbance of the fluorographic image. It is concluded that quantitative fluorography can only be applied to unstained gels of a uniform acrylamide concentration. 相似文献
94.
When the conditions for detecting proteins by ammoniacal silver staining (B. R. Oakley, D. R. Kirsch, and N. R. Morris (1980) Anal. Biochem. 105, 361-363.) following gel electrophoresis were varied, it was noted that glutaraldehyde pretreatment was necessary for maximal staining, which could not be explained simply as the result of "fixation." Further studies indicated that glutaraldehyde enhancement of protein staining with this silver reagent was probably due to oxidation of the aldehyde groups by silver ions, resulting in metallic silver depositions within the gel which act as nucleation sites for additional metallic silver localization in the protein bands upon the addition of formaldehyde developer. This proposed mechanism is consistent with the Tollen's reaction, as well as some aspects of the photographic process. Consistent with this notion, silver-staining intensities are directly related to mole percentage lysine of various standard proteins. 相似文献
95.
Mineral uptake by winter wheat (Trilicum aestivum L. cv. Martonvasari 8) was studied throughout the life cycle. Accumulation of macronutrients (i.e. total nitrogen, phosphorus, potassium, sodium, magnesium and calcium) and the water content of roots and shoots of plants grown in complete nutrient solution were higher than those of plants grown in two types of soils. The supply of macronutrients was in some cases limiting for soil-grown plants as revealed by a comparison of available and accumulated amounts of these nutrients. Their supply was abundant, however, for solution-grown plants. This led to a doubling of grain yield for the latter plants with a three fold increase in accumulation of dry matter and a five-fold increase in fresh weight. The efficiency ratios of solution-grown plants to soil-grown plants were approximately 1 for N and Na, 0.5 for Mg and Ca, and 0.3 for P and K. 相似文献
96.
Daniel G. Doughtie Philip J. Conklin K.Ranga Rao 《Journal of invertebrate pathology》1983,42(2):249-258
Adult grass shrimp were exposed to four concentrations (0.5, 1.0, 2.0 and 4.0 ppm) of hexavalent chromium for 28 days. At the end of the exposure period, over 50% of the surviving shrimp possessed cuticular lesions that had many of the gross characteristics of “shell disease.” These lesions were usually associated with articulations of the appendages and abdomen. Furthermore, it was found that at increasing levels of chromium exposure, there was a proportionate increase in the loss of limbs such that nearly 50% of the limbs were lost in grass shrimp exposed to the highest test concentration of chromium. Histological and ultrastructural examination of numerous lesions demonstrated a range of degenerative features within the subcuticular epithelium that included cytoplasmic vacuolization, mitochondrial swelling, chromatin emargination, and the presence of unusual nuclear inclusions that appear to indicate direct chromium toxicity. Additionally, a marked retardation in new epicuticle and exocuticle formation was observed in viable tissues associated with lesions in late premolt shrimp. It is proposed that chromium interferes with the normal functions of subcuticular epithelium, particularly cuticle formation, and subsequently causes structural weaknesses or perforations to develop in the cuticle of newly moted shrimp. Because of these chromium-induced exoskeletal deficiencies, a viaduct for pathogenic organisms (e.g., bacteria) and direct chromium influx is formed that perpetuates lesion development. 相似文献
97.
Jane E. Langridge-Smith Joseph H. Sellin Michael Field 《The Journal of membrane biology》1983,72(1-2):131-139
Summary In intact ileal mucosa, uptake of SO4 across the brush border membrane requires the presence of Na and is saturable, withK1/2=1.3mm at 140mm Na (P.L. Smith, S.A. Orellana & M. Field, 1981.J. Membrane Biol.
63:199–206). The present study examines the substrate specificities and transport stoichiometry of the Na-dependent SO4 uptake process. The effects of variations in medium anion and cation composition on lumen-to-epithelium influx of SO4 (J
me
SO4
) were determined under short-circuit conditions.J
me
SO4
is inhibited by thiosulfate, but not by phosphate, methylsulfate, vanadate or taurocholate. Cl is weakly inhibitory. Uptake of SO4 is poorly supported by Li, and is unaffected by K, indicating a specific dependence on Na. At low SO4 concentration (0.22mm),J
me
SO4
is a hyperbolic function of medium Na concentration; the corresponding Hill plot is linear with a slope of 1.0, suggesting a transport stoichiometry of 1 Na: 1 SO4. At high SO4 concentration (6.7mm), the Na-dependent SO4 velocity curve is sigmoidal and yields a Hill plot which is again linear but has a slope of 1.56, suggesting transport of more than 1 Na per SO4. SO4 uptake in presence of Na exhibits a dependence on medium pH. At 0.22mm SO4 and 140mm Na,J
me
SO4
was doubled by lowering pH from 7.4 to 6.8. However, at 6.7mm SO4 and 140mm Na, changing pH had no effect onJ
me
SO4
over the range 6.8 to 8.5. The pH dependence ofJ
me
SO4
at 6.7mm SO4 was restored when medium Na was lowered to 3mm, suggesting that pH sensitivity is a function of the concentration of preformed NaSO
4
–
ion pair. The results suggest that SO4 influx across the ileal brush border occurs by electroneutral Na+/NaSO
4
–
or Na+/H+/SO
4
2–
cotransport, the former being favored by high concentrations of Na and SO4. 相似文献
98.
Piero Cammarano Filomena Mazzei Paola Londei Angela Teichner Mario de Rosa Agata Gambacorta 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1983,740(3):300-312
Ribosomal subunits of Caldariella acidophila (max.growth temp., 90°C) have been compared to subunits of Bacillus acidocaldarius (max. growth temp., 70°C) and Escherichia coli (max. growth temp., 47°C) with respect to (a) bihelical content of rRNA; (b) G·C content of bihelical domains and (c) tightness of rRNA-protein interactions. The principal results are as follows. 1. Subunits of C. acidophila ribosomes (Tm = 90–93°C) exhibit considerable thermal tolerance over their B. acidocaldarius (Tm = 77°C) and E. coli counterparts (Tm = 72°C). 2. Based on the ‘melting’ hyperchromicities of the intact ribosomal subunits a 51–55% fraction of the nucleotides appears to participate in hydrogen-bonded base pairing regardless of ribosome source, whereas a larger fraction, 67–70%, appears to be involved in hydrogen bonding in the naked rRNA species. 3. The G·C content of bihelical domains of both free and ribosome-bound rRNA increases with increasing thermophily; based on hyperchromicity dispersion spectra of intact subunits and free rRNA, the bihelical parts of C. acidophila rRNA are estimated to contain 63–64% G·C, compared to 58.5% G·C for B. acidocaldarius and 55% G·C for E. coli. 4. The increment in ribosome Tm values with increasing thermophily is greater than the increase in Tm for the free rRNA, indicating that within ribosomes bihelical domains of the thermophile rRNA species are stabilized more efficiently than their mesophile counterparts by proteins or/ and other component(s). 5. The efficiency of the rRNA-protein interactions in the mesophile and thermophile ribosomes has been probed by comparing the releases, with LiCl-urea, of the rRNA species from the corresponding ribosomal subunits stuck to a Celite column through their protein moiety; it has been established that the release of C. acidophila rRNA from the Celite-bound ribosomes occurs at salt-urea concentrations about 4-fold higher than those required to release rRNA from Celite-bound E. coli ribosomes. 6. Compared to E. coli, the C. acidophila 50 and 30 S ribosomal subunits are considerably less susceptible to treatment designed to promote ribosome unfolding through depletion of magnesium ions. 相似文献
99.
Estimation of pyruvate decarboxylation in perfused rat skeletal muscle 总被引:12,自引:0,他引:12
P Schadewaldt U Münch M Prengel W Staib 《Biochemical and biophysical research communications》1983,116(2):456-461
A total of 46 E. coli strains showing mannose-resistant, P-blood-group independent hemagglutination of human erythrocytes were tested for binding to neuraminic acid. Nine of the strains completely lost their hemagglutination activity after the erythrocytes were treated with neuraminidase. To characterize the receptor structure, different neuraminic acid containing glycoproteins, their desialylated derivatives and neuraminyl oligosaccharides were tested for hemagglutination inhibition. These studies showed that the nine strains had binding specificity for alpha 2-3 linked neuraminic acid. 相似文献
100.
We measured single strand breaks (SSB) in pancreas DNA produced by N-nitrosobis(2-oxopropyl)amine (BOP) in hamsters fed purified diets containing added sodium selenite (Se) at 0.0, 0.1 and 5.0 ppm. There were fewer SSB in those given the 5.0 ppm Se diet throughout the experiment. One hour after dosing with BOP (20 mg/kg), there were 2.26 ± 0.47, 2.83 ± 0.43 and 1.74 ± 0.43 SSB per 108 daltons (mean ± S.E.M.) respectively in the three groups. The SSB were repaired faster in the 5.0 ppm Se-fed group. The approximate half-lives of the SSB were 33, 30 and 8 days, respectively. In the hamsters fed 5.0 ppm Se there was a small, statistically significant increase in pancreatic DNA synthesis. Autoradiographic analysis indicated that this was repair synthesis. In a second experiment, hamsters were fed one of the three diets prior to and for 2 days after administration of a single dose of BOP (20 mg/kg). They were then fed the 5.0 ppm Se diet for 5 days. The number of SSB was compared with those in hamsters fed their original diet for 7 days after BOP dosing. There was a statistically significant difference in the number of SSB in the hamsters fed 0.1 ppm Se before and for 2 days after BOP. In these hamsters there were 1.21 ± 0.24 SSB per 108 daltons compared with 3.19 ± 0.4 (mean ± S.E.M.). These results suggest high levels of dietary Se stimulate the repair of carcinogen-induced DNA damage. 相似文献