全文获取类型
收费全文 | 21943篇 |
免费 | 2286篇 |
国内免费 | 17篇 |
专业分类
24246篇 |
出版年
2022年 | 127篇 |
2021年 | 268篇 |
2020年 | 149篇 |
2019年 | 214篇 |
2018年 | 266篇 |
2017年 | 204篇 |
2016年 | 402篇 |
2015年 | 643篇 |
2014年 | 699篇 |
2013年 | 964篇 |
2012年 | 1226篇 |
2011年 | 1219篇 |
2010年 | 784篇 |
2009年 | 789篇 |
2008年 | 1079篇 |
2007年 | 1107篇 |
2006年 | 1068篇 |
2005年 | 1070篇 |
2004年 | 1094篇 |
2003年 | 1086篇 |
2002年 | 1086篇 |
2001年 | 401篇 |
2000年 | 341篇 |
1999年 | 367篇 |
1998年 | 324篇 |
1997年 | 282篇 |
1996年 | 237篇 |
1995年 | 226篇 |
1994年 | 233篇 |
1993年 | 248篇 |
1992年 | 311篇 |
1991年 | 323篇 |
1990年 | 277篇 |
1989年 | 247篇 |
1988年 | 264篇 |
1987年 | 253篇 |
1986年 | 245篇 |
1985年 | 291篇 |
1984年 | 244篇 |
1983年 | 219篇 |
1982年 | 249篇 |
1981年 | 207篇 |
1980年 | 194篇 |
1979年 | 193篇 |
1978年 | 196篇 |
1977年 | 176篇 |
1976年 | 172篇 |
1975年 | 139篇 |
1974年 | 183篇 |
1973年 | 174篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
M R Stroud S B Levery M E Salyan C E Roberts S Hakomori 《European journal of biochemistry》1992,203(3):577-586
A Lewis-b-active glycosphingolipid containing a repetitive type-1 chain carbohydrate core was isolated from human colonic adenocarcinoma cell line Colo205. This glycosphingolipid was purified by HPLC and preparative high-performance thin-layer chromatography and its structure elucidated by positive-ion fast-atom-bombardment mass spectrometry with collision-induced disassociation, 1H-NMR spectroscopy and methylation analysis. The glycosphingolipid was found to be a trifucosylated derivative of this novel carbohydrate core, having the following structure: [formula; see text]. 相似文献
52.
Extrapolation from in vitro tests to human risk: experience with sodium fluoride clastogenicity 总被引:1,自引:0,他引:1
Genotoxic effects observed in vitro, only at high doses or high levels of cytotoxicity, will be false positives if such conditions are not achieved or cannot be tolerated in vivo. However, for such effects to be disregarded there must be a threshold dose or level of cytotoxicity below which genotoxicity is absent. Sodium fluoride (NaF) has previously been shown to be clastogenic in vitro in Syrian hamster cells and human fibroblasts. We have extended these studies in human fibroblasts and included a positive control (mitomycin C, MMC) which is clastogenic in vivo and carcinogenic, and a chemically related control (NaCl). Cytotoxicity was measured as mitotic inhibition and cell death (loss of clonogenicity). The results are used to illustrate the problems associated with quantitative extrapolation from in vitro tests to human risk, as follows. (1) There appears to be a threshold response (clastogenicity vs. dose) with NaF at around 10 micrograms/ml (48 h exposure) but a more definitive conclusion must await elucidation of the mechanisms of clastogenicity. (2) NaCl is weakly clastogenic at 1000 times the threshold dose for NaF. The mechanisms are unlikely to be similar. (3) No clastogenicity was detected with NaF below about 30% mitotic inhibition but the relationship between clastogenicity and mitotic inhibition was similar for NaF and MMC. (4) There was no obvious threshold in the relationship between clastogenicity and cell killing with NaF. MMC was less clastogenic than NaF at equitotoxic doses. Observations 3 and 4 preclude the possibility of regarding the clastogenicity of NaF as a false positive by virtue of associated cytotoxicity. 相似文献
53.
The effect of sodium chloride and temperature on the rate and extent of growth of Clostridium botulinum type A in pasteurized pork slurry 总被引:6,自引:0,他引:6
A selective medium was used to enumerate Clostridium botulinum growing in the presence of natural spoilage organisms in a model cured pork slurry. The growth responses of a mixed spore inoculum of six strains of Cl. botulinum type A were studied at 15 degrees, 20 degrees and 27 degrees C with 1.5, 2.5, 3.5 or 4.5% (w/v) salt added (aw range 0.961-0.990). Gompertz and logistic curves, which have a sigmoid shape, were fitted to the data and lag times, growth rates, generation times and time to maximum growth rates were derived. Variation in germination rates of the spores occasionally gave a falsely extended lag time resulting in an exceptionally high estimate for growth rate. Products containing 4.5% (w/v) NaCl would be capable of supporting growth of proteolytic strains of Cl. botulinum, even at 15 degrees C, although the lag period would be extended. In products where absence of Cl. botulinum cannot be assured additional preservative measures are essential. The information obtained provides a framework to investigate the effects of a wider range of additives or variables on the growth responses of Cl. botulinum. 相似文献
54.
Osteoblasts synthesize and respond to transforming growth factor-type beta (TGF-beta) in vitro 总被引:29,自引:11,他引:18
P G Robey M F Young K C Flanders N S Roche P Kondaiah A H Reddi J D Termine M B Sporn A B Roberts 《The Journal of cell biology》1987,105(1):457-463
Transforming growth factor-type beta (TGF-beta) has been identified as a constituent of bone matrix (Seyedin, S. M., A. Y. Thompson, H. Bentz, D. M. Rosen, J. M. McPherson, A. Conti, N. R. Siegel, G. R. Gallupi, and K. A. Piez, 1986, J. Biol. Chem. 261:5693-5695). We used both developing bone and bone-forming cells in vitro to demonstrate the cellular origin of this peptide. TGF-beta mRNA was detected by Northern analysis in both developing bone tissue and fetal bovine bone-forming cells using human cDNA probes. TGF-beta was shown to be synthesized and secreted by metabolically labeled bone cell cultures by immunoprecipitation from the medium. Further, TGF-beta activity was demonstrated in conditioned media from these cultures by competitive radioreceptor and growth promotion assays. Fetal bovine bone cells (FBBC) were found to have relatively few TGF-beta receptors (5,800/cell) with an extremely low Kd of 2.2 pM (high binding affinity). In contrast to its inhibitory effects on the growth of many cell types including osteosarcoma cell lines, TGF-beta stimulated the growth of subconfluent cultures of FBBC; it had little effect on the production of collagen by these cells. We conclude that bone-forming cells are a source for the TGF-beta that is found in bone, and that these cells may be modulated by this factor in an autocrine fashion. 相似文献
55.
Summary The activated dimonophosphate of 3-deoxyadenosine (cordycepin) undergoes oligomerization to produce a new family of pyrophosphate-linked oligomers in which the average repeating unit involves a nine-atom structural group. The presence of a poly(U) template increase the relative yields of higher oligomers, although the template-free reaction is itself extremely efficient.For the previous paper in this series see Schwartz et al. (1987) 相似文献
56.
57.
Sequence homology in the metalloproteins; purple acid phosphatase from beef spleen and uteroferrin from porcine uterus 总被引:1,自引:0,他引:1
D F Hunt J R Yates J Shabanowitz N Z Zhu T Zirino B A Averill S T Daurat-Larroque J G Shewale R M Roberts K Brew 《Biochemical and biophysical research communications》1987,144(3):1154-1160
The primary structures of purple acid phosphatase and uteroferrin, two iron-binding glycoproteins isolated from beef spleen and porcine uterine fluids, respectively, have been examined by a combination of tandem mass spectrometry and classical Edman sequencing methods. Reported here are amino acid sequence data covering more than 90% of the primary structures for these two proteins. The sequence data reveal an unexpectedly high degree of homology, greater than 90%, for these two proteins. 相似文献
58.
Enzymatic hydrolysis of short-chain lecithin/long-chain phospholipid unilamellar vesicles: sensitivity of phospholipases to matrix phase state 总被引:2,自引:0,他引:2
Short-chain lecithin/long-chain phospholipid unilamellar vesicles (SLUVs), unlike pure long-chain lecithin vesicles, are excellent substrates for water-soluble phospholipases. Hemolysis assays show that greater than 99.5% of the short-chain lecithin is partitioned in the bilayer. In these binary component vesicles, the short-chain species is the preferred substrate, while the long-chain phospholipid can be treated as an inhibitor (phospholipase C) or poor substrate (phospholipase A2). For phospholipase C Bacillus cereus, apparent Km and Vmax values show that bilayer-solubilized diheptanoylphosphatidylcholine (diheptanoyl-PC) is nearly as good a substrate as pure micellar diheptanoyl-PC, although the extent of short-chain lecithin hydrolysis depends on the phase state of the long-chain lipid. For phospholipase A2 Naja naja naja, both Km and Vmax values show a greater range: in a gel-state matrix, diheptanoyl-PC is hydrolyzed with micellelike kinetic parameters; in a liquid-crystalline matrix, the short-chain lecithin becomes comparable to the long-chain component. Both enzymes also show an anomalous increase in specific activity toward diheptanoyl-PC around the phase transition temperature of the long-chain phospholipid. Since the short-chain lecithin does not exhibit a phase transition, this must reflect fluctuations in head-group area or vertical motions of the short-chain lecithin caused by surrounding long-chain lecithin molecules. These results are discussed in terms of a specific model for SLUV hydrolysis and a general explanation for the "interfacial activation" observed with water-soluble phospholipases. 相似文献
59.
The relationship between structure and activity of acid-extracted and purified acrosin obtained from cauda epididymal hamster spermatozoa was studied. A four-step purification procedure of acrosin was used; it included 1.) acid extraction, 2.) gel filtration over Sephadex G-100 resin, 3.) ion exchange on CM-Sepharose CL-6B, and 4.) affinity chromatography on proflavin-Sepharose 4B. Analysis of the purified enzyme by high-performance liquid chromatography (300 SW + I-125) revealed a molecular weight of 44,000, which was identical to that obtained for acid-extracted acrosin. Slab-gel electrophoresis under nondenaturing conditions showed only one active band, as revealed with a highly sensitive assay using N alpha-benzyloxycarbonyl-L-lysine thiobenzyl ester as substrate. The radiation inactivation size of acid extracted acrosin was calculated to be 8400. This small unit could represent the active polypeptide portion of a larger monomer molecule or could represent the size of active subunits. Because acrosin is autocatalytic and highly active during fertilization, it is suggested that the active portion of the completely processed form of the enzyme is of small molecular weight. 相似文献
60.
Cycles in cannibalistic egg-larval interactions 总被引:5,自引:0,他引:5
Alan Hastings 《Journal of mathematical biology》1987,24(6):651-666
A model of a cannibalistic larval-egg interaction such as occurs in Tribolium is developed which leads to a system of nonlinear Volterra integral equations. I determine the local stability properties of the unique equilibrium point of the model. A Hopf bifurcation analysis shows that the model always undergoes a subcritical bifurcation when stability is lost. Numerical solutions confirm the presence of multiple attractors over a range of parameter values. The form of the cycles observed in the numerical solutions is analogous to that observed in laboratory populations of Tribolium. 相似文献