全文获取类型
收费全文 | 122724篇 |
免费 | 14497篇 |
国内免费 | 1751篇 |
出版年
2021年 | 1273篇 |
2020年 | 1028篇 |
2019年 | 1206篇 |
2018年 | 1505篇 |
2017年 | 1372篇 |
2016年 | 1822篇 |
2015年 | 2419篇 |
2014年 | 2949篇 |
2013年 | 3490篇 |
2012年 | 4199篇 |
2011年 | 3963篇 |
2010年 | 2596篇 |
2009年 | 2502篇 |
2008年 | 3121篇 |
2007年 | 2995篇 |
2006年 | 2958篇 |
2005年 | 2615篇 |
2004年 | 2467篇 |
2003年 | 2453篇 |
2002年 | 2342篇 |
2001年 | 9796篇 |
2000年 | 9661篇 |
1999年 | 7316篇 |
1998年 | 1698篇 |
1997年 | 1843篇 |
1996年 | 1620篇 |
1995年 | 1486篇 |
1994年 | 1405篇 |
1993年 | 1305篇 |
1992年 | 4858篇 |
1991年 | 4587篇 |
1990年 | 4030篇 |
1989年 | 4054篇 |
1988年 | 3636篇 |
1987年 | 3111篇 |
1986年 | 2802篇 |
1985年 | 2709篇 |
1984年 | 1988篇 |
1983年 | 1740篇 |
1982年 | 1241篇 |
1981年 | 995篇 |
1979年 | 1767篇 |
1978年 | 1366篇 |
1977年 | 1199篇 |
1976年 | 1035篇 |
1975年 | 1157篇 |
1974年 | 1166篇 |
1973年 | 1167篇 |
1972年 | 1037篇 |
1971年 | 956篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Presence and localization of proteins immunologically related to erythrocyte protein 4.1 in human skin 总被引:1,自引:0,他引:1
Analogues of human erythrocyte protein 4.1 have been examined in the human skin by immunochemical techniques using anti-human erythrocyte protein 4.1 antibodies. Immunoblot analysis revealed that human epidermis contains 4.1-like proteins of 80 kDa and 78 kDa that cross react with anti-protein 4.1 antibodies. Analysis with immunofluorescence microscopy revealed that the plasma membrane of the human epidermal keratinocyte was stained intensively in the basal cells, whereas spinous cells were moderately stained. It is noted that eccrine sweat gland cells and ductal cells were also stained in the peripheral cytoplasma. Taken together, these results demonstrate that 4.1-like proteins are present in human epidermal keratinocytes, eccrine sweat gland cells and ductal cells. The present findings enable us to suggest that a membrane skeletal protein lattice might exist in these cells. 相似文献
82.
83.
A tertiary structure prediction is described using Monte Carlo simulated annealing for the peptide fragment corresponding to residues 16-36 of bovine pancreatic trypsin inhibitor (BPTI). The simulation starts with randomly chosen initial conformations and is performed without imposing experimental constraints using energy functions given for generic interatomic interactions. Out of 20 simulation trials, seven conformations show a sheet-like structure--two strands connected by a turn--although this sheet-like structure is not as rigid as that observed in native BPTI. It is also shown that these conformations are mostly looped and exhibit a native-like right-handed twist. Unlike the case with the C-peptide of RNase A, no conspicuous alpha-helical structure is found in any of the final conformations obtained in the simulation. However, the lowest-energy conformation does not resemble exactly the native structure. This indicates that the rigid beta-sheet conformation of native BPTI merely corresponds to a local minimum of the energy function if the fragment with residues 16-36 is isolated from the native protein. A statistical analysis of all 20 final conformations suggests that the tendency for the peptide segments to form extended beta-strands is strong for those with residues 18-24, and moderate for those with residues 30-35. The segment of residues 25-29 does not tend to form any definite structure. In native BPTI, the former segments are involved in the beta-sheet and the latter in the turn. A folding scenario is also speculated from this analysis. 相似文献
84.
A variant of the FitzHugh-Nagumo model is proposed in order to fully make use of the computational properties of intraneuronal dynamics. The mechanisms of threshold and refractory periods resulting from the double dynamical processes are qualitatively studied through computer simulation. The results show that the variant neuron model has the property that its threshold, refractory period and response amplitude are dynamically adjustable. This paper has also discussed some problems relating to collective property, learning and implementation of the neural network based on the neuron model proposed. It is noted that the implicit way to describe threshold and refractory period is advantageous to adaptive learning in neural networks and that molecular electronics probably provides an effective approach to implementing the above neuron model. 相似文献
85.
86.
Water soluble polyethyleneimine derivatives containing nucleic acid bases were found to interact with polynucleotides, DNA, RNA. The conformational change by formation of complex was observed by CD spectra and was discussed with the hypochromicity in UV spectra. The rates of interactions between nucleic acid bases in polymers were slow as shown by UV spectra, but the conformational changes of the polynucleotides were fast as shown by CD spectra. In the case of the uracil derivative (PEI-Hse-Ura), high value of CD spectra [theta] 2.80 = -8.0 x 10(-4) for the complex with DNA might be caused by psi type conformation of DNA. 相似文献
87.
Impaired secretion and fluid-phase endocytosis in the End4 mutant of Chinese hamster ovary cells 总被引:6,自引:0,他引:6
R H Wang P A Colbaugh C Y Kao E A Rutledge R K Draper 《The Journal of biological chemistry》1990,265(33):20179-20187
Mutant V.24.1 defines the End4 complementation group of temperature-sensitive Chinese hamster ovary cell mutants selected for resistance to protein toxins. We investigated the secretory pathway in the mutant cells and found: 1) The hemagglutinin of influenza virus failed to reach the plasma membrane and was retained in a form sensitive to endoglycosidase H at the restrictive temperature. 2) Transferrin receptors synthesized at the restrictive temperature remained sensitive to endoglycosidase H. 3) Secretion of total soluble protein into the medium was strongly reduced at high temperature. These data indicate that V.24.1 cells are defective in secretion at the restrictive temperature. To see what effect the lesion had on the endocytic pathway, we measured the accumulation and recycling of the fluid-phase marker horseradish peroxidase. Accumulation was inhibited by 50% while recycling was barely affected, suggesting that the rate of fluid-phase endocytosis was reduced. We previously showed that the clathrin-coated pit pathway of endocytosis was not affected in the mutant, indicated by a normal transferrin cycle (Colbaugh, P. A., Stookey, M., and Draper, R. K. (1989) J. Cell Biol. 108, 2211-2219). Thus, the secretory lesion correlates with reduced fluid-phase endocytosis without impairing the clathrin-dependent pathway of receptor-mediated endocytosis. We also investigated the delivery of endocytosed material to lysosomes and found that delivery was partially, but not completely, impaired in the mutant. This suggests that endocytosed material can enter lysosomes, although slowly, in the absence of a functional secretory pathway. 相似文献
88.
89.
In a continuing study of microbial secondary metabolism in
simulated microgravity, we have examined gramicidin S (GS) production by
Bacillus brevis strain Nagano in NASA High Aspect Rotating Vessels
(HARVs), which are designed to simulate some aspects of microgravity. Growth
and GS production were found to occur under simulated microgravity. When
performance under simulated microgravity was compared with that under normal
gravity conditions in the bioreactors, GS production was found to be
unaffected by simulated microgravity. The repressive effect of glycerol in
flask fermentations was not observed in the HARV. Thus the negative effect of
glycerol on specific GS formation is dependent on shear and/or vessel
geometry, not gravity.
Received: 7 August 1996 / Accepted: 17 September 1996 相似文献
90.