全文获取类型
收费全文 | 5041篇 |
免费 | 375篇 |
出版年
2022年 | 61篇 |
2021年 | 119篇 |
2020年 | 53篇 |
2019年 | 72篇 |
2018年 | 73篇 |
2017年 | 64篇 |
2016年 | 118篇 |
2015年 | 210篇 |
2014年 | 257篇 |
2013年 | 274篇 |
2012年 | 422篇 |
2011年 | 389篇 |
2010年 | 227篇 |
2009年 | 208篇 |
2008年 | 334篇 |
2007年 | 294篇 |
2006年 | 311篇 |
2005年 | 262篇 |
2004年 | 288篇 |
2003年 | 262篇 |
2002年 | 241篇 |
2001年 | 41篇 |
2000年 | 28篇 |
1999年 | 43篇 |
1998年 | 60篇 |
1997年 | 43篇 |
1996年 | 32篇 |
1995年 | 47篇 |
1994年 | 27篇 |
1993年 | 18篇 |
1992年 | 26篇 |
1991年 | 31篇 |
1990年 | 22篇 |
1989年 | 17篇 |
1988年 | 21篇 |
1987年 | 20篇 |
1986年 | 16篇 |
1985年 | 25篇 |
1984年 | 29篇 |
1983年 | 27篇 |
1982年 | 23篇 |
1981年 | 35篇 |
1980年 | 33篇 |
1979年 | 17篇 |
1978年 | 20篇 |
1977年 | 21篇 |
1976年 | 22篇 |
1974年 | 19篇 |
1973年 | 18篇 |
1970年 | 16篇 |
排序方式: 共有5416条查询结果,搜索用时 15 毫秒
21.
22.
Exogenous gangliosides enhance the interaction of fibronectin with ganglioside-deficient cells 总被引:14,自引:0,他引:14
Kenneth M. Yamada David R. Critchley Peter H. Fishman Joel Moss 《Experimental cell research》1983,143(2):295-302
The major cell-surface glycoprotein fibronectin mediates a variety of cellular adhesive interactions that have been reported to be competitively inhibited by gangliosides. These effects suggest a possible function of gangliosides as receptors for fibronectin. To test this hypothesis more directly, we examined the interaction of endogenous fibronectin with a ganglioside-deficient cell line, NCTC 2071. These cells, which grow in serum-free medium, synthesized fibronectin. The fibronectin did not bind to these cells, but instead bound diffusely to the culture substratum. When the cells were cultured in medium containing ganglioside, the fibronectin became bound to the cell surface in fibrillar strands. The order of effectiveness of purified gangliosides was GT1b greater than GD1a greater than GM1 greater than GM2 greater than GM3. The effect with mixed gangliosides was accompanied by a restoration of cellular capacity to bind and to respond to cholera toxin. Treatment of the cells with several phospholipids did not alter fibronectin binding. Our results support the hypothesis that gangliosides can help mediate the binding of fibronectin to fibroblasts. 相似文献
23.
24.
Joel Yellin 《Journal of mathematical biology》1983,17(2):191-216
The dynamical behavior of multi-allele, one-locus systems is analyzed under population regulation. Weak selection is assumed. It is shown that for sufficiently large times, t, the nth time derivative of the population number N(t) is of order n}+1 in the selection coefficients. These order relations imply there is an asymptotic “quasi-equilibrium” in which population size and mean fitness change slowly relative to changes in gene frequencies. Consistent with the results of other authors, in quasi-equilibrium the mean fitness is second-order in the selection coefficients. In an effort to understand dynamic behavior beyond the immediate neighborhood of equilibrium, the topology of mean fitness surfaces is explored. In general, population regulation leads to regions of decreasing mean fitness in which there are important changes in gene frequencies. To illustrate this and other related phenomena, I analyze models in which there is logarithmic population control, and in which genotypic fitnesses Wi(x) are linear in the allele frequencies x. Exact solutions for mean fitness W(x) are obtained for two- and three-allele systems with symmetric fertilities and mortalities. 相似文献
25.
FLAGELLAR ELONGATION AND SHORTENING IN CHLAMYDOMONAS : The Use of Cycloheximide and Colchicine to Study the Synthesis and Assembly of Flagellar Proteins 总被引:81,自引:56,他引:25 下载免费PDF全文
Flagella can be removed from the biflagellate Chlamydomonas and the cells begin to regenerate flagella almost immediately by deceleratory kinetics. Under usual conditions of deflagellation, more than 98% of all flagella are removed. Under less drastic conditions, cells can be selected in which one flagellum is removed and the other left intact. When only one of the two flagella is amputated, the intact flagellum shortens by linear kinetics while the amputated one regenerates. The two flagella attain an equal intermediate length and then approach their initial length at the same rate. A concentration of cycloheximide which inhibits protein synthesis permits less than one-third of each flagellum to form when both flagella are amputated. When only one is amputated in cycloheximide, shortening proceeds normally and the degree of elongation in the amputated flagellum is greater than if both were amputated in the presence of cycloheximide. The shortening process is therefore independent of protein synthesis, and the protein from the shortening flagellum probably enters the pool of precursors available for flagellar formation. Partial regeneration of flagella occurs in concentrations of cycloheximide inhibitory to protein synthesis suggesting that some flagellar precursors are present. Cycloheximide and flagellar pulse-labeling studies indicate that precursor is used during the first part of elongation, is resynthesized at mid-elongation, and approaches its original level as the flagella reach their initial length. Colchicine completely blocks regeneration without affecting protein synthesis, and extended exposure of deflagellated cells to colchicine increases the amount of flagellar growth upon transfer to cycloheximide. When colchicine is applied to cells with only one flagellum removed, shortening continues normally but regeneration is blocked. Therefore, colchicine can be used to separate the processes of shortening and elongation. Radioautographic studies of the growth zone of Chlamydomonas flagella corroborate previous findings that assembly is occurring at the distal end (tip growth) of the organelle. 相似文献
26.
27.
P. C. Vincent G. Borner A. D. Chanana E. P. Cronkite M. L. Greenberg D. D. Joel L. M. Schiffer P. A. Stryckmans 《Cell proliferation》1969,2(3):235-247
Some of the time parameters of the cell cycle in bovine thoracic duct lymphocytes have been estimated by analysing labeled mitoses curves, and by double labeling. The two methods gave similar estimates of Ts. Thus, Ts measured directly from labeled mitoses curves varied from 4 to 6 hr, while the estimates from double labeling were 4.8 and 4.5 hr. T G measured directly from labeled mitoses curves was 5 hr, and estimates of TG from the values of Ts ranged from 6.3 to 7.7 hr. The present data confirm the short generative cycle of large thoracic duct lymphocytes. Extracorporeal irradiation of the lymph (ECIL) had no detectable effect on the cell cycle or the fractional production rate of the lymphocytes. However, the calculated absolute production was reduced following ECIL, due to a decrease in the absolute number of cells present. The grain count over mitoses after ECIL was approximately one-half that found before ECIL. 相似文献
28.
Philip N. Sawyer Joel Levine Roger Mazlen Ignatius Valmont 《The Journal of general physiology》1961,45(2):181-196
Experiments giving evidence of active Na and Cl ion fluxes across large canine blood vessel walls (aorta, vena cava) in vitro have been presented. The information has been obtained using ion tracer techniques after Ussing and with diffusion cells of the Hogben type. The available data suggest that the membranes are satisfactorily oxygenated by the bathing solutions saturated with oxygen at atmospheric pressure. Evidence is offered which indicates that active ion transport does occur across the aorta and vena cava in in vitro experiments. Under the conditions of the experiment net Na and Cl flux takes place from intima to adventitia across the aorta, and from adventitia to intima across the vena cava at low measured potential differences. The possible relationships of derangement of active ion transport mechanisms, produced by electric currents and tissue injury potential differences, to intravascular thrombosis are alluded to. It would appear that sodium and chloride fluxes across large blood vessel walls in vitro occur at least in part as the result of metabolic processes and cannot be explained simply on the basis of diffusion across a semipermeable membrane. 相似文献
29.
30.
How two species interact during and after colonization influences which of them will be present in each stage of succession.
In the tolerance model of ecological succession in a patchy environment, empty patches can be colonized by any species, but
the ability to tolerate reduced resource levels determines which species will exclude the other. Here, we analyze a meta-population
model of the possible roles of competition in colonization and succession, using non-linear Markov chains as a mathematical
framework. Different kinds of competition affect the final equilibrial, abundances of the species involved in qualitatively
different ways. An explicit criterion is given to determine which interactions have stronger effects on the final equilibrial
levels of the weaker, species. Precise conditions are stated for the co-existence of both species. Both species are more likely
to co-exist in the presence of an intermediate disturbance frequency. 相似文献