全文获取类型
收费全文 | 5007篇 |
免费 | 215篇 |
国内免费 | 102篇 |
出版年
2024年 | 10篇 |
2023年 | 64篇 |
2022年 | 89篇 |
2021年 | 105篇 |
2020年 | 136篇 |
2019年 | 110篇 |
2018年 | 133篇 |
2017年 | 111篇 |
2016年 | 108篇 |
2015年 | 119篇 |
2014年 | 242篇 |
2013年 | 281篇 |
2012年 | 174篇 |
2011年 | 274篇 |
2010年 | 200篇 |
2009年 | 251篇 |
2008年 | 222篇 |
2007年 | 202篇 |
2006年 | 213篇 |
2005年 | 218篇 |
2004年 | 201篇 |
2003年 | 176篇 |
2002年 | 161篇 |
2001年 | 95篇 |
2000年 | 69篇 |
1999年 | 96篇 |
1998年 | 114篇 |
1997年 | 98篇 |
1996年 | 86篇 |
1995年 | 91篇 |
1994年 | 112篇 |
1993年 | 77篇 |
1992年 | 82篇 |
1991年 | 71篇 |
1990年 | 91篇 |
1989年 | 49篇 |
1988年 | 50篇 |
1987年 | 57篇 |
1986年 | 22篇 |
1985年 | 37篇 |
1984年 | 48篇 |
1983年 | 26篇 |
1982年 | 36篇 |
1981年 | 30篇 |
1980年 | 20篇 |
1979年 | 20篇 |
1978年 | 12篇 |
1976年 | 11篇 |
1975年 | 6篇 |
1974年 | 6篇 |
排序方式: 共有5324条查询结果,搜索用时 149 毫秒
1.
《Developmental cell》2021,56(16):2329-2347.e6
- Download : Download high-res image (154KB)
- Download : Download full-size image
2.
《Cryobiology》2018
Corals are sensitive to minute changes in their environments, and their continued existence is substantially threatened by the increasing number of destructive anthropogenic activities and unprecedented rates of climate change. Although cryopreservation has been successfully to preserve mammalian gametes for decades, coral cryopreservation was attempted for the first time less than 15 years ago, and freezing protocols exist for only a handful of coral species. The present study developed a cryopreservation protocol for the sperm of the common Indo-Pacific reef-builder Acropora humilis. Colonies of reefs of Sattahip Bay, Chonburi Province, Thailand were collected from 3 m depth with a mesh net during a spawning event. Immediately after collection, the sperm were isolated and subjected to a two-step freezing method featuring DMSO, polyethylene glycol, or methanol as the cryoprotectant. Viability and motility were assessed via a bioluminescence technique and a “computer-assisted semen analysis, and it was found that a 15-min equilibration with 2 M DMSO followed by cooling at 41.7 °C was the optimum cryopreservation protocol for A. humilis sperm. The post-thaw sperm achieved 45% fertilization success, and 35% of the fertilized eggs developed into blastopore larvae. The present optimized protocol can therefore facilitate the preservation of sperm for future propagation efforts of this species and provide an experimental platform for optimizing cryopreservation protocols for gametes of other scleractinian coral species. 相似文献
3.
Georg Steinhauser Johannes H. Sterba Karin Poljanc Max Bichler Karl Buchtela 《Journal of trace elements in medicine and biology》2006,20(3):119-153
In this study, 18 partly commercially available samples of rock salt from Austria, Germany, Pakistan, Poland, Switzerland, and Ukraine were investigated with respect to their content of trace elements using instrumental neutron activation analysis. Elements detected were Al, Ba, Br, Ca, Ce, Cl, Co, Cr, Cs, Eu, Fe, Hf, La, Mn, Na, Rb, Sb, Sc, Sm, Sr, Ta, Tb, Th, and Zn, some of them only in individual cases. An estimation of the bioavailability of these trace elements was performed by dissolving an equivalent of the sodium chloride samples in diluted hydrochloric acid (simulating stomach acid), filtering off the insoluble components, and analyzing the evaporated filtrate. It could be shown that in most cases bioactive trace elements like Fe can be found in rock salt in the form of almost insoluble compounds and are therefore not significantly bioavailable, whereas thorium, for example, was partly bioavailable in two cases. A significant contribution to the recommended daily intake of metal trace elements by using rock salt for nutrition can be excluded. 相似文献
4.
Reuben S.E. Young Andrew P. Bowman Kaylyn D. Tousignant Berwyck L.J. Poad Jennifer H. Gunter Lisa K. Philp Colleen C. Nelson Shane R. Ellis Ron M.A. Heeren Martin C. Sadowski Stephen J. Blanksby 《Journal of lipid research》2022,63(6):100223
The cellular energy and biomass demands of cancer drive a complex dynamic between uptake of extracellular FAs and their de novo synthesis. Given that oxidation of de novo synthesized FAs for energy would result in net-energy loss, there is an implication that FAs from these two sources must have distinct metabolic fates; however, hitherto, all FAs have been considered part of a common pool. To probe potential metabolic partitioning of cellular FAs, cancer cells were supplemented with stable isotope-labeled FAs. Structural analysis of the resulting glycerophospholipids revealed that labeled FAs from uptake were largely incorporated to canonical (sn-) positions on the glycerol backbone. Surprisingly, labeled FA uptake also disrupted canonical isomer patterns of the unlabeled lipidome and induced repartitioning of n-3 and n-6 PUFAs into glycerophospholipid classes. These structural changes support the existence of differences in the metabolic fates of FAs derived from uptake or de novo sources and demonstrate unique signaling and remodeling behaviors usually hidden from conventional lipidomics. 相似文献
5.
F F Smith J R Mertz I Krebs L L Tres C B Chae Z Zakeri J Engelhardt D Hoover M Tenniswood A L Kierszenbaum 《Molecular reproduction and development》1992,33(4):363-372
We have previously reported that a heterodimeric protein secreted by rat Sertoli cells is antigenically related to a protein associated with outer dense fibers of the sperm tail. Therefore, we have explored the possibility that Sertoli and spermatogenic cells express a similar gene encoding a homologous protein. A Sertoli cell heterodimeric protein cDNA probe recognizes specific mRNA in pachytene and round spermatids fractionated by centrifugal elutriation; however, this specific mRNA was less prominent than in cultured Sertoli cells. In agreement with these observations, in situ hybridization experiments show that Sertoli cells are predominantly engaged in active heterodimeric protein mRNA synthesis, while meiotic prophase spermatocytes and spermatids also show significant but less abundant specific mRNA. Immunoblotting experiments demonstrate that, while Sertoli cells synthesize a heterodimeric protein consisting of two disulfide-linked components with molecular masses of 45 and 35 kD, both primary spermatocytes and round spermatids synthesize single 30 kD monomers not associated by disulfide linkage but recognized by antisera to Sertoli cell heterodimeric protein. Immunoblotting and immunogold electron microscopic studies show that antisera to Sertoli cell heterodimeric protein recognize a protein associated with outer dense fibers. This immunoreactivity was abolished by a 5-min pronase treatment, without affecting the integrity of outer dense fibers. Results of this study and previous studies demonstrate that both Sertoli and spermatogenic cells express a similar gene and that an antigenically related product encoded by this gene becomes associated with outer dense fibers during their assembly at spermiogenesis. 相似文献
6.
Lawrence Que Jr. 《Journal of biological inorganic chemistry》2004,9(6):684-690
The oxygen activation mechanisms proposed for nonheme iron systems generally follow the heme paradigm in invoking the involvement of iron-peroxo and iron-oxo species in their catalytic cycles. However, the nonheme ligand environments allow for end-on and side-on dioxygen coordination and impart greater flexibility in the modes of dioxygen activation. The currently available evidence for nonheme iron-peroxo and iron-oxo intermediates is summarized and discussed in light of the ongoing discussion on the nature of the oxidant(s) in heme enzymes. 相似文献
7.
Charles E. Wenner John C. Cheney L. David Tomei 《Journal of cellular biochemistry》1981,15(2):161-168
The introduction of either PGF2α (10?7 M) or TPA (10?7 M) stimulated, ouabain-sensitive 86Rb+ influx at 30 min in postconfluent 3T3-4 mouse fibroblast cultures by 117% and 124%, respectively. Both TPA and PGF2α at these concentrations stimulated the incorporation of 3H-TdR into DNA. TPA had the greatest stimulatory effect, which was similar to that obtained with 10% fetal calf serum. In accord with the idea that modulation of membrane processes such as Na+/K+ pump activity in fibroblasts may reflect important events related to the initiation of DNA synthesis, it was observed that in both 3T3-4 and C3H-1 0T½ cells there were parallel increases in 3H-TdR incorporation and ouabain-sensitive 86Rb+ influxes with 10?7 M TPA, whereas PGF2α stimulated a significant increase in 3H-TdR incorporation in 3T3-4 but not C3H-10T½ cells and only marginal increases in ouabain-sensitive 86Rb+ influx in both. Therefore, although there appears to be a close correlation between Na+/K+ pump activation and subsequent S-phase entry following TPA stimulation, a similar correlation for PGF2α cannot be confirmed. 相似文献
8.
Harry Le Vine Pedro Cuatrecasas 《Biochimica et Biophysica Acta (BBA)/General Subjects》1981,672(3):248-261
A cytosolic, macromolecular factor required for the cholera toxin-dependent activation of pigeon erythrocyte adenylate cyclase and cholera toxin-dependent ADP-ribosylation of a membrane-bound 43 000 dalton polypeptide has been purified 1100-fold from horse erythrocyte cytosol using organic solvent precipitation and heat treatment. This factor, 13 000 daltons, does not absorb to anionic or cationic exchange resins, is sensitive to trypsin or 10% trichloroacetic acid and is not extractable by diethyl ether. Activation of adenylate cyclase by cholera toxin requires the simultaneous presence of ATP (including possible trace GTP), NAD+, dithiothreitol, cholera toxin, membranes and the cytosolic macromolecular factor. Reversal of cholera toxin activation of adenylate cyclase, and of the toxin-dependent ADP-ribosylation, requires the presence of the cytosolic factor. The ability of the purified cytosolic factor to influence the hormonal sensitivity of liver membrane adenylate cyclase may provide clues to its physiological functions. 相似文献
9.
Michael E. Salvucci Archie R. Portis Jr. William L. Ogren 《Photosynthesis research》1985,7(2):193-201
Ribulosebisphosphate carboxylase/oxygenase (EC 4.1.1.39) (rubisco) must be fully activated in order to catalyze the maximum rates of photosynthesis observed in plants. Activation of the isolated enzyme occurs spontaneously, but conditions required to observe full activation are inconsistent with those known to occur in illuminated chloroplasts. Genetic studies with a nutant of Arabidopsis
thaliana incapable of activating rubisco linked two chloroplast polypeptides to the activation process in vivo. Using a reconstituted light activation system, it was possible to demonstrate the participation of a chloroplast protein in rubisco activation. These results indicate that a specific chloroplast enzyme, rubisco activase, catalyzes the activation of rubisco in vivo. 相似文献
10.