全文获取类型
收费全文 | 49712篇 |
免费 | 4546篇 |
国内免费 | 30篇 |
出版年
2022年 | 201篇 |
2021年 | 758篇 |
2020年 | 452篇 |
2019年 | 567篇 |
2018年 | 701篇 |
2017年 | 647篇 |
2016年 | 1039篇 |
2015年 | 1751篇 |
2014年 | 1934篇 |
2013年 | 2543篇 |
2012年 | 3217篇 |
2011年 | 3277篇 |
2010年 | 2126篇 |
2009年 | 1903篇 |
2008年 | 2705篇 |
2007年 | 2812篇 |
2006年 | 2742篇 |
2005年 | 2627篇 |
2004年 | 2527篇 |
2003年 | 2399篇 |
2002年 | 2298篇 |
2001年 | 538篇 |
2000年 | 444篇 |
1999年 | 607篇 |
1998年 | 729篇 |
1997年 | 547篇 |
1996年 | 469篇 |
1995年 | 427篇 |
1994年 | 411篇 |
1993年 | 440篇 |
1992年 | 475篇 |
1991年 | 411篇 |
1990年 | 402篇 |
1989年 | 389篇 |
1988年 | 364篇 |
1987年 | 326篇 |
1986年 | 350篇 |
1985年 | 375篇 |
1984年 | 453篇 |
1983年 | 372篇 |
1982年 | 477篇 |
1981年 | 490篇 |
1980年 | 384篇 |
1979年 | 318篇 |
1978年 | 332篇 |
1977年 | 293篇 |
1976年 | 326篇 |
1975年 | 220篇 |
1974年 | 274篇 |
1973年 | 271篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
911.
912.
913.
Post David M.; Frost Thomas M.; Kitchell James F. 《Journal of plankton research》1995,17(8):1621-1632
Changes in zooplankton populations during the experimental acidilicationof Little Rock Lake provided an opportunity to examine specificmechanisms underlying the morphological responses of bosminidsto changing predation pressure. Two large copepods, Epischuralacustris and Mesocyclops edax, disappeared from the lake'sacidified basin in 1986 and 1989, respectively, while a smallercopepod predator, Tropocyclops extensus, increased during laterstages of acidification. The two bosminid species showed distinctlydifferent responses coinciding with the changes in copepod predation.Bosmina longirostris exhibited a significant decrease in mucrolength with the decline of M.edax and E.lacustris. Its meanbody and antennule length, however, did not change. We suggestthat the decoupling of B.longirostris mucro length and antennulelength may have been related to the persistence of the smallercopepod predator, T.extensus. Eubosmina tubicen showed no apparentresponse to declines in M.edax and E.lacustris abundance ineither mean mucro, antennule or body length. Allometric analysesindicated, however, that mucro length was related to size-dependentcopepod predation for both B.longirostris and E.tubicen. 相似文献
914.
Isotopic fractionation during nitrate uptake by phytoplankton grown in continuous culture 总被引:7,自引:0,他引:7
The isotopic fractionation associated with uptake of NO3by six species of phytoplankton (two diatoms, one cryptophyte,one chlorophyte and two haptophytes) was measured at a varietyof steady-state growth rates in nitrogen-limited continuousculture. The magnitude of the isotopic fractionation factor( 相似文献
915.
Anthony L. Moore Ann L. Umbach James N. Siedow 《Journal of bioenergetics and biomembranes》1995,27(4):367-377
A major characteristic of plant mitochondria is the presence of a cyanide-insensitive alternative oxidase which catalyzes the reduction of oxygen to water. Current information on the properties of the oxidase is reviewed. Conserved amino acid motifs have been identified which suggest the presence of a hydroxo-bridged di-iron center in the active site of the alternative oxidase. On the basis of sequence comparison with other di-iron center proteins, a structural model for the active site of the alternative oxidase has been developed that has strong similarity to that of methane monoxygenase. Evidence is presented to suggest that the alternative oxidase of plant mitochondria is the newest member of the class II group of di-iron center proteins. 相似文献
916.
David M. Rhoads Charles S. Levings III James N. Siedow 《Journal of bioenergetics and biomembranes》1995,27(4):437-445
URF13 is the product of a mitochondrial-encoded gene (T-urfl3) found only in maize plants containing the Texas male-sterile cytoplasm (cms-T), and it is thought to be responsible for both cytoplasmic male sterility and the susceptibility ofcms-T maize to the fungal pathogensBipolaris maydis race T andPhyllosticta maydis. Mitochondria isolated fromcms-T maize are uniquely sensitive to pathotoxins (T-toxin) produced by these fungi and to methomyl (a commercial insecticide). URF13 acts as a receptor that specifically binds T-toxin to produce hydrophilic pores in the inner mitochondrial membrane. When expressed inEscherichia coli cells, URF13 also forms hydrophilic pores in the plasma membrane if exposed to T-toxin or methomyl. Topological studies established that URF13 contains three membrane-spanning -helices, two of which are amphipathic and can contribute to pore formation. Chemical crosslinking of URF13 was used to demonstrate the existence of URF13 oligomers incms-T mitochondria andE. coli cells. The ability of the carboxylate-specific reagent,N,N-dicyclohexycarbodiimide, to cross-link URF13 was used in conjunction with site-directed mutagenesis to establish that the URF13 tetramer has a central core consisting of a four--helical bundle which undergoes a conformational change after interaction with T-toxin or methomyl. Overall, the experimental evidence indicates that URF13 functions as a ligand-gated, pore-forming T-toxin receptor incms-T mitochondria. 相似文献
917.
Richard J. Bold Jin Ishizuka Srinivasan Rajaraman †J. Regino Perez-Polo Courtney M. Townsend Jr. James C. Thompson 《Journal of neurochemistry》1995,64(6):2622-2628
Abstract: Carcinoid tumors are a group of neuroendocrine neoplasms distributed widely throughout the body but most commonly occurring in the gut. These tumors retain many characteristics of their neural crest origin, including secretion of neuroactive peptides and responsiveness to neurotrophic substances. Nerve growth factor (NGF), a neurotrophic protein involved in maintenance and differentiation of peripheral sympathetic and sensory neurons, regulates growth of several neural tumor cells by inducing a differentiated phenotype and subsequent inhibition of cell growth rate. We examined the actions of NGF in a functioning human pancreatic carcinoid cell line (termed BON). NGF has no effect on the cytoarchitecture or constitutive secretion of bioamines in this carcinoid cell line. NGF, however, stimulates the in vitro cellular proliferation of BON cells. BON cells possess mRNA for the NGF receptors (p75LNGFR and p140trkA ) and membrane-associated tyrosine kinase activity is increased in response to NGF. Both the mitogenic activity of NGF, as well as the receptor-linked tyrosine kinase activity, can be abrogated in BON cells by the trkA inhibitor K-252a and specific anti-NGF antibody. Our studies demonstrate that NGF is a mitogen for this carcinoid cell line without effect on cellular phenotype or cytoarchitecture. NGF may play a role in the development and progression of human carcinoid tumors. 相似文献
918.
Dido Vassilacopoulou James A. Ripellino Nikolaos Tezapsidis Vivian Y. H. Hook Nikolaos K. Robakis 《Journal of neurochemistry》1995,64(5):2140-2146
Abstract: The amyloid β peptide (Aβ) of Alzheimer disease is derived from the proteolytic processing of the amyloid precursor proteins (APPs), which are considered type I transmembrane proteins. Here we report that the soluble fraction of isolated adrenal medullary chromaffin granules (CG), a model neuronal secretory vesicle system, contains an antigen that immunochemically and on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was indistinguishable from full-length APP. A truncated APP fragment with intact Aβ sequence was also detected in the soluble fraction of CG. In vitro experiments showed that full-length APP was solubilized from CG membranes at 37°C as a function of pH, with a peak of activity between pH 8.5 and pH 9.0. Solubilization of full-length APP was inhibited by several protease inhibitors, including aprotinin, cystatin, and iodoacetamide, by the divalent cations Ca2+ and Zn2+ , and by preheating of the membranes. These results are consistent with and suggest the involvement of an enzymatic mechanism in the solubilization of potentially amyloidogenic full-length APP. Production of Aβ from a transmembrane APP predicts a proteolytic cleavage within the lipid bilayer, a site relatively inaccessible to proteases. Thus, the detected soluble, potentially amyloidogenic, full-length APP may be a substrate for the proteases producing Aβ. The detection of soluble APP with intact Aβ sequence in secretory vesicles is consistent with the extracellular topology of amyloid depositions. 相似文献
919.
An in vitro binding assay involving egg plasma membrane vesicles (PMVs) of Fucus serratus L. and proteins contained in a KCl extract of sperm has been used to identify a sperm protein involved in egg binding. High-performance gel filtration (HPGF) separated the sperm KCl extract into several major fractions, and a protein (apparent M, 60 kDa) was identified as being involved in binding to the egg PMVs. This protein ran on denaturing sodium dodecyl sulfate (SDS)gels with an apparent molecular weight of 27 kDa. This suggests that either the native form of the protein is a dimer or the molecular weight on HPGF is an artifact caused by high ionic strength buffer promoting hydrophobic interactions. When KCl-sol-uble proteins were separated by SDS-polyacrylamide gel electrophoresis (PAGE), blotted onto nitrocellulose, and incubated with biotinylated egg PMVs, these bound to a band at 27 kDa, confirming the role of this protein. Addition of the Fucus sperm extract or HPGF fractions containing the binding protein to eggs in the absence of sperm induced the release of polysaccharides onto the egg cell surface. This labeling was patchy, in contrast to the uniform release of polysaccharides observed when sperm were added to eggs. The monoclonal antibody (MAb) FS17 was raised against the 27-kDa sperm protein. It labeled the sperm body and both flagella by immunofluorescence, though the sperm had to he permeabilized to observe labeling, suggesting that the epitope recognized is not exposed at the cell surface. Addition of FS17 to the KCl extract in the binding assay reduced subsequent binding of egg PMVs. Removal of the 27-kDa protein recognized by FS17 from the sperm extract prevented the binding of egg PMVs in the binding assay and the triggering of the patchy release of polysaccharides when added to eggs. Overall the results suggest that the 27-kDa sperm protein is involved in binding to the egg plasma membrane and can trigger partial activation of the egg . 相似文献
920.
James R. Manhart Greta A. Fryxell M. Clia Villac Linda Y. Segura 《Journal of phycology》1995,31(3):421-427
The region of the nuclear ribosomal DNA (rDNA) operon containing the small subunit (SSU), internal transcribed spacer 1 (ITS1), and a portion of the 5.8s rDNA gene was sequenced in one isolate each of Pseudo-nitzschia multiseries (Hasle) Hasle and Pseudo-nitzschia pungens (Grunow in Cleve & Möller) Hasle. The SSUs of these two species were highly similar, differing only in 14 point mutations and one insertion/deletion in 1774 bp. The ITS1 sequences were more variable, with 57 point mutations and three insertion/deletions in 257 bp. There were no differences in 44 bp of the 5.8S sequences. Restriction fragment patterns (RFPs) for the restriction endonucleases HaeIII, Hha1, and Rsa1 for 13 isolates of P. multiseries from the Atlantic, Pacific, and Gulf coasts of the United States and 16 isolates of P. pungens from the three coasts of the United States, in addition to Japan and China, were compared. There were differences between the RFPs of P. multiseries and P. pungens that corresponded to sites mapped by the DNA sequences, but no infraspecific variation in RFPs was observed for either species. The differences in RFPs correlate with morphological, immunological, and other rDNA differences and support the recognition of these taxa as separate species. 相似文献