全文获取类型
收费全文 | 4418篇 |
免费 | 453篇 |
国内免费 | 2篇 |
出版年
2022年 | 51篇 |
2021年 | 118篇 |
2020年 | 62篇 |
2019年 | 75篇 |
2018年 | 90篇 |
2017年 | 90篇 |
2016年 | 164篇 |
2015年 | 213篇 |
2014年 | 265篇 |
2013年 | 291篇 |
2012年 | 374篇 |
2011年 | 351篇 |
2010年 | 228篇 |
2009年 | 182篇 |
2008年 | 278篇 |
2007年 | 269篇 |
2006年 | 233篇 |
2005年 | 216篇 |
2004年 | 177篇 |
2003年 | 159篇 |
2002年 | 156篇 |
2001年 | 47篇 |
2000年 | 29篇 |
1999年 | 33篇 |
1998年 | 33篇 |
1997年 | 20篇 |
1996年 | 23篇 |
1994年 | 18篇 |
1993年 | 26篇 |
1992年 | 26篇 |
1991年 | 25篇 |
1990年 | 21篇 |
1989年 | 23篇 |
1988年 | 23篇 |
1987年 | 22篇 |
1986年 | 22篇 |
1985年 | 29篇 |
1984年 | 26篇 |
1983年 | 20篇 |
1982年 | 23篇 |
1981年 | 20篇 |
1980年 | 18篇 |
1979年 | 19篇 |
1978年 | 20篇 |
1977年 | 15篇 |
1976年 | 19篇 |
1974年 | 26篇 |
1973年 | 19篇 |
1972年 | 16篇 |
1968年 | 14篇 |
排序方式: 共有4873条查询结果,搜索用时 62 毫秒
31.
Julie Marcus 《The Australian journal of anthropology》1987,17(3):185-197
Olive Pink studied anthropology at the University of Sydney under Professor A.P. Elkin. Although she did fieldwork among the Northern Aranda and Wailbri of Central Australia, she became disenchanted with anthropology and lived a reclusive life in Alice Springs. In this paper I present a brief outline of her life, particularly during the 1930's I point to the problems she encountered and suggested that she needs to be relocated within her discipline. 相似文献
32.
Phospholipid metabolism in human neutrophils activated by N-formyl-methionyl-leucyl-phenylalanine. Degranulation is not required for release of arachidonic acid: studies with neutrophils and neutrophil-derived cytoplasts. 下载免费PDF全文
E M Wynkoop M J Broekman H M Korchak A J Marcus G Weissmann 《The Biochemical journal》1986,236(3):829-837
Neutrophils respond to chemoattractants by aggregating, degranulating, remodelling of phospholipids and releasing arachidonic acid. To determine whether ligand-induced remodelling of phospholipids depends on redistribution of intracellular organelles (degranulation), we compared phospholipid remodelling of human neutrophils with that of neutrophil-derived cytoplasts. Cytoplasts, organelle-depleted vesicles of cytosol surrounded by plasmalemma, cannot degranulate. Without a stimulus, [3H]arachidonate was incorporated preferentially into phosphatidylinositol (PI) and phosphatidylcholine (PC). Exposure of cytoplasts and neutrophils prelabelled with [3H]arachidonate or [14C]glycerol to fMet-Leu-Phe (10(-7) M) induced rapid changes in distribution of label and mass of individual phospholipids: [3H]arachidonate in phosphatidic acid (PA) increased 500% (120 s), [14C]glycerol incorporation and mass of PA approached 200% of unstimulated values, and [3H]arachidonate in PI decreased continuously; these data are compatible with activity of a PI/PA cycle. However, the mass of PI in both preparations and [14C]glycerol label in intact neutrophils increased initially (5 s), suggesting net synthesis and mobilization of more than one pool of PI. Heterogeneity of PC pools was also observed: [3H]arachidonate was lost from PC immediately upon addition of stimulus, whereas mass and [14C]glycerol values increased. Thus, net phospholipid synthesis, redistribution of arachidonate and activation of the PI/PA cycle are immediate responses of the neutrophil to receptor occupancy by chemoattractants. Furthermore, the similarity in response to fMet-Leu-Phe of neutrophils and granule-free cytoplasts indicates that these processes are independent of degranulation. 相似文献
33.
Although recombination cannot increase under conditions of random mating or complete selfing in regimes of constant selection, with mixed random mating and selfing, selection for increased recombination can occur. For some fitness regimes there may be selection for reduced recombination with both low and high degrees of selfing but selection for increased recombination with moderate degrees of selfing. With some fitness regimes there is a historical effect: depending on which equilibrium a population starts from, there may be selection for either increased or decreased recombination. In other cases the direction of selection may be determined by the present state of individuals within the population. If recombination is already fairly limited, there may be selection for further reduction. If recombination is already fairly frequent, there may be selection for increased recombination. For certain symmetric viability systems there may be an intermediate value of the recombination fraction between 0 and 0.5 toward which the population will evolve. Although it is not yet possible to classify precisely those fitness matrices that can exhibit selection for increased recombination, it does appear that selection for increased recombination can occur only if at least two of the double homozygotes are less fit than would be expected on the basis of a comparison of the fitnesses of the single and double heterozygotes on an additive scale. 相似文献
34.
The enzyme 3′-AMP nucleotidase was purified 2,500- to 5,000-fold from extracts of an acetone powder of wheat (Triticum aestivum) embryonic axes germinated for 40 hours. Sodium dodecyl sulfate acrylamide gel electrophoresis and chromatography on Biogel-P100 indicate that the enzyme is monomeric with a molecular weight of 39,000. Extracts of embryos germinated up to 6 hours have only 1% of the 40-hour level of enzyme activity. To see if the increase to 40 hours represents de novo synthesis, extracts were compared for their ability to react with a rabbit antibody prepared against the enzyme. In immunodiffusion tests, 40-hour extracts showed a strong precipitin line coincident with that of the purified enzyme, whereas no precipitation was observed with 1-hour extracts. When the enzyme present in 40-hour extracts was partially inactivated by EDTA, it still blocked the ability of the antibody to inhibit enzyme activity. Extracts of 1-hour embryos, in contrast, were not able to block the inhibitory activity of the antibody. Embryos allowed to take up 35SO4 between 40 and 46 hours of germination synthesized 35S-labeled 3′-nucleotidase. In contrast, no radioactive protein synthesized by embryos during the first 6 hours of germination coincided on gel electrophoresis with the enzyme. These results indicate that the increase in 3′-nucleotidase activity is a consequence of de novo synthesis of the enzyme. 相似文献
35.
The requirement for the synthesis of RNA and DNA in early germination of wheat (Triticum aestivum var Newana) embryonic axes has been studied by incubating embryos in the presence of appropriate inhibitors and monitoring both embryo growth and the rates of specific metabolic processes. Experiments with 5-fluorouridine showed that both rRNA and DNA synthesis could be curtailed by 60 to 70% without affecting embryo growth to 24 hours. Similarly, the presence of mitomycin C and methotrexate inhibited DNA synthesis 70%, with only a small effect on growth. Experiments with a range of concentrations of cordycepin and α-amanitin indicated that mRNA synthesis could be curtailed by 30 to 40% within the first 8 hours of germination with only a small effect on embryo growth. Thus, at least the initial phases of seed embryo germination are not closely linked to the synthesis of mRNA, rRNA, or DNA. Maximal sensitivity of embryo growth was obtained with cycloheximide and 2-(4-methyl-2,6-dinitroanilino)-N-methyl propionamide, supporting the idea that protein synthesis is the macromolecular process most closely linked to early germination. 相似文献
36.
History of total nasal reconstruction with particular emphasis on the folded forehead flap technique 总被引:1,自引:0,他引:1
The history of plastic surgery is identified throughout the centuries with the history of rhinoplasty. The Indian Koomas first and later the Italian surgeons found valid solutions to the problems caused by partial loss of the nasal pyramid. However, the idea of rebuilding, with a single forehead flap, the tip and columella and providing at the same time a lining of skin for the newly formed nose goes back to the middle of the nineteenth century. The Italian Natale Petrali (1842) and the Germans Johann Friedrich Dieffenbach (1845) and Ernst Blasius (1848) contend for precedence in carrying out this important procedure still used today, which, barring postoperative contracture, represented a great advance in successful total rhinoplasty. 相似文献
37.
In order to regulate cell volume during hyperosmotic stress, the intertidal copepod Tigriopus californicus, like other aquatic crustaceans, rapidly accumulates high levels of intracellular alanine, proline, and glycine. Glutamate-pyruvate transaminase (GPT; EC 2.6.1.2), which catalyzes the final step of alanine synthesis, is genetically polymorphic in T. californicus populations at Santa Cruz, California. Spectrophotometric studies of homogenates derived from a homozygous isofemale line of each of the two common GPT alleles indicated that the GPTF allozyme has a significantly higher specific activity than the GPTS allozyme. Under conditions of hyperosmotic stress, individual adult copepods of GPTF and GPTF/S genotypes accumulated alanine, but not glycine or proline, more rapidly than GPTS homozygotes. When young larvae were subjected to the same hyperosmotic conditions, GPTS larvae suffered a significantly higher mortality than GPTF or GPTF/S larvae. These results suggest that the biochemical differences among GPT allozymes result in specific physiological variation among GPT genotypes and that this physiological variation is manifested in differential genotypic survivorships under some naturally occurring environmental conditions.This work was supported in part by a grant from the Lerner Fund for Marine Research of the American Museum of Natural History, an NIH Training Grant in Integrative Biology, and NIH Grants GM 28016 and GM 10452. 相似文献
38.
39.
40.
The glutamate permeation system in Escherichia coli K-12 consists of three genes: gltC, gltS, and gltR. The genes gltC and gltS are very closely linked, and are located between the pyrE and tna loci, in the following order: tna, gltC, gltS, pyrE; gltR is located near the metA gene. The three glt genes constitute a regulatory system in which gltR is the regulator gene responsible for the formation of repressor, gltS is the structural gene of the glutamate permease, and gltC is most probably the operator locus. The synthesis of glutamate permease is partially repressed in wild-type K-12 strains, resulting in the inability of these strains to utilize glutamate as the sole source of carbon. Derepression due to mutation at the gltC locus enables growth on glutamate as a carbon source both at 30 C and at 42 C. Temperature-sensitive gltR mutants capable of utilizing glutamate for growth at 42 C but not at 30 C were found to be derepressed for glutamate permease when grown at 42 C and partially repressed (wild-type phenotype) upon growth at 30 C. These mutants produce an altered thermolabile repressor which can be inactivated by mild heat treatment (10 min at 44 C) in the absence of growth. 相似文献