全文获取类型
收费全文 | 1334篇 |
免费 | 122篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2022年 | 15篇 |
2021年 | 29篇 |
2020年 | 19篇 |
2019年 | 16篇 |
2018年 | 18篇 |
2017年 | 26篇 |
2016年 | 56篇 |
2015年 | 67篇 |
2014年 | 91篇 |
2013年 | 83篇 |
2012年 | 107篇 |
2011年 | 133篇 |
2010年 | 75篇 |
2009年 | 71篇 |
2008年 | 92篇 |
2007年 | 98篇 |
2006年 | 80篇 |
2005年 | 61篇 |
2004年 | 60篇 |
2003年 | 67篇 |
2002年 | 65篇 |
2001年 | 10篇 |
2000年 | 6篇 |
1999年 | 12篇 |
1998年 | 17篇 |
1997年 | 8篇 |
1996年 | 11篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1993年 | 7篇 |
1992年 | 5篇 |
1991年 | 1篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 2篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1973年 | 1篇 |
1971年 | 2篇 |
1969年 | 1篇 |
排序方式: 共有1456条查询结果,搜索用时 15 毫秒
1.
Myriam R. Hirt Marlee Tucker Thomas Müller Benjamin Rosenbaum Ulrich Brose 《Ecology and evolution》2020,10(14):7094-7105
- Realized trophic niches of predators are often characterized along a one‐dimensional range in predator–prey body mass ratios. This prey range is constrained by an “energy limit” and a “subdue limit” toward small and large prey, respectively. Besides these body mass ratios, maximum speed is an additional key component in most predator–prey interactions.
- Here, we extend the concept of a one‐dimensional prey range to a two‐dimensional prey space by incorporating a hump‐shaped speed‐body mass relation. This new “speed limit” additionally constrains trophic niches of predators toward fast prey.
- To test this concept of two‐dimensional prey spaces for different hunting strategies (pursuit, group, and ambush predation), we synthesized data on 63 terrestrial mammalian predator–prey interactions, their body masses, and maximum speeds.
- We found that pursuit predators hunt smaller and slower prey, whereas group hunters focus on larger but mostly slower prey and ambushers are more flexible. Group hunters and ambushers have evolved different strategies to occupy a similar trophic niche that avoids competition with pursuit predators. Moreover, our concept suggests energetic optima of these hunting strategies along a body mass axis and thereby provides mechanistic explanations for why there are no small group hunters (referred to as “micro‐lions”) or mega‐carnivores (referred to as “mega‐cheetahs”).
- Our results demonstrate that advancing the concept of prey ranges to prey spaces by adding the new dimension of speed will foster a new and mechanistic understanding of predator trophic niches and improve our predictions of predator–prey interactions, food web structure, and ecosystem functions.
2.
The cyanobacterial photosystem, I complex from Synechococcus sp. PCC6301 contains polypeptides of apparent Mr of 70,000, 18,000, 17,700, 16,000 and 10,000. Procedures were developed for the purification of the Mr 17,700 and 10,000 polypeptides. Amino acid analyses showed the absence of cystine and cysteine from these polypeptides. Amino-terminal sequences of 98 residues for the Mr 17,700 polypeptide and of 42 residues for the Mr 10,000 polypeptide were determined. Studies of pigment distribution within the photosystem I complex indicated that the binding of chlorophyll a and -carotene is in part dependent on the presence of these polypeptides.Abbreviations PSI
photosystem I
- P700
reaction center of PSI
- SDS
sodium dodecylsulfate
- TBS
tris-buffered saline
- TTBS
TBS containing Tween-20 相似文献
3.
Chronic cold exposure of rats (9 days at 5°C) induces an alteration of the fatty acid composition of phospholipids in brown adipose tissue. The alteration is due to an increase of the unsaturation degree of these lipids. The phenomenon can be reproduced by 10–7 mole. h–1 administration of noradrenaline for 9 days in rats kept at 25°C. Thus, phospholipid alteration in brown fat of cold exposed rats is most probably a consequence of the increase of sympathetic tone which occurs in this tissue during exposure to cold. 相似文献
4.
Gauthier Janel Lacroix Renée Coté Alain Doyon Julien Drolet Michel 《Applied psychophysiology and biofeedback》1985,10(2):139-159
In order to assess the relative effectiveness of finger warming and temporal blood volume pulse reduction biofeedback in the treatment of migraine, 22 female migraine patients were assigned to one of three experimental conditions: temporal artery constriction feedback, finger temperature feedback, or waiting list. Biofeedback training consisted of 12 sessions over a 6-week period. All patients completed 5 weeks of daily self-monitoring of headache activity (frequency, duration, and intensity) and medication before and after treatment. Treatment credibility was assessed at the end of Sessions 1, 6, and 12. Results showed that temporal constriction and finger temperature biofeedback were equally effective in controlling migraine headaches and produced greater benefits than the waiting list condition. Power analyses indicated that very large sample sizes would have been required to detect any significant differences between the two treatment groups. No significant relationships were found between levels of therapeutic gains and levels of thermal or blood volume pulse self-regulation skills. Likewise, treatment outcome was not found to be related to treatment credibility. Further analyses revealed that changes in headache activity and medication were associated with changes in vasomotor variability. Because blood volume pulse variability was not significantly affected by biofeedback training, questions about its role in the therapeutic mechanism are raised.This research was supported in part by grants from the Quebec Ministry of Education and the Quebec Ministery of Social Affairs to the first author, and an award from the Medical Research Council of Canada to the second author. The authors are indebted to Drs. Frank Andrasik, Howard Barbaree, Edward Blanchard, Martin Ford, and Patrick McGrath, as well as to two anonymous reviewers, for their helpful comments on an earlier draft of this paper. 相似文献
5.
D Landry L Doyon J Poudrier M Lafontaine M Pelletier S Montplaisir 《Journal of immunology (Baltimore, Md. : 1950)》1990,144(3):836-843
Human thymic dendritic cells (DC) have previously been shown to be intimately associated with thymocytes in situ and in culture. We report that thymic DC express LFA-3 and ICAM-1 adhesion molecules and may spontaneously associate with autologous thymocytes within mitogen-independent clusters. Moreover, the accessory activity of isolated human thymic DC was investigated in Con A-stimulation assays. By proliferation experiments, measured as [3H]TdR incorporation, we demonstrated that irradiated thymic DC strongly increase the mitogen-induced activation of autologous PBL as well as of unfractionated thymocytes. More interestingly, in coculture assays performed with purified thymocyte subsets, we have found that thymic DC greatly enhance the Con A proliferation of CD1- CD3bright thymocytes whereas the accessory activity toward the CD1+ CD3- thymocytes was very weak. Inhibition experiments demonstrated that the DC accessory activity is inhibited by anti-DR-related and anti-IL-2R mAb. However, blocking assays with anti-CD11b, anti-CD11c, anti-LFA-3, and anti-ICAM1 mAb showed that the accessory function obtained is similar to that with untreated cultures. We conclude that isolated human thymic DC may present potent DR- and IL-2-dependent accessory activity mainly directed toward the CD1- CD3bright thymocyte subpopulation, suggesting that thymic DC may be involved in the in vivo proliferation of mature thymocytes. 相似文献
6.
Caterina Crescimanno Jean-Michel Foidart Agns Noël Myriam Polette Erik Maquoi Philippe Birembaut Eugenia Baramova Peter Kaufmann Mario Castellucci 《Experimental cell research》1996,227(2):240
Implantation and placental development are dependent upon trophoblast invasion of the endometrium. While the villous trophoblast does not display invasive behavior, the extravillous cytotrophoblast is highly invasive. By cloning BeWo choriocarcinoma cells, we have isolated two distinct clones that share similarities with villous and extravillous cytotrophoblasts. When cultured at the surface of a type I collagen gel, BeWo MC-1 cells were not invasive, whereas BeWo MC-2 cells rapidly invaded this matrix. When injected subcutaneously in nude mice, BeWo MC-1 cells developed a localized tumor and BeWo MC-2 cells developed larger tumors with micrometastases. Gelatinase A expression and minute amounts of gelatinase B were detected in the parental cell line and in both clones. However, the parental and the BeWo MC-2 cells secreted 5- to 10-fold more gelatinase A than the BeWo MC-1 cells. Laminin and matrigel stimulated the production of gelatinase A in BeWo MC-2 cells. Type I collagen promoted the conversion of the 72-kDa progelatinase A in an active enzyme only in parental BeWo and in BeWo MC-2 cells. These clones provide an interesting model for studying the complex mechanisms regulating implantation as well as the controlled invasiveness during implantation compared to tumor invasion. 相似文献
7.
Insertional mutagenesis to isolate acetate-requiring mutants in Chlamydomonas reinhardtii 总被引:1,自引:0,他引:1
Abstract An arg 7 mutant of the green alga Chlamydomonas reinhardtii was transformed with pARG7.8, a plasmid bearing the wild-type ARG 7 gene. Out of 4100 arg+ transformants selected on an arginine-free medium supplemented with acetate, nine failed to grow on acetate-free medium (ac− mutants). The results of the genetic and molecular analysis of several ac− mutants are in agreement with the hypothesis that they originated from insertion of the incoming plasmid into the nuclear genome. These mutants should constitute valuable tools for isolating the corresponding wild-type genes after plasmid rescue into Escherichia coli . 相似文献
8.
Magali Basille Bruno J. Gonzalez Laurence Desrues †Myriam Demas ‡Alain Fournier Hubert Vaudry 《Journal of neurochemistry》1995,65(3):1318-1324
Abstract: The presence of receptors for the novel neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) has been recently demonstrated in the external granule cell layer of the cerebellum, a germinative matrix that generates the majority of cerebellar interneurons. In the present study, we have taken advantage of the possibility of obtaining a culture preparation that is greatly enriched in immature cerebellar granule cells to investigate the effect of PACAP on the adenylyl cyclase and phospholipase C transduction pathways. The two molecular forms of PACAP, i.e., 27-(PACAP27) and 38-(PACAP38) amino-acid forms of PACAP, induced a dose-dependent stimulation of cyclic AMP production in granule cells. The potencies of PACAP27 and PACAP38 were similar (ED50 = 0.12 ± 0.01 and 0.23 ± 0.07 n M , respectively), whereas vasoactive intestinal polypeptide (VIP) was ∼100 times less potent. PACAP27 and PACAP38 also induced a dose-dependent stimulation of polyphosphoinositide breakdown (ED50 = 19.1 ± 6.3 and 13.4 ± 6.0 n M , respectively), whereas VIP had no effect on polyphosphoinositide metabolism. The effect of PACAP38 on inositol phosphate formation was significantly reduced by U-73122 and by pertussis toxin, indicating that activation of PACAP receptors causes stimulation of a phospholipase C through a pertussis toxin-sensitive G protein. In contrast, forskolin and dibutyryl cyclic AMP did not affect PACAP-induced stimulation of inositol phosphates. Taken together, the present results demonstrate that PACAP stimulates independently the adenylyl cyclase and the phospholipase C transduction pathways in immature cerebellar granule cells. These data favor the concept that PACAP may play important roles in the control of proliferation and/or differentiation of cerebellar neuroblasts. 相似文献
9.
Myriam Calonje Concepción García Mendoza Amelia Pérez Cabo Monique Novaes-Ledieu 《Current microbiology》1995,30(2):111-115
Significant differences in gross wall chemical composition were detected in four commercial Agaricus bisporus strains. All were grown under the same conditions and their walls prepared by a mild method of breakage. A more detailed analysis of the wall fractions, isolated by means of their distinct solubilities, also showed striking structural differences among the four strains studied. The detected differences, not only in the overall composition of the wall but also in the polysaccharide structure, could assist in the characterization of strains and/or varieties of the commercial basidiomycete A. bisporus. 相似文献
10.
Summary Cytogenetic studies on lymphocytes from a girl aged 3 years and 10 months revealed a ring chromosome 15. Several banding methods showed the r(15) chromosome not to have any apparent deletion of the long arm. The silver staining technique for nucleolar organizer regions showed an NOR positive region (band p12). In only a few cells was a chromosome 15 missing. The size of the r(15) was found to be constant. Comparison with 11 previous reported cases in the literature shows that the clinical manifestations in the different patients with ring chromosome 15 are constant although not clinically identifiable and it appears likely to attribute them to a significantly retarded intrauterine and postnatal growth instead of presumed deficiency in the long arm and mosaic configurations. 相似文献