全文获取类型
收费全文 | 5031篇 |
免费 | 370篇 |
专业分类
5401篇 |
出版年
2023年 | 16篇 |
2022年 | 38篇 |
2021年 | 93篇 |
2020年 | 41篇 |
2019年 | 56篇 |
2018年 | 78篇 |
2017年 | 63篇 |
2016年 | 135篇 |
2015年 | 249篇 |
2014年 | 260篇 |
2013年 | 319篇 |
2012年 | 372篇 |
2011年 | 341篇 |
2010年 | 234篇 |
2009年 | 195篇 |
2008年 | 281篇 |
2007年 | 274篇 |
2006年 | 252篇 |
2005年 | 267篇 |
2004年 | 263篇 |
2003年 | 219篇 |
2002年 | 202篇 |
2001年 | 50篇 |
2000年 | 39篇 |
1999年 | 50篇 |
1998年 | 67篇 |
1997年 | 54篇 |
1996年 | 49篇 |
1995年 | 56篇 |
1994年 | 53篇 |
1993年 | 47篇 |
1992年 | 33篇 |
1991年 | 36篇 |
1990年 | 26篇 |
1989年 | 30篇 |
1988年 | 35篇 |
1987年 | 32篇 |
1986年 | 25篇 |
1985年 | 24篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 17篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1979年 | 25篇 |
1977年 | 29篇 |
1976年 | 20篇 |
1971年 | 14篇 |
1959年 | 14篇 |
1954年 | 11篇 |
排序方式: 共有5401条查询结果,搜索用时 9 毫秒
251.
The cyclic lipopeptide fengycin, produced by Bacillus subtilis, exhibits its antimicrobial capabilities by altering the integrity of the cell membrane of plant pathogens. Previous work has correlated fengycin activity with membrane characteristics, such as sterol content. This work focused on the influence of fengycin on supported lipid bilayers containing varying levels of ergosterol. Total internal reflection fluorescence (TIRF) microscopy was used to visualize and distinguish ordered (Lβ/Lo) and disordered (Lα/Ld) domains in the model membranes following exposure to low (50 μg) and high (500 μg) fengycin doses. Application of an initial low dose of fengycin to 0% and 3% ergosterol-containing bilayers resulted in redistribution of Lα/Lβ and Lo/Ld domains, respectively, which the bilayers compensated and corrected for over time. These membranes were unable to tolerate a second 50 μg dose or a single high fengycin dose. The 6% ergosterol bilayers were able to tolerate sequential low doses of fengycin. Exposure of these bilayers to the high fengycin dose caused a decrease in the number of Lo domains, albeit less than that seen in the 0% and 3% ergosterol bilayers. Bilayers containing 12% ergosterol, exhibited the least amount of change after fengycin exposure. These were the only bilayer to exhibit an increase in area taken up by ordered domains. These results suggest fengycin may preferentially act on the Lβ or Lo phase, the area in which ergosterol resides. Bilayers containing low levels of ergosterol appear to be more sensitive to the lipopeptide, suggesting ergosterol plays a role in buffering perturbations caused by fengycin. 相似文献
252.
253.
Kaitlin M. Baudier Catherine L. D'Amelio Elisabeth Sulger Michael P. O'Connor Sean O'Donnell 《Ecography》2019,42(4):730-739
Endothermic animals do not always have a single adaptive internal temperature; some species exhibit plastic homeostasis, adaptively allowing body temperature to drop when thermoregulatory costs are high. Like large‐bodied endotherms, some animal societies exhibit collective thermal homeostasis. We tested for plasticity of thermoregulation in the self‐assembled temporary nests (bivouacs) of army ants. We measured core bivouac temperatures under a range of environmental conditions and at different colony developmental (larval vs pupal brood) stages. Contrary to previous assertions, bivouacs were not perfect thermoregulators in all developmental stages. Instead, bivouacs functioned as superorganismal facultative endotherms, using a combination of site choice and context‐dependent metabolic heating to adjust core temperatures across an elevational cline in ambient temperature. When ambient temperature was low, the magnitude of metabolic heating was dependent on colony developmental stage: pupal bivouacs were warmer than larval bivouacs. At cooler high elevations, bivouacs functioned like some endothermic animals that intermittently lower their body temperatures to conserve energy. Bivouacs potentially conserved energy by investing less metabolic heating in larval brood because the high costs of impaired worker development may require more stringent thermoregulation of pupae. Our data also suggest that site choice played an important role in bivouac cooling under high ambient temperatures at low elevations. Climate warming may expand upper elevational range limits of Eciton burchellii parvispinum, while reducing the availability of cool and moist bivouac sites at lower elevations, potentially leading to future low‐elevation range contraction. 相似文献
254.
255.
Juana Martinez Kritika Subramanian Daniel Margolis Elisabeth O'Dwyer Joseph Osborne Yuliya Jhanwar Himanshu Nagar Nicholas Williams Arindam RoyChoudhury Gabriela Madera John Babich Sandra Huicochea Castellanos 《Translational oncology》2022,15(1)
BackgroundThe primary objective was to compare the overall diagnostic performance, presented as detection rate of 68Ga-PSMA-HBED-CC positron emission tomography/magnetic resonance imaging (PSMA PET/MRI) versus conventional, multiparametric MRI (mpMRI) in a population of patients with biochemically recurrent prostate cancer. In conjunction with this analysis, secondary objectives included the evaluation of the detection rate stratified by PSA levels and primary treatment modality.MethodsA total of 165 PSMA PET MRI were performed from April 2018 to May 2021, of whom 108 were presenting for biochemical recurrent disease. The PSMA PET vertex to thigh were read by two different board-certified nuclear medicine physicians while the MRI head and neck, chest, abdomen, and pelvis (with dedicated, PI-RADS compliant multiparametric prostate MRI) were read by two board certified diagnostic radiologists.AnalysisPSMA PET/MRI had a higher detection rate than mpMRI when evaluating patients with biochemical recurrence (BCR) with similar results demonstrated when sub-analysis was performed using PSA levels, primary treatment modality, and time since androgen deprivation therapy. Our study also showed PSMA PET/MRI had a higher sensitivity than mpMRI.DiscussionOur findings demonstrate that PSMA PET/MRI is a better imaging modality in the detection of disease in the setting of BCR when compared to MRI alone. Combined utility with PSMA PET/MRI is a powerful tool which can aid in not only the detection of disease, but also guide in treatment planning for prostate cancer patients. 相似文献
256.
Sylvie Chastant Elisabeth Christians Evelyne Campion Jean-Paul Renard 《Molecular reproduction and development》1996,44(4):423-432
HSP 70.1 is one of the first genes to be expressed in the mouse embryo at the time of zygotic genome activation. We studied the regulation of this gene, using a transgene associating HSP 70.1 promoter and the firefly luciferase reporter gene, which allows the precise quantification of HSP 70.1 level of expression on individual embryos. In the present work, we show first that the level of HSP 70.1 expression at the two-cell stage is significantly higher (around two-fold) in embryos whose maternal cytoplasm is from C3H strain than with BALB/c strain. We verified that this difference is not an artefact of the use of transgenic embryos, of the time of first cleavage, or of in vitro culture. This regulation of HSP 70.1 level of expression is controlled by strain-specific maternal modifiers and is independent of replication, syngamy, and mitosis. Following nuclear transfer, reactivation of HSP 70.1 is also subjected to the same epigenetic influence. Only the strain-of-origin of the recipient cytoplast modulates the level of HSP 70.1 reprogrammation; the origin of donor nucleus is not significant, demonstrating the reversibility of this strain effect. These results point out the importance of the quality of recipient cytoplast in the intensity of gene reprogrammation, which may be of importance for nuclear transfer efficiency. © 1996 Wiley-Liss, Inc. 相似文献
257.
The effect of exposure of single rat pituitary cells to 50 Hz sine wave magnetic fields of various strengths on the intracellular free Ca2+ concentration ([Ca2+]i) was studied by using dual-emission microfluorimetry, using indo-1 as probe. A 30 min exposure of the cells to vertical 50 μT peak magnetic field triggered a long-lasting increase in [Ca2+]i from a basal value of about 185 ± 4 nM to 326 ± 41 nM (S.E.; n = 150). The vertical and horizontal components of the static magnetic field were 57 and 15 μT, respectively. The 50 Hz ambient magnetic field was always below 0.1 μT rms. The effect was observed both at 25 ± 2 °C and at 37 ± 2 °C. Responsive cells, for which [Ca2+]i rose to values above 309 nM, were identified as lactotrophs and represented 29% of the total pituitaries. [Ca2+]i increase, for the most part, was due to Ca2+ influx through voltage-dependent dihydropiridine-sensitive calcium channels inhibited by PN 200-110. However, neither Ca2+ channel blockers nor removal of Ca2+ from the external medium during exposure completely prevented the field-induced [Ca2+]i increase. Additional experiments using an MTT colorimetric assay showed that alteration of Ca2+ homeostasis of lactotrophs was associated with impairment of some mitochondrial processes. © Wiley-Liss, Inc. 相似文献
258.
Elisabeth Haberlehner 《International Review of Hydrobiology》1988,73(5):537-546
Underwater observations of the three common fish species Alburnus alburnus (L., 1758), Rutilus rutilus (L., 1758) and Scardinius erythrophthalmus (L., 1758) in a small part of the Danube flood area near Vienna have been made from June to October 1985 and 1986. Particular emphasis has been laid on the partitioning of feeding resources and habitat. It has been established that these species prefer different water zones. Alburnus a. and Scardinius e. are “obligate” schoolers (BREDER, 1967) whereas Rutilus r. is a facultative schooler. While feeding Rutilus r. and Scardinius e. form “aggregations” (BREDER, 1959) and both adopt equivalent feeding techniques. Furthermore Alburnus a. is a quick moving species living near the surface and chasing mostly in schools. While inter- and intraspecific aggression is rarely observed for Rutilus r. and Scardinius e., Alburnus a. often show interspecific actions. 相似文献
259.
260.
Li K Pasternak C Härtig E Haberzettl K Maxwell A Klug G 《Nucleic acids research》2004,32(15):4563-4575
The expression of many genes of facultatively photosynthetic bacteria of the genus Rhodobacter is controlled by the oxygen tension. Among these are the genes of the puf and puc operons, which encode proteins of the photosynthetic apparatus. Previous results revealed that thioredoxins are involved in the regulated expression of these operons, but it remained unsolved as to the mechanisms by which thioredoxins affect puf and puc expression. Here we show that reduced TrxA of Rhodobacter capsulatus and Rhodobacter sphaeroides and oxidized TrxC of R.capsulatus interact with DNA gyrase and alter its DNA supercoiling activity. While TrxA enhances supercoiling, TrxC exerts a negative effect on this activity. Furthermore, inhibition of gyrase activity strongly reduces puf and puc expression. Our results reveal a new signaling pathway by which oxygen can affect the expression of bacterial genes. 相似文献