全文获取类型
收费全文 | 14883篇 |
免费 | 1798篇 |
国内免费 | 3篇 |
出版年
2021年 | 330篇 |
2020年 | 180篇 |
2019年 | 243篇 |
2018年 | 270篇 |
2017年 | 254篇 |
2016年 | 445篇 |
2015年 | 698篇 |
2014年 | 683篇 |
2013年 | 777篇 |
2012年 | 983篇 |
2011年 | 973篇 |
2010年 | 614篇 |
2009年 | 532篇 |
2008年 | 763篇 |
2007年 | 777篇 |
2006年 | 634篇 |
2005年 | 628篇 |
2004年 | 629篇 |
2003年 | 555篇 |
2002年 | 508篇 |
2001年 | 273篇 |
2000年 | 273篇 |
1999年 | 253篇 |
1998年 | 167篇 |
1997年 | 141篇 |
1996年 | 134篇 |
1995年 | 126篇 |
1994年 | 126篇 |
1993年 | 102篇 |
1992年 | 215篇 |
1991年 | 220篇 |
1990年 | 177篇 |
1989年 | 170篇 |
1988年 | 167篇 |
1987年 | 158篇 |
1986年 | 165篇 |
1985年 | 174篇 |
1984年 | 152篇 |
1983年 | 118篇 |
1982年 | 125篇 |
1981年 | 86篇 |
1980年 | 93篇 |
1979年 | 126篇 |
1978年 | 112篇 |
1977年 | 91篇 |
1976年 | 96篇 |
1975年 | 80篇 |
1974年 | 89篇 |
1973年 | 95篇 |
1972年 | 72篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
1.
Protein metal-binding sites. 总被引:2,自引:0,他引:2
Metal ions have a role in a variety of important functions in proteins including protein folding, assembly, stability, conformational change, and catalysis. The presence or absence of a given metal ion is crucial to the conformation or activity of over one third of all proteins. Recent developments have been made in the understanding and design of metal-binding sites in proteins, an important and rapidly advancing area of protein engineering. 相似文献
2.
D Roberts 《Acta anatomica》1978,101(2):160-169
Retention of a retromandibular space has necessitated developing a system of mandibular protrusion in hominid species. It is possible that mandibular protrusion can be effected by a single muscle-the lateral pterygoid-and the motion controlled by the excentrically placed mandibular suspensory ligaments. The elasticity of the ligaments produces an integrity maintenance force between the articular condyle and eminence which is normally of minimal size. Excessive craniofacial flexion, or the retention of a juvenile configuration of the mandible, could result in increasing this integrity maintenance force and cause crepitation and clicking. Ajustment of the ligaments could reduce these pathological manifestations. 相似文献
3.
4.
C F Roberts 《Biochimica et biophysica acta》1970,201(2):267-283
5.
6.
7.
Rebecca R. Sharitz Susan A. Wineriter Michael H. Smith Edwin H. Liu 《American journal of botany》1980,67(9):1297-1303
Biochemical phenotsypes of four taxa of Typha from the eastern United States were determined by starch gel electrophoresis. The isozyme banding patterns of T. latifolia, T. angustifolia and T. domingensis are distinct and allow unambiguous species identification when morphological characters are inadequate or unsuitable. The fourth form, T. glauca, is not an F1 hybrid, but it does appear to be intermediate between T. latifolia and T. angustifolia. The status of T. glauca and evolutionary relationships among the four forms may now be clarified by additional sampling because of the distinct and relatively invariant isozyme banding patterns which are described. 相似文献
8.
Measurements of cytoplasmic and vacuolar pH in Neurospora using nitrogen-15 nuclear magnetic resonance spectroscopy 总被引:3,自引:0,他引:3
The nitrogen-15 chemical shift of the N1 (tau)-nitrogen of 15N-labeled histidine and the half-height line widths of proton-coupled resonances of the delta- and omega,omega'-nitrogens of 15N-labeled arginine and of the alpha-nitrogens of 15N-labeled alanine and proline were measured in intact mycelia of Neurospora crassa to obtain to estimates of intracellular pH. For intracellular 15N-labeled histidine, the N1 (tau)-nitrogen chemical shift was 200.2 ppm. In vitro measurements showed that the chemical shift was slightly affected by the presence of phosphate, with which the basic amino acids may be associated in vivo. These considerations indicate a pH of 5.7-6.0 for the environment of intracellular histidine. The half-height line widths of the delta- and omega,omega'-nitrogens of [15N]arginine were 15 and 26 Hz, respectively. In vitro studies showed that these line widths also are influenced by the presence of phosphate, and, after suitable allowance for this, the line widths indicate pH 6.1-6.5 for intracellular arginine. The half-height line widths for intracellular alanine and proline were 17 and 12 Hz, respectively, which are consistent with an intracellular pH of 7.1-7.2. Pools of histidine and arginine are found principally in the vacuole of Neurospora, most likely in association with polyphosphates. Proline and alanine are cytoplasmic. The results reported here are consistent with these localizations and indicate that the vacuolar pH is 6.1 +/- 0.4 while the cytoplasmic pH is 7.15 +/- 0.10. Comparisons of these estimates with those obtained by other techniques and their implications for vacuolar function are discussed. 相似文献
9.
John G. Neuhoff Katherine L. Long Rebecca C. Worthington 《Evolution and human behavior》2012,33(4):318-322
Listeners consistently perceive approaching sounds to be closer than they actually are and perceptually underestimate the time to arrival of looming sound sources. In a natural environment, this underestimation results in more time than expected to evade or engage the source and affords a “margin of safety” that may provide a selective advantage. However, a key component in the proposed evolutionary origins of the perceptual bias is the appropriate timing of anticipatory motor behaviors. Here we show that listeners with poorer physical fitness respond sooner to looming sounds and with a larger margin of safety than listeners with better physical fitness. The anticipatory perceptual bias for looming sounds is negatively correlated with physical strength and positively correlated with recovery heart rate (a measure of aerobic fitness). The results suggest that the auditory perception of looming sounds may be modulated by the response capacity of the motor system. 相似文献
10.
Cyrtandra (Gesneriaceae) is a genus of flowering plants with over 800 species distributed throughout Southeast Asia and the Pacific Islands. On the Hawaiian Islands, 60 named species and over 89 putative hybrids exist, most of which are identified on the basis of morphology. Despite many previous studies on the Hawaiian Cyrtandra lineage, questions regarding the reconciliation of morphology and genetics remain, many of which can be attributed to the relatively young age and evidence of hybridization between species. We utilized targeted enrichment, high‐throughput sequencing, and modern phylogenomics tools to test 31 Hawaiian Cyrtandra samples (22 species, two putative hybrids, four species with two samples each, one species with four samples) and two outgroups for species relationships and hybridization in the presence of incomplete lineage sorting (ILS). Both concatenated and species‐tree methods were used to reconstruct species relationships, and network analyses were conducted to test for hybridization. We expected to see high levels of ILS and putative hybrids intermediate to their parent species. Phylogenies reconstructed from the concatenated and species‐tree methods were highly incongruent, most likely due to high levels of incomplete lineage sorting. Network analyses inferred gene flow within this lineage, but not always between taxa that we expected. Multiple hybridizations were inferred, but many were on deeper branches of the island lineages suggesting a long history of hybridization. We demonstrated the utility of high‐throughput sequencing and a phylogenomic approach using 569 loci to understanding species relationships and gene flow in the presence of ILS. 相似文献