首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2320篇
  免费   194篇
  国内免费   3篇
  2021年   31篇
  2020年   17篇
  2019年   30篇
  2018年   19篇
  2017年   28篇
  2016年   42篇
  2015年   64篇
  2014年   69篇
  2013年   102篇
  2012年   119篇
  2011年   129篇
  2010年   75篇
  2009年   64篇
  2008年   90篇
  2007年   108篇
  2006年   85篇
  2005年   92篇
  2004年   105篇
  2003年   90篇
  2002年   98篇
  2001年   25篇
  2000年   25篇
  1999年   41篇
  1998年   46篇
  1997年   27篇
  1996年   21篇
  1995年   22篇
  1994年   22篇
  1993年   25篇
  1992年   30篇
  1991年   26篇
  1990年   20篇
  1989年   27篇
  1988年   23篇
  1987年   16篇
  1985年   21篇
  1984年   26篇
  1982年   35篇
  1981年   27篇
  1980年   31篇
  1979年   16篇
  1978年   27篇
  1977年   20篇
  1976年   26篇
  1974年   24篇
  1973年   21篇
  1972年   17篇
  1961年   17篇
  1959年   15篇
  1957年   16篇
排序方式: 共有2517条查询结果,搜索用时 31 毫秒
951.
The starfish SALMFamide neuropeptides S1 (GFNSALMFamide) and S2 (SGPYSFNSGLTFamide) are the prototypical members of a family of neuropeptides that act as muscle relaxants in echinoderms. Comparison of the bioactivity of S1 and S2 as muscle relaxants has revealed that S2 is ten times more potent than S1. Here we investigated a structural basis for this difference in potency by comparing the bioactivity and solution conformations (using NMR and CD spectroscopy) of S1 and S2 with three chimeric analogs of these peptides. A peptide comprising S1 with the addition of S2's N-terminal tetrapeptide (Long S1 or LS1; SGPYGFNSALMFamide) was not significantly different to S1 in its bioactivity and did not exhibit concentration-dependent structuring seen with S2. An analog of S1 with its penultimate residue substituted from S2 (S1(T); GFNSALTFamide) exhibited S1-like bioactivity and structure. However, an analog of S2 with its penultimate residue substituted from S1 (S2(M); SGPYSFNSGLMFamide) exhibited loss of S2-type bioactivity and structural properties. Collectively, our data indicate that the C-terminal regions of S1 and S2 are the key determinants of their differing bioactivity. However, the N-terminal region of S2 may influence its bioactivity by conferring structural stability in solution. Thus, analysis of chimeric SALMFamides has revealed how neuropeptide bioactivity is determined by a complex interplay of sequence and conformation.  相似文献   
952.
953.
954.
The impact of storm-flow on river biofilm architecture was investigated using transmission (TEM) and scanning (SEM) electron microscopy. TEM resin substrata were colonized under light-grown (LG) or dark-grown (DG) conditions for 33 weeks in the Clywedog River, North Wales, prior to exposure to ambient-flow (approx. 60 cm·s?1) or storm-flow (approx. 235 cm·s?1+ river sediment) in a laboratory flume. Line transect methodology was used to quantify information from TEM ultrathin sections of LG material. In the LG ambient-flow biofilm, bacteria were more abundant directly adjacent to the substratum and were noticeably denser directly under the adnate diatom Cocconeis. Higher in the biofilm, the bacteria were loosely dispersed in the matrix between other cells. Cyanobacteria occurred most frequently as single cells but were also found in large “palisade” formations adjacent to the substratum. Significant horizontal and vertical nearest-neighbor associations were noted for both bacteria and cyanobacteria. Cells of Cocconeis were common adjacent to the substratum, providing shelter to, and often elevated upon, an “organic pad” of bacteria, cyanobacteria, and densely staining exopolysaccharide. Cyanobacteria and Cocconeis were resistant to removal by storm-flow, but Cocconeis frustules were sometimes damaged. Bacteria in the LG storm-flow samples were less common adjacent to the substratum and were sometimes more dispersed higher in the biofilm than in ambient-flow samples. We suggest that storm-flow hydrodynamic forces may redistribute bacteria adjacent to the substratum into higher areas of the biofilm. In addition, bacteria and the exopolysaccharide matrix were sometimes removed down to the substratum by storm-flow, unless beneath Cocconeis. The DG biofilm consisted almost entirely of bacteria. Storm-flow only removed surface growth from DG biofilms, and SEM revealed peritrich stalk abrasion and “blow-down.” Pre-disturbance biofilm architecture appears to influence the form of destruction. We suggest that the “microcosms” of Cocconeis and their underlying cells not only serve as an inoculum to recolonize the surface when conditions permit but enhance immigration by interrupting flow patterns across the surface.  相似文献   
955.
Closely related species often show adjacent geographic distributions, albeit with some overlap. This contiguity is thought to result from secondary contact between (spatially separated) diverging groups or from parapatric speciation. Fights between males of closely related species will affect their chance to mate with females of the other species, which in turn may promote their spatial segregation and drive their speciation. Stigmaeopsis miscanthi is a social spider mite that lives in a group within self-woven nests on leaves of Chinese silver grass. This mite shows lethal male–male fight as a means to maintain a harem, and has two forms showing differences in the levels of male–male aggression, diapause intensity in females and the relative length of the first to third legs. The two forms show parapatric distributions. We found that males of one form readily engage themselves in lethal fight with males of the other form, thereby acquiring the nests and gaining access to females of this other form. Males of the aggressive form tend to win the fights with males of the other form. Their first legs are longer which may provide them with a better weapon and which also indicate a larger body width. However, another determinant of who wins the fight is the length of the third legs which can be a proxy for body length. Based on these results, we hypothesize that male killing behavior is one of the mechanisms maintaining parapatry (instead of sympatry) of the two spider mite forms apart from difference in diapause attributes.  相似文献   
956.
The concentrations of available arsenic, copper, lead, and zinc in the soil, and the concentration of these elements in vegetal tissues were measured. The most common species at the sites were studied. All the species that were analyzed took up pollutants and could indicate polluted soil. However, all the studied species did not fit to map pollution. The birch (Betula pendula) and most of the fungi species had still quite low concentrations in their tissues compared with the available concentrations in the soil. No direct correlation between the pollution content in the soil and in the vegetation tissues could be observed. Specimens of Thlaspi caerulescens were accumulating zinc. Of the four fungi species collected, only Thelephora caryophyllea seemed to accumulate actively in the studied pollutants. Moreover, it was possible to use an arsenic test with the fungi, which is also interesting from the perspective of preliminary assessment of the degree of pollution. A qualitative judgement of the soil pollution is possible by examining the plant material. To obtain a more quantitative and complete mapping, the method has to be developed further and completed with other investigations when vegetation is missing.  相似文献   
957.
The neuropeptides S1 (GFNSALMFamide) and S2 (SGPYSFNSGLTFamide), which share sequence similarity, were discovered in the starfish Asterias rubens and are prototypical members of the SALMFamide family of neuropeptides in echinoderms. SALMFamide neuropeptides act as muscle relaxants and both S1 and S2 cause relaxation of cardiac stomach and tube foot preparations in vitro but S2 is an order of magnitude more potent than S1. Here we investigated a structural basis for this difference in potency using spectroscopic techniques. Circular dichroism spectroscopy showed that S1 does not have a defined structure in aqueous solution and this was supported by 2D nuclear magnetic resonance experiments. In contrast, we found that S2 has a well-defined conformation in aqueous solution. However, the conformation of S2 was concentration dependent, with increasing concentration inducing a transition from an unstructured to a structured conformation. Interestingly, this property of S2 was not observed in an N-terminally truncated analogue of S2 (short S2 or SS2; SFNSGLTFamide). Collectively, the data obtained indicate that the N-terminal region of S2 facilitates peptide self-association at high concentrations, which may have relevance to the biosynthesis and/or bioactivity of S2 in vivo.  相似文献   
958.
As a first step towards understanding microbial dissolution processes, our research focuses on characterizing attachment features that form between a Pseudomonas sp. bacteria and the Fe(III)‐(hydr)oxide minerals hematite and goethite. Microbial growth curves in Fe‐limited growth media indicated that the bacteria were able to obtain Fe from the Fe(III)‐(hydr)oxidesfor use in metabolic processes. A combination of scanning electron microscopy, epifluorescence, and Tapping Mode? atomic‐force microscopy showed that the bacteria colonized some fraction of mineralogical aggregates. These aggregates were covered by bacteria and were linked together by relatively open biofilms consisting of networks of fiber‐like attachment features intertwined through thin films of amorphous‐looking organic material. The biofilm material encompassed numerous individual bacteria, as well as minéralogie particles. We hypothesize that the bacteria first attached to mineral aggregates, perhaps via their flagella, forming colonies. Following initial attachment, the bacteria exuded additional attachment features in the form of fine, branching fibrils intertwined through thin films. The detailed structures of these attachment features were highlighted by Phase Imaging atomic‐force microscopy, which served as a real‐time contrast enhancement technique and showed some poorly defined sensitivity to different surface materials, most probably related to differences in stiffness or viscoelasticity. Although the mechanism of the microbially enhanced dissolution remains unknown, we hypothesize that the bacteria may have produced micro environments conducive to dissolution through the use of observed extracellular materials.  相似文献   
959.
960.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号