全文获取类型
收费全文 | 1403篇 |
免费 | 79篇 |
出版年
2021年 | 29篇 |
2020年 | 14篇 |
2019年 | 20篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 26篇 |
2015年 | 41篇 |
2014年 | 36篇 |
2013年 | 45篇 |
2012年 | 49篇 |
2011年 | 51篇 |
2010年 | 49篇 |
2009年 | 45篇 |
2008年 | 58篇 |
2007年 | 70篇 |
2006年 | 48篇 |
2005年 | 44篇 |
2004年 | 25篇 |
2003年 | 21篇 |
2002年 | 31篇 |
1999年 | 17篇 |
1998年 | 16篇 |
1997年 | 17篇 |
1996年 | 16篇 |
1995年 | 15篇 |
1994年 | 13篇 |
1993年 | 20篇 |
1991年 | 20篇 |
1990年 | 17篇 |
1989年 | 13篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1984年 | 14篇 |
1982年 | 15篇 |
1980年 | 14篇 |
1976年 | 15篇 |
1975年 | 12篇 |
1974年 | 16篇 |
1973年 | 18篇 |
1972年 | 20篇 |
1968年 | 12篇 |
1965年 | 37篇 |
1964年 | 19篇 |
1963年 | 32篇 |
1962年 | 23篇 |
1961年 | 33篇 |
1960年 | 21篇 |
1959年 | 27篇 |
1958年 | 27篇 |
1957年 | 12篇 |
排序方式: 共有1482条查询结果,搜索用时 15 毫秒
91.
Ant inquilines are obligate social parasites, usually lacking a sterile worker caste, which are dependent on their hosts for survival and reproduction. Social parasites are rare among the fungus‐gardening ants (Myrmicinae: tribe Attini) and only four species are known until now, all being inquilines from the Higher Attini. We describe Mycocepurus castrator sp.n. , the first inquiline social parasite to be discovered in the Lower Attini. Our study of the parasite's behaviour and life history supports the conclusion drawn from external morphology: Mycocepurus castrator is an evolutionarily derived inquiline parasite of Mycocepurus goeldii. Inquilines are of great interest to evolutionary biology because it is debated if they originated via sympatric or allopatric speciation. We discuss the life history evolution, behaviour and morphology of socially parasitic, fungus‐growing ants. 相似文献
92.
93.
Glomalean mycorrhizal fungi from tropical Australia 总被引:6,自引:0,他引:6
A comparison of different methods for isolation of vesicular-arbuscular mycorrhizal (VAM) fungi into open-pot cultures was
undertaken as part of a study of the diversity of these fungi. Four different isolation techniques using spores separated
from soil, soil trap cultures, root samples, or transplanted seedlings grown in intact soil cores were used to obtain as many
fungi as possible from each site. Isolation methods were compared using paired samples from the same locations within natural
(savanna, rocky hill, wetland, rainforest) and disturbed (minesite) habitats in a seasonally dry tropical region in the Northern
Territory of Australia. There were large differences in (i) the efficiency (rate of increase in mycorrhizal colonisation),
(ii) the proportion of successful cultures, (iii) fungal diversity (number of fungal species in each culture) and (iv) specificity
(identity of species isolated) between these four procedures. However, the less-efficient procedures generally resulted in
a higher proportion of cultures of one fungus, which could be used without further isolation steps. Most species of Scutellospora, Acaulospora and Gigaspora were obtained primarily from field-collected spores, but only 50% of these culture attempts were successful. Spores from
these initial cultures produced mycorrhizas much more rapidly and successfully when used to start second-generation cultures.
Several species of fungi, rarely recovered as living spores from field soils, were dominant in many trap cultures started
from soil or roots. Most of these fungi were Glomus species, that were first distinguished by colonisation patterns in roots and eventually identified after sporulation in second-
or third-generation trap cultures. These experiments demonstrated that glomalean fungi in the habitats sampled belonged to
two functional categories, based on whether or not spores were important propagules. The "non-sporulating" fungi were dominant
in many trap cultures, which suggests that these fungi had higher total inoculum levels in soils than other fungi. Pot-culturing
methods provided additional information on fungal diversity which complemented spore occurrence data obtained using the same
soil samples and provided valuable new information about the biology of these fungi.
Accepted: 26 December 1998 相似文献
94.
95.
Koning R van den Worm S Plaisier JR van Duin J Pieter Abrahams J Koerten H 《Journal of molecular biology》2003,332(2):415-422
The icosahedrally symmetrized structure of bacteriophage MS2 as determined by cryo-electron microscopy (EM) reveals the presence of genomic RNA that attaches to coat-protein dimers. Earlier X-ray diffraction studies revealed similar interactions between the unique operator hairpin of the MS2 genomic RNA and the coat-protein dimer. This observation leads us to conclude that not only the operator, but also many other RNA sequences in the genome of MS2, are able to bind to the coat-protein dimer. A substantial number of potential coat-protein-dimer binding sites are present in the genome of MS2 that can account for the observed RNA densities in the EM map. Moreover, it appears that these stem-loop structures are able to bind in a similar fashion to the coat protein dimer as the wild-type operator hairpin. The EM map also shows additional density between the potential operator-binding sites, linking the RNA stem-loops together to form an icosahedral network around the 3 and 5-fold axes. This RNA network is bound to the inside of the MS2 capsid and probably influences both capsid stability and formation, supporting the idea that capsid formation and RNA packaging are intimately linked to each other. 相似文献
96.
97.
Drosophila innate immunity: a genomic view of pathogen defense 总被引:1,自引:0,他引:1
98.
Transient exposure of hydrophobic surface in the photoactive yellow protein monitored with Nile Red 总被引:2,自引:0,他引:2 下载免费PDF全文
Hendriks J Gensch T Hviid L van Der Horst MA Hellingwerf KJ van Thor JJ 《Biophysical journal》2002,82(3):1632-1643
In this study we have investigated binding of the fluorescent hydrophobicity probe Nile Red to the photoactive yellow protein (PYP), to characterize the exposure and accessibility of hydrophobic surface upon formation of the signaling state of this photoreceptor protein. Binding of Nile Red, reflected by a large blue shift and increase in fluorescence quantum yield of the Nile Red emission, is observed exclusively when PYP resides in its signaling state. N-terminal truncation of the protein allows assignment of the region surrounding the chromophore as the site where Nile Red binds to PYP. We also observed a pH dependence of the affinity of Nile Red for pB, which we propose is caused by pH dependent differences of the structure of the signaling state. From a comparative analysis of the kinetics of Nile Red binding and transient absorption changes in the visible region we can conclude that protonation of the chromophore precedes the exposure of a hydrophobic surface near the chromophore binding site, upon formation of the signaling state. Furthermore, the data presented here favor the view that the signaling state is structurally heterogeneous. 相似文献
99.
Animal cells internalize specific extracellular macromolecules (ligands) by using specialized cell surface receptors that operate through a complex and highly regulated process known as receptor-mediated endocytosis, which involves the binding, internalization, and transfer of ligands through a series of distinct intracellular compartments. For the uptake of a variety of carbohydrate-containing macromolecules, such as glycoproteins, animal cells use specialized membrane-bound lectins as endocytic receptors that recognize different sugar residues or carbohydrate structures present on various ligands. The hepatic asialoglycoprotein receptor, which recognizes glycoconjugates containing terminal galactose or N-acetylgalactosamine residues, was the first membrane lectin discovered and has been a classical system for studying receptor-mediated endocytosis. Studies of how the asialoglycoprotein receptor functions have led to the discovery of two functionally distinct, parallel pathways of clathrin-mediated endocytosis (called the State 1 and State 2 pathways), which may also be utilized by all the other endocytic recycling receptor systems. Another endocytic membrane lectin, the hyaluronan/chondroitin sulfate receptor, which has recently been purified and cloned, is responsible for the turnover in mammals of these glycosaminoglycans, which are important components of extracellular matrices. We discuss the characteristics and physiological importance of these two proteins as examples of how lectins can function as endocytic receptors. 相似文献
100.
Intracellular Glucose Concentration in Derepressed Yeast Cells Consuming Glucose Is High Enough To Reduce the Glucose Transport Rate by 50% 下载免费PDF全文
Bas Teusink Jasper A. Diderich Hans V. Westerhoff Karel van Dam Michael C. Walsh 《Journal of bacteriology》1998,180(3):556-562
In Saccharomyces cerevisiae cells exhibiting high-affinity glucose transport, the glucose consumption rate at extracellular concentrations above 10 mM was only half of the zero trans-influx rate. To determine if this regulation of glucose transport might be a consequence of intracellular free glucose we developed a new method to measure intracellular glucose concentrations in cells metabolizing glucose, which compares glucose stereoisomers to correct for adhering glucose. The intracellular glucose concentration was 1.5 mM, much higher than in most earlier reports. We show that for the simplest model of a glucose carrier, this concentration is sufficient to reduce the glucose influx by 50%. We conclude that intracellular glucose is the most likely candidate for the observed regulation of glucose import and hence glycolysis. We discuss the possibility that intracellular glucose functions as a primary signal molecule in these cells. 相似文献