全文获取类型
收费全文 | 3410篇 |
免费 | 406篇 |
国内免费 | 234篇 |
出版年
2024年 | 19篇 |
2023年 | 111篇 |
2022年 | 129篇 |
2021年 | 154篇 |
2020年 | 168篇 |
2019年 | 159篇 |
2018年 | 152篇 |
2017年 | 117篇 |
2016年 | 131篇 |
2015年 | 141篇 |
2014年 | 167篇 |
2013年 | 243篇 |
2012年 | 121篇 |
2011年 | 157篇 |
2010年 | 110篇 |
2009年 | 163篇 |
2008年 | 158篇 |
2007年 | 154篇 |
2006年 | 158篇 |
2005年 | 195篇 |
2004年 | 182篇 |
2003年 | 133篇 |
2002年 | 112篇 |
2001年 | 62篇 |
2000年 | 57篇 |
1999年 | 48篇 |
1998年 | 51篇 |
1997年 | 38篇 |
1996年 | 37篇 |
1995年 | 33篇 |
1994年 | 35篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 20篇 |
1990年 | 24篇 |
1989年 | 11篇 |
1988年 | 14篇 |
1987年 | 20篇 |
1986年 | 13篇 |
1985年 | 28篇 |
1984年 | 20篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 20篇 |
1980年 | 16篇 |
1979年 | 15篇 |
1978年 | 26篇 |
1977年 | 18篇 |
1976年 | 11篇 |
1975年 | 11篇 |
排序方式: 共有4050条查询结果,搜索用时 218 毫秒
71.
A mass spectral fragmentation pattern of permethyl 6-C-glycosylflavones is proposed from the MS data of permethyl derivatives mono-O-deuteriomethylated in the 2″-, 3″-, 4″- or 6″-positions. The synthesis of these compounds via O″-glycosyl-6-C-glucosylflavones is described. 相似文献
72.
The triterpenoid saponins identified in plants during the period 1973 to 1978 inclusive are reviewed. Their natural occurrence, chemistry and biological activities are discussed. A compilation of all saponins, the structures of which have been fully elucidated, is included. 相似文献
73.
Jayr de Paiva Campello Sebastião Ferreira Fonseca Ching-Jer Chang Ernest Wenkert 《Phytochemistry》1975,14(1):243-248
Sitosterol and the following terpenic compounds have been isolated from the bark of Podocarpus lambertius: 3β-hydroxytotarol, 4β-carboxynortotarol, and macrophyllic and lambertic acids. The leaves yielded sitosterol, stigmastan-3β,5α-diol-6-one, isopimaric acid, phyllocladene, isophyllocladene, 8,9-abieten-15-ol and 17-isophyllocladenol. 相似文献
74.
Two enantiomeric cuparene-type sesquiterpenoids, (R)-(−)-cuparene (1) and (R)-(−)-δ-cuparenol (2), have been isolated from the liverwort, Bazzania pompeana. The structures and absolute configurations of the two compounds have been determined. 相似文献
75.
BRIAN P. EYDEN 《The Journal of eukaryotic microbiology》1975,22(3):336-344
SYNOPSIS. Light and electron microscope observations on Dunaliella primolecta Butcher from logarithmic and stationary phases of batch cultures are correlated. Except for the lack of a cell wall the fine structure has typical volvocid features. The transition from logarithmic to stationary phase is marked by changes in content and size of cytoplasmic vacuoles, accumulation of cytoplasmic lipid, accumulation of starch in the plastid matrix, and by the formation of autophagosome-like bodies. The organelles in stationary-phase flagellates are closely packed together because of the cytoplasmic lipid and starch-distended chloroplast. Organisms from logarithmic phase have an abundant ribosome-packed groundplasm supporting the organelles. In the cytochemical demonstration of acid phosphatase activity, Golgi cisternae and smooth and coated Golgi vesicles contain Gomori reaction product. The possible roles of the Golgi apparatus in this flagellate are discussed. 相似文献
76.
The new clerodane diterpene 1 was isolated from the aerial parts of Baccharis tucumanensis (Compositae). Its structure has been determined from spectral data combined with chemical transformations. 相似文献
77.
Takashi Torigoe 《Primates; journal of primatology》1985,26(2):182-194
Seventy-four primates species (24 genera of six families) were presented with a nylon rope and a wooden cube, and their subsequent
manipulations were recorded in detail. Five hundreds and six manipulation patterns were distinguished on the basis of the
actions performed, body-parts used and relations to other objects. Inter-specific comparisons revealed three groups: (1) lemurs,
marmosets, spider monkeys and leaf-eaters; (2) Old World monkeys except leaf-eaters; and (3) cebus monkeys and apes. The first
group had the smallest repertoire of manipulations, in which only a few types of actions and body-parts were involved. The
second and third groups had more varied modes of manipulation. Actions such as Roll, Rub and Slide, and use of fingers characterized
these groups. Except for the lesser ape, their manipulations were frequently related with other objects. Moreover, actions
such as Drape, Drop, Strike, Swing and Throw were typical of the third group. The factors producing such inter-specific differences
in manipulations and the relations to tool use are discussed. 相似文献
78.
《Cell》2022,185(23):4409-4427.e18
79.
TROY G. MURPHY TIFFANY T. PHAM 《Biological journal of the Linnean Society. Linnean Society of London》2012,106(3):673-681
Coloration plays an important role in sexual and social communication, and in many avian species both males and females maintain elaborate colours. Recent research has provided strong support for the hypothesis that elaborate female traits can be maintained by sexual or social selection; however, most research on female ornamentation has focused on pigment‐based colours, and less is known about how structural colours are maintained. Both sexes of the turquoise‐browed motmot (Eumomota superciliosa) have a blue‐green racket‐tipped tail, and it remains unknown if tail coloration serves as a sexual or social signal in one or both sexes. Here, we describe sexual dichromatism in the blue‐green portion of the tail racket, and we test for a relationship between coloration and condition, as indicated by growth bars. Tail colour of both sexes has a similar spectral shape, and there is significant, although moderate, sexual dichromatism: males are brighter than females, and males have marginally greater blue‐green saturation than females. The length of feather grown per day is positively related to overall feather brightness, but this relationship is only present in males. The relationship between male coloration and condition suggests that tail colour has the potential to convey information about individual quality during mate choice or contest competition. The lack of a similar relationship in females suggests that female tail colour does not convey the same condition‐dependent information that we suggest may be reflected by male colour. Female tail colour may therefore reflect other aspects of condition, be involved in other (non‐condition‐dependent) forms of communication, or be expressed as a non‐functional byproduct of genetic correlation between the sexes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 673–681. 相似文献
80.
Núria Cerdà‐Costa Francesc Xavier Gomis‐Rüth 《Protein science : a publication of the Protein Society》2014,23(2):123-144
The cleavage of peptide bonds by metallopeptidases (MPs) is essential for life. These ubiquitous enzymes participate in all major physiological processes, and so their deregulation leads to diseases ranging from cancer and metastasis, inflammation, and microbial infection to neurological insults and cardiovascular disorders. MPs cleave their substrates without a covalent intermediate in a single‐step reaction involving a solvent molecule, a general base/acid, and a mono‐ or dinuclear catalytic metal site. Most monometallic MPs comprise a short metal‐binding motif (HEXXH), which includes two metal‐binding histidines and a general base/acid glutamate, and they are grouped into the zincin tribe of MPs. The latter divides mainly into the gluzincin and metzincin clans. Metzincins consist of globular ~130–270‐residue catalytic domains, which are usually preceded by N‐terminal pro‐segments, typically required for folding and latency maintenance. The catalytic domains are often followed by C‐terminal domains for substrate recognition and other protein–protein interactions, anchoring to membranes, oligomerization, and compartmentalization. Metzincin catalytic domains consist of a structurally conserved N‐terminal subdomain spanning a five‐stranded β‐sheet, a backing helix, and an active‐site helix. The latter contains most of the metal‐binding motif, which is here characteristically extended to HEXXHXXGXX(H,D). Downstream C‐terminal subdomains are generally shorter, differ more among metzincins, and mainly share a conserved loop—the Met‐turn—and a C‐terminal helix. The accumulated structural data from more than 300 deposited structures of the 12 currently characterized metzincin families reviewed here provide detailed knowledge of the molecular features of their catalytic domains, help in our understanding of their working mechanisms, and form the basis for the design of novel drugs. 相似文献