全文获取类型
收费全文 | 16355篇 |
免费 | 1359篇 |
国内免费 | 16篇 |
专业分类
17730篇 |
出版年
2023年 | 64篇 |
2022年 | 160篇 |
2021年 | 341篇 |
2020年 | 192篇 |
2019年 | 235篇 |
2018年 | 328篇 |
2017年 | 275篇 |
2016年 | 508篇 |
2015年 | 762篇 |
2014年 | 796篇 |
2013年 | 1063篇 |
2012年 | 1269篇 |
2011年 | 1256篇 |
2010年 | 801篇 |
2009年 | 787篇 |
2008年 | 1019篇 |
2007年 | 961篇 |
2006年 | 954篇 |
2005年 | 832篇 |
2004年 | 842篇 |
2003年 | 795篇 |
2002年 | 734篇 |
2001年 | 191篇 |
2000年 | 163篇 |
1999年 | 169篇 |
1998年 | 168篇 |
1997年 | 120篇 |
1996年 | 124篇 |
1995年 | 113篇 |
1994年 | 97篇 |
1993年 | 104篇 |
1992年 | 127篇 |
1991年 | 91篇 |
1990年 | 95篇 |
1989年 | 84篇 |
1988年 | 70篇 |
1987年 | 61篇 |
1986年 | 65篇 |
1985年 | 62篇 |
1984年 | 74篇 |
1983年 | 64篇 |
1982年 | 49篇 |
1981年 | 59篇 |
1980年 | 54篇 |
1979年 | 64篇 |
1978年 | 41篇 |
1977年 | 41篇 |
1976年 | 46篇 |
1975年 | 43篇 |
1974年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
The hydrocarbon fraction of leaf waxes of three halophytes of the Chenopodiaceae common to Mediterranean salt marshes (Suaeda vera, Sarcocornia fruticosa and Halimione portulacoides) revealed the presence of a minor series of odd and even chains 1-chloro-n-alkanes ranging from C(19) to C(29). The identification of these new chlorinated plant constituents was based on a combination of mass spectrometry data with selective chlorine detection (CPG-AED) and was confirmed by comparison with authentic standards. The qualitative and quantitative distributions of these 1-chloro-n-alkanes varied inter-specifically. Homologues with an odd carbon-chain were predominant in all species but maximised at C(25) and C(27) in S. vera and S. fruticosa, and at C(27) and C(29) in H. portulacoides. Remarkably, 1-chloro-nonacosane was an abundant homologue only in this latter species. Leaves of S. vera contained 4 to 7 times more of total chloroalkanes than leaves of the other two species. These compounds accounted for 10, 4 and 1% of the hydrocarbon fraction of leaf waxes of S. vera, S. fruticosa and H. portulacoides, respectively. Attempts to link the occurrence of these chloroalkanes with other classes of leaf waxes (n-alkenes, n-aldehydes and n-alcohols) did not allowed a clear precursor-product relationship to be established. The biological functions as well as the mode of synthesis of alkylchlorides in (halophyte) plants remain unknown but undoubtedly deserve further attention. 相似文献
982.
Identification of potent inhibitors of Helicoverpa armigera gut proteinases from winged bean seeds 总被引:2,自引:0,他引:2
Giri AP Harsulkar AM Ku MS Gupta VS Deshpande VV Ranjekar PK Franceschi VR 《Phytochemistry》2003,63(5):523-532
Dry mature seeds of winged bean (Psophocarpus tetragonolobus L., DC.) (WB) contain several proteinase inhibitors. Two-dimensional gel analysis of WB seed protein followed by activity visualization using a gel-X-ray film contact print technique revealed at least 14 trypsin inhibitors (TIs) in the range of 28-6 kD. A total of seven inhibitors (WBTI-1 to 7) were purified by heat treatment and gel filtration followed by elution from preparative native gels. Based on their biochemical characterization such as molecular mass, pI, heat stability, and susceptibility to inactivation by reducing agents, WBTI-1 to 4 are Kunitz type inhibitors while WBTI-5 to 7 are classified as Bowman-Birk type serine proteinase inhibitors. Although Kunitz type TIs (20-24 kD) of WB have been reported, the smaller TIs that belong to the Bowman-Birk type have not been previously characterized. Seven major TIs isolated from WB seed were individually assessed for their potential to inhibit the gut proteinases (HGP) of Helicoverpa armigera, a pest of several economically important crops, which produces at least six major and several minor trypsin/chymotrypsin/elastase-like serine proteinases in the gut. WBTI-1 (28 kD) was identified as a potent inhibitor of HGP relative to trypsin and among the other WBTIs; it inhibited 94% of HGP activity while at the same concentration it inhibited only 22% of trypsin activity. WBTI-2 (24 kD) and WBTI-4 (20 kD) inhibited HGP activity greater than 85%. WBTI-3,-5,-6 and-7 showed limited inhibition of HGP as compared with trypsin. These results indicate that WBTIs have different binding potentials towards HGP although most of the HGP activity is trypsin-like. We also developed a simple and versatile method for identifying and purifying proteinase inhibitors after two-dimensional separation using the gel-X-ray film contact print technique. 相似文献
983.
Two novel 9,11-secosterols, aplidiasterols A (3β,6β,11-trihydroxy-9,11-seco-5α-cholest-7-en-9-one, 1) and B (3β,5α,6β,11-tetrahydroxy-9,11-secocholest-7-en-9-one, 2), along with the known secosterols 3 and 4, were isolated from the Mediterranean ascidian Aplidium conicum and their structures were determined by spectroscopic data. Aplidiasterols A and B were found to be cytotoxic against rat glioma (C6) and murine monocyte/macrophage (J774) tumor cells in vitro. Compounds 1-4 represent the first example of secosterols isolated from tunicates. 相似文献
984.
The goal of generalized logical analysis is to model complex biological systems, especially so-called regulatory systems,
such as genetic networks. This theory is mainly characterized by its capacity to find all the steady states of a given system
and the functional positive and negative circuits, which generate multistationarity and a cycle in the state sequence graph,
respectively. So far, this has been achieved by exhaustive enumeration, which severely limits the size of the systems that
can be analysed. In this paper, we introduce a mathematical function, called image function, which allows the calculation
of the value of the logical parameter associated with a logical variable depending on the state of the system. Thus the state
table of the system is represented analytically. We then show how all steady states can be derived as solutions to a system
of steady-state equations. Constraint programming, a recent method for solving constraint satisfaction problems, is applied
for that purpose. To illustrate the potential of our approach, we present results from computer experiments carried out on
very large randomly-generated systems (graphs) with hundreds, or even thousands, of interacting components, and show that
these systems can be solved using moderate computing time. Moreover, we illustrate the approach through two published applications,
one of which concerns the computation times of all steady states for a large genetic network. 相似文献
985.
Spitz et al (2003[this issue of Cell]) describe the properties of a novel cis-regulatory DNA element, the global control region (GCR), which regulates gene expression over distances of several hundred kilobases at the mouse HoxD complex. The GCR provides an explanation for the colinear genetic linkage and expression of individual Hox genes within developing limbs. 相似文献
986.
Desmoglein 4 in hair follicle differentiation and epidermal adhesion: evidence from inherited hypotrichosis and acquired pemphigus vulgaris 总被引:14,自引:0,他引:14
Kljuic A Bazzi H Sundberg JP Martinez-Mir A O'Shaughnessy R Mahoney MG Levy M Montagutelli X Ahmad W Aita VM Gordon D Uitto J Whiting D Ott J Fischer S Gilliam TC Jahoda CA Morris RJ Panteleyev AA Nguyen VT Christiano AM 《Cell》2003,113(2):249-260
Cell adhesion and communication are interdependent aspects of cell behavior that are critical for morphogenesis and tissue architecture. In the skin, epidermal adhesion is mediated in part by specialized cell-cell junctions known as desmosomes, which are characterized by the presence of desmosomal cadherins, known as desmogleins and desmocollins. We identified a cadherin family member, desmoglein 4, which is expressed in the suprabasal epidermis and hair follicle. The essential role of desmoglein 4 in skin was established by identifying mutations in families with inherited hypotrichosis, as well as in the lanceolate hair mouse. We also show that DSG4 is an autoantigen in pemphigus vulgaris. Characterization of the phenotype of naturally occurring mutant mice revealed disruption of desmosomal adhesion and perturbations in keratinocyte behavior. We provide evidence that desmoglein 4 is a key mediator of keratinocyte cell adhesion in the hair follicle, where it coordinates the transition from proliferation to differentiation. 相似文献
987.
Higgins VJ Beckhouse AG Oliver AD Rogers PJ Dawes IW 《Applied and environmental microbiology》2003,69(8):4777-4787
Genome-wide expression analysis of an industrial strain of Saccharomyces cerevisiae during the initial stages of an industrial lager fermentation identified a strong response from genes involved in the biosynthesis of ergosterol and oxidative stress protection. The induction of the ERG genes was confirmed by Northern analysis and was found to be complemented by a rapid accumulation of ergosterol over the initial 6-h fermentation period. From a test of the metabolic activity of deletion mutants in the ergosterol biosynthesis pathway, it was found that ergosterol is an important factor in restoring the fermentative capacity of the cell after storage. Additionally, similar ERG10 and TRR1 gene expression patterns over the initial 24-h fermentation period highlighted a possible interaction between ergosterol biosynthesis and the oxidative stress response. Further analysis showed that erg mutants producing altered sterols were highly sensitive to oxidative stress-generating compounds. Here we show that genome-wide expression analysis can be used in the commercial environment and was successful in identifying environmental conditions that are important in industrial yeast fermentation. 相似文献
988.
Neuronal groups projecting widely in the brain are being experimentally associated to attention and mood changes. Those groups are known to exert a modulatory effect over other larger groups. On the other hand, some people think of the brain functions as being performed by specialized modular systems. In this work, we propose an architecture of modular nature to explore a particular decision process. We show the importance of the modulatory effect of a special evaluation segment in that process. 相似文献
989.
The bacteriocin, lacticin 3147, increased isoleucine transamination by Lactococcus lactis IFPL359 in a cheese model system. The formation of -keto--methyl-n-valeric acid and 2-hydroxy-3-methyl-valeric acid increased by three times in cheese slurries at 12 °C and cheese aroma intensity increased as well, which corresponded with a higher 2-methylbutanal formation. 相似文献
990.