全文获取类型
收费全文 | 797篇 |
免费 | 102篇 |
专业分类
899篇 |
出版年
2021年 | 9篇 |
2019年 | 7篇 |
2018年 | 10篇 |
2017年 | 12篇 |
2016年 | 16篇 |
2015年 | 33篇 |
2014年 | 23篇 |
2013年 | 38篇 |
2012年 | 40篇 |
2011年 | 43篇 |
2010年 | 22篇 |
2009年 | 22篇 |
2008年 | 39篇 |
2007年 | 44篇 |
2006年 | 42篇 |
2005年 | 38篇 |
2004年 | 34篇 |
2003年 | 33篇 |
2002年 | 35篇 |
2000年 | 16篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 6篇 |
1996年 | 5篇 |
1995年 | 10篇 |
1994年 | 6篇 |
1993年 | 7篇 |
1992年 | 9篇 |
1991年 | 8篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 13篇 |
1984年 | 16篇 |
1983年 | 19篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1974年 | 7篇 |
1973年 | 6篇 |
1971年 | 5篇 |
1970年 | 7篇 |
1969年 | 7篇 |
1967年 | 6篇 |
1965年 | 9篇 |
排序方式: 共有899条查询结果,搜索用时 15 毫秒
11.
Generalist predator's niche shifts reveal ecosystem changes in an experimentally fragmented landscape 下载免费PDF全文
Julian Resasco Kika T. Tuff Saul A. Cunningham Brett A. Melbourne Andrew L. Hicks Seth D. Newsome Kendi F. Davies 《Ecography》2018,41(7):1209-1219
Habitat fragmentation can alter the trophic structure of communities and environmental conditions, thus driving changes in biodiversity and ecosystem functions. Quantifying niches of generalist predators can reveal how fragmentation alters ecosystems. In a habitat fragmentation experiment, we used stable isotopes of a generalist predator skink to test predictions from spatial theory on trophic structure and to quantify abiotic changes associated with fragmentation among continuous forest, fragments, and matrix habitats. We predicted that in fragments and the matrix, isotopic niches would shift due to decreases in skink trophic positions (δ15N) from reductions in trophic structure of arthropod food webs and abiotic changes over time (δ13C) relative to continuous forest. Contrary to theoretical predictions, we did not find evidence of reductions in trophic structure with fragmentation. In fact, skink δ15N values were higher in the matrix and fragments than continuous forest, likely due to changes in distributions of a detritivorous prey species. In addition, δ13C values in the matrix decreased over years since fragmentation due to abiotic changes associated with matrix tree maturation. We show how isotopic niches are influenced by fragmentation via shifts in biotic and abiotic processes. The potential for either or both spatial and abiotic effects of fragmentation present a challenge for theory to better predict ecological changes in fragmented landscapes. 相似文献
12.
Susan E. Jensen Donald W. S. Westlake Saul Wolfe 《Applied microbiology and biotechnology》1984,20(3):155-160
Summary Four enzymes required for the biosynthesis of pencillins and cephalosporins by Streptomyces clavuligerus have been immobilized on an anion exchange resin. The capabilities of the system have been studied by circulation of reaction mixtures through the immobilized enzyme reactor. Within 30 min, all of the substrate -(l--aminoadipyl)-l-cysteinyl-d-valine is consumed and converted to a mixture of penicillins and cephalosporins. After 60 min the major antibiotic products are (iso)penicillin N and desacetylcephalosporin C. The activity of the immobilized enzyme reactor activity is stable to storage at temperatures below 4°C but activity is lost on repeated use. 相似文献
13.
Miodownik Saul; Melendez Jose; Carlon Vittoria Arslan; Burda Brian 《Journal of applied physiology》1998,84(6):2177-2182
Themethanol-burning lung model has been used as a technique for generatinga predictable ratio of carbon dioxide production (CO2) to oxygen consumption(O2) or respiratoryquotient (RQ). Although an accurate RQ can be generated, quantitativelypredictable and adjustableO2 andCO2 cannot be generated. Wedescribe a new burner device in which the combustion rate of methanolis always equal to the infusion rate of fuel over an extended range ofO2 concentrations. This permitsthe assembly of a methanol-burning lung model that is usable withO2 concentrations up to 100% and provides continuously adjustable and quantitativeO2 (69-1,525 ml/min)and CO2 (46-1,016ml/min) at a RQ of 0.667. 相似文献
14.
Purkis Sam J. Gleason Arthur C. R. Purkis Charlotte R. Dempsey Alexandra C. Renaud Philip G. Faisal Mohamed Saul Steven Kerr Jeremy M. 《Coral reefs (Online)》2019,38(3):467-488
Coral Reefs - With compelling evidence that half the world’s coral reefs have been lost over the last four decades, there is urgent motivation to understand where reefs are located and their... 相似文献
15.
Ramírez-Salinas Gema L. García-Machorro J. Quiliano Miguel Zimic Mirko Briz Verónica Rojas-Hernández Saul Correa-Basurto J. 《Journal of molecular modeling》2015,21(11):1-14
Journal of Molecular Modeling - There is a wide variety of ion channel types with various types of blockers, making research in this field very complicated. To reduce this complexity, it is... 相似文献
16.
Saul Bader 《The Journal of cell biology》1959,5(2):217-229
The amounts of DNA in interphase nuclei were compared with the amounts of DNA in metaphase and anaphase figures in Feulgen-stained tissue sections of 5 specimens of the human ovarian papillary serous adenocarcinoma. The relative amounts of DNA per cell were determined by cytophotometric measurements of interphase nuclei at a single wavelength and of mitotic figures by the two wavelength method. The 5 specimens conformed to the stem cell concept of cell proliferation since anaphase distributions of amounts of DNA were restricted to a narrow range of DNA values indicating the successful mitosis of a single cell type (stem cell) out of several cell types whose presence were suggested by the wide spread of interphase and metaphase values. In addition, the data indicated that, in some instances, only the amounts of DNA in anaphase figures can reliably identify the stem cell. Changes in the frequency of dividing cells having doubled amounts of DNA, and/or the presence of cells resulting from endoreduplication can distort the interphase distribution of amounts of DNA and thus give rise to a modal DNA interphase value which is not the same as the DNA value of the stem cell (anaphase figures). 相似文献
17.
Abstract We show that the classical Metropolis Monte Carlo (MMC) algorithm converges very slowly when applied to the primitive electrolyte environment for a high charge-density polyelectrolyte. This slowness of convergence, which is due to the large density inhomogeneity around the polyelectrolyte, produces noticeable errors in the ion distribution functions for MMC runs of 1.3 × 106 trial steps started from nonequilibrium distributions. We report that an algorithm which we call DSMC (for density-scaled Monte Carlo) overcomes this problem and provides relatively rapid convergence in this application. We suggest that DSMC should be well-suited for other Monte Carlo simulations on physical systems where large density inhomogeneities occur. 相似文献
18.
Abstract Glycerol-stabilised cell extracts of Streptomyces clavuligerus contain an enzyme activity which synthesises ACV from the individual amino acids L -α-aminoadipic acid, L -cysteine and L -valine. Enzyme activity was optimum in reaction mixtures containing 1 mM ATP together with an ATP regenerating system. The ACV synthetase enzyme formed ACV analogs when provided with L - carboxymethylcysteine in place of L -α-aminoadipic acid or when provided with L - allo isoleucine or L -α-aminobutyrate in place of L -valine. Multistep conversion of individual amino acids to penicillin and cephalosporin antibiotics was restricted as a result of the inhibitory effects of L -α-aminoadipic acid and L -cysteine on isopenicillin N synthetase. 相似文献
19.
Background
The chloroplast of eukaryotic microalgae such as Chlamydomonas reinhardtii is a potential platform for metabolic engineering and the production of recombinant proteins. In industrial biotechnology, inducible expression is often used so that the translation or function of the heterologous protein does not interfere with biomass accumulation during the growth stage. However, the existing systems used in bacterial or fungal platforms do not transfer well to the microalgal chloroplast. We sought to develop a simple inducible expression system for the microalgal chloroplast, exploiting an unused stop codon (TGA) in the plastid genome. We have previously shown that this codon can be translated as tryptophan when we introduce into the chloroplast genome a trnWUCA gene encoding a plastidial transfer RNA with a modified anticodon sequence, UCA.Results
A mutated version of our trnWUCA gene was developed that encodes a temperature-sensitive variant of the tRNA. This allows transgenes that have been modified to contain one or more internal TGA codons to be translated differentially according to the culture temperature, with a gradient of recombinant protein accumulation from 35 °C (low/off) to 15 °C (high). We have named this the CITRIC system, an acronym for cold-inducible translational readthrough in chloroplasts. The exact induction behaviour can be tailored by altering the number of TGA codons within the transgene.Conclusions
CITRIC adds to the suite of genetic engineering tools available for the microalgal chloroplast, allowing a greater degree of control over the timing of heterologous protein expression. It could also be used as a heat-repressible system for studying the function of essential native genes in the chloroplast. The genetic components of CITRIC are entirely plastid-based, so no engineering of the nuclear genome is required.20.
Saul AB 《Neuro-Signals》2008,16(2-3):209-225
The timing of the retinal input to the lateral geniculate nucleus is highly modified in lagged cells. Evidence is reviewed for how the responses of these cells are generated, how their structure and function differs from their nonlagged neighbors, and what their projections to cortex might do. 相似文献