全文获取类型
收费全文 | 3054篇 |
免费 | 341篇 |
国内免费 | 2篇 |
专业分类
3397篇 |
出版年
2021年 | 42篇 |
2019年 | 36篇 |
2018年 | 29篇 |
2017年 | 39篇 |
2016年 | 62篇 |
2015年 | 90篇 |
2014年 | 121篇 |
2013年 | 153篇 |
2012年 | 169篇 |
2011年 | 172篇 |
2010年 | 99篇 |
2009年 | 71篇 |
2008年 | 124篇 |
2007年 | 144篇 |
2006年 | 119篇 |
2005年 | 123篇 |
2004年 | 115篇 |
2003年 | 102篇 |
2002年 | 99篇 |
2001年 | 85篇 |
2000年 | 97篇 |
1999年 | 82篇 |
1998年 | 49篇 |
1997年 | 40篇 |
1996年 | 38篇 |
1995年 | 23篇 |
1994年 | 29篇 |
1993年 | 25篇 |
1992年 | 54篇 |
1991年 | 46篇 |
1990年 | 39篇 |
1989年 | 44篇 |
1988年 | 53篇 |
1987年 | 42篇 |
1986年 | 43篇 |
1985年 | 57篇 |
1984年 | 30篇 |
1982年 | 23篇 |
1981年 | 22篇 |
1980年 | 22篇 |
1979年 | 37篇 |
1978年 | 27篇 |
1977年 | 43篇 |
1976年 | 31篇 |
1975年 | 22篇 |
1974年 | 35篇 |
1973年 | 38篇 |
1972年 | 32篇 |
1970年 | 26篇 |
1969年 | 20篇 |
排序方式: 共有3397条查询结果,搜索用时 15 毫秒
21.
22.
23.
24.
M P Heyn P J Bauer N A Dencher 《Biochemical and biophysical research communications》1975,67(3):897-903
The polyamine and ribosome contents during the logarithmic phase of growth were much higher than those in the late logarithmic or stationary phase of growth. On the other hand, the Mg2+ content did not vary markedly throughout the different growth phases. These data, together with the results of a previous communication (1), suggest that increased polyamines during the logarithmic phase of growth may play significant roles not only in the neutralization of the negative charges of increased ribosomes but also in the increase of the velocity of polypeptide chain elongation by binding to the ribosomes. The polyphenylalanine synthetic activity of ribosomes from the logarithmic phase of growth was higher than that of ribosomes obtained from the stationary phase of growth in the presence of optimal concentrations of spermidine. 相似文献
25.
Gap junction remodeling and altered connexin43 expression in the failing human heart 总被引:15,自引:0,他引:15
Kostin S Rieger M Dammer S Hein S Richter M Klövekorn WP Bauer EP Schaper J 《Molecular and cellular biochemistry》2003,242(1-2):135-144
Gap junctions (GJ) are important determinants of cardiac conduction and the evidence has recently emerged that altered distribution of these junctions and changes in the expression of their constituent connexins (Cx) may lead to abnormal coupling between cardiomyocytes and likely contribute to arrhythmogenesis. However, it is largely unknown whether changes in the expression and distribution of the major cardiac GJ protein, Cx43, is a general feature of diverse chronic myocardial diseases or is confined to some particular pathophysiological settings. In the present study, we therefore set out to investigate qualitatively and quantitatively the distribution and expression of Cx43 in normal human myocardium and in patients with dilated (DCM), ischemic (ICM), and inflammatory cardiomyopathies (MYO). Left ventricular tissue samples were obtained at the time of cardiac transplantation and investigated with immunoconfocal and electron microscopy. As compared with the control group, Cx43 labeling in myocytes bordering regions of healed myocardial infarction (ICM), small areas of replacement fibrosis (DCM) and myocardial inflammation (MYO) was found to be highly disrupted instead of being confined to the intercalated discs. In all groups, myocardium distant from these regions showed an apparently normal Cx43 distribution at the intercalated discs. Quantitative immunoconfocal analyis of Cx43 in the latter myocytes revealed that the Cx43 area per myocyte area or per myocyte volume is significantly decreased by respectively 30 and 55% in DCM, 23 and 48% in ICM, and by 21 and 40% in MYO as compared with normal human myocardium. In conclusion, focal disorganization of GJ distribution and down-regulation of Cx43 are typical features of myocardial remodeling that may play an important role in the development of an arrhythmogenic substrate in human cardiomyopathies. 相似文献
26.
Llorenç Milà i Canals Christian Bauer Jochen Depestele Alain Dubreuil Ruth Freiermuth Knuchel Gérard Gaillard Ottar Michelsen Ruedi Müller-Wenk Bernt Rydgren 《The International Journal of Life Cycle Assessment》2007,12(1):5-15
Background, Aim and Scope
Land use by agriculture, forestry, mining, house-building or industry leads to substantial impacts, particularly on biodiversity
and on soil quality as a supplier of life support functions. Unfortunately there is no widely accepted assessment method so
far for land use impacts. This paper presents an attempt, within the UNEP-SETAC Life Cycle Initiative, to provide a framework
for the Life Cycle Impact Assessment (LCIA) of land use.
Materials and Methods:
This framework builds from previous documents, particularly the SETAC book on LCIA (Lindeijer et al. 2002), developing essential
issues such as the reference for occupation impacts; the impact pathways to be included in the analysis; the units of measure
in the impact mechanism (land use interventions to impacts); the ways to deal with impacts in the future; and bio-geographical
differentiation.
Results:
The paper describes the selected impact pathways, linking the land use elementary flows (occupation; transformation) and parameters
(intensity) registered in the inventory (LCI) to the midpoint impact indicators and to the relevant damage categories (natural
environment and natural resources). An impact occurs when the land properties are modified (transformation) and also when
the current man-made properties are maintained (occupation).
Discussion:
The size of impact is the difference between the effect on land quality from the studied case of land use and a suitable reference
land use on the same area (dynamic reference situation). The impact depends not only on the type of land use (including coverage
and intensity) but is also heavily influenced by the bio-geographical conditions of the area. The time lag between the land
use intervention and the impact may be large; thus land use impacts should be calculated over a reasonable time period after
the actual land use finishes, at least until a new steady state in land quality is reached.
Conclusions:
Guidance is provided on the definition of the dynamic reference situation and on methods and time frame to assess the impacts
occurring after the actual land use. Including the occupation impacts acknowledges that humans are not the sole users of land.
Recommendations and Perspectives:
The main damages affected by land use that should be considered by any method to assess land use impacts in LCIA are: biodiversity
(existence value); biotic production potential (including soil fertility and use value of biodiversity); ecological soil quality
(including life support functions of soil other than biotic production potential). Bio-geographical differentiation is required
for land use impacts, because the same intervention may have different consequences depending on the sensitivity and inherent
land quality of the environment where it occurs. For the moment, an indication of how such task could be done and likely bio-geographical
parameters to be considered are suggested. The recommendation of indicators for the suggested impact categories is a matter
of future research. 相似文献
27.
Clades that predate the origin of biomes that they inhabit provide unique opportunities to examine both when major environmental transitions occurred, and how lineages adapted to these changes. The isolated island continent Australia has undergone a profound environmental transition through the Miocene, from relatively mesic to predominantly arid; however, we have much to learn about both the timing of this change, and how organisms may have responded to it. The family Carphodactylidae is an ancient Gondwanan group of geckos that occurs across all major Australian biomes. A multilocus (ND2, Rag-1, c-mos) phylogenetic and dating analysis of the most ecologically diverse clade within this group, the genus Nephrurus (sensuBauer, 1990) reveals that two of three morphological taxa historically recognized (the 'spiny knob-tails' and 'Underwoodisaurus') are relatively species depauperate, pleisomorphic basal grades that diversified through the late Oligocene and early Miocene, and are now absent from most of the arid biome. Based on their deep divergence and morphological distinctiveness we recognize two lineages (milii and sphyrurus) as monotypic genera, the later of which is named herein (Uvidicolus nov. gen). In contrast, a third morphological group, the 'smooth knob-tails,' is a monophyletic group of five exclusively arid zone burrowing species that has radiated relatively recently (mid-Miocene). Our phylogeny indicates that successful colonization of this novel and challenging biome by Nephrurus correlates with an initial shift to terrestriality and adaptation to at least seasonally arid conditions around the early Miocene, and the eventual evolution and subsequent mid-Miocene radiation of a lineage specialized for burrowing. 相似文献
28.
Zimmermann D Reuss R Westhoff M Gessner P Bauer W Bamberg E Bentrup FW Zimmermann U 《Journal of experimental botany》2008,59(11):3157-3167
A high-precision pressure probe is described which allows non-invasive online-monitoring of the water relations of intact leaves. Real-time recording of the leaf water status occurred by data transfer to an Internet server. The leaf patch clamp pressure probe measures the attenuated pressure, P(p), of a leaf patch in response to a constant clamp pressure, P(clamp). P(p) is sensed by a miniaturized silicone pressure sensor integrated into the device. The magnitude of P(p) is dictated by the transfer function of the leaf, T(f), which is a function of leaf patch volume and ultimately of cell turgor pressure, P(c), as shown theoretically. The power function T(f)=f(P(c)) theoretically derived was experimentally confirmed by concomitant P(p) and P(c) measurements on intact leaflets of the liana Tetrastigma voinierianum under greenhouse conditions. Simultaneous P(p) recordings on leaflets up to 10 m height above ground demonstrated that changes in T(f) induced by P(c) changes due to changes of microclimate and/or of the irrigation regime were sensitively reflected in corresponding changes of P(p). Analysis of the data show that transpirational water loss during the morning hours was associated with a transient rise in turgor pressure gradients within the leaflets. Subsequent recovery of turgescence during the afternoon was much faster than the preceding transpiration-induced water loss if the plants were well irrigated. Our data show the enormous potential of the leaf patch clamp pressure probe for leaf water studies including unravelling of the hydraulic communication between neighbouring leaves and over long distances within tall plants (trees). 相似文献
29.
30.
Exonuclease V from Saccharomyces cerevisiae. A 5'----3'-deoxyribonuclease that produces dinucleotides in a sequential fashion 总被引:3,自引:0,他引:3
A novel deoxyribonuclease, exonuclease V, has been purified approximately 30,000-fold from Saccharomyces cerevisiae. Exonuclease V is localized in the nucleus. The nuclease degrades single-stranded, but not double-stranded, DNA from the 5'-end. The products of exonuclease action are dinucleotides, except the 3'-terminal tri- and tetranucleotides which are not degraded. Studies with synthetic oligo- and polynucleotides with specified sequence elements showed that exonuclease V cleaves off dinucleotides as primary digestion products. Thus, the polymers (pT)9pA(pT)n and (pT)10pA(pT)n yielded pTpA and pApT as digestion products, respectively. Removal of the 5'-terminal phosphate from the DNA substrate results in reduced binding of the enzyme to the substrate. In addition, the initial hydrolytic cut by exonuclease V on the dephosphorylated substrate produces a mixture of dinucleoside monophosphates and trinucleoside diphosphates. The enzyme is processive in action. 相似文献