全文获取类型
收费全文 | 1111篇 |
免费 | 127篇 |
国内免费 | 67篇 |
出版年
2023年 | 20篇 |
2022年 | 26篇 |
2021年 | 37篇 |
2020年 | 34篇 |
2019年 | 52篇 |
2018年 | 49篇 |
2017年 | 31篇 |
2016年 | 51篇 |
2015年 | 35篇 |
2014年 | 47篇 |
2013年 | 81篇 |
2012年 | 36篇 |
2011年 | 46篇 |
2010年 | 42篇 |
2009年 | 53篇 |
2008年 | 71篇 |
2007年 | 59篇 |
2006年 | 46篇 |
2005年 | 53篇 |
2004年 | 47篇 |
2003年 | 38篇 |
2002年 | 39篇 |
2001年 | 38篇 |
2000年 | 28篇 |
1999年 | 19篇 |
1998年 | 24篇 |
1997年 | 17篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 13篇 |
1993年 | 10篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1972年 | 4篇 |
1970年 | 2篇 |
排序方式: 共有1305条查询结果,搜索用时 31 毫秒
81.
Standard approach to membrane transport generated by osmotic andhydrostatic pressures, developed by Kedem and Katchalsky, is based onprinciples of thermodynamics of irreversible processes. In this paper wepropose an alternative technique. We derive transport equations from fewfairly natural assumptions and a mechanistic interpretation of the flows.In particular we postulate that a sieve-type membrane permeability isdetermined by the pore sizes and these are random within certain range.Assuming that an individual pore is either permeable or impermeable tosolute molecules, the membrane reflection coefficient depends on the ratioof permeable and impermeable pores. Considering flows through permeableand impermeable pores separately, we derive equations for the total volumeflux, solute flux and the solvent flux across the membrane. Comparing themechanistic equations to the Kedem-Katchalsky equations we find the formereasier to interpret physically. Based on the mechanistic equations we alsoderive a correlation relation for the membrane transport parameters L
p,, and . This relation eliminates the need for experimentaldetermination of all three phenomenological parameters, which in somecases met with considerable difficulties. 相似文献
82.
The non-random mixing of biomembrane components, especially saturated phospholipids, exhibits important consequences in molecular biology. Particularly, the distribution of lipids within natural and model membranes is strongly determined by the selective association processes. These processes of phospholipids take place due to the cooperative modes in multiparticle systems as well as the specific lipid-lipid interactions both in the hydrophobic core and in the region of the polar headgroups. We demonstrated that the investigation of the selective association processes of saturated phospholipids might contribute to the insight of the lipid domains appearance inside the bilayer membranes. The association probabilities of like-pairs and cross-pairs from a binary mixture of saturated phospholipids were tested for both parallel and anti-parallel alignments of the polar headgroups. The present model confirms the experimental evidence for saturated phospholipids to have a high tendency for association in parallel configuration of the electric dipole moments of the polar headgroups whether the cross-sectional area of the polar headgroup is in an usual range of 25-55 2. There are three major lipid domains in a binary mixture of saturated phospholipids: (i) lipid domains in non-mixed phase of the first mixture component, in parallel alignment of the polar headgroups; (ii) lipid domains in non-mixed phase of the second mixture component, in anti-parallel alignment of the polar headgroups; (iii) lipid domains in mixed phase. We think that the selective association processes of phospholipids are neither exclusively, nor only involved in promoting the lipid domains appearance through bilayer phospholipid membranes. 相似文献
83.
B. Delesalle R. Buscail J. Carbonne T. Courp V. Dufour S. Heussner A. Monaco M. Schrimm 《Coral reefs (Online)》1998,17(2):121-132
The export of carbon and carbonate from coral reefs was investigated through a multidisciplinary investigation of the hydrological,
geochemical, sedimentological and biological features of Tiahura reef on the northwestern coast of Moorea Island (French Polynesia).
The hydrology of the fore-reef is characterised by prevailing longshore western currents and a strong thermocline. As revealed
by turbidity structures (benthic and intermediate nepheloid layers) and by the amount of particles collected by near-bottom
sediment traps, horizontal and downslope advections of particles dominate over offshore vertical transport. The exported material
is rich in carbonate (ca. 80%) and poor in organic matter (ca. 4%). Sedimentation rates at 430 m depth, i.e. definitive export,
reached 209.6 mg m-2 d-1 (dry weight). Estimates of carbon and carbonates export for Tiahura reef also reported here represent respectively 47% and
21% of the organic and inorganic carbon produced within the reef. 相似文献
84.
In a seminatural manipulation experiment with artificial irrigation we followed throughfall and forest floor solution chemistry collected underneath aphid infested and uninfested Norway spruce. Solutions underneath infested trees showed significantly higher concentrations of dissolved organic carbon (DOC) but lower concentrations of dissolved organic nitrogen (DON), NO3 -N, and NH4 -N in throughfall solutions and of NH4 -N in forest floor solutions. Average concentrations were 40.5% (DON), 27.5% (NO3 -N), and 46.2% lower (NH4 -N) underneath infested trees in throughfall solutions, and 19.5% (DON), 9.4% (NO3 -N), and 42.0% (NH4 -N) lower in forest floor solutions. Differences in throughfall were more pronounced than in forest floor leachates. It is likely that honeydew is fuelling the metabolism of micro-organisms and thus critically affects above and below ground nutrient cycles. We emphasize the importance of linking the biology of herbivores and micro-organisms with geochemical processes. 相似文献
85.
Douglas A. Burns 《Biogeochemistry》1998,40(1):73-96
Models of the effects of atmosphericN deposition in forested watersheds have notadequately accounted for the effects of aquatic andnear-stream processes on the concentrations and loadsof NO
in surface waters. This studycompared the relative effects of aquatic andnear-stream processes with those from the terrestrialecosystem on the retention and transport ofNO
in two contrasting stream reaches ofthe Neversink River, a forested watershed in theCatskill Mountains of New York that receives among thehighest load of atmospheric N deposition in thenortheastern United States. Stream water samples werecollected every two hours and ground-water andtributary samples were collected daily at base flowconditions during four 48-hour periods from April toOctober 1992, and NO
mass balances werecalculated for each site. Results indicated diurnalvariations in stream NO
concentrations inboth reaches during all four sampling periods; this isconsistent with uptake of NO
byphotoautotrophs during daylight hours. Mass-balanceresults revealed significant stream reach losses ofNO
at both sites during all samplingperiods. The diurnal variations in NO
concentrations and the retention of NO
relative to terrestrial contributions to the streamreaches were greater downstream than upstream becausephysical factors such as the head gradients ofinflowing ground water and the organic matter contentof sediment are more favorable to uptake anddenitrification downstream. The mass retention ofNO
increased as the mean 48-hr streamdischarge increased at each site, indicating that theresponsible processes are dependent onNO
supply. Low stream temperatures duringthe April sampling period, however, probably reducedthe rate of retention processes, resulting in smallerlosses of NO
than predicted from streamdischarge alone. Water samples collected from thestream, the hyporheic zone, and the alluvial groundwater at sites in both reaches indicated that the neteffect of hyporheic processes on downstreamNO
transport ranged from conservativemixing to complete removal by denitrification. Therelative effects of biological uptake anddenitrification as retention mechanisms could not bequantified, but the results indicate that bothprocesses are significant. These results generallyconfirm that aquatic and near-stream processes causesignificant losses of NO
in the NeversinkRiver, and that the losses by these processes atdownstream locations can exceed the NO
contributions to the stream from the terrestrialenvironment during summer and fall base-flowconditions. Failure to consider these aquatic andnear-stream processes in models of watershed responseto atmospheric N deposition could result inunderestimates of the amount of NO
leaching from forested ecosystems and to an inabilityto unequivocally relate geographic differences inNO
concentrations of stream waters tocorresponding differences in terrestrial processes. 相似文献
86.
S.R. Georgetti F.T.M.C. Vicentini C.Y. Yokoyama M.F. Borin A.C.C. Spadaro M.J.V. Fonseca 《Journal of applied microbiology》2009,106(2):459-466
Aims: To evaluate the soybean polyphenol glucosides bioconversion to aglycone forms by different β-glucosidases-producing filamentous fungi to enhance their antioxidant activity.
Methods and Results: Soybean defatted flour was submitted to solid-state fermentation with Aspergillus niger , Aspergillus niveus and Aspergillus awamori . The fungi studied produced approximately the same β-glucosidase activity units amount when p- nitrophenyl-β- d -glucopyranoside was used as substrate for the assay. However, electrophoretic analysis, using 4-methylumbellipheryl-β- d -glucopyranoside as substrate, showed that β-glucosidase produced by A. niveus was more active. Fermented methanolic extracts showed an increase in polyphenol and genistein contents and antioxidant activities. The highest genistein content was found in soybean fermented by A. niveus . Methanolic extracts of the soybean fermented by the different fungi showed a similar capacity of scavenging H2 O2 generated in vivo by the tumour promoter 12- O- tetradecanoyl phorbol-13-acetate.
Conclusions: A. niveus synthesized a β-glucosidase with higher specificity to hydrolyse genistin β-glycosidic bond than those produced by A . awamori and A. niger .
Significance and Impact of the Study: The utilization of these β-glucosidases-producing fungi in soybean fermentation processes resulted in the obtaining of methanolic extracts with different antioxidant potentials that could be used either therapeutically or as an antioxidant in nonphysiological oxidative stress conditions, as the one induced in skin by UV radiation. 相似文献
Methods and Results: Soybean defatted flour was submitted to solid-state fermentation with Aspergillus niger , Aspergillus niveus and Aspergillus awamori . The fungi studied produced approximately the same β-glucosidase activity units amount when p- nitrophenyl-β- d -glucopyranoside was used as substrate for the assay. However, electrophoretic analysis, using 4-methylumbellipheryl-β- d -glucopyranoside as substrate, showed that β-glucosidase produced by A. niveus was more active. Fermented methanolic extracts showed an increase in polyphenol and genistein contents and antioxidant activities. The highest genistein content was found in soybean fermented by A. niveus . Methanolic extracts of the soybean fermented by the different fungi showed a similar capacity of scavenging H
Conclusions: A. niveus synthesized a β-glucosidase with higher specificity to hydrolyse genistin β-glycosidic bond than those produced by A . awamori and A. niger .
Significance and Impact of the Study: The utilization of these β-glucosidases-producing fungi in soybean fermentation processes resulted in the obtaining of methanolic extracts with different antioxidant potentials that could be used either therapeutically or as an antioxidant in nonphysiological oxidative stress conditions, as the one induced in skin by UV radiation. 相似文献
87.
D. A. PELTZER R. B. ALLEN G. M. LOVETT D. WHITEHEAD D. A. WARDLE 《Global Change Biology》2010,16(2):732-746
There has been a rapidly developing literature on the effects of some of the major drivers of global change on carbon (C) sequestration, particularly carbon dioxide (CO2) enrichment, land use change, nitrogen (N) deposition and climate change. However, remarkably little attention has been given to one major global change driver, namely biological invasions. This is despite growing evidence that invasive species can dramatically alter a range of aboveground and belowground ecosystem processes, including those that affect C sequestration. In this review, we assess the evidence for the impacts of biological invaders on forest C stocks and C sequestration by biological invaders. We first present case studies that highlight a range of invader impacts on C sequestration in forest ecosystems, and draw on examples that involve invasive primary producers, decomposers, herbivores, plant pathogens, mutualists and predators. We then develop a conceptual framework for assessing the effects of invasive species on C sequestration impacts more generally, by identifying the features of biological invaders and invaded ecosystems that are thought to most strongly regulate C in forests. Finally we assess the implications of managing invasive species on C sequestration. An important principle that emerges from this review is that the direct effects of invaders on forest C are often smaller and shorter‐term than their indirect effects caused by altered nutrient availability, primary productivity or species composition, all of which regulate long‐term C pools and fluxes. This review provides a conceptual basis for improving our general understanding of biological invaders on ecosystem C, but also points to a paucity of primary data that are needed to determine the quantitative effects of invaders on ecosystem processes that drive C sequestration. 相似文献
88.
Synchrony of spatial populations: heterogeneous population dynamics and reddened environmental noise
Many species exhibit widespread spatial synchrony in population fluctuations. This pattern is of great ecological interest
and can be a source of concern when a species is rare or endangered. Moran’s theorem suggests that if two (or more) populations
sharing a common linear density-dependence in the renewal process are disturbed with correlated noise, they will become synchronized
with correlation matching the noise correlation. In this report, correlation of nonidentical populations that are described
by linear and stationary autoregressive processes is analyzed. We show that the expected spatial synchrony between two populations
can be decomposed into two multiplicative components. One is the demographic component related to the values of the autoregressive
coefficients and the noise color. The other is the spatial correlation of the environmental colored noise. The main results
are consistent with the predictions of previous experiments and simulations, and the importance of this report is to provide
theoretical support. 相似文献
89.
90.
Aims: To investigate the intracellular ethanol accumulation in yeast cells by using laser tweezers Raman spectroscopy (LTRS). Methods and Results: Ethanol accumulation in individual yeast cells during aerobic fermentation triggered by excess glucose was studied using LTRS. Its amount was obtained by comparing intracellular and extracellular ethanol concentrations during initial process of ethanol production. We found that (i) yeasts start to produce ethanol within 3 min after triggering aerobic fermentation, (ii) average ratio of intracellular to extracellular ethanol is 1·54 ± 0·17 during the initial 3 h after addition of 10% (w/v) excess glucose and (iii) the accumulated intracellular ethanol is released when aerobic fermentation is stimulated with decreasing glucose concentration. Conclusions: Intracellular ethanol accumulation occurs in initial stage of a rapid aerobic fermentation and high glucose concentration may attribute to this accumulation process. Significance and Impact of the Study: This work demonstrates LTRS is a real‐time, reagent‐free, in situ technique and a powerful tool to study kinetic process of ethanol fermentation. This work also provides further information on the intracellular ethanol accumulation in yeast cells. 相似文献