全文获取类型
收费全文 | 935篇 |
免费 | 68篇 |
国内免费 | 108篇 |
出版年
2024年 | 4篇 |
2023年 | 11篇 |
2022年 | 12篇 |
2021年 | 14篇 |
2020年 | 22篇 |
2019年 | 30篇 |
2018年 | 36篇 |
2017年 | 24篇 |
2016年 | 36篇 |
2015年 | 17篇 |
2014年 | 35篇 |
2013年 | 59篇 |
2012年 | 36篇 |
2011年 | 44篇 |
2010年 | 32篇 |
2009年 | 44篇 |
2008年 | 38篇 |
2007年 | 46篇 |
2006年 | 31篇 |
2005年 | 48篇 |
2004年 | 41篇 |
2003年 | 27篇 |
2002年 | 38篇 |
2001年 | 30篇 |
2000年 | 12篇 |
1999年 | 21篇 |
1998年 | 17篇 |
1997年 | 22篇 |
1996年 | 13篇 |
1995年 | 19篇 |
1994年 | 21篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 5篇 |
1990年 | 14篇 |
1989年 | 19篇 |
1988年 | 11篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 17篇 |
1984年 | 22篇 |
1983年 | 15篇 |
1982年 | 15篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1974年 | 2篇 |
1973年 | 3篇 |
排序方式: 共有1111条查询结果,搜索用时 31 毫秒
1.
Iron-sulfur core extrusions from spinach [( 2Fe-2S]) and Clostridium pasteurianum (2[4Fe-4S]) ferredoxins in aqueous Triton X-100/urea containing excess benzenethiol yield quantitatively [FenSn(SPh)4]2- with n = 2 and n = 4, respectively. The iron-sulfur cluster can be separated from the corresponding apoprotein by rapid passage of the extrusion mixture over a small anaerobic column of Whatman DE-52 anion-exchange cellulose. Essentially quantitative recovery of [FenSn (SPh)4]2- is achieved in the eluate. The apoprotein remaining on the column can be eluted with 0.5 M NaCl. Most of the residual Triton X-100 and benzenethiol can be removed by passage of the apoprotein eluate over a small column of Bio-Beads SM-2, a hydrophobic polystyrene adsorbent. Apoprotein recovery is comparable to that obtained by other chromatographic methods. At least with spinach ferredoxin, the apoprotein prepared in this fashion can be reconstituted. The procedures developed in this work are potentially most applicable to selective removal of [2Fe-2S] and [4Fe-4S] centers from a multicenter enzyme without irreversible denaturation. 相似文献
2.
Sulfur bacteria such as Beggiatoa or Thiomargarita have a particularly high capacity for storage because of their large size. In addition to sulfur and nitrate, these bacteria also store phosphorus in the form of polyphosphate. Thiomargarita namibiensis has been shown to release phosphate from internally stored polyphosphate in pulses creating steep peaks of phosphate in the sediment and thereby inducing the precipitation of phosphorus-rich minerals. Large sulfur bacteria populate sediments at the sites of recent phosphorite formation and are found as fossils in ancient phosphorite deposits. Therefore, it can be assumed that this physiology contributes to the removal of bioavailable phosphorus from the marine system and thus is important for the global phosphorus cycle. We investigated under defined laboratory conditions which parameters stimulate the decomposition of polyphosphate and the release of phosphate in a marine Beggiatoa strain. Initially, we tested phosphate release in response to anoxia and high concentrations of acetate, because acetate is described as the relevant stimulus for phosphate release in activated sludge. To our surprise, the Beggiatoa strain did not release phosphate in response to this treatment. Instead, we could clearly show that increasing sulfide concentrations and anoxia resulted in a decomposition of polyphosphate. This physiological reaction is a yet unknown mode of bacterial polyphosphate usage and provides a new explanation for high phosphate concentrations in sulfidic marine sediments. 相似文献
3.
The green marine macroalga Ulva lactuca L. was found to be able to utilize HCO3? from sea water in two ways. When grown in flowing natural sea water at 16°C under constant dim irradiance, photosynthesis at pH8.4 was suppressed by acetazolamide but unaffected by 4,4′-diisothiocyanostilbene-2,2′-disulphonate. These responses indicate that photosynthetic HCO3? utilization was via extracellular carbonic anhydrase (CA) -mediated dehydration followed by CO2 uptake. The algae were therefore described as being in a ‘CA state’. If treated for more than 10 h in a sea water flow-through system at pH9.8, these thalli became insensitive to acetazolamide but sensitive to 4,4′-diisothiocyanostilbene-2,2′-disulphonate. This suggests the involvement of an anion exchanger (AE) in the direct uptake of HCO3?, and these plants were accordingly described as being in an ‘AE state’. Such thalli showed an approximately 10-fold higher apparent affinity for HCO3? (at pH9.4) than those in the ‘CA state’, while thalli of both states showed a very high apparent affinity for CO2. These results suggest that the two modes of HCO3? utilization constitute two ways in which inorganic carbon may enter the Ulva lactuca cells, with the direct entry of HCO3?, characterizing the ‘AE state’, being inducible and possibly functioning as a complementary uptake system at high external pH values (e.g. under conditions conducive to high photosynthetic rates). Both mechanisms of entry appear to be connected to concentrating CO2 inside the cell, probably via a separate mechanism operating intracellularly. 相似文献
4.
5.
S. R. Ghosh 《Hydrobiologia》1979,62(1):89-92
Augmentation of plankton production in Indian fishponds by application of manure and inorganic fertilisers was studied in laboratory experiments. The effects of the salinity of the medium were also examined.Laboratory trials were made with mustard oil cake and urea on equivalent nutrient bases in media with varying degrees of salinity (0–30 ppt at 2 ppt. intervals). It was observed that Closterium, Fragilaria, Pinnularia and Gyrosigma grew well in the higher salinity range between 24 to 30 ppt. whereas lower salinities ranging from a trace to 8 ppt. are suitable for improved production of Anabaena, Synedra, Navicula, Amphipleura, Amphora and Nitzschia. Comparatively better production of plankton was recorded with urea than with mustard oil cake. 相似文献
6.
7.
A stable carbon isotope study of dissolved inorganic carbon cycling in a softwater lake 总被引:3,自引:0,他引:3
Andrew L. Herczeg 《Biogeochemistry》1987,4(3):231-263
The dissolved inorganic carbon (DIC) cycle in a softwater lake was studied using natural variations of the stable isotopes of carbon,12C and13C. During summer stratification there was a progressive decrease in epilimnion DIC concentration with a concomitant increase in 13CDIC), due to preferential uptake of12C by phytoplankton and a change in the dominant CO2 source from inflow andin situ oxidation to invasion from the atmosphere. There was an increase in hypolimnion DIC concentration throughout summer with a concomitant general decrease in 13CDIC from oxidation of the isotopically light particulate organic carbon that sank down through the thermocline from the epilimnion.Mass balance calculations of DI12C and DI13C in the epilimnion for the summer (June 23–September 25) yield a mean rate of net conversion of DIC to organic carbon (Corg) of 430 ± 150 moles d-1 (6.5 ± 1.8 m moles m-2 d-1. Net CO2 invasion from the atmosphere was 420 ± 120 moles d-1 (6.2 ± 1.8 m moles m-2 d-1) with an exchange coefficient of 0.6 ± 0.3m d-1. These results imply that at least for the summer months the phytoplankton obtained about 90% of their carbon from atmosphere CO2. About 50% of CO2 invasion and conversion to Corg for the summer occurred during a two week interval in mid-summer.DIC concentration increased in the hypolimnion at a rate of 350 ± 70 moles DIC d-1 during summer stratification. The amount of DIC added to the hypolimnion was equivalent to 75 ± 20% of net conversion of DIC to Corg in the euphotic zone over spring and summer implying rapid degradation of POC in the hypolimnion. The 13C of DIC added to the deep water (-22.) was too heavy to have been derived from oxidation of particulate organic carbon alone. About 20% of the added DIC must have diffused from hypolimnetic sediments where relatively heavy CO2 (-7) was produced by a combination of POC oxidation and as a by-product of methanogenesis. 相似文献
8.
The phosphate metabolism of Platymonas subcordiformis was investigated by 31P-NMR spectroscopy with special attention on the effect of external pH. Glycolyzing cells and cells energized by respiration or photosynthesis gave spectra dependent upon their metabolic state. The transition from deenergized to energized states is accompanied by a shift of cytoplasmic pH from 7.1–7.4, an increase of ATP level and-in well energized cells-the appearance of a new signal tentatively assigned to phosphoarginine.The spectra remain stable over a wide range of external pH. Cytoplasmic pH is well regulated in respiring cells for external pH in the range 5.3–12.3. The typical 0.4 units difference of internal pH in energized as compared to deenergized cells is not affected by external pH in the range 6–12. The intensity of a signal attributed to PEP is markedly increased at high external pH. pH regulation is less efficient below external pH of 6 in deenergized cells. Below pH 3.8 oxidative phosphorylation ceases. Upon raising cytoplasmic pH to 7.4 in deenergized cells polyphosphate chains start to disintegrate.Abbreviations PEP
Phosphoenolpyruyate
- P
i
inorganic phosphate
- PP
i
inorganic pyrophosphate
- poly P
polyphosphates
- PP-1, PP-2, PP-3
terminal, second, and third phosphate residue of polyphosphates
- PP-4
core phosphate residues of polyphosphates
- pH
i
, pH
o
internal (cytoplasmic) and external pH
- NTP/NDP
nucleotide triphosphate/-diphosphate
- S/N
signal to noise ratio 相似文献
9.
Absidia cylindrospora and Rhizomucor pusillus, causal agents of phycomycoses, were cultured on sterile natural keratins in a mineral solution and the keratin degradation products analyzed. The excess of sulphur was removed by oxidation to inorganic sulphate and thiosulphate, which were the main products of sulphitolysis of keratin. The proteolytic activity of the two fungi depended on the nature of the keratin substrate. Human scalp hair was the most favoured keratin substrate by both the fungi. 相似文献
10.
J. Van Den Ende 《Plant and Soil》1991,133(1):65-74
The products of activities of calcium and sulphate were calculated for solutions of 75 glasshouse soils. The majority of these products was found to be higher than the solubility product of gypsum, thus indicating that these soil solutions were possibly supersaturated. In another investigation, soil solutions were examined to determine whether such high activity products could be really attributed to supersaturation. By means of ultracentrifuging of solutions of glasshouse soils, it could be established that the solutions were practically free of sulphate-bearing colloidal particles. Some solutions contained calcium-bearing colloidal particles, but the quantities of calcium contained in these particles were too small to substantially influence the calcium activity. Addition of gypsum crystals to soil solutions led to crystallization of so much calcium and sulphate that the products of the activities of calcium and sulphate dropped from values that can be listed as high to values approaching the solubility product of gypsum. The results obtained demonstrate the occurrence of supersaturation of soil solutions with respect to gypsum. It is further postulated that the presence of humic substances in the soil solution is responsible for this supersaturation. The possible occurrence of supersaturation with respect to gypsum in soils other than glasshouse soils is discussed. 相似文献