全文获取类型
收费全文 | 11294篇 |
免费 | 699篇 |
国内免费 | 1790篇 |
出版年
2024年 | 58篇 |
2023年 | 225篇 |
2022年 | 346篇 |
2021年 | 395篇 |
2020年 | 442篇 |
2019年 | 451篇 |
2018年 | 382篇 |
2017年 | 347篇 |
2016年 | 403篇 |
2015年 | 338篇 |
2014年 | 556篇 |
2013年 | 923篇 |
2012年 | 474篇 |
2011年 | 608篇 |
2010年 | 432篇 |
2009年 | 671篇 |
2008年 | 722篇 |
2007年 | 661篇 |
2006年 | 537篇 |
2005年 | 571篇 |
2004年 | 400篇 |
2003年 | 381篇 |
2002年 | 347篇 |
2001年 | 265篇 |
2000年 | 230篇 |
1999年 | 211篇 |
1998年 | 168篇 |
1997年 | 187篇 |
1996年 | 174篇 |
1995年 | 188篇 |
1994年 | 188篇 |
1993年 | 162篇 |
1992年 | 148篇 |
1991年 | 122篇 |
1990年 | 143篇 |
1989年 | 124篇 |
1988年 | 86篇 |
1987年 | 70篇 |
1986年 | 92篇 |
1985年 | 101篇 |
1984年 | 103篇 |
1983年 | 52篇 |
1982年 | 62篇 |
1981年 | 43篇 |
1980年 | 37篇 |
1979年 | 39篇 |
1978年 | 28篇 |
1977年 | 44篇 |
1976年 | 16篇 |
1974年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
Porin of the outer membrane of Rhodobacter capsulatus St. Louis (ATCC 23782) was isolated and reconstituted into lipid bilayer membranes. The porin was obtained either by the sodium dodecyl sulfate treatment of cell envelopes (SDS-porin) or by saline extraction of whole cells (NaCl-porin). Nanomolar concentrations of both porin preparations resulted in a strong conductance increase of the lipid bilayer membranes by many orders of magnitude. At small protein concentrations the conductance increased in a stepwise fashion, the average single channel conductance being about 0.35 nS in 0.1 M KCl for SDS-porin and NaCl-porin as well. The single channel conductance was a linear function of the specific conductance of the aqueous phase. The results were consistent with the assumption that the porin formed large water-filled transmembrane channels in the membrane. From the average value of the single channel conductance in 0.1 M KCl an effective channel diameter of about 1.5 nm was estimated for both types of porins.Abbreviations EDTA
ethylenediamine tetraacetic acid
- SDS
sodium dodecyl sulfate 相似文献
42.
F. Widdel 《Archives of microbiology》1987,148(4):286-291
Sulfate-reducing bacteria with oval to rod-shaped cells (strains AcRS1, AcRS2) and vibrio-shaped cells (strains AcRM3, AcRM4, AcRM5) differing by size were isolated from anaerobic marine sediment with acetate as the only electron donor. A vibrio-shaped type (strain AcKo) was also isolated from freshwater sediment. Two strains (AcRS1, AcRM3) used ethanol and pyruvate in addition to acetate, and one strain (AcRS1) grew autotrophically with H2, sulfate and CO2. Higher fatty acids or lactate were never utilized. All isolates were able to grow in ammonia-free medium in the presence of N2. Nitrogenase activity under such conditions was demonstrated by the acetylene reduction test. The facultatively lithoautotrophic strain (AcRS1), a strain (AcRS2) with unusually large cells (2×5 m), and a vibrio-shaped strain (AcRM3) are described as new Desulfobacter species, D. hydrogenophilus, D. latus, and D. curvatus, respectively. 相似文献
43.
Summary M1 is a virulent bacteriophage of Methanobacterium thermoautotrophicum strain Marburg. Restriction enzyme analysis of the linear, 30.4 kb phage DNA led to a circular map of the 27.1 kb M1 genome. M1 is thus circularly permuted and exhibits terminal redundancy of approximately 3 kb. Packaging of M1 DNA from a concatemeric precursor initiates at the pac site which was identified at coordinate 4.6 kb on the circular genome map. It proceeds clockwise for at least five packaging rounds. Headful packaging was also shown for M2, a phage variant with a 0.7 kb deletion at coordinate 23.25 on the map. 相似文献
44.
Hajime Tokuda 《Journal of bioenergetics and biomembranes》1989,21(6):693-704
The marine bacteriumVibrio alginolyticus was found to possess the respiratory Na+ pump that generates an electrochemical potential of Na+, which plays a central role in bioenergetics ofV. alginolyticus, as a direct result of respiration. Mutants defective in the Na+ pump revealed that one of the two kinds of NADH: quinone oxidoreductase requires Na+ for activity and functions as the Na+ pump. The Na+ pump composed of three subunits was purified and reconstituted into liposomes. Generation of membrane potential by the reconstituted proteoliposomes required Na+. The respiratory Na+ pump coupled to the NADH: quinone oxidoreductase was found in wide varieties of Gramnegative marine bacteria belonging to the generaAlcaligenes, Alteromonas, andVibrio, and showed a striking similarity in the mode of electron transfer and enzymic properties. Na+ extrusion seemed to be coupled to a dismutation reaction, which leads to the formation of quinol and quinone from semi-quinone radical. 相似文献
45.
By using sodium thioglycolate to dissolve the high amount of excreted stalk material in axenic cultures of the chemolithoautotrophic iron bacterium Gallionella ferruginea, the ultrastructure of Gallionella cells from pure cell suspensions could be studied without any loss of viability or disturbance by dense ferric stalk fibers, and compared with Thiobacillus ferrooxidans, also grown chemolithoautotrophically with ferrous iron as energy source. Both organisms were chemically fixed or freeze-etched. Particular structural differences between these iron-bacteria could be ascertained. G. ferruginea possesses intracytoplasmic membranes and soluble d-ribulose-1,5-bisphosphate-carboxylase, whereas T. ferrooxidans contains carboxysomes but no intracytoplasmic membranes; Gallionella forms poly--hydroxybutyrate and glycogen as storage material; T. ferrooxidans produces only glycogen. Both organisms also differ from each other with respect to the freeze fracture behaviour of the cell envelope layers. Whereas the cells of T. ferrooxidans exhibit a characteristic double cleavage, exposing the plasmic fracture face and exoplasmic fracture face of the outer membrane and cytoplasmic membrane, the exceptionally thin multilayered cell envelope of G. ferruginea revealed a particularly intimate association between the layers, resulting in a visualisation of the supramolecular organisation of only the inner fracture face of the cytoplasmic membrane. The results are discussed predominantly in relation to the extremely distinct environments of both organisms. 相似文献
46.
Methanogenium organophilum, a non-autotrophic methanogen able to use primary and secondary alcohols as hydrogen donors, was grown on ethanol. Per mol of methane formed, 2 mol of ethanol were oxidized to acetate. In crude extract, an NADP+-dependent alcohol dehydrogenase (ADH) with a pH optimum of about 10.0 catalyzed a rapid (5 mol/min·mg protein; 22°C) oxidation of ethanol to acetaldehyde; after prolonged incubation also acetate was detectable. With NAD+ only 2% of the activity was observed. F420 was not reduced. The crude extract also contained F420: NADP+ oxidoreductase (0.45 mol/min·mg protein) that was not active at the pH optimum of ADH. With added acetaldehyde no net reduction of various electron acceptors was measured. However, the acetaldehyde was dismutated to ethanol and acetate by the crude extract. The dismutation was stimulated by NADP+. These findings suggested that not only the dehydrogenation of alcohol but also of aldehyde to acid was coupled to NADP+ reduction. If the reaction was started with acetaldehyde, formed NADPH probably reduced excess aldehyde immediately to ethanol and in this way gave rise to the observed dismutation. Acetate thiokinase activity (0.11 mol/min·mg) but no acetate kinase or phosphotransacetylase activity was observed. It is concluded that during growth on ethanol further oxidation of acetaldehyde does not occur via acetylCoA and acetyl phosphate and hence is not associated with substrate level phosphorylation. The possibility exists that oxidation of both ethanol and acetaldehyde is catalyzed by ADH. Isolation of a Methanobacterium-like strain with ethanol showed that the ability to use primary alcohols also occurs in genera other than Methanogenium.Non-standard abbreviations ADH
alcohol dehydrogenase
- Ap5ALi3
P1,P5-Di(adenosine-5-)pentaphosphate
- DTE
dithioerythritol (2,3-dihydroxy-1,4-dithiolbutane)
- F420
N-(N-l-lactyl--l-glutamyl)-l-glutamic acid phosphodiester of 7,8-dimethyl-8-hydroxy-5-deazariboflavin-5-phosphate
-
Mg.
Methanogenium
- OD578
optical density at 578 nm
- PIPES
1,4-piperazine-diethanesulfonic acid
- TRICINE
N-(2-hydroxy-1,1-bis[hydroxymethyl]methyl)-glycine
- Tris
2-amino-2-hydroxy-methylpropane-1,3-diol
- U
unit (mol substrate/min) 相似文献
47.
Archaeoglobus fulgidus is an extremely thermophilic archaebacterium that can grow at the expense of lactate oxidation with sulfate to CO2 and H2S. The organism contains coenzyme F420, tetrahydromethanopterin, and methanofuran which are coenzymes previously thought to be unique for methanogenic bacteria. We report here that the bacterium contains methylenetetrahydromethanopterin: F420 oxidoreductase (20 U/mg), methenyltetrahydromethanopterin cyclohydrolase (0.9 U/mg), formyltetrahydromethanopterin: methanofuran formyltransferase (4.4 U/mg), and formylmethanofuran: benzyl viologen oxidoreductase (35 mU/mg). Besides these enzymes carbon monoxide: methyl viologen oxidoreductase (5 U/mg), pyruvate: methyl viologen oxidoreductase (0.7 U/mg), and membranebound lactate: dimethylnaphthoquinone oxidoreductase (0.1 U/mg) were found. 2-Oxoglutarate dehydrogenase, which is a key enzyme of the citric acid cycle, was not detectable. From the enzyme outfit it is concluded that in A. fulgidus lactate is oxidized to CO2 via a modified acetyl-CoA/carbon monoxide dehydrogenase pathway involving C1-intermediates otherwise only used by methanogenic bacteria.Non-standard abbreviations APS
adenosine 5-phosphosulfate
- BV
benzyl viologen
- DCPIP
2,6-dichlorophenolindophenol
- DMN
2,3-dimethyl-1,4-naphthoquinone
- DTT
DL-1,4-dithiothreitol
- H4F
tetrahydrofolate
- H4MPT
tetrahydromethanopterin
- CH2
H4MPT, methylene-H4MPT
- CH
H4MPT, methenyl-H4MPT
- Mes
morpholinoethane sulfonic acid
- MFR
methanofuran
- Mops
morpholinopropane sulfonic acid
- MV
methyl viologen
- Tricine
N-tris(hydroxymethyl)-methylglycine
- U
mol product formed per min 相似文献
48.
Laboratory studies with the southern green stink bug, Nezara viridula (L.) (Hemiptera: Pentatomidae), feeding on seeds or fruits of some alternate host plants, demonstrated differences in nymphal survival and development time, and body weight at adult emergence. Lowest mortalities (25%) were observed for nymphs on fruits of dakota mustard [Brassica kaber (DC.)] and siberian motherwort (Leonurus sibiricus L.). On fruits of castor bean (Ricinus communis L.) nymph mortality was ca. 60%, and on mature seeds of L. sibiricus ca. 75%. Total development time from second instar to adult ranged from ca. 26 days on B. kaber to ca. 42 days on R. communis. Fresh body weight at adult emergence was greatest for nymphs on L. sibiricus and lowest on R. communis. Adult survival after 30 days and mean longevity was greater on L. sibiricus than on the remaining foods, being drastically reduced on R. communis. In general, reproductive performance on all foods was similar, except R. communis, on which no female reproduced. Female weight gain of N. viridula was generally greater on L. sibiricus On R. communis, adults lost weight during their first week and showed the lowest weight gain at the end of day 29. Field observations coupled with these laboratory studies suggest that in the Warta area (northern Paraná state, Brazil, 23° 11 S), L. sibiricus and B. kaber are important alternate food plants in the seasonal phenology of N. viridula. R. communis, however, is of poor nutritional quality, serving primarily as a refuge.
Résumé L'alimentation au laboratoire de N. viridula avec les graines et les fruits de diverses plantes a entraîné différentes mortalités larvaires, différentes vitesses de développement et des différences dans les poids des adultes à l'émergence. Les mortalités les plus faibles (25%) on été observées avec les fruits de Brassica kaber DC. et de Leonurus sibiricus L.Avec les fruits de Ricinus communis L., la mortalité larvaire était de 60%; sur graines mûres de L. sibiricus, elle était de 75%. La durée du développement du second stade à l'adulte a varié de 26 jours sur B. kaber à 42 jours sur R. communis. Le poids frais des adultes à l'émergence était le plus élevé avec L. sibiricus et le plus faible avec de R. communis. La longévité des adultes au-delà de 30 jours était la plus élevée avec L. sibiricus, et était fortement réduite avec R. communis. Dans l'ensemble, la reproduction était de même importance avec les différents aliments, à l'exception de R. communis avec lequel les femelles ne se reproduisaient pas. Le gain de poids des femelles était généralement supérieur avec L. sibiricus; avec R. communis les adultes ont perdu du poids pendant la première semaine et ont présenté le plus faible gain à la fin du 29ème jour.Des observations dans la nature, associées à ces expériences de laboratoire, laissent supposer que dans la zone de Warta (au Nord de l'état de Paraná, 23° 11 S, au Bresil), L. sibiricus et B. kaber sont des plantes hôtes alternatives importantes dans la phénologie saisonnière de N. viridula. Cependant, R. communis, de faible valeur alimentaire, sert avant tout de refuge.相似文献
49.
H. M. Koop M. Valentijn-Benz A. V. Nieuw Amerongen P. A. Roukema J. De Graaff 《Antonie van Leeuwenhoek》1989,55(3):277-290
Twenty-seven oral strains of the genera Actinomyces (5), Bacteroides (3), and Streptococcus (19) were tested for aggregation by human whole saliva, as well as the effect of culture medium, Ca-ions, and bacteria concentration thereupon. Of the media tested, GF-broth gave rise to less interference by autoaggregation or higher aggregation titers than BHI and TSB, and was used throughout this study. In most cases, Ca-ions (1 mM) only enhanced the rate of induced aggregation, whereas raising the bacteria concentration increased the rate of both induce- and autoaggregation. The final titers, ranging from 1–64, were hardly affected by these parameters, except those of S. rattus HG 59 and S. mutans HG 199, which were respectively increased and decreased by Ca-ions. Saliva-induced aggregation was observed for 21 strains of A. viscosus, A. naeslundii, A. israelii, B. gingivalis, B. intermedius, S. cricetus, S. mutans, S. rattus, S. sanguis, and S. sobrinus, mostly within 15 min to 3 h. Seventeen of these strains also showed autoaggregation, usually well after the onset of induced aggregation. Any potential induced aggregation of B. gingivalis HG 91 was always masked by autoaggregation, as well as that of the S. mutans strains under a particular set of conditions. The aggregation rate and titer varied considerably in a mutually unrelated and strain-dependent way. These microtiterplate data were matched by the 5 spectrophotometric patterns observed for saliva-bacterial interaction, which moreover, gave the better differentiation between induced and autoaggregation. In conclusion, most strains tested can show rapid saliva-induced aggregation in a strain-dependent way, yet strongly affected by the experimental conditions and interference from autoaggregation. 相似文献
50.
Glass-beads (diam. = 250 μm) were buried 10 cm deep in the sediment of a stream. After an exposure of eight weeks, bacterial
densities on the beads varied between 2.7 × 105 and 2.4 × 107/cm2, and the length of the fungal mycelium between 0.2 and 5.3 mm/cm2. Bacterial densities did not show any correlation with the DOC content of the water, but were positively correlated with
respiration on the beads. Fungal mycelium was negatively correlated with water temperature. Acid hydrolysis of stream-exposed
beads released sugars and amino acids, whose combined carbon content exceeded that of the microbial cells by a factor of at
least 4. Gut extracts of Gammarus tigrinus and Tipula caloptera released amino acids and sugars from stream-exposed beads. 相似文献