全文获取类型
收费全文 | 62441篇 |
免费 | 5171篇 |
国内免费 | 31篇 |
专业分类
67643篇 |
出版年
2022年 | 429篇 |
2021年 | 878篇 |
2020年 | 548篇 |
2019年 | 653篇 |
2018年 | 858篇 |
2017年 | 796篇 |
2016年 | 1413篇 |
2015年 | 2483篇 |
2014年 | 2610篇 |
2013年 | 3442篇 |
2012年 | 4368篇 |
2011年 | 4369篇 |
2010年 | 2857篇 |
2009年 | 2498篇 |
2008年 | 3634篇 |
2007年 | 3695篇 |
2006年 | 3491篇 |
2005年 | 3494篇 |
2004年 | 3447篇 |
2003年 | 3213篇 |
2002年 | 3183篇 |
2001年 | 765篇 |
2000年 | 609篇 |
1999年 | 787篇 |
1998年 | 886篇 |
1997年 | 615篇 |
1996年 | 621篇 |
1995年 | 604篇 |
1994年 | 587篇 |
1993年 | 621篇 |
1992年 | 566篇 |
1991年 | 496篇 |
1990年 | 424篇 |
1989年 | 430篇 |
1988年 | 427篇 |
1987年 | 357篇 |
1986年 | 358篇 |
1985年 | 414篇 |
1984年 | 468篇 |
1983年 | 398篇 |
1982年 | 477篇 |
1981年 | 426篇 |
1980年 | 371篇 |
1979年 | 256篇 |
1978年 | 307篇 |
1977年 | 303篇 |
1976年 | 239篇 |
1975年 | 226篇 |
1974年 | 252篇 |
1973年 | 240篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Big Moose Basin: simulation of response to acidic deposition 总被引:2,自引:1,他引:1
George F. Davis John J. Whipple Steven A. Gherini Carl W. Chen Robert A. Goldstein Arland H. Johannes Peter W.H Chan Ronald K. Munson 《Biogeochemistry》1987,3(1-3):141-161
The ILWAS model has been enhanced for application to multiple-lake hydrologic basins. This version of the model has been applied to the Big Moose basin, which includes Big Moose Lake and its tributary streams, lakes, and watersheds. The basin, as defined, includes an area of 96 km2, with over 20 lakes and ponds, and 70 km of streams. Hydrologic and chemical calibrations have been made using data from seven sampling stations. When total atmospheric sulfur loading to the basin is halved, the model predicts, after four years of simulation, a decreasing sulfate concentration and to a lesser extent a rising alkalinity at Big Moose Lake outlet. At the end of four years, the results show an increase in pH of 0.1 to 0.5 pH units depending upon season. 相似文献
122.
Denitrification with methanol in the presence of high salt concentrations and at high pH levels 总被引:3,自引:0,他引:3
Jan Peter van der Hoek Paul J. M. Latour Abraham Klapwijk 《Applied microbiology and biotechnology》1987,27(2):199-205
Summary In the combined ion exchange/biological denitrification process for nitrate removal from ground water, in which nitrate is removed by ion exchange, the resins are regenerated in a closed circuit by a biological denitrification reactor. This denitrification reactor eliminates nitrate from the regenerant. Methanol is used as electron donor for biological denitrification. To obtain sufficient regeneration of the resins within a reasonable time, high NaCl or NaHCO3 concentrations (10–30 g/l) in the regenerant are necessary. High NaHCO3 concentrations affected the biological denitrification in three ways: a) a slight decrease in denitrification capacity (30%) was observed; b) the yield coefficient and CH3OH/NO3
-–N ratio decreased. When high NaHCO3 concentrations (above 10g NaHCO3/l) were used, the yield coefficient was 0.10–0.13 g VSS/g NO3
-–N and the CH3OH/NO3
-–N ratio was 2.00–2.03 g/g; c) high NaHCO3 concentrations influenced nitrite production. Nitrite is an intermediate product of biological denitrification and with rising NaHCO3 concentrations nitrite accumulation was suppressed. This was explained by the effect of high NaHCO3 concentrations on the pH in the microenvironment of the denitrifying organisms. High NaCl concentrations also resulted in a slight decrease in denitrification capacity, but the second and third effects were not observed in the presence of high NaCl concentrations.Although the pH in the regenerant will rise as a result of biological denitrification, the capacity of a denitrification reactor did not decrease significantly when a pH of 8.8–9.2 was reached. 相似文献
123.
Summary Washed microsomal preparations (100 000 xg sediment) from the yeast Sporopachydermia cereana that had been grown on trimethylamine N-oxide as sole nitrogen source catalysed the NAD(P)H-dependent reduction of trimethylamine N-oxide to trimethylamine. Under anaerobic conditions, this was the sole reaction product, but under aerobic conditions only small amounts of trimethylamine accumulated, most being further metabolized to methylamine and formaldehyde (no detectable dimenthylamine accumulated due to its rapid turnover). In the absence of NAD(P)H, no formation of amines or formaldehyde from trimethylamine N-oxide was detected. The trimethylamine N-oxide reductase activity was inhibited by quinacrine, Cu2+ ions, triethylamine N-oxide (apparent K
i 0.43 mM) and dimethyl sulphoxide (K
i 0.94 mM). Chlorate and nitrate failed to inhibit the enzyme. The K
m for trimethylamine N-oxide was 29 M. Triethylamine N-oxide was also reduced by the microsomal preparation with the formation of acetaldehyde, and this reduction was sensitive to the same inhibitors as trimethylamine N-oxide, suggesting that both amine oxides are metabolized by the same enzyme(s). It is concluded that trimethylamine N-oxide is metabolized in this yeast via an NAD(P)H-dependent reductase.Abbreviations TMAO
triemthylamine N-oxide 相似文献
124.
Nigel W. Kerby Gordon W. Niven Peter Rowell William D. P. Stewart 《Applied microbiology and biotechnology》1987,25(6):547-552
Summary Mutant strains of the N2-fixing cyanobacterium bacterium Anabaena variabilis resistant to 6-fluorotryptophan or to ethionine were isolated. Many of these strains liberated amino acids into their media in the absence of 6-fluorotryptophan and ethionine. Nitrogenase activity was higher in mutant strains than in the parent strain. Mutant strains were immobilised in calcium alginate and sustained photoproduction of amino acids has been demonstrated.Abbreviations ETH
ethionine
- FT
6-fluorotryptophan
- Hepes
4-(2-hydroxyethyl)-1, piperazine ethanesulphonic acid
- PEP
phosphoenolpyruvate
- DAHP
3-deoxy-d-arabinoheptulosonate 7-phosphate
- chl a
chlorophyll a 相似文献
125.
Janet L. Taylor Jonathan D. G. Jones Steve Sandler Gunhild M. Mueller John Bedbrook Pamela Dunsmuir 《Molecular & general genetics : MGG》1987,210(3):572-577
Summary The Serratia marcescens chiA gene encodes a secreted chitinase activity which contributes to the fungal growth inhibition exhibited by this bacterium. The coding region from the chiA gene was fused to the promoter and 3 polyadenylation region of the Agrobacterium nopaline synthase gene. Site-directed mutagenesis of specific nucleotides surrounding the initiating AUG of the coding sequence of this chimeric gene resulted in up to an eight-fold increase in the amount of chitinase protein detected in transformed plant tissue. Analysis of the chiA mRNA indicated that these nucleotides also affected mRNA levels. At least 50% of the chitinase protein produced in transformed tobacco cells was the same molecular weight as the S. marcescen secreted protein. 相似文献
126.
127.
Electron probe microanalysis of transverse sections was used to provide semiquantitative estimates of Al and other cations
in the leaf tissues of some Al-accumulating plants of the cerrado vegetation of central Brazil. Concentrations of Al were
generally higher than those of other cations in the phloem elements of these plants growing on dystrophic as well as fertile
acid soils. When one of the Al-accumulating species,Vochysia thyrsoidea, was grown in a calcareous soil, the concentration of Al in the phloem elements of leaves was lower than that of Ca and K
though the cotyledons showed higher concentrations of Al than those of other cations. 相似文献
128.
JoAnne S. Richards Gerard J. Hickey Shiuan Chen John E. Shively Peter F. Hall Dana Gaddy-Kurten Richard Kurten 《Steroids》1987,50(4-6):393-409
To determine the molecular basis for changes in aromatase (P450arom) activity in rat ovarian follicles and corpora lutea, seven clones for rat P450arom cDNA have been identified and isolated from a rat granulosa cell λgtll cDNA expression library using a 62 mer deoxyoligonucleotide probe (derived from an amino acid sequence of purified human placental aromatase) and a human placental P450arom cDNA probe. One of the rat P450arom cDNA clones contained an insert 1.2 kb in size. Both the human 1.8 kb cDNA and the rat 1.2 kb cDNA probes hybridized to a single species of P450arom mRNA that was 2.6 kb in size. Northern blot analysis revealed that corpora lutea isolated on day 15 of pregnancy contained high amounts of P450arom mRNA, whereas granulosa cells of antral follicles of hormonally primed, hypophysectomized rats (i.e., those from which mRNA was isolated to construct the cDNA library) contained only low amounts of P450arom mRNA. The lower amounts of P450arom in granulosa cells of preovulatory follicles in the estradiol-follicle-stimulating hormone primed hypophysectomized rats were unexpected because follicles incubated in medium containing testosterone substrate produce more estradiol than do corpora lutea isolated on day 15 of pregnancy and incubated under similar conditions. Additional studies will determine the hormonal events responsible for the elevated amounts and constitutive maintenance of P450arom mRNA and aromatase activity in luteal cells in vivo and in vitro. 相似文献
129.
Peter Sackl und Georg Rauer 《Journal of Ornithology》1987,128(4):491-494
Zuammenfassung Im niederösterreichischen Waldviertel (200–700 m üNN) stieg der relative Anteil einjähriger von 17,9 auf 50% aller revierverteidigenden Hausrotschwanz- mit zunehmender Meereshöhe. Die Zunahme betrifft vor allem einjährige dercairei-Morphe, während dieparadoxus-Morphe in geringerer Anzahl und gleichförmiger als in allen Höhenbereichen auftrat.
The portion of first-year males in breeding populations of Black Redstarts (Phoenicurus ochruros) and their altitudinal distribution in the Waldviertel, Lower Austria
Summary In 1986, between April 13 and July 5, breeding territories of male Black Redstarts were mapped in Lower Austria (200–700 m NN), with regard to their age (first-year or older) and the plumage types of first-year (femalelike or adult malelike). Due to a greater percentage of femalelike subadult (cairei-type) in higher altitudes, the proportion of first-year was found to increase from 17,9 per cent at 200–300 m to 50 per cent above 600 m. On the contrary adultlike first-year (paradoxus-type) are evenly distributed in smaller numbers over the whole altitudinal range.相似文献
130.
Peter J. Syapin Terry Ritchie Lorna Noble Ernest P. Noble 《Journal of neurochemistry》1987,48(4):1285-1290
The relationship between the stability of potential neurochemical markers and autolysis time was studied at 4 degrees C and 25 degrees C using postmortem brain samples from two rat strains. In general, qualitatively similar results were obtained with either N/Nih or Sprague-Dawley rats; however, quantitative differences were often observed, particularly in regard to benzodiazepine receptor changes. For every enzyme activity or binding property examined, no significant change was found when brains were kept at 4 degrees C for up to 72 h prior to freezing at -70 degrees C. Na,K-ATPase and low-affinity Ca-ATPase activities were also stable in brains kept at 25 degrees C for up to 72 h. Mg-ATPase activity was reduced in brains kept at 25 degrees C for 24 and 48 h. [3H]Guanidinoethylmercaptosuccinic acid [( 3H]GEMSA) binding to enkephalin convertase in the cytosol was not significantly changed in brains kept at 25 degrees C; however, a small increase was seen for [3H]GEMSA binding to the membrane fraction at 24, but not 48 and 72 h postmortem. [3H]Quinuclidinyl benzilate [( 3H]QNB) binding to muscarinic cholinergic receptors decreased in brains kept at 25 degrees C for 72 h. Opioid receptor binding also decreased in brains kept at 25 degrees C. Using [3H]2-D-alanine-5-D-leucine enkephalin to label delta opioid receptors, a statistically significant decrease in binding was observed as early as 6 h postmortem, and was completely abolished after 72 h at 25 degrees C. In contrast, [3H]naloxone binding was unchanged after 24 h at 25 degrees C, but was decreased after 48 and 72 h.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献