全文获取类型
收费全文 | 1021篇 |
免费 | 59篇 |
国内免费 | 34篇 |
专业分类
1114篇 |
出版年
2023年 | 11篇 |
2022年 | 21篇 |
2021年 | 17篇 |
2020年 | 34篇 |
2019年 | 31篇 |
2018年 | 21篇 |
2017年 | 23篇 |
2016年 | 24篇 |
2015年 | 26篇 |
2014年 | 25篇 |
2013年 | 82篇 |
2012年 | 23篇 |
2011年 | 29篇 |
2010年 | 27篇 |
2009年 | 30篇 |
2008年 | 33篇 |
2007年 | 37篇 |
2006年 | 39篇 |
2005年 | 32篇 |
2004年 | 31篇 |
2003年 | 53篇 |
2002年 | 51篇 |
2001年 | 43篇 |
2000年 | 34篇 |
1999年 | 18篇 |
1998年 | 19篇 |
1997年 | 27篇 |
1996年 | 19篇 |
1995年 | 21篇 |
1994年 | 17篇 |
1993年 | 30篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 17篇 |
1989年 | 18篇 |
1988年 | 19篇 |
1987年 | 13篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 10篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1976年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有1114条查询结果,搜索用时 0 毫秒
41.
The processes that occur at the micro‐scale site of calcification are fundamental to understanding the response of coral growth in a changing world. However, our mechanistic understanding of chemical processes driving calcification is still evolving. Here, we report the results of a long‐term in situ study of coral calcification rates, photo‐physiology, and calcifying fluid (cf) carbonate chemistry (using boron isotopes, elemental systematics, and Raman spectroscopy) for seven species (four genera) of symbiotic corals growing in their natural environments at tropical, subtropical, and temperate locations in Western Australia (latitudinal range of ~11°). We find that changes in net coral calcification rates are primarily driven by pHcf and carbonate ion concentration []cf in conjunction with temperature and DICcf. Coral pHcf varies with latitudinal and seasonal changes in temperature and works together with the seasonally varying DICcf to optimize []cf at species‐dependent levels. Our results indicate that corals shift their pHcf to adapt and/or acclimatize to their localized thermal regimes. This biological response is likely to have critical implications for predicting the future of coral reefs under CO2‐driven warming and acidification. 相似文献
42.
Base cation biogeochemistry and weathering under oak and pine: a controlled long-term experiment 总被引:2,自引:0,他引:2
Large earthen-walled lysimeters at the San Dimas Experimental Forest in southern California present a unique opportunity to assess vegetation effects on biogeochemical processes and cation release by weathering in controlled soil-vegetation systems where archived samples of soil parent material are available for comparison. The lysimeters were filled in 1937 with homogenized fine sandy loam derived on site from the weathering of diorite, and planted in 1946 with scrub oak (Quercus dumosa) and Coulter pine (Pinus coulteri). Changes in base cation contents were measured in above-ground biomass, and total and exchangeable soil pools to a depth of 1 meter. All cations in the non-exchangeable soil pool decreased relative to the initial fill material, indicating release by weathering. Sodium and K were depleted from both exchangeable and non-exchangeable pools of the soils. Plant uptake of Na was minimal, whereas K storage in vegetation exceeded the loss from the exchangeable soil pool. In both soil-vegetation systems, but especially for oak, there was an increase in exchangeable Ca and Mg. For all base cations, storage in above-ground biomass was greater for oak, whereas losses by weathering from the non-exchangeable soil pool were greater under pine. Strong evidence supports biocycling as a controlling mechanism resulting in greater Ca and Mg release by weathering under pine. In addition, decreases in non-exchangeable Ca and Mg were strongly correlated to decrease in Si under oak, whereas no correlation was observed under pine. We conclude that weathering reactions or stoichiometry differed between vegetation types.Corresponding author 相似文献
43.
A multi‐component catalyst Ni‐VOx/AC (VOx is comprised of V2O5 and VO2, x = 2.18) was synthesized by a wet impregnation method. The synthesized Ni‐VOx/AC shows a superior catalytic effect on de/hydrogenation of Mg. The MgH2+Ni‐VOx/AC composites can absorb 6.2 wt.‐% hydrogen within only 1 min at 150 °C under a hydrogen pressure of 2 MPa and desorb 6.5 wt.‐% hydrogen within 10 min at 300 °C under an initial hydrogen pressure of 1 KPa, which overcomes a critical barrier for practical use of Mg as a hydrogen storage material. A significant decrease of activation energy (Ea) indicates that Ni‐VOx/AC catalyst is highly efficient for Mg de/hydrogenation, which may be ascribed to the synergistic effect of bimetals (metal oxides) and nanocarbon. 相似文献
44.
45.
Release of the inhibition of messenger RNA translation in extracts of interferon-treated Ehrlich ascites tumor cells by added transfer RNA 总被引:10,自引:0,他引:10
S L Gupta M L Sopori P Lengyel 《Biochemical and biophysical research communications》1974,57(3):763-770
In an extract of Ehrlich ascites tumor (EAT) cells which had been “preincubated” for 45 min to lower endogenous protein synthesis (S30C) the translation of exogenous encephalomyocarditis (EMC) viral mRNA proceeds at a constant rate for over 90 min. In a similarly treated extract of interferon-treated EAT cells (S30INT) the translation proceeds at a lower rate than in the S30C for about 30 min and then stops. The impairment of the translation in the S30INT is mediated by one or more inhibitors. After the cessation of translation the viral mRNA in the S30INT is in large polysomes. The size of these changes little (if any) during a further 15 min incubation. The addition of mouse tRNA (but not ribosomal RNA or tRNA) to the S30INT after the cessation of viral mRNA translation results in the restart of translation at a rate close to that in the S30C. This effect of tRNA is diminished by pactamycin, which inhibits peptide chain initiation but not elongation. These results indicate that addition of tRNA allows the elongation of incomplete peptide chains and the initiation of new chains. The need for added tRNA may be due to the fact that in S30INT the amino acid acceptance of some of the endogenous tRNA species (but not of added tRNAs) is impaired. This impairment is pronounced for leucine and very slight, if any, for five other amino acids tested (i.e. isoleucine, methionine, phenylalanine, threonine, and valine). 相似文献
46.
47.
Ca2+ - and Mg2+ -dependent ATPase activity (EC 3.6.1.3) in a plasma membrane-enriched fraction increased rapidly after in vivo application of physiologically active concentrations of triacontanol (TRIA) to the roots of barley ( Hordeum vulgare L. cv. Conquest) seedlings. Ca2+ - and Mg2+ -dependent ATPase activity was 64 and 85% higher, respectively, in the roots of seedlings germinated in the presence of growth-promoting concentrations of TRIA compared to controls. The increase in vivo was concentration dependent, with the greatest increase obtained at 2.3 n M TRIA. Maximal stimulation of ATPase activity of excised tissue treated with TRIA coincided with the temperature at which the barley was grown. At this temperature the plasma membrane is primarily in a mixed gel/liquid crystalline state. Pretreatment of barley roots with cyclohexamide did not alter ATPase stimulation by TRIA. Two to three times more [14 C]-TRIA (mg membrane protein)−1 was found associated with plasma membrane-enriched vesicles treated with TRIA than with vesicles enriched for mitochondrial membranes or for vesicles enriched for tonoplast, Golgi and rough endoplasmic reticulum. Both Ca2+ - and Mg2+ -dependent ATPase activity increased by 40–60% within 30 min of the addition of 2.3 n M TRIA to cell-free extracts of barley roots. The addition of octacosanol, the C28 analogue of TRIA, to cell-free extracts did not affect metal-dependent ATPase activity. Consistent with many studies in the green-house, simultaneous additions of equimolar amounts of TRIA and octacosanol to cell-free extracts resulted in inhibition of ATPase stimulation by TRIA. TRIA may directly affect plasma membrane function in barley roots. 相似文献
48.
Kazimierz S. Kasprzak Michael P. Waalkes Lionel A. Poirier 《Biological trace element research》1987,13(1):253-273
Interactions between the physiologically essential metals calcium, magnesium, and zinc and the carcinogenic metals nickel
and cadmium were investigated to help elucidate the mechanisms of action of the carcinogenic metals. Bioassay studies revealed
several significant findings, including: (1) the ability of magnesium and calcium to inhibit nickel-induced elevation of pulmonary
adenoma incidence in strain A mice; (2) the ability of magnesium, but not of calcium, to prevent cadmium-induced subcutaneous
sarcoma formation; and (3) the ability of magnesium, but not of calcium, to inhibit nickel-induced muscle tumor formation.
Biochemical studies indicated a direct relationship between the antitumorigenic potential of magnesium and the capacity of
this metal to: (1) inhibit nickel and cadmium uptake by the target tissues in vivo; (2) inhibit nickel-induced disturbances
in DNA synthesis in vivo; (3) inhibit nuclear and cytosolic uptake of nickel by the target tissue cells in vivo; and (4) inhibit
nickel and cadmium binding to DNA in vitro. Calcium, which in most cases did not prevent carcinogenesis, had no consistent
influence on the uptake of carcinogenic metals or their biochemical effects in the target tissues. Magnesium and zinc, but
not calcium, were also found to attenuate the acute toxic effects of nickel, indicating a possible correlation between prevention
of acute effects and reduction in tumorigenicity. Zinc, which antagonizes cadmium tumorigenicity in the rat testis, was found
to reduce markedly cadmium uptake into isolated testicular interstitial cells. Also, zinc was found to inhibit strongly cadmium
binding to DNA in vitro. 相似文献
49.
Growth inhibition of plants suffering from Al toxicity is generally accompanied by impaired root development which can be quantitatively described by reduced specific root length (m g-1 dry root). In addition, the uptake of nutrients such as Mg and Ca is inhibited. Increased supply of either Mg or Ca can significantly diminish the negative effect of Al on root development and improve the Mg or Ca nutrition of the plants. The positive effect of Ca is well established but the effect of Mg has been observed in only a few plan species. Therefore, the effects of increasing Mg and Ca supply on Al toxicity in plants of seven monocots and eight dicots have been now examined in nutrient solution experiments. In general, Mg appears to be more effective than Ca in alleviating Al toxicity with the monocots, whereas the reverse is true for the dicots. Increased concentrations of Mg and Ca in solution seem to protect the plants against Al toxicity by improving the Mg or Ca nutrition and by alleviating the toxic effect of Al on root development. 相似文献
50.
J. A. van der Velden† F. A. T. Spanings † G. Flik C. Zegers Z. I. Kolar S. E.Wendelaar Bongat† 《Journal of fish biology》1991,39(1):83-91
The growth rate and magnesium concentration in scales, bone and muscle of freshwater tilapia, Oreochronüs mossambicus (Peters), initially weighing between 70 and 300 g, were followed during low-magnesium feeding. The growth rate decreased in fish on low-magnesium diets, but no changes were observed in the magnesium concentration in the scales, bone or muscle. No changes were observed in calcium or sodium concentrations in these tissues. We conclude that adult tilapia fed a low-magnesium diet manage, in contrast to other fish species, to maintain their magnesium balance and must therefore obtain magnesium from the water. 相似文献