全文获取类型
收费全文 | 38812篇 |
免费 | 3680篇 |
国内免费 | 26篇 |
出版年
2022年 | 297篇 |
2021年 | 626篇 |
2020年 | 374篇 |
2019年 | 488篇 |
2018年 | 612篇 |
2017年 | 540篇 |
2016年 | 776篇 |
2015年 | 1549篇 |
2014年 | 1595篇 |
2013年 | 2108篇 |
2012年 | 2632篇 |
2011年 | 2623篇 |
2010年 | 1689篇 |
2009年 | 1560篇 |
2008年 | 2169篇 |
2007年 | 2270篇 |
2006年 | 2113篇 |
2005年 | 2163篇 |
2004年 | 2076篇 |
2003年 | 1835篇 |
2002年 | 1842篇 |
2001年 | 477篇 |
2000年 | 369篇 |
1999年 | 493篇 |
1998年 | 542篇 |
1997年 | 337篇 |
1996年 | 296篇 |
1995年 | 276篇 |
1994年 | 288篇 |
1993年 | 295篇 |
1992年 | 309篇 |
1991年 | 285篇 |
1990年 | 252篇 |
1989年 | 309篇 |
1988年 | 265篇 |
1987年 | 265篇 |
1986年 | 232篇 |
1985年 | 280篇 |
1984年 | 262篇 |
1983年 | 256篇 |
1982年 | 321篇 |
1981年 | 313篇 |
1980年 | 239篇 |
1979年 | 207篇 |
1978年 | 218篇 |
1977年 | 198篇 |
1976年 | 179篇 |
1975年 | 149篇 |
1974年 | 189篇 |
1973年 | 163篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
101.
102.
A sequential coring approach was used to measure root biomass and production over 1 year in four different communities within the Great Dismal Swamp. A second method, an implanted bag technique, was also used to measure root production, and values were generally lower using this technique. On all sites, fine roots were the most dynamic root component. Both biomass (1,887 g/m2) and production (354–989 g m 2 yr-1) were highest on the mixed hardwood site, the least flooded site, and second highest on the cedar site, the site with the longest duration of soil saturation (1,033 g/m2 and 274–366 g m-2 yr-1). The maple-gum (696 g/m2 and 59–91 g m-2 yr-1) and cypress (824 g/m2 and 68–308 g m-2 yr-1) sites had similarly low amounts of biomass and rates of production. Environmental parameters that influenced production include frequency and duration of flooding, and soil type. Peaks in belowground production were observed on the most productive sites (mixed hardwood and cedar) in summer and late fall-winter; the other two sites exhibited little seasonal variability. The least flooded stand appears to allocate a greater percentage of net primary production belowground than the more extensively flooded stands. The ratio of above- and belowground allocation appears to change in response to a flooding gradient. This has major implications for ecosystem functions as carbon allocation patterns determine the array of litter types generated (leaves vs. roots) which affect decomposition rates and nutrient availability. 相似文献
103.
Very few basal attachments of the spiralled fenestrate bryozoan Archimedes are known. A newly discovered specimen is interpreted to have grown on a thin cylindrical ephemeral substratum that extended above the sediment-water interface, allowing paired spirals to develop and to extend in opposite directions when the colony was very small. 相似文献
104.
105.
Douglas A. Frank 《Oikos》2008,117(11):1718-1724
The importance of top predators in controlling ecological processes in large, intact ecosystems is unclear. In grasslands that support abundant ungulates, top–down control by predators may be particularly important, because of the tight biogeochemical linkages of ungulate prey with plants and soil microbes. Here, I examined the effects of the recent reintroduction of the gray wolf Canis lupus on ecosystem processes in Yellowstone National Park, where herds of grazing ungulates previously have been shown to stimulate several processes, including soil net nitrogen (N) mineralization. Rates of ungulate grazing intensity and soil net N mineralization were compared before and after wolf reintroduction in grasslands ranging five‐fold in aboveground production. Grazing intensity and grassland net N mineralization declined after wolf reintroduction, a likely partial function of fewer ungulates; wolf predation has been one of several factors implicated in causing the decline in Yellowstone ungulates. In addition, the spatial pattern of grazing and net N mineralization changed after reintroduction. A shift in the spatial patterns of grazer‐associated processes is consistent with a growing body of work indicating that wolves have changed habitat use patterns of ungulates in Yellowstone National Park. These findings suggest widespread wolf effects on ungulate prey, plants, and microbial activity that have spatially reorganized grassland energy and nutrient dynamics in Yellowstone Park. 相似文献
106.
Biochemical Evidence for Formate Transfer in Syntrophic Propionate-Oxidizing Cocultures of Syntrophobacter fumaroxidans and Methanospirillum hungatei 下载免费PDF全文
Frank A. M. de Bok Maurice L. G. C. Luijten Alfons J. M. Stams 《Applied microbiology》2002,68(9):4247-4252
The hydrogenase and formate dehydrogenase levels in Syntrophobacter fumaroxidans and Methanospirillum hungatei were studied in syntrophic propionate-oxidizing cultures and compared to the levels in axenic cultures of both organisms. Cells grown syntrophically were separated from each other by Percoll gradient centrifugation. In S. fumaroxidans both formate dehydrogenase and hydrogenase levels were highest in cells which were grown syntrophically, while the formate-H2 lyase activities were comparable under the conditions tested. In M. hungatei the formate dehydrogenase and formate-H2 lyase levels were highest in cells grown syntrophically, while the hydrogenase levels in syntrophically grown cells were comparable to those in cells grown on formate. Reconstituted syntrophic cultures from axenic cultures immediately resumed syntrophic growth, and the calculated growth rates of these cultures were highest for cells which were inoculated from the axenic S. fumaroxidans cultures that exhibited the highest formate dehydrogenase activities. The results suggest that formate is the preferred electron carrier in syntrophic propionate-oxidizing cocultures of S. fumaroxidans and M. hungatei. 相似文献
107.
The inhibitory effects of potassium sorbate on the bioenergetics, phenylalanine uptake, protein synthesis, and certain aspects of cell regulation were examined in putrefactive anaerobe 3679. Undissociated sorbic acid appeared to act as a protonophore by lowering the intracellular pH and dissipating the proton motive force of the membrane. Sorbate inhibited the uptake of phenylalanine, decreased the rate of protein synthesis, and altered patterns of phosphorylated nucleotide accumulation, resulting in increased intracellular concentrations of GTP, ppGpp, and an unidentified compound (possibly pppGpp). The addition of a noninhibitory amount of tetracycline released the inhibition of growth by sorbate. Based on these results, we concluded that the inhibition of putrefactive anaerobe 3679 by sorbate resulted from a stringent-type regulatory response induced by the protonophoric activity of sorbic acid. 相似文献
108.
Roy E. Weber Frank B. Jensen Raymond P. Cox 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1987,157(2):145-152
Summary The allosteric effects of the erythrocytic nucleoside triphosphates (NTP) and of proton concentrations were investigated by precise measurement of Hb–O2 equilibria of tench hemoglobin (including extreme, high and low saturation ranges) and analysed in terms of the MWC two state model and the Adair four step oxygenation theory.At low concentrations (NTP/Hb ratio=1.0, and pH>7.3) ATP, GTP and protons decrease Hb–O2 affinity by increasing the allosteric constantL and reducingK
T, the association constant1 of the deoxy, tense state of the Hb, without significantly affecting that (K
R) of the oxy state, increasing the free energy of cooperativity (G). High concentrations of these effectors, however, also reduceK
R. The greater sensitivity of the half-saturation O2 tension (P
50) of the Hb to GTP than to ATP at the same concentration, correlates with greater effects of GTP on bothK
T andK
R. The pH and NTP dependence of the four Adair association constants and the calculated fractional populations of Hb molecules in different stages of oxygenation show that the autochthonous NTP effectors and protons stabilize the T structure and postpone the TR transition basic to cooperativity in fish Hb.The possible implications of the findings for aquatic respiration are discussed.Abbreviations
ATP
adenosine triphosphate
-
DPG
2,3-diphosphoglycerate (glycerate-2,3-bisphosphate)
-
GTP
guanosine triphosphate
-
IHP
inositol hexaphosphate
-
NTP
nucleoside triphosphates
In this paperK
T andK
R are defined as theassociation equilibrium constants instead of dissociation constants (as originally defined by Monod et al. 1965) to facilitate comparison with the Adair constants 相似文献
109.
Frank B. Jensen 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1987,157(1):51-60
Summary Exercise-stress in tench resulted in severe acidoses in both the red cells and the extracellular fluid in vivo. These coincident pH decreases conformed to the in vitro pHi-pHe relationship for tench blood in the oxygenated state. The extracellular acidosis was primarily respiratory in winter and metabolic in spring and summer. This was due to more effective buffering of metabolic protons in winter by an elevation in [HCO
3
–
] levels, rather than to differences in the lactic- and carbonic acid loads. A good correspondence was found between buffered metabolic protons and increases in [lactate].There was no evidence for -adrenergic red cell swelling and associated red cell pH changes in tench both after exercise and adrenaline infusion. Arterial O2 transport was, however, improved in exercise by pronounced increases in
.Large increases in plasma potassium concentration and small elevations of chloride and calcium levels occurred in exercise. Hematocrit and blood [Hb] also increased, probably due to an adrenergic release of erythrocytes from the spleen, but these increases were small and appeared unimportant for blood O2 transport.Seasonal differences were found in exercise-induced changes in [lactate], in the magnitude of electrolyte and
changes, as well as in resting values for pHe, pHi, [HCO
3
–
], [Cl–] and [Ca++]. The origin and importance of these are discussed. 相似文献
110.
Thermodynamic analysis of precisely measured oxygen equilibria of tench (Tinca tinca) hemoglobin and their dependence on ATP and protons 总被引:1,自引:1,他引:0
Frank B. Jensen Roy E. Weber 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1987,157(2):137-143
Summary Precise oxygen equilibria including extreme, high and low saturation values were determined for hemoglobin (Hb) from the freshwater teleostTinca tinca at three temperatures, each at two pH levels and in the presence and absence of the erythrocytic cofactor ATP, at twofold molar excess over Hb.Analysis of the data in terms of Adair's successive oxygenation theory shows that in the absence of ATP, each of the four oxygenation steps are exothermic, but that net heat release decreases as pH falls from 8.2 to 7.4. ATP greatly depresses the temperature sensitivity of oxygenation particularly at physiological erythrocytic pH, where endothermic cofactor dissociation finds expression in a reverse temperature sensitivity for binding of the 3rd oxygen molecule to the tetrameric Hb.Enthalpy (H
i) and entropy (S
i) changes of oxygenation vary with oxygenation step, i, as well as with pH and ATP addition, but the variations of H
i are similar to those of S
i reflecting enthalpy-entropy compensation.The data show that the cooperative effects in tench Hb can be dominated either by entropic or enthalpic contributions, depending on the experimental condition and the oxygenation step. 相似文献