全文获取类型
收费全文 | 6062篇 |
免费 | 692篇 |
国内免费 | 2篇 |
专业分类
6756篇 |
出版年
2022年 | 59篇 |
2021年 | 98篇 |
2020年 | 64篇 |
2019年 | 76篇 |
2018年 | 89篇 |
2017年 | 77篇 |
2016年 | 134篇 |
2015年 | 232篇 |
2014年 | 292篇 |
2013年 | 272篇 |
2012年 | 418篇 |
2011年 | 426篇 |
2010年 | 270篇 |
2009年 | 216篇 |
2008年 | 321篇 |
2007年 | 284篇 |
2006年 | 257篇 |
2005年 | 259篇 |
2004年 | 236篇 |
2003年 | 207篇 |
2002年 | 211篇 |
2001年 | 94篇 |
2000年 | 97篇 |
1999年 | 90篇 |
1998年 | 73篇 |
1997年 | 51篇 |
1996年 | 47篇 |
1995年 | 59篇 |
1994年 | 46篇 |
1993年 | 50篇 |
1992年 | 80篇 |
1991年 | 62篇 |
1990年 | 78篇 |
1989年 | 76篇 |
1988年 | 80篇 |
1987年 | 65篇 |
1986年 | 59篇 |
1985年 | 59篇 |
1984年 | 57篇 |
1983年 | 45篇 |
1982年 | 51篇 |
1981年 | 50篇 |
1980年 | 34篇 |
1979年 | 57篇 |
1978年 | 45篇 |
1976年 | 49篇 |
1975年 | 51篇 |
1974年 | 59篇 |
1973年 | 76篇 |
1972年 | 53篇 |
排序方式: 共有6756条查询结果,搜索用时 0 毫秒
1.
M N Berry R B Gregory A R Grivell D C Henly C D Nobes J W Phillips P G Wallace 《Biochimica et biophysica acta》1988,936(3):294-306
The lipophilic triphenylmethylphosphonium cation (TPMP+) has been employed to measure delta psi m, the electrical potential across the inner membrane of the mitochondria of intact hepatocytes. The present studies have examined the validity of this technique in hepatocytes exposed to graded concentrations of inhibitors of mitochondrial energy transduction. Under these conditions, TPMP+ uptake allows a reliable measure of delta psi m in intracellular mitochondria, provided that the ratio [TPMP+]i/[TPMP+]e is greater than 50:1 and that at the end of the incubation more than 80% of the hepatocytes exclude Trypan blue. Hepatocytes, staining with Trypan blue, incubated in the presence of Ca2+, do not concentrate TPMP+. The relationships between delta psi m and two other indicators of cellular energy state, delta GPc and Eh, or between delta psi m and J0, were examined in hepatocytes from fasted rats by titration with graded concentrations of inhibitors of mitochondrial energy transduction. Linear relationships were generally observed between delta psi m and delta GPc, Eh or J0 over the delta psi m range of 120-160 mV, except in the presence of carboxyatractyloside or oligomycin, where delta psi m remained constant. Both the magnitude and the direction of the slope of the observed relationships depended upon the nature of the inhibitor. Hepatocytes from fasted rats synthesized glucose from lactate or fructose, and urea from ammonia, at rates which were generally linear functions of the magnitude of delta psi m, except in the presence of oligomycin or carboxyatractyloside. Linear relationships were also observed between delta psi m and the rate of formation of lactate in cells incubated with fructose and in hepatocytes from fed rats. The linear property of these force-flow relationships is taken as evidence for the operation of thermodynamic regulatory mechanisms within hepatocytes. 相似文献
2.
Summary Worldwide, invasive weeds threaten agricultural, natural and urban ecosystems. In Australia's agricultural and grazing regions, invasive species often establish across extensive areas where weed management is hampered by an inability to detect the location and timing of an outbreak. In these vast landscapes, an effective detection and monitoring system is required to delineate the extent of the invasion and identify spatial and temporal factors associated with weed establishment and thickening. In this study, we utilize a time series of remote sensing imagery to detect the spatial and temporal patterns of Prickly Acacia ( Acacia nilotica ) invasion in the Mitchell grass plains of North Queensland. We develop a spectral index from Landsat images which is applied to images from 1989 to 2004, in combination with a classification mask, to identify locations and monitor changes in Prickly Acacia density across 29 000 km2 of Mitchell grass plains. The approach identified spectral and temporal signatures consistent with Prickly Acacia infestation on 1.9% of this landscape. Field checking of results confirmed presence of the weed in previously unrecorded locations. The approach may be used to evaluate future spread, or outcomes of management strategies for Prickly Acacia in this landscape and could be employed to detect and monitor invasions in other extensive landscapes. 相似文献
3.
During an ultrastructural study of small-intestinal mucosa from a patient suffering from alpha-chain disease organisms were identified within the epithelial cytoplasm which showed the fine structural features of the coccidian group. Though coccidiosis is well recognized as causing a diarrhoeal and often lethal illness in animals it has been neglected as a cause of disease in man. Thus this finding may be significant and warrants further investigation into its possible role in the pathogenesis of alpha-chain disease. 相似文献
4.
T J Wallace E M Kodsi T B Langston M R Gergis W M Grogan 《The Journal of biological chemistry》2001,276(35):33165-33174
Site-directed mutagenesis is used to identify amino acid residues that dictate reported differences in substrate specificity between rat hepatic neutral cytosolic cholesteryl ester hydrolase (hncCEH) and rat lung carboxylesterase (LCE), proteins differing by only 4 residues in their primary sequences. Beginning with LCE, the substitution Met(423) --> Ile(423) alone or in combination with other mutations increased activity with p-nitrophenylcaprylate (PNPC) relative to more hydrophilic p-nitrophenylacetate (PNPA), typical of hncCEH. The substitution Thr(444) --> Met(444) was necessary but not sufficient for expression of cholesteryl esterase activity in COS-7 cells. The substitution Asn(506) --> Ser(506), creating a potential phosphorylation site, uniformly increased activity with both PNPA and PNPC, was necessary but not sufficient for expression of cholesteryl esterase activity and conferred susceptibility to activation by cAMP-dependent protein kinase, a property of hncCEH. The 3 mutations in combination were necessary and sufficient for expression of cholesteryl esterase activity by the mutated LCE. The substitution Gln(186) --> Arg(186) selectively reduced esterase activity with PNPA and PNPC but was not required for cholesteryl esterase activity. Homology modeling from x-ray structures of acetylcholinesterases is used to propose three-dimensional models for hncCEH and LCE that provide insight into the effects of these mutations on substrate specificity. 相似文献
5.
Summary Detopped root systems of tobacco plants (Nicotiana tabacum L. var. Virginia Gold) were exposed to air at different temperatures (14 to 23°C). The volume of xylem exudate was related
to temperature and decreasing the temperature resulted in a lag in the appearance of exudate. For 4 and 8 days of exudation
the amounts of cations transported in the exudate were temperature dependent. The Q10 values were higher than usually encountered in biological reactions; the mean value was around 6.
This work was supported in part by National Science Foundation Grant GB8170.
University of California 相似文献
6.
G. K. Wallace 《Biological cybernetics》1989,61(3):233-240
The patterns of EMG activity in the biceps and triceps muscles were recorded during horizontal oscillatory movements of the forearm. Subjects showed increased frequency of oscillation as they voluntarily reduced movement amplitude. EMG burst duration was significantly correlated with wavelength of oscillation in every case. In almost half the cases burst intensity was also positively correlated with wavelength. Subjects seemed to be using one or both these methods to control amplitude. A model was developed in three stages which satisfactorily accounted for the data. 相似文献
7.
8.
9.
The complete nucleotide sequence of the mitochondrial (mt) genome was determined for specimens of the coral species Montipora
cactus (Bernard 1897) and Anacropora
matthai (Pillai 1973), representing two morphologically distinct genera of the family Acroporidae. These sequences were compared
with the published mt genome sequence for the confamilial species, Acropora tenuis (Dana 1846). The size of the mt genome was 17,887 bp and 17,888 bp for M. cactus and A. matthai. Gene content and organization was found to be very similar among the three Acroporidae mt genomes with a group I intron
occurring in the NADH dehyrogenase 5 (nad5) gene. The intergenic regions were also similar in length among the three corals. The control region located between the
small ribosomal RNA (ms) and the cytochrome oxidase 3 (cox3) gene was significantly smaller in M. cactus and A. matthai (both 627 bp) than in A. tenuis (1086 bp). Only one set of repeated sequences was identified at the 3′-end of the control regions in M. cactus and A. matthai. A lack of the abundant repetitive elements which have been reported for A. tenuis, accounts for the relatively short control regions in M. cactus and A. matthai. Pairwise distances and relative rate analyses of 13 protein coding genes, the group I intron and the largest intergenic
region, igr3, revealed significant differences in the rate of molecular evolution of the mt genome among the three species, with an extremely
slow rate being seen between Montipora and Anacropora. It is concluded that rapid mt genome evolution is taking place in genus Acropora relative to the confamilial genera Montipora and Anacropora although all are within the relatively slow range thought to be typical of Anthozoa. 相似文献
10.