全文获取类型
收费全文 | 79245篇 |
免费 | 5529篇 |
国内免费 | 16篇 |
专业分类
84790篇 |
出版年
2023年 | 501篇 |
2022年 | 677篇 |
2021年 | 1330篇 |
2020年 | 1050篇 |
2019年 | 1266篇 |
2018年 | 2141篇 |
2017年 | 1865篇 |
2016年 | 2577篇 |
2015年 | 3303篇 |
2014年 | 3501篇 |
2013年 | 4694篇 |
2012年 | 5544篇 |
2011年 | 5367篇 |
2010年 | 3406篇 |
2009年 | 2867篇 |
2008年 | 4203篇 |
2007年 | 4054篇 |
2006年 | 3899篇 |
2005年 | 3261篇 |
2004年 | 3279篇 |
2003年 | 2925篇 |
2002年 | 2755篇 |
2001年 | 2049篇 |
2000年 | 2040篇 |
1999年 | 1549篇 |
1998年 | 794篇 |
1997年 | 605篇 |
1996年 | 580篇 |
1995年 | 559篇 |
1994年 | 463篇 |
1993年 | 443篇 |
1992年 | 844篇 |
1991年 | 785篇 |
1990年 | 694篇 |
1989年 | 669篇 |
1988年 | 602篇 |
1987年 | 611篇 |
1986年 | 529篇 |
1985年 | 534篇 |
1984年 | 441篇 |
1983年 | 402篇 |
1982年 | 329篇 |
1981年 | 272篇 |
1979年 | 411篇 |
1978年 | 290篇 |
1975年 | 269篇 |
1974年 | 322篇 |
1973年 | 324篇 |
1972年 | 280篇 |
1970年 | 263篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
121.
Lactic acid purification was directly done from fermentation utilizing a fluidized bed column refilled with a strong anionic exchange resin. The purpose of this work was to study the influence of two important design parameters, bed-diameter (D) and bed-height (H), in the lactic acid binding and elution capacity of the matrix. By changing the settled bed height from 2.5 to 5 cm for each diameter of column analyzed it was possible to obtain an 50% increase in the binding capacity of the resin in all experiments. This fact was attributed to a higher contact time between the culture broth and the anionic resin produced by the increase of back mixing and lactic acid residence time. 相似文献
122.
Summary
Kluyveromyces fragilis produces polygalacturonase (PG) on a lactose medium. Although the enzyme is normally repressed at high aeration levels, significant amounts of PG can be produced under such conditions when pectin is added as inducer. The productivity and yield of cell mass were not significantly affected by the presence of inducer, suggesting potential applications to current single cell protein processes from whey. 相似文献
123.
124.
A simple electrophoretic method is introduced allowing to isolate five fractions of skeletal muscle ST-system vesicles. In a previous study differences in lipid content, 3H-ouabain binding and in presence of triads in individual fractions (Lehotsky et. al. 1986) were analysed. In the present study biochemical characterization was extended, and (in accordance with previous results) major differences were observed to exist between fraction 1 and fractions 3 and 4. SDS-PAGE showed that fractions 3 and 4 were enriched in a protein with m.w. 100 kD, these fractions showing the highest specific activities of (Mg2+ + Ca2+)-ATPase and oxalate-supported Ca2+-uptake; activities of Mg2+-ATPase and surface membrane marker enzymes were the lowest in these fractions. On the other hand, in fraction 1 the highest activities of Mg2+-ATPase and marker enzymes of the surface membrane were observed together with a decreased content of the 100 kD protein and activities of Ca2+ transport. It could be concluded that the method is suitable to differentiate between relatively pure SR (fractions 3 and 4) and fractions rich in sarcolemma or T-tubules components (fractions 1 and 5). 相似文献
125.
Serum catalase enzyme activity in liver diseases 总被引:1,自引:0,他引:1
Serum catalase activity was moderately increased in fatty liver, acute alcoholic hepatitis and in the decompensated form of cardiac circulatory failure. It showed significant increase in acute yellow atrophy and in toxic hepatitis while no changes were detected in liver cirrhosis and viral hepatitis. Serum catalase activity showed a good correlation (r = 0.820) with the serum glutamate dehydrogenase activity. In accordance with our results, the inexpensive assay of serum catalase activity is suggested for the detection of severe liver cell damage. 相似文献
126.
Calcium in hippocampus following lidocaine induced seizures: an electron cytochemical study 总被引:1,自引:0,他引:1
The effect of lidocaine seizures on cellular accumulation of calcium was studied in hippocampal subfields CA1 and CA3 and the dentate gyrus of rats, using the combined oxalate-pyroantimonate method. The specificity of the reaction was ascertained by EGTA treatment and X-ray microanalysis. In control rats, calcium was visualized between myelin lamellae of axons, in synaptic vesicles and in some lysosomes. Two hours after onset of lidocaine seizures selective neuronal degenerations appeared in hippocampal subfields CA1 and CA3 but not in the dentate gyrus. Calcium deposits were present in numerous mitochondria of pyramidal cells and, occasionally, also of neuroglial cells. Many of these mitochondria exhibited ultrastructural alterations. Calcium uptake was most prominent in the CA3 sector but was also present in the CA1 subfield as well as the dentate gyrus. Intracellular calcium uptake, in consequence, is not the unique attribute of selectively vulnerable hippocampal neurons. 相似文献
127.
Hippocampal EEG was recorded in behaving rats in which the brain stem afferents to the septal region were previously damaged. In these animals rhythmic slow activity (RSA or theta) was continuously present, including drinking and immobility. The average frequency of RSA, however, was significantly higher during running (8-9 Hz) than during drinking or awake motionless state (6-8 Hz). In normal rats irregular sharp waves, rather than RSA were present during drinking and immobility. The results suggest that brain stem afferents are necessary to suppress the rhythmic firing of septal "pacemaker" cells. 相似文献
128.
Between 1981–83 the gut contents ofDaphnia galeata, D. cucullata, Eudiaptomus gracilis, andCyclops vicinus were examined with light and scanning electron microscope to obtain information on the feeding of these species in Lake Balaton.
The twoDaphnia species feed mainly on abioseston, and it is assumed that their primary nutrient source was organic matter adsorbed onto
the surfaces of the abioseston granules plus bacteria and detritus.E. gracilis feeds on algae, showing a preference for green algae and diatoms.C. vicinus is also a prodigious consumer of algae in Lake Balaton, utilizing the whole size spectrum of phytoplankton.
Concerning the trophic relationships between phytoplankton and zooplankton in Lake Balaton, that between diatoms and bothE. gracilis andC. vicinus is the most conspicouos. Convincing evidence for an extensive utilization of blue-green algae was not found. Though there
is no firm evidence yet, it is likely that theDaphnia are dependent on organic matter adsorbed on the abioseston. 相似文献
129.
Preliminary data on the skeletal biology of 78 Taino skeletons belonging to Juan Dolio, an archaeological site of the Maguana
province, 80 Km. east of S. Domingo, are presented. The minimum number of individuals, sex, age, stature, and morphologic
and morphometric characters were determined. Dental wear and pathology of cranial and post-cranial bones were also recorded. 相似文献
130.
Is the binding of magnesium (II) to calmodulin significant? An investigation by magnesium-25 nuclear magnetic resonance 总被引:8,自引:0,他引:8
Previous reports on the interaction between calmodulin (CaM) and Mg2+ range from no binding to a binding constant of 10(4) M-1 [for a summary, see Cox, J. A., Comte, M., Malnoe, A., Berger, D., & Stein, E. A. (1984) Met. Ions Biol. Syst. 17, 215-273]. In order to resolve the controversy, we used 25Mg NMR to study the binding of Mg2+ to apo-CaM, CaM.Ca2(2)+ (in which sites III and IV are occupied by Ca2+), CaM.La2(3)+ (in which sites I and II are occupied by La3+), and the two tryptic fragments of calmodulin, TR1C (containing sites I and II of CaM) and TR2C (containing sites III and IV of CaM). In each system, a "titration set" and a "temperature set" were obtained, and the spectral data were analyzed by total band-shape analysis to calculate the association constant (Ka) and off-rate (koff). As in the case of Ca2+ binding, sites I and II and sites III and IV were treated as two sets of equivalent sites, and a Ca2+/Mg2+ competition experiment suggested that Mg2+ competes with Ca2+ for the same sites. For both CaM.Ca2(2)+ and TR1C, moderately large Ka (2000 and 3500 M-1, respectively) and moderate off-rates (koff = 2300 and 3000 s-1, respectively, at 25 degrees C) were observed. For both CaM.La2(3)+ and TR2C, binding of Mg2+ was weaker by a factor of ca. 10 (Ka = 300 and 200 M-1, respectively) while the off-rates were also moderate (koff = 3500 and 2200 s-1, respectively).(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献