全文获取类型
收费全文 | 2589篇 |
免费 | 296篇 |
出版年
2023年 | 17篇 |
2021年 | 33篇 |
2020年 | 19篇 |
2019年 | 29篇 |
2018年 | 44篇 |
2017年 | 30篇 |
2016年 | 48篇 |
2015年 | 93篇 |
2014年 | 105篇 |
2013年 | 125篇 |
2012年 | 153篇 |
2011年 | 167篇 |
2010年 | 84篇 |
2009年 | 72篇 |
2008年 | 114篇 |
2007年 | 107篇 |
2006年 | 108篇 |
2005年 | 92篇 |
2004年 | 104篇 |
2003年 | 104篇 |
2002年 | 83篇 |
2001年 | 59篇 |
2000年 | 63篇 |
1999年 | 53篇 |
1998年 | 28篇 |
1997年 | 26篇 |
1996年 | 34篇 |
1995年 | 24篇 |
1994年 | 24篇 |
1993年 | 21篇 |
1992年 | 47篇 |
1991年 | 47篇 |
1990年 | 45篇 |
1989年 | 41篇 |
1988年 | 49篇 |
1987年 | 52篇 |
1986年 | 29篇 |
1985年 | 36篇 |
1984年 | 25篇 |
1983年 | 31篇 |
1982年 | 23篇 |
1981年 | 19篇 |
1980年 | 19篇 |
1979年 | 32篇 |
1978年 | 25篇 |
1977年 | 22篇 |
1975年 | 17篇 |
1974年 | 28篇 |
1972年 | 19篇 |
1967年 | 17篇 |
排序方式: 共有2885条查询结果,搜索用时 234 毫秒
31.
A young, male, free-ranging Pacific white-sided dolphin (Lagenorhynchus obliquidens) was found disoriented and died after being held in captivity for several months. Malacic lesions in several areas of the brain were associated with helminth eggs. The appearance and location of these eggs suggested they were of the genus Nasitrema. 相似文献
32.
Summary The biomass production and the plasmid stability of immobilizedE. coli cells in K-carrageenan gel beads were investigated in continuous cultures. Several factors, such as inoculum size, gel bead volume and gel concentration were examined in order to increase the cell concentration inside the immobilized cell reactor, and therefore to increase the overall productivity. 相似文献
33.
34.
Frankia sp. HFP ArI3 (host plant Alnus rubra Bong.) was grown in defined medium and the culture solution was analyzed for the presence of various cytokinins and related compounds. N6- (Δ2-isopentenyl) adenosine was the only cytokinin detected by both high performance liquid chromatography and gas chromatography-mass spectrometry, at levels of approximately 1 ng/ml culture medium. 相似文献
35.
M Nasri F Berry S Sayadi D Thomas J N Barbotin 《Journal of general microbiology》1988,134(8):2325-2331
The maintenance of the plasmid vectors pTG201 and pTG206 (which both carry the Pseudomonas putida xylE gene) and pB lambda H3 in Escherichia coli hosts was studied in free and immobilized continuous cultures. pTG201, containing the strong lambda PR promoter, was more quickly lost than plasmid pTG206, containing the tetracycline resistance gene promoter. The instability of pTG201 seems to be related to high expression of the cloned xylE genet. Fluctuations in the proportion of pTG201-containing cells were observed in the free system, suggesting the appearance of adaptive descendants (with and without plasmid) from the initial strains. The loss of plasmid vectors from E. coli cells and the fluctuations in the proportion of plasmid-containing cells could be prevented by immobilizing plasmid-containing bacteria in carrageenan gel beads. 相似文献
36.
Boronic acid derivatives of good peptide substrates of the serine proteases cause slow-binding inhibition, manifested as biphasic binding (Kettner and Shenvi: J. Biol Chem. 259:15106-15114, 1984). These inhibitors are thought to act as reaction-intermediate analogs. Three peptide boronic acids--Ac-Pro-boro-Val-OH, DNS-Ala-Pro-boro-Val-OH, and Ac-Ala-Ala-Pro-boro-Val-OH--were chosen for far-ultraviolet circular dichroism (CD) studies in order to determine whether the second phase involves a conformational change of pancreatic elastase. The dipeptide is a simple competitive inhibitor (Ki = 0.27 microM) and the latter are slow-binding inhibitors (Ki = 16.4 and 0.25 nM, respectively). Spectral deconvolution and correction for the formation of antiparallel beta-sheet by the peptide inhibitor itself indicate that there is no significant change in the secondary structure of the enzyme in either the initial or final inhibitor complex. A kinetic experiment confirmed that the slow-binding step was not associated with a CD spectral change, and that therefore a protein conformational change was not responsible for the slow binding. 相似文献
37.
Identification and characterization of a pregnane steroid recognition site that is functionally coupled to an expressed GABAA receptor 总被引:2,自引:0,他引:2
Conclusion Based on the pharmacological and biochemical evidence to date, especially that derived from the recombinantly expressed receptor studies, the suggestion that a novel GBRC-linked steroid recognition site exists becomes a cogent argument. The high affinity of the steroid site for certain naturally occurring metabolites of progesterone and glucocorticoids favors a physiologic role for these steroids in the regulation of brain excitability. Clearly, investigations of such a regulatory role is warranted. If present, it provides an important example of endocrine control of a major inhibitory neurotransmitter in the CNS. Moreover, as we gain a greater understanding of the molecular organization of the GBRC, the putative steroid site provides a novel target for the rational design of therapeutic agents for the treatment of anxiety, epilepsy, and insomnia.Special issue dedicated to Dr. Eugene Roberts. 相似文献
38.
Differential down-regulation of protein kinase C subspecies in KM3 cells 总被引:13,自引:0,他引:13
The down-regulation of protein kinase C (PKC) subspecies in KM3 cells (a pre-B, pre-T cell line) has been examined. The PKC from KM3 cells was resolved into two subspecies, type II (mainly beta II) and type III (alpha), upon hydroxyapatite column chromatography. Biochemical and immunocytochemical analysis revealed that, when these cells were treated with 12-O-tetradecanoylphorbol 13-acetate (TPA), the time course of down-regulation of the PKC subspecies was different; type II PKC was translocated and depleted from the cell more quickly than type III enzyme. The results suggest that each PKC subspecies plays a different role in the cellular response to TPA and probably to other external stimuli. 相似文献
39.
A R Grivell M N Berry D C Henly J W Phillips P G Wallace B J Gannon D W Henderson T M Mukherjee J G Swift 《Experimental cell research》1986,165(1):11-28
The controlled centrifugation of isolated rat hepatocytes at 260 000 g results in the formation of membrane-bounded cell fragments that we have termed 'cytospheres'. A method is described for the isolation of these cytospheres. Cytospheres are spherical, have a mean diameter of 9.2 +/- 3.2 microns (SD) and a protein content of 225 +/- 12 mg/g wet wt. About 3% of the protein from the original isolated hepatocyte suspension is recoverable. Transmission electron microscopy (TEM) shows cytospheres to possess a trilaminar membrane, and a finely granular hyaloplasm generally devoid of organelles, filaments and microtubules. Freeze-fracture studies reveal a membrane structure typical of a plasma membrane. Ouabain and wheat germ agglutinin (WGA)-binding studies indicate that the original orientation of the plasma membrane is maintained throughout the formation of the cytospheres. The cytospheres have also been characterized biochemically. Cytospheres are enriched in the enzymes normally associated with the hyaloplasm, whereas the activities of enzymes localized in organelles are greatly diminished. Lipid analysis of the cytosphere membrane indicates that it is derived from the plasma membrane of the hepatocyte. Cytospheres are sensitive to changes in the osmolarity and ionic composition of their environment. Cytospheres should therefore prove a useful preparation for the study of hyaloplasm metabolism and of plasma membrane receptor and permeability properties. 相似文献
40.
Effects of pH on Activity and Activation of Ribulose 1,5-Bisphosphate Carboxylase at Air Level CO(2) 总被引:9,自引:8,他引:1 下载免费PDF全文
The effects of pH on catalysis and activation characteristics of spinach ribulose 1,5-bisphosphate (RuBP) carboxylase were examined at air level of CO2. Catalysis at limiting CO2 was independent of pH over the range of pH 8.2 to 8.8 However, the kinetics of activation and the apparent equilibrium between the activated and inactivated forms of the enzyme were strongly dependent upon the pH and the presence or absence of the substrate RuBP. When incubated at air level of CO2 at pH 8.2 in the absence of RuBP, the enzyme activation state was approximately 75% of that achieved with saturating CO2 at that pH. The extent of activation increased with pH reaching 100% at pH values of 8.6 or higher. Adding RuBP to the activation medium after equilibrium activation state had been established decreased the apparent equilibrium activation level at pH values below 8.6. This effect was reversed at pH values above 8.6. Activation of inactive enzyme by CO2 and Mg2+ was inhibited dramatically at pH values below 8.6 and less so at pH values above 8.6. Studies showed that binding of RuBP to the inactive form of the enzyme was pH dependent with tighter binding occurring at lower pH values. It is suggested that the tight binding of RuBP to the inactive enzyme tends to decrease the equilibrium concentration of the activated form at pH values less than 8.6. These studies indicate that stromal pH could have a strong effect on the activation state of this enzyme in vivo, and possible feedback interactions which might adjust the apparent Vmax to match the rate of RuBP regeneration are discussed. 相似文献