全文获取类型
收费全文 | 1880篇 |
免费 | 80篇 |
国内免费 | 1篇 |
出版年
2022年 | 7篇 |
2021年 | 11篇 |
2020年 | 10篇 |
2019年 | 13篇 |
2018年 | 22篇 |
2017年 | 15篇 |
2016年 | 46篇 |
2015年 | 71篇 |
2014年 | 61篇 |
2013年 | 103篇 |
2012年 | 90篇 |
2011年 | 127篇 |
2010年 | 71篇 |
2009年 | 56篇 |
2008年 | 103篇 |
2007年 | 118篇 |
2006年 | 119篇 |
2005年 | 101篇 |
2004年 | 83篇 |
2003年 | 113篇 |
2002年 | 95篇 |
2001年 | 19篇 |
2000年 | 21篇 |
1999年 | 25篇 |
1998年 | 26篇 |
1997年 | 12篇 |
1996年 | 16篇 |
1995年 | 22篇 |
1994年 | 18篇 |
1993年 | 16篇 |
1992年 | 23篇 |
1991年 | 30篇 |
1990年 | 24篇 |
1989年 | 24篇 |
1988年 | 13篇 |
1987年 | 16篇 |
1986年 | 20篇 |
1985年 | 13篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1978年 | 14篇 |
1977年 | 11篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1974年 | 11篇 |
1973年 | 8篇 |
排序方式: 共有1961条查询结果,搜索用时 31 毫秒
41.
42.
Diversity concept in ecology 总被引:1,自引:0,他引:1
Hierarchy of systems organization is used as a framework in advancing methodological guidelines for posing correct questions related to ecological diversity.Diversity if defined in general terms as a property of a set of elements dependent on and determines: by the epistemological perspective. Ontological diversity, because it is indefinite, is regarded as unmeasurable.Ecological diversity — be it species, spatial, reproductive or trophic, is a particular case of diversity of matter and must be precisely defined on the studied level of system organization.Diversity measurements combining more than one level of organization are information-void, for data become irretrievable as a result of such a treatment.Hypotheses explaining diversity sources are briefly surveyed. An integrated model interrelating them is constructed as a result of some basic views on the structure of matter coupled with the empirical knowledge of involved factors. The model reflects hierarchies of systems, their dynamics, and the complexities of factors affecting diversity. The model's properties suggest that it is conceptually related to an imaginary model of organization of ecological systems. Parameters recognized as invariant components of this complexity are: habitat differentiation, ecological specialization of species, ambiental changes, and integration of this system.Finally, it is shown that the peculiar shape of the species abundances curve is a necessary consequence of stratification of the system structure, which is an intrinsic attribute of hierarchical organization. 相似文献
43.
RNA synthesis during morphogenesis of the fungusMucor racemosus 总被引:6,自引:0,他引:6
Bacteroides succinogenes produces acetate and succinate as major products of carbohydrate fermentation. An investigation of the enzymes involved indicated that pyruvate is oxidized by a flavin-dependent pyruvate cleavage enzyme to acetyl-CoA and CO2. Active CO2 exchange is associated with the pyruvate oxidation system. Reduction of flavin nucleotides is CoASH-dependent and does not require ferredoxin. Acetyl-CoA is further metabolized via acetyl phosphate to acetate and ATP. Reduced flavin nucleotide is used to reduce fumarate to succinate by a particulate flavin-specific fumarate reductase reaction which may involve cytochrome b. Phosphoenolpyruvate (PEP) is carboxylated to oxalacetate by a GDP-specific PEP carboxykinase. Oxalacetate, in turn, is converted to malate by a pyridine nucleotide-dependent malate dehydrogenase. The organism has a NAD-dependent glyceraldehyde-3-phosphate dehydrogenase. The data suggest that reduced pyridine nucleotides generated during glycolysis are oxidized in malate formation and that the electrons generated during pyruvate oxidation are used to reduce fumarate to succinate. 相似文献
44.
Jerzy W. Jurka 《Journal of theoretical biology》1977,68(4):515-520
Some properties of transitions generated during an imprecise replication of nucleic acids are described here. A possible role of these transitions in the evolution of the genetic code with respect to the evolution of the tertiary structure of proteins is suggested. The data supporting this hypothesis are obtained from the analysis of certain regularities present in the genetic code and in the proportions of amino acid residues buried in the interior of globular proteins. 相似文献
45.
Marian Orlowski Elizabeth Wilk Stevens Pearce Sherwin Wilk 《Journal of neurochemistry》1979,33(2):461-469
Abstract— An enzyme with the specificity of a prolyl endopeptidase was purified about 880-fold from rabbit brain. The enzyme hydrolyzes peptidylprolyl-peptide and peptidylprolyl-amino acid bonds. Several biologically active peptides such as angiotensin, bradykinin, neurotensin. substance P and thyrotropin releasing hormone are degraded by hydrolysis of the bond between the carboxyl group of proline and the adjacent amino acid or ammonia respectively. The enzyme is activated by dithiothreitol and inhibited by heavy metals and thiol blocking agents. The serine protease inhibitor phenylmethanesulfonylfluoride has no effect on activity; however, inhibition was obtained with diisopropylfluorophosphate. Prolyl endopeptidase has a molecular weight of about 66,000 and a pH optimum of about 8.3. A new chromogenic substrate, N -benzyloxycarbonylglycyl-L-prolylsulfamethoxazole, was used for determination of enzyme activity. The substrate is hydrolyzed to N -benzyloxycarbonylglycyl-L-proline and free sulfamethoxazole which can be conveniently determined by a colorimetric procedure. 相似文献
46.
Ewa Stach-Chilf Jerzy B. Warchol Prof. Dr. Christoph Pilgrim 《Cell and tissue research》1981,219(2):417-423
Summary The neurohypophysis of donor mice was implanted under the renal capsule of the recipients. The pituicytes survived while the neurosecretory axons disappeared. The ultrastructure of the glial cells was observed seven and nine weeks after transplantation. There were no signs of phagocytotic activity although remnants of axons were still present at seven weeks. The numerous processes of the pituicytes form a network with intercellular spaces wide in younger and narrower in older implants. The cells are connected by desmosomes and gap junctions. Pituicytes as well as blood vessels preserve their organotypic appearance. The transplant thus represents an experimental model for investigations on pituicytes in vivo in the absence of neurosecretory axons.Fellow of the Alexander von Humboldt-Stiftung. On leave from Department of Histology and Embryology, Institute of Biostructure, Academy of Medicine, Pozna, Poland 相似文献
47.
Summary The nervous system of the most primitive fresh-water Turbellaria of the order Catenulida was investigated. Neurosecretory
cells were found to occur in it. These cells have a structure similar to that of nerve cells, they only differ by the presence
of neurosecretion granules and a more developed golgi complex. In the simplest Catenulida,Stenostomun, neurosecretory granules are found both in the brain cells and in the trunks in the nerve fibres where they are rather randomly
distributed. Another representative of this family, the more highly organizedCatenula, exhibits specialized brain and nerve trunk regions containing neurosecretion granules.
This work was supported in part by the Committee on Cytobiology of the Polish Academy of Sciences 相似文献
48.
49.
F Henao B de Foresta S Orlowski A Cuenda C Gutiérrez-Merino P Champeil 《European journal of biochemistry》1991,202(2):559-567
We investigated the effect of the local anesthetic procaine on the activity of the calcium pump protein of sarcoplasmic reticulum (SR) vesicles. Procaine slowed down the rate of calcium uptake by SR vesicles without enhancing the vesicles' passive permeability. This slowing of the unidirectional pumping rate was reflected by the inhibition of the maximal rate of the transport-coupled Ca(2+)-ATPase activity. The inhibition was dependent on Mg2+ concentration; at optimal (i.e. low) concentrations of magnesium, half-maximal inhibition occurred with procaine concentrations close to 15-20 mM. Inhibition of ATPase was not mediated by a change in the properties of the bulk lipid phase. Procaine moderately reduced the true affinity of ATPase for ATP, whereas equilibrium binding of calcium to ATPase in the absence of ATP was virtually not modified by procaine. In fast-kinetics studies, we explored the various intermediate steps in the ATPase catalytic cycle, in order to determine which of them were targets for inhibition by procaine. We found that procaine slowed down ATPase dephosphorylation, an effect which is at least partly responsible for the observed inhibition of overall ATPase activity. In contrast, procaine accelerated the calcium-induced transconformation of unphosphorylated ATPase in the absence of ATP, and altered neither the rate of the Ca(2+)-dependent phosphorylation of ATPase, nor the rate of the dissociation of Ca2+ from phosphorylated ATPase towards the SR lumen, a critical step, the rate of which was measured by a novel fast-filtration method. These results are discussed with respect to the possible site(s) of binding of this amphiphile on the ATPase, and in relation to the contribution of individual steps in the catalytic cycle to the rate limitation of unperturbed SR ATPase activity. 相似文献
50.
Reconstruction and analysis of human alu genes 总被引:39,自引:0,他引:39