全文获取类型
收费全文 | 377篇 |
免费 | 32篇 |
出版年
2022年 | 4篇 |
2021年 | 6篇 |
2020年 | 5篇 |
2019年 | 4篇 |
2018年 | 6篇 |
2017年 | 4篇 |
2016年 | 12篇 |
2015年 | 15篇 |
2014年 | 15篇 |
2013年 | 14篇 |
2012年 | 30篇 |
2011年 | 21篇 |
2010年 | 18篇 |
2009年 | 14篇 |
2008年 | 23篇 |
2007年 | 18篇 |
2006年 | 14篇 |
2005年 | 14篇 |
2004年 | 10篇 |
2003年 | 17篇 |
2002年 | 15篇 |
2001年 | 9篇 |
2000年 | 9篇 |
1999年 | 3篇 |
1997年 | 6篇 |
1995年 | 4篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 6篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1967年 | 2篇 |
1937年 | 4篇 |
1936年 | 4篇 |
1934年 | 2篇 |
1932年 | 2篇 |
1906年 | 2篇 |
排序方式: 共有409条查询结果,搜索用时 15 毫秒
401.
Nancy Ries 《American anthropologist》1997,99(4):873-874
The Russian Concept of Work: Suffering, Drama, and Tradition in Pre- and Post-Revolutionary Russia. Anna Feldman Leibovich. Westport, CT: Praeger, 1995. 166 pp. 相似文献
402.
403.
Systematic mappings of the effects of protein mutations are becoming increasingly popular. Unexpectedly, these experiments often find that proteins are tolerant to most amino acid substitutions, including substitutions in positions that are highly conserved in nature. To obtain a more realistic distribution of the effects of protein mutations, we applied a laboratory drift comprising 17 rounds of random mutagenesis and selection of M.HaeIII, a DNA methyltransferase. During this drift, multiple mutations gradually accumulated. Deep sequencing of the drifted gene ensembles allowed determination of the relative effects of all possible single nucleotide mutations. Despite being averaged across many different genetic backgrounds, about 67% of all nonsynonymous, missense mutations were evidently deleterious, and an additional 16% were likely to be deleterious. In the early generations, the frequency of most deleterious mutations remained high. However, by the 17th generation, their frequency was consistently reduced, and those remaining were accepted alongside compensatory mutations. The tolerance to mutations measured in this laboratory drift correlated with sequence exchanges seen in M.HaeIII’s natural orthologs. The biophysical constraints dictating purging in nature and in this laboratory drift also seemed to overlap. Our experiment therefore provides an improved method for measuring the effects of protein mutations that more closely replicates the natural evolutionary forces, and thereby a more realistic view of the mutational space of proteins. 相似文献
404.
The substrate requirement of phospholipids for hydrolysis with phospholipase C from Bacillus cereus was studied with synthetic lipids well-defined in structure and configuration. For optimal activity, the glycerol molecule must contain three substituents: phosphocholine in sn-3-, an ester bond in sn-2- and an ether- or ester bond in sn-1-position. The length of the ester or ether chains is of minor importance. Any deviation from these structural requirements results in a large decrease in the hydrolysis rate. These essential structural and configurational elements for optimal activity for the B. cereus enzyme are perfectly combined in the platelet activating factor, 1-O-hexadecyl-2-acetyl-sn-glycero-3- phosphocholine. This molecule is one of the best substrates for hydrolysis with the bacterial phospholipase C. 相似文献
405.
Constitutive acid phosphatase was purified from yeast cells grown in a medium supplied with 100 mM phosphate. Specific activity of the pure enzyme was 63.5 mumol/min X mg. The enzyme contains 42.5% of protein, 55% of mannose and 2.5% of N-acetylglucosamine. The carbohydrate chains are N-glycosidically linked to the protein. The pure enzyme shows non-linearity in the Lineweaver-Burk plot, thus indicating the presence of two enzyme forms with Km values of about 0.65 mM and 8.5 mM. The pH optimum of the enzyme is at pH 3.3. The enzyme is much more sensitive to heat denaturation than the repressible acid phosphatase. 相似文献
406.
There is often an interest in knowing, for a given ligand concentration, how many protein molecules have one, two, three, etc. ligands bound in a specific manner. This is a question that cannot be addressed using conventional ensemble techniques. Here, a mathematical method is presented for separating specific from nonspecific binding in nonensemble studies. The method provides a way to determine the distribution of specific binding stoichiometries at any ligand concentration when using nonensemble (e.g., single-molecule) methods. The applicability of the method is demonstrated for ADP binding to creatine kinase using mass spectroscopy data. A major advantage of our method, which can be applied to any protein-ligand system, is that no previous information regarding the mechanism of ligand interaction is required. 相似文献
407.
Background, Aim and Scope The objective of this life cycle assessment (LCA) study is to develop LCA models for energy systems in order to assess the
potential environmental impacts that might result from meeting energy demands in buildings. The scope of the study includes
LCA models of the average electricity generation mix in the USA, a natural gas combined cycle (NGCC) power plant, a solid
oxide fuel cell (SOFC) cogeneration system; a microturbine (MT) cogeneration system; an internal combustion engine (ICE) cogeneration
system; and a gas boiler.
Methods LCA is used to model energy systems and obtain the life cycle environmental indicators that might result when these systems
are used to generate a unit energy output. The intended use of the LCA analysis is to investigate the operational characteristics
of these systems while considering their potential environmental impacts to improve building design using a mixed integer
linear programming (MILP) optimization model.
Results The environmental impact categories chosen to assess the performance of the energy systems are global warming potential (GWP),
acidification potential (AP), tropospheric ozone precursor potential (TOPP), and primary energy consumption (PE). These factors
are obtained for the average electricity generation mix, the NGCC, the gas boiler, as well as for the cogeneration systems
at different part load operation. The contribution of the major emissions to the emission factors is discussed.
Discussion The analysis of the life cycle impact categories indicates that the electrical to thermal energy production ratio has a direct
influence on the value of the life cycle PE consumption factors. Energy systems with high electrical to thermal ratios (such
as the SOFC cogeneration systems and the NGCC power plant) have low PE consumption factors, whereas those with low electrical
to thermal ratios (such as the MT cogeneration system) have high PE consumption factors. In the case of GWP, the values of
the life cycle GWP obtained from the energy systems do not only depend on the efficiencies of the systems but also on the
origins of emissions contributing to GWP. When evaluating the life cycle AP and TOPP, the types of fuel as well as the combustion
characteristics of the energy systems are the main factors that influence the values of AP and TOPP.
Conclusions An LCA study is performed to eraluate the life cycle emission factors of energy systems that can be used to meet the energy
demand of buildings. Cogeneration systems produce utilizable thermal energy when used to meet a certain electrical demand
which can make them an attractive alternative to conventional systems. The life cycle GWP, AP, TOPP and PE consumption factors
are obtained for utility systems as well as cogeneration systems at different part load operation levels for the production
of one kWh of energy output.
Recommendations and Perspectives Although the emission factors vary for the different energy systems, they are not the only factors that influence the selection
of the optimal system for building operations. The total efficiencies of the system play a significant part in the selection
of the desirable technology. Other factors, such as the demand characteristics of a particular building, influence the selection
of energy systems.
The emission factors obtained from this LCA study are used as coefficients of decision variables in the formulation of an
MILP to optimize the selection of energy systems based on environmental criteria by taking into consideration the system efficiencies,
emission characteristics, part load operation, and building energy demands. Therefore, the emission factors should not be
regarded as the only criteria for choosing the technology that could result in lower environmental impacts, but rather one
of several factors that determine the selection of the optimum energy system.
ESS-Submission Editor: Arpad Horvath (horvath@ce.berkeley.edu) 相似文献
408.
Ohne Zusammenfassung 相似文献
409.
Ries Stanley; Widders Irvin; Savithiry Srinivasan; Wert Violet; Baughan Rebecca 《Journal of experimental botany》1994,45(3):367-371
Several plant species (snapbeans, soybeans, tomatoes, and maize)were shown to respond within 360 s to pinching a single,fully expanded leaf with a clinical haemostat. The responsewas measured by an increase, as compared to controls, in Ca2+, Mg2+ and K+ concentrations in the exudate from the stump ofstems excised at the first leaf (maize) or cotyledonary nodeat various times after pinching. The enhanced apoplastic cationconcentrations were shown to be transient, diminishing withtime. Usually no response could be measured if excision wasdelayed more than 3 min. The number of pinches affected themagnitude of the response, with one pinch sufficient in mostcases. Our data suggest that transient changes in extracellularcations are involved in signal trans-duction or as a secondmessenger in response to perturbations. Key words: Cations, signalling, Ca2+, Mg2+, K+ 相似文献