全文获取类型
收费全文 | 5974篇 |
免费 | 473篇 |
国内免费 | 3篇 |
出版年
2023年 | 26篇 |
2022年 | 25篇 |
2021年 | 92篇 |
2020年 | 88篇 |
2019年 | 114篇 |
2018年 | 168篇 |
2017年 | 145篇 |
2016年 | 221篇 |
2015年 | 388篇 |
2014年 | 386篇 |
2013年 | 444篇 |
2012年 | 478篇 |
2011年 | 463篇 |
2010年 | 309篇 |
2009年 | 284篇 |
2008年 | 365篇 |
2007年 | 337篇 |
2006年 | 320篇 |
2005年 | 307篇 |
2004年 | 287篇 |
2003年 | 234篇 |
2002年 | 236篇 |
2001年 | 131篇 |
2000年 | 108篇 |
1999年 | 81篇 |
1998年 | 41篇 |
1997年 | 27篇 |
1996年 | 35篇 |
1995年 | 20篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 29篇 |
1991年 | 19篇 |
1990年 | 26篇 |
1989年 | 19篇 |
1988年 | 11篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1981年 | 5篇 |
1979年 | 12篇 |
1977年 | 8篇 |
1976年 | 8篇 |
1975年 | 10篇 |
1974年 | 9篇 |
1972年 | 5篇 |
1971年 | 6篇 |
1969年 | 11篇 |
1968年 | 9篇 |
排序方式: 共有6450条查询结果,搜索用时 206 毫秒
1.
2.
A continuous production of fructooligosaccharides from sucrose was investigated by fructosyltransferase immobilized on a high porous resin, Diaion HPA 25. The optimum pH (5.5) and temperature (55°C) of the enzyme for activity was unaltered by immobilization, and the immobilized enzyme became less sensitive to the pH change. The optimal operation conditions of the immobilized enzyme column for maximizing the productivity were as follows: 600 g/L of sucrose feed concentration, flow rate of superficial space velocity 2.7 h?1. When the enzyme column was run at 50°C, about 8% loss of the initial activity of immobilized enzyme was observed after 30 days of continuous operation, during which high productivity of 1174 g/L·h was achieved. The kinds of products obtained using the immobilized enzyme were almost the same as those using soluble enzymes or free cells. 相似文献
3.
Theoretical calculation of the sugar concentrations during enzymatic production of fructooligosaccharides 总被引:1,自引:0,他引:1
Summary In a batch production of fructooligosaccharides from sucrose, the concentrations of residual sucrose, glucose and fructooligosaccharides at a given reaction time(t) and initial sucrose concentration(S0) were theoretically calculated by the following correlation equations: Glucose(t) = 0.0653 S0 × ln(t); Fructooligosaccharides(t) = 0.1636 S0 × ln(t); Sucrose(t)=S0 - Glucose(t) + FOS(t). 相似文献
4.
Il-Young Paik Hwa-Eun Jin Ah-Ram Suh Hee-Tae Roh Sang-Hoon Suh 《Biochemical and biophysical research communications》2009,383(1):103-107
To investigate the effects of hydration status on oxidative DNA damage and exercise performance, 10 subjects ran on a treadmill until exhaustion at 80% VO2max during four different trials [control (C), 3% dehydration (D), 3% dehydration + water (W) or 3% dehydration + sports drink (S)]. Dehydration significantly decreased exercise time to exhaustion (D < C and S). Plasma MDA levels were significantly higher at pre-exercise in D than C. Plasma TAS was significantly lower at pre-exercise in C and S than in D, and was significantly lower in S than D at 60 min of recovery. Dehydration significantly increased oxidative DNA damage during exercise, but fluid replacement with water or sports drink alleviated it equally. These results suggest that (1) dehydration impairs exercise performance and increases DNA damage during exercise to exhaustion; and (2) fluid replacement prolongs exercise endurance and attenuates DNA damage. 相似文献
5.
Won S. Woo 《Phytochemistry》1974,13(12):2887-2889
6.
Yi-Wen Chiu Sangwon Suh Stephan Pfister Stefanie Hellweg Annette Koehler 《The International Journal of Life Cycle Assessment》2012,17(1):16-24
Purpose
Though the development of biofuel has attracted numerous studies for quantifying potential water demand applying life cycle thinking, the impacts of biofuel water consumption still remain unknown. In this study, we aimed to quantify ecological impact associated with corn-based bioethanol water consumption in Minnesota in responding to different refinery expansion scenarios by applying a life cycle impact assessment method. 相似文献7.
Eduardo A. B. Almeida Marcio R. Pie Seán G. Brady Bryan N. Danforth 《Journal of Biogeography》2012,39(3):526-544
Aim The evolutionary history of bees is presumed to extend back in time to the Early Cretaceous. Among all major clades of bees, Colletidae has been a prime example of an ancient group whose Gondwanan origin probably precedes the complete break‐up of Africa, Antarctica, Australia and South America, because modern lineages of this family occur primarily in southern continents. In this paper, we aim to study the temporal and spatial diversification of colletid bees to better understand the processes that have resulted in the present southern disjunctions. Location Southern continents. Methods We assembled a dataset comprising four nuclear genes of a broad sample of Colletidae. We used Bayesian inference analyses to estimate the phylogenetic tree topology and divergence times. Biogeographical relationships were investigated using event‐based analytical methods: a Bayesian approach to dispersal–vicariance analysis, a likelihood‐based dispersal–extinction–cladogenesis model and a Bayesian model. We also used lineage through time analyses to explore the tempo of radiations of Colletidae and their context in the biogeographical history of these bees. Results Initial diversification of Colletidae took place at the Late Cretaceous (≥ 70 Ma). Several (6–14) lineage exchanges between Australia and South America via Antarctica during the Late Cretaceous and Eocene epochs could explain the disjunctions observed between colletid lineages today. All biogeographical methods consistently indicated that there were multiple lineage exchanges between South America and Australia, and these approaches were valuable in exploring the degree of uncertainty inherent in the ancestral reconstructions. Biogeographical and dating results preclude an explanation of Scrapterinae in Africa as a result of vicariance, so one dispersal event is assumed to explain the disjunction in relation to Euryglossinae. The net diversification rate was found to be highest in the recent history of colletid evolution. Main conclusions The biogeography and macroevolutionary history of colletid bees can be explained by a combination of Cenozoic vicariance and palaeoclimatic changes during the Neogene. The austral connection and posterior break‐up of South America, Antarctica and Australia resulted in a pattern of disjunct sister lineages. Increased biome aridification coupled with floristic diversification in the southern continents during the Neogene may have contributed to the high rates of cladogenesis in these bees in the last 25–30 million years. 相似文献
8.
Stoichiometries, equilibrium constants and optical extinction coefficients of calcium-antipyrylazo III (An) complexing are determined with the analytical method described in article I of this series. Spcctrophotometric Ca titrations of An at the wavelengths 595 and 710 nm indicate overall dissociation equilibrium constants for the complexes CaAn, CaAn2 and Ca2An to be 4.5 × 10?4 M, 1.1 × 10?8 M2 and 1.5× 10?6 M2, respectively, extrapolated to zero ionic strength. Ca titrations of solutions containing An plus acetylcholine receptor protein give clear evidence that An binds to the protein to a large extent in the presence of Ca2+; furthermore, addition of acetylcholine results in release of protein-bound Ca and An. This is the first reported indication that antipyrylazo III binds to biological material and questions the usefulness of this dye as a Ca indicator in biological systems. 相似文献
9.
Won Jae Choi 《Applied microbiology and biotechnology》2009,84(2):239-247
Enantiopure epoxides are high value-added synthons for the production of pharmaceuticals, agrochemicals, as well as versatile
fine chemicals and have broad scope of market demand for their applications. A major challenge in conventional organic synthesis
is to generate such compounds in high enantiopurity with reasonable yield. Among possible chemical and biological technologies
for enantiopure epoxide preparation, enzymatic kinetic resolution has been paid much attention with respect to its high enantioselectivity.
Epoxide hydrolase (EH) has shown promising characteristics for the preparation of enantiopure epoxides and vicinal diols during
enantioselective hydrolysis of racemic epoxides. EH is readily available from microbial resources thus it is being employed
for biohydrolysis of a variety of epoxides. Recent technical progress in EH-catalyzed enantioselective hydrolysis is summarized
in terms of exploration of novel EH, its functional improvement, high throughput assay, and preparative scale resolution process. 相似文献
10.
Seunghun Lee Won Yeong Park Hoonchul Chang Bumju Kim Won Hyuk Jang Seonghan Kim Younghoon Shin Myoung Joon Kim Kyung Hwa Lee Eui Hyun Kim Euiheon Chung Ki Hean Kim 《Journal of biophotonics》2020,13(1)
Delineation of brain tumor margins during surgery is critical to maximize tumor removal while preserving normal brain tissue to obtain optimal clinical outcomes. Although various imaging methods have been developed, they have limitations to be used in clinical practice. We developed a high‐speed cellular imaging method by using clinically compatible moxifloxacin and confocal microscopy for sensitive brain tumor detection and delineation. Moxifloxacin is a Food and Drug Administration (FDA) approved antibiotic and was used as a cell labeling agent through topical administration. Its strong fluorescence at short visible excitation wavelengths allowed video‐rate cellular imaging. Moxifloxacin‐based confocal microscopy (MBCM) was characterized in normal mouse brain specimens and visualized their cytoarchitecture clearly. Then, MBCM was applied to both brain tumor murine models and two malignant human brain tumors of glioblastoma and metastatic cancer. MBCM detected tumors in all the specimens by visualizing dense and irregular cell distributions, and tumor margins were easily delineated based on the cytoarchitecture. An image analysis method was developed for automated detection and delineation. MBCM demonstrated sensitive delineation of brain tumors through cytoarchitecture visualization and would have potentials for human applications, such as a surgery‐guiding method for tumor removal. 相似文献