全文获取类型
收费全文 | 4333篇 |
免费 | 388篇 |
国内免费 | 1篇 |
出版年
2023年 | 23篇 |
2022年 | 27篇 |
2021年 | 66篇 |
2020年 | 36篇 |
2019年 | 52篇 |
2018年 | 89篇 |
2017年 | 62篇 |
2016年 | 108篇 |
2015年 | 174篇 |
2014年 | 218篇 |
2013年 | 219篇 |
2012年 | 317篇 |
2011年 | 280篇 |
2010年 | 195篇 |
2009年 | 166篇 |
2008年 | 236篇 |
2007年 | 253篇 |
2006年 | 229篇 |
2005年 | 233篇 |
2004年 | 187篇 |
2003年 | 216篇 |
2002年 | 191篇 |
2001年 | 54篇 |
2000年 | 42篇 |
1999年 | 40篇 |
1998年 | 54篇 |
1997年 | 38篇 |
1996年 | 38篇 |
1995年 | 31篇 |
1994年 | 27篇 |
1993年 | 31篇 |
1992年 | 27篇 |
1991年 | 39篇 |
1990年 | 52篇 |
1989年 | 29篇 |
1988年 | 36篇 |
1987年 | 31篇 |
1986年 | 21篇 |
1985年 | 22篇 |
1984年 | 31篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 28篇 |
1980年 | 19篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 15篇 |
1974年 | 19篇 |
1968年 | 15篇 |
1967年 | 18篇 |
排序方式: 共有4722条查询结果,搜索用时 15 毫秒
51.
Quantum Requirement for Photosynthesis in Chlorophyll-Deficient Plants with Unusual Lamellar Structures 总被引:1,自引:0,他引:1
Neither an over-all deficiency of chlorophyll, nor an increased enzymatic capacity for maximal rates, nor an unusual lamellar structure was found to change the number of quanta required for the evolution of one molecule of oxygen in healthy aurea mutants of tobacco. The average minimal quantum number remains 10 (efficiency 0.1) as in many algae and typical higher plants. Most of the time the optimal efficiency depends on the availability of some far-red radiation, particularly in the blue region of the spectrum where blue light alone is rather inefficient. These results fit an explanation offered earlier in connection with the hydrogen or acetate photometabolism of algae in far-red light. 相似文献
52.
Georg Steinbacher Otto Wettstein Josef Scholze Wolfgang Makatsch Kurt Bauer Harald Duchrow Klaus Warncke Peter Dancker E. Bezzel H. Remold und Helmut Sick 《Journal of Ornithology》1959,100(1):103-112
Ohne Zusammenfassung 相似文献
53.
54.
55.
Lars Nitschke Karin Heeger Hans Bender Georg E. Schulz 《Applied microbiology and biotechnology》1990,33(5):542-546
Summary The -cyclodextrin glycosyltransferase (-CGTase) gene was isolated from a -library prepared from Bacillus circulans strain no. 8. It was subcloned into plasmid pTZ and expressed by its endogenous regulatory sequences in Escherichia coli JM 103. The structural gene was sequenced and showed an open reading frame for a polypeptide of 718 amino acid residues. The recombinant -CGTase had the same enzymatic properties as the extracellular CGTase (684 amino acid residues, corresponding to a mol. wt. of 74416) produced by B. circulans strain no. 8. The amino acid sequence showed the highest homology (74.6% identical amino acids) with the CGTase of B. circulans strain F-2, which had been erroneously described as an amylase. The homology with the enzyme from the alkalophilic Bacillus sp. strain no. 1011 was 71.4%. The amino acid sequence derived will be used for elucidating the three-dimensional structure of the enzyme.
Offprint requests to: H. Bender 相似文献
56.
Microwave irradiation of rats at 2.45 GHz activates pinocytotic-like uptake of tracer by capillary endothelial cells of cerebral cortex 总被引:5,自引:0,他引:5
Far-field exposures of male albino rats to 2.45-GHz microwaves (10-microseconds pulses, 100 pps) at a low average power density (10 mW/cm2; SAR approximately 2 W/kg) and short durations (30-120 min) resulted in increased uptakes of tracer through the blood-brain barrier (BBB). The uptake of systemically administered rhodamine-ferritin complex by capillary endothelial cells (CECs) of the cerebral cortex was dependent on power density and on duration of exposure. At 5 mW/cm2, for example, a 15-min exposure had no effect. Near-complete blockade of uptake resulted when rats were treated before exposure to microwaves with a single dose of colchicine, which inhibits microtubular function. A pinocytotic-like mechanism is presumed responsible for the microwave-induced increase in BBB permeability. 相似文献
57.
58.
59.
M J Wasicko J E Melton J A Neubauer N Krawciw N H Edelman 《Journal of applied physiology》1990,68(1):53-58
To determine if depression of central respiratory output during progressive brain hypoxia (PBH) can be generalized to other brain stem outputs, we examined the effect of PBH on the tonic (tSCS) and inspiratory-synchronous (iSCS) components of preganglionic superior cervical sympathetic (SCS) nerve activity. Peak phrenic and SCS activity were measured in nine anesthetized, paralyzed, peripherally chemodenervated, vagotomized cats. PBH was produced by inhalation of 0.5% CO in 40% O2 while blood pressure and end-tidal CO2 were maintained constant. A progressive reduction in arterial O2 content from 14.3 +/- 0.6 to 4.5 +/- 0.3 vol% caused a 79 +/- 7% depression of peak phrenic activity and an 84 +/- 10% reduction of iSCS activity, but tSCS activity increased 42 +/- 21%. During CO2 rebreathing, iSCS activity increased in parallel with peak phrenic activity while tSCS activity was unchanged. The slopes of the CO2 responses of both phrenic (6.3 +/- 1.2%max/mmHg) and iSCS (4.6 +/- 0.8%max/mmHg) activity were unaffected by PBH. In four of nine hypocapnic and three of nine hypoxic studies, inspiratory activity in the SCS nerve was observed even after completely silencing the phrenic neurogram.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
60.
Shells of probable former living communities ofDreissena polymorpha were found within sediments of the shallow polytrophic to hypertrophic hard water Lake Breitling (Havel-Lake system, Germany).
Corresponding sediments have been deposited between approximately 1940 and 1970 and reflect increasing eutrophication and
heavy metal pollution of the Lake during this period (Schettler, 1992). Single shells from various sediment depths were analysed
by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) along a line on the outer part of the shell. The
response of these freshwater mussels to increasing heavy metal pollution is clearly reflected in the distribution of Pb, Cu,
Cd and Zn within their valves. In general, Cd, Cu, Pb and Zn contents are lower, and the distribution more even, in the outer
parts of the deepest (oldest) shells compared to shells from higher in the cored sediments. Notably higher contents of Cu,
Pb and Zn were recorded from the central (umbonal) part of the more recent shells, but this behaviour is not recorded for
Cd. Metabolic changes brought on by worsening environmental conditions are proposed to explain this phenomena. Acidity produced
during anaerobic metabolism can be neutralised by dissolution of the carbonate part of the shell. Copper, Zn and Pb, which
show an affinity for the organic component of the shell, may thus accumulate by repeated dissolution and reprecipitation of
the shell during the lifetime of an individual organism. Cadmium, which is bound mainly in the aragonite of the shells, is
released during the dissolution of carbonate and is not concentrated in the umbonal area of the shell. 相似文献