全文获取类型
收费全文 | 3937篇 |
免费 | 337篇 |
国内免费 | 2篇 |
专业分类
4276篇 |
出版年
2023年 | 19篇 |
2022年 | 29篇 |
2021年 | 59篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 83篇 |
2017年 | 53篇 |
2016年 | 92篇 |
2015年 | 163篇 |
2014年 | 206篇 |
2013年 | 204篇 |
2012年 | 292篇 |
2011年 | 260篇 |
2010年 | 177篇 |
2009年 | 156篇 |
2008年 | 220篇 |
2007年 | 240篇 |
2006年 | 208篇 |
2005年 | 220篇 |
2004年 | 172篇 |
2003年 | 204篇 |
2002年 | 175篇 |
2001年 | 42篇 |
2000年 | 33篇 |
1999年 | 32篇 |
1998年 | 48篇 |
1997年 | 38篇 |
1996年 | 32篇 |
1995年 | 32篇 |
1994年 | 22篇 |
1993年 | 27篇 |
1992年 | 15篇 |
1991年 | 34篇 |
1990年 | 40篇 |
1989年 | 23篇 |
1988年 | 32篇 |
1987年 | 26篇 |
1986年 | 17篇 |
1985年 | 18篇 |
1984年 | 22篇 |
1983年 | 19篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 19篇 |
1978年 | 15篇 |
1977年 | 16篇 |
1974年 | 14篇 |
1969年 | 13篇 |
1968年 | 13篇 |
1967年 | 15篇 |
排序方式: 共有4276条查询结果,搜索用时 15 毫秒
11.
Cell-free extracts from leaves of Rhus typhina L. (sumach) were found to transfer the 1-O-galloyl moiety of l,6-di-O-galloyl-β-d-glucose to the 2-position of the same compound, yielding 1,2,6-tri-O-galloyl-β-d-glucose and leaving 6-O-galloylglucose as the deacylated by-product. The enzyme catalyzing this ‘disproportionation’ was purified almost 1700-fold. It had a molecular weight of approx. 56 000, a K m value of 11.5 mM, was stable between pH 4.5 and 6.5, and most active at pH 5.9 and 40° C. The systematic name “1,6-di-O-galloyl-glucose: 1,6-di-O-galloylglucose 2-O-galloyltransferase” (EC 2.3.1.) was proposed for this new enzyme whose detection provided evidence that, in addition to β-glucogallin (1-O-galloyl-β-d-glucose), higher substituted glucose esters also have the potential to serve as acyl donors in the biosynthesis of gallotannins. 相似文献
12.
Georg Schlayer 《Planta》1971,98(4):294-299
Summary In epidermal cells of the cytoledons of sugar beets, Beta vulgaris L., the DNA content per cell can be increased 2–4 fold by means of compensatory growth and other measures of better nutrition, by application of stronger light, or by addition of more moisture to the soil. It decreases with deviations from the optimal growth temperature (22° C) and after lack of nitrogen in the nutritive solution. Differences in DNA content, representing mainly differences in the level of endopolyploidy, result in corresponding differences in the number of plastids in the cells. 相似文献
13.
Peter Schubert Hansjörg Teschemacher Georg W. Kreutzberg Albert Herz 《Histochemistry and cell biology》1970,22(3):277-288
Summary The intracerebral distribution patterns of 14C-morphine, 3H-dihydromorphine, and 3H-fentanyl after intraventricular injection were studied autoradiographically in rats and rabbits. The extent of permeation into the ventricular wall was measured at different times after injection. The hydrophilic morphine and dihydromorphine could be demonstrated within the tissue up to 4 hours. They seemed to be retained within the gray matter and hindered in crossing fiber bundles. On the other hand, the lipophilic fentanyl was quickly removed from the brain but remained relatively longer demonstrable within the white matter. Also, after intrathecal injection of 14C-morphine a time dependent spread from the injection site was observed. The use of autoradiography in pharmacological experiments as described was found advantageous. Thus, it is possible to correlate directly, the time course of the pharmacological effect and the respective distribution pattern of the drug applied. This may lead to better information about the probable sites of drug action. 相似文献
14.
Zusammenfassung Die bemerkenswerte Sternbildung von Agrobacterium luteum Stamm A61 (Moll et al., 1967; Ahrens, 1968) ist auf polare Fimbrien zurückzuführen, die um die Sternaggregate ausgebreitet sind. Es wird angenommen, daß sich Schwärmzellen an den Fimbrien verfangen und durch Kontraktion rasch zum Stern gezogen werden. Nach elektronenoptischen Präparaten sind die Fimbrien bis zu 10,5 lang und können die beobachtete Sternbildung gut erklären. Agrobacterium luteum Stamm B14 besitzt ebenfalls lange polare Fimbrien. Beide Stämme tragen peritriche Geißeln, deren Struktur sich deutlich von den dünnen und unregelmäßigen Fimbrien unterscheidet.
The function of fimbriae in the peculiar star formation of Agrobacterium luteum
Summary The remarkable way of star formation in Agrobacterium luteum strain A61 (Moll et al., 1967; Ahrens, 1968) was found to be due to polar fimbriae which are spread around the star-shaped aggregates. It is assumed that swarmers adhere to the fimbriae and, by contraction, are swiftly pulled towards the star. As seen in electron microscopic preparations, the fimbriae are up to 10,5 long and may well explain the star formation observed. Agrobacterium luteum strain B 14 also possesses long polar fimbriae. Both strains have peritrichous flagella, the structure of which being clearly distinct from the delicate and irregular fimbriae.相似文献
15.
Georg Heinrich 《Protoplasma》1964,58(3):402-425
Ohne Zusammenfassung 相似文献
16.
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
17.
18.
19.
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
20.
PET1402, a nuclear gene required for proteolytic processing of cytochrome oxidase subunit 2 in yeast
Mathias Bauer Meinhardt Behrens Karlheinz Esser Georg Michaelis Elke Pratje 《Molecular & general genetics : MGG》1994,245(3):272-278
The nuclear mutation pet ts1402 prevents proteolytic processing of the precursor of cytochrome oxidase subunit 2 (cox2) in Saccharomyces cerevisiae. The structural gene PET1402 was isolated by genetic complementation of the temperature-sensitive mutation. DNA sequence analysis identified a 1206-bp open reading frame, which is located 215 by upstream of the PET122 gene. The DNA sequence of PET1402 predicts a hydrophobic, integral membrane protein with four transmembrane segments and a typical mitochondrial targeting sequence. Weak sequence similarity was found to two bacterial proteins of unknown function. Haploid cells containing a null allelle of PET1402 are respiratory deficient. 相似文献