全文获取类型
收费全文 | 3914篇 |
免费 | 336篇 |
国内免费 | 1篇 |
出版年
2023年 | 19篇 |
2022年 | 29篇 |
2021年 | 58篇 |
2020年 | 34篇 |
2019年 | 40篇 |
2018年 | 83篇 |
2017年 | 53篇 |
2016年 | 91篇 |
2015年 | 162篇 |
2014年 | 205篇 |
2013年 | 202篇 |
2012年 | 291篇 |
2011年 | 259篇 |
2010年 | 174篇 |
2009年 | 155篇 |
2008年 | 220篇 |
2007年 | 241篇 |
2006年 | 208篇 |
2005年 | 220篇 |
2004年 | 172篇 |
2003年 | 204篇 |
2002年 | 175篇 |
2001年 | 41篇 |
2000年 | 29篇 |
1999年 | 31篇 |
1998年 | 46篇 |
1997年 | 37篇 |
1996年 | 31篇 |
1995年 | 30篇 |
1994年 | 22篇 |
1993年 | 27篇 |
1992年 | 15篇 |
1991年 | 34篇 |
1990年 | 41篇 |
1989年 | 24篇 |
1988年 | 30篇 |
1987年 | 26篇 |
1986年 | 16篇 |
1985年 | 17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4251条查询结果,搜索用时 15 毫秒
11.
It is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda. 相似文献
12.
13.
14.
Manfred Keller Andras Seregi Georg Hertting Rolf Jackisch 《Neurochemistry international》1987,10(4):433-443
Prostaglandin (PG) and thromboxane B2 (TXB2) biosynthesis was studied in cultured astrocytes from neonatal rat brain hemispheres. After two weeks of cultivation, prostanoids were formed with the spectrum: PGD2 > TXB2 > PGF2 > PGE2, as measured by specific radioimmunoassays. Under basal conditions PGD2 biosynthesis (9.55 ng/mg protein/15 min) was in the same order of magnitude as the sum of the other prostanoids. The formation of prostanoids was stimulated in a concentration dependent manner (up to 6–10 fold) by the calcium ionophore A 23187 (0.01–10 μM) as well as by melittin (0.01–5 μg/ml), phospholipase A2 (10–40 U/ml) and phospholipase C (0.01–1 U/ml). Basal and evoked PG and TXB2 biosynthesis depended on the availability of Ca2+, as demonstrated in Ca2+ free incubation medium containing Na2EDTA (1 μM), or with verapamil (100 μM) and 3,4,5-trimethoxybenzoic acid-8-(diethylamino)-octylester-HCl (TMB-8, 1–100 μM). Indomethacin (10 μM), mepacrine (100 μM) and p-bromophenacylbromide (50 μ M) inhibited basal and evoked PG formation. Thin-layer chromatography (TLC) detection after incubation of the cells with [3H]arachidonic acid (1 μCi/ml, for 60 min) confirmed the results obtained by radioimmunoassay. Incubation of [3H]arachidonic acid labelled cells with inonophore or phospholipases, followed by lipid extraction and TLC, showed that A 23187 liberated [3H]arachidonic acid predominantly from phosphatidylethanolamine, whereas phospholipase A2 and C reduced mainly the labelling of the phosphatidyl-inositol/-choline fraction. Potassium depolarization of the cells did not enhance prostanoid formation. Similarly, drugs with affinity to - or β-adrenoceptors, or to dopamine-, 5-hydroxytryptamine-, muscarine-, histamine-, glutamate-, aspartate-, GABA, adenosine- and opioid-receptors failed to stimulate prostanoid biosynthesis. Also compounds like angiotensin, bradykinin and thrombin were ineffective in this respect.
In conclusion, our results confirm that cultured astrocytes possess the complete pattern of enzymes necessary for prostanoid formation and hence might play a crucial role in brain prostanoid biosynthesis. Stimulation of prostanoid biosynthesis involves Ca2+-dependent activation of phospholipase A2, cyclooxygenase reaction and further PG metabolism. However, the endogenous stimulus for enhanced prostanoid synthesis in the brain still has to be established. 相似文献
15.
Spinach-leaf ferredoxin was identified as a calcium-binding protein by 45Ca autoradiography on nitrocellulose membranes and with the cationic carbocyanine dye 1-ethyl-2-[3-(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)-2-methylpropenyl] naphtho[1,2-d]thiazolium bromide (stains-all). Binding of 45Ca was observed at pH 6.8 and pH 7.8 and in the presence of 5 mM and 20 mM MgCl2. At the higher MgCl2 concentration the Ca2+-binding capacity is reduced. Only micromolar concentrations of LaCl3, however, are required to achieve a similar effect. Both the oxidized and reduced forms of ferredoxin bind calcium.Abbreviations PAGE
polyacrylamide gel electrophoresis
- SDS
sodium dodecyl sulfate
- stains-all
1-ethyl-2-[3-(1-ethylnaphtho[1,2-d]thiazolin-2-ylidene)-2-methylpropenyl] naptho[1,2-d]thiazolium bromide 相似文献
16.
Peter Sackl und Georg Rauer 《Journal of Ornithology》1987,128(4):491-494
Zuammenfassung Im niederösterreichischen Waldviertel (200–700 m üNN) stieg der relative Anteil einjähriger von 17,9 auf 50% aller revierverteidigenden Hausrotschwanz- mit zunehmender Meereshöhe. Die Zunahme betrifft vor allem einjährige dercairei-Morphe, während dieparadoxus-Morphe in geringerer Anzahl und gleichförmiger als in allen Höhenbereichen auftrat.
The portion of first-year males in breeding populations of Black Redstarts (Phoenicurus ochruros) and their altitudinal distribution in the Waldviertel, Lower Austria
Summary In 1986, between April 13 and July 5, breeding territories of male Black Redstarts were mapped in Lower Austria (200–700 m NN), with regard to their age (first-year or older) and the plumage types of first-year (femalelike or adult malelike). Due to a greater percentage of femalelike subadult (cairei-type) in higher altitudes, the proportion of first-year was found to increase from 17,9 per cent at 200–300 m to 50 per cent above 600 m. On the contrary adultlike first-year (paradoxus-type) are evenly distributed in smaller numbers over the whole altitudinal range.相似文献
17.
18.
Marina Lotti Michael Noah Marina Stöffler-Meilicke Georg Stöffler 《Molecular & general genetics : MGG》1989,216(2-3):245-253
Summary Ribosomal proteins L4, L5, L20 and L25 have been localized on the surface of the 50S ribosomal subunit of Escherichia coli by immuno-electron microscopy. The two 5S RNA binding proteins L5 and L25 were both located at the central protuberance extending towards its base, at the interface side of the 50S particle. L5 was localized on the side of the central protuberance that faces the L1 protuberance, whereas L25 was localized on the side that faces the L7/L12 stalk. Proteins L4 and L20 were both located at the back of the 50S subunit; L4 was located in the vicinity of proteins L23 and L29, and protein L20 was localized between proteins L17 and L10 and is thus located below the origin of the L7/L12 stalk. 相似文献
19.
Sigrun Feldmann Georg A. Sprenger Hermann Sahm 《Applied microbiology and biotechnology》1989,31(2):152-157
Summary Grown anaerobically on d-xylose, Klebsiella planticola ATCC 33531 produced acetate, formate, lactate, CO2 and ethanol as major end-products. A Mu-insertion mutant which lacked pyruvate-formate-lyase showed among its fermentation products more than 70% d-lactate with residual acetate, 2,3-butanediol, and traces of ethanol, formate, and CO2. After the introduction of a plasmid carrying the gene for the enzyme pyruvate decarboxylase from Zymomonas mobilis, this Klebsiella mutant became an efficient ethanol producer. The recombinant strain produced 387 mM ethanol from 275 mM xylose in 80 h, about 83% of the theoretical maximal yield. Furthermore, this mutant consumed more than double the amount of xylose (41 g/l) compared to the wild-type, due to reduced production of inhibiting acids during growth.Dedicated to Professor Dr. Zähner on the occasion of his 60th birthday 相似文献
20.
Extracts of denitrifying bacteria grown anaerobically with phenol and nitrate catalyzed an isotope exchange between 14CO2 and the carboxyl group of 4-hydroxybenzoate. This exchange reaction is ascribed to a novel enzyme, phenol carboxylase, initiating the anaerobic degradation of phenol by para-carboxylation to 4-hydroxybenzoate. Some properties of this enzyme were determined by studying the isotope exchange reaction. Phenol carboxylase was rapidly inactivated by oxygen; strictly anoxic conditions were essential for preserving enzyme activity. The exchange reaction specifically was catalyzed with 4-hydroxybenzoate but not with other aromatic acids. Only the carboxyl group was exchanged; [U-14C]phenol was not exchanged with the aromatic ring of 4-hydroxybenzoate. Exchange activity depended on Mn2+ and inorganic phosphate and was not inhibited by avidin. Ortho-phosphate could not be substituted by organic phosphates nor by inorganic anions; arsenate had no effect. The pH optimum was between pH 6.5–7.0. The specific activity was 100 nmol 14CO2 exchange · min-1 · mg-1 protein. Phenol grown cells contained 4-hydroxybenzoyl CoA synthetase activity (40 nmol · min-1 · mg-1 protein). The possible role of phenol carboxylase and 4-hydroxybenzoyl CoA synthetase in anaerobic phenol metabolism is discussed. 相似文献