全文获取类型
收费全文 | 95575篇 |
免费 | 4474篇 |
国内免费 | 16篇 |
专业分类
100065篇 |
出版年
2024年 | 265篇 |
2023年 | 326篇 |
2022年 | 337篇 |
2021年 | 990篇 |
2020年 | 2865篇 |
2019年 | 4445篇 |
2018年 | 4750篇 |
2017年 | 4994篇 |
2016年 | 5275篇 |
2015年 | 5282篇 |
2014年 | 5161篇 |
2013年 | 6500篇 |
2012年 | 5587篇 |
2011年 | 5654篇 |
2010年 | 5213篇 |
2009年 | 3327篇 |
2008年 | 4120篇 |
2007年 | 3759篇 |
2006年 | 3538篇 |
2005年 | 3285篇 |
2004年 | 3083篇 |
2003年 | 2885篇 |
2002年 | 2877篇 |
2001年 | 1912篇 |
2000年 | 2056篇 |
1999年 | 1113篇 |
1998年 | 535篇 |
1997年 | 412篇 |
1996年 | 465篇 |
1995年 | 389篇 |
1994年 | 434篇 |
1993年 | 380篇 |
1992年 | 403篇 |
1991年 | 335篇 |
1990年 | 307篇 |
1989年 | 282篇 |
1988年 | 270篇 |
1987年 | 280篇 |
1985年 | 322篇 |
1984年 | 394篇 |
1983年 | 341篇 |
1982年 | 331篇 |
1981年 | 317篇 |
1980年 | 284篇 |
1979年 | 286篇 |
1978年 | 281篇 |
1977年 | 259篇 |
1976年 | 249篇 |
1975年 | 296篇 |
1974年 | 235篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Wolfgang Witt Peter Hampel Klaus Böcker Angelika Mertsching 《Archives of microbiology》1989,151(2):154-158
Divalent cations activate the lysophospholipase and transacylase reactions catalyzed by the same enzymes in the yeast Saccharomyces cerevisiae. The activation was observed at neutral pH, but not at the pH optimum of lysophospholipase/transacylase, near 3.5. Adenine nucleotides, especially AMP and ADP, are strong inhibitors of the same group of enzymes. Half maximal inhibition by AMP was found at a concentration of about 20 M. The inhibition by nucleotides in low concentrations is enhanced by divalent cations. 相似文献
3.
4.
Octopamine receptor subclasses were first proposed to explain differences in the pharmacological profiles of a range of physiological responses to octopamine obtained in the extensor-tibiae neuromuscular preparation of the locust. Thus, OCTOPAMINE1 receptors which inhibit an endogenous myogenic rhythm, increase intracellular calcium levels. Also OCTOPAMINE2 receptors which modulate neuromuscular transmission in this preparation, increase the level of adenylate cyclase activity. The current status of this classification is reviewed by examining the pharmacology of responses to octopamine in a range of preparations. It is concluded that the distinction between OCTOPAMINE1 and OCTOPAMINE2 receptor types is still valid, but that OCTOPAMINE2 receptors exhibit some tissue specific variations. Studies on a clonedDrosophila octopamine/tyramine (phenolamine) receptor are discussed and illustrate many of the difficulties presently encountered in making a definitive classification of octopamine receptors. These include the possibilities that single receptors may activate multiple second messenger systems and that different agonists may differentially couple the same receptor to different second messenger systems. 相似文献
5.
Summary Versatile three-step procedures for syntheses of seven racemi-fluoro-a-amino acids are described. Alkylation oftert-butyl N-(diphenylmethylene) glycinate with 1-bromo-2-fluoroalkanes gave N-protected aminoacid esters both in anhydrous medium using lithium-diisopropylamide as base at low temperature or in a two phase system of 50% aqueous sodium hydroxide and methylene chloride with triethylbenzylammonium chloride as the phase transfer catalyst at room temperature. Subsequent two-step deprotection with citric acid and hydrochloric acid gave the title compounds in 13–33% overall yields.Dedicated to Professor Dr.mult., Dr.h.c. Alois Haas on the occasion of his 65th birthday 相似文献
6.
Distribution and properties of pea (Pisum sativum L.) cells, competent forAgrobacterium-mediated transformation were analysed byin situ histochemical detection of GUS (-glucuronidase) activity, 4 d after inoculation with engineeredAgrobacterium tumefaciens. The vector system consisted of the hypervirulent disarmed strain EHA101 and the binary plasmid pIBGUS, carrying an intron-containing, 35S-promotor drivengusA (oruidA) gene and two selectable marker genes. Cells competent for transformation were mainly restricted to the dedifferentiating cells neighbouring the vascular system of cotyledon and epicotyl explants. A standardized assay was developed, allowing determination and quantification of factors influencing number and distribution of competent cells. In etiolated seedlings, competence for transformation decreased with the distance of the epicotyl explant from the shoot apex and was specifically induced by the exogenous application of auxins. Transient expression ofgusA afterAgrobacterium-mediated DNA transfer was dramatically reduced upon application of cell-cycle and DNA replication inhibitors aphidicolin, colchicine and nalidixic acid. GUS expression after direct DNA transfer of double-stranded plasmid DNA (via PEG into protoplasts or via particle bombardment of epicotyl segments) was independent of cell-division/DNA replication.A GUS-positive mutant of EHA101 was constructed to allowin situ analysis of attaching bacteria within the plant tissue. Attachment and invasion was inhibited by well-developed cuticula but was restored after chloroform treatment of the tissue surface. Moreover, no correlation was found between distribution of attaching bacteria and the pattern of transformation-competent cells. 相似文献
7.
Arthopyrenia endobrya from Southern Brazil is illustrated and described as a new species of lichens. The thallus is composed of filamentous green algae loosely surrounded by fungal hyphae. Both symbionts grow endophytically within the leaf cells of two species ofLejeuneaceae (Hepaticae). The algae and hyphae penetrate the cell walls of the host by means of fine perforations. The ascocarps develop between the leaves and perforate them with their apical region. The classification as a member of the genusArthopyrenia is preliminary. 相似文献
8.
Jörgen Jensen Susanne Holmgren 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(2):184-188
Summary The release of substance P-like immunoreactive material (SPLI) from the vascularly perfused stomach of the rainbow trout, Oncorhynchus mykiss, was studied. In most cases, SPLI was detected in the collected vascular perfusate during experimental resting conditions. Distensions of the stomach, accomplished by a water-filled intragastric balloon, produced an initial rapid relaxation of the stomach, followed by a slow further relaxation and a stimulation of contractile activity. The amount of SPLI in the vascular perfusate was significantly elevated during the distension period. Tetrodotoxin had no effect on the response to distension or on the release of SPLI during distension, indicating release from tetrodotoxin-insensitive neurons or endocrine cells. The results suggest that a substance P-like peptide may be involved in the contractile response and/or in the maintenance of muscular tone during gastric distensions in the rainbow trout. Infusion of capsaicin had no effect on the release of SPLI. However, capsaicin caused an increase in vascular flow, an effect that could be repeated on a second infusion of capsaicin, indicating that the action may not be specific to sensory neurons.Abbreviations 5-HT
5-Hydroxytryptamine
- RIA
radioimmunoassay
- SP
substance P
- SPLI
substance P-like immunoreactive material
- TTX
tetrodotoxin 相似文献
9.
Summary Ectomycorrhizae were found in root samples of the treesAfzelia quanzensis Welw. andBrachystegia spiciformis Benth. (Caesalpiniaceae), collected in the coastal miombo type woodland 50 km west of Dar-es-Salaam, Tanzania. Root nodules
with a structure resembling that of nitrogen-fixing root nodules of other leguminous plants were observed in theA. quanzensis material. The climate of the locality is rather dry, and strongly seasonal. In the tropics, ectomycorrhizae have previously
been found only in humid or rain forest climate zones. 相似文献
10.