全文获取类型
收费全文 | 53226篇 |
免费 | 3775篇 |
国内免费 | 135篇 |
出版年
2021年 | 369篇 |
2020年 | 329篇 |
2019年 | 357篇 |
2018年 | 845篇 |
2017年 | 810篇 |
2016年 | 1248篇 |
2015年 | 1895篇 |
2014年 | 2054篇 |
2013年 | 2624篇 |
2012年 | 3241篇 |
2011年 | 3030篇 |
2010年 | 1864篇 |
2009年 | 1432篇 |
2008年 | 2530篇 |
2007年 | 2535篇 |
2006年 | 2482篇 |
2005年 | 2336篇 |
2004年 | 2277篇 |
2003年 | 2110篇 |
2002年 | 2120篇 |
2001年 | 1547篇 |
2000年 | 1642篇 |
1999年 | 1308篇 |
1998年 | 500篇 |
1997年 | 411篇 |
1996年 | 388篇 |
1995年 | 358篇 |
1994年 | 383篇 |
1993年 | 345篇 |
1992年 | 803篇 |
1991年 | 781篇 |
1990年 | 750篇 |
1989年 | 703篇 |
1988年 | 661篇 |
1987年 | 585篇 |
1986年 | 574篇 |
1985年 | 536篇 |
1984年 | 473篇 |
1983年 | 438篇 |
1982年 | 390篇 |
1981年 | 386篇 |
1979年 | 492篇 |
1978年 | 364篇 |
1977年 | 308篇 |
1975年 | 365篇 |
1974年 | 355篇 |
1973年 | 330篇 |
1972年 | 358篇 |
1971年 | 320篇 |
1968年 | 311篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
951.
Hector M. Alvarez Frank Mayer Dirk Fabritius A. Steinbüchel 《Archives of microbiology》1996,165(6):377-386
An oleaginous hydrocarbon-degrading Rhodococcus opacus strain (PD630) was isolated from a soil sample. The cells were able to grow on a variety of substrates and to produce large
amounts of three different types of intracellular inclusions during growth on alkanes, phenylalkanes, or non-hydrocarbon substrates.
Electron microscopy revealed large numbers of electron-transparent inclusions with a sphere-like structure. In addition, electron-dense
inclusions representing polyphosphate and electron-transparent inclusions with an elongated disc-shaped morphology occurred
in small amounts. The electron-transparent inclusions of alkane- or gluconate-grown cells were composed of neutral lipids
(98%, w/w), phospholipids (1.2%, w/w), and protein (0.8%, w/w). The major component of the cellular inclusions was triacylglycerols;
minor amounts of diacylglycerols and probably also some free fatty acids were also present. Free fatty acids and/or fatty
acids in acylglycerols in cells of R. opacus amounted up to 76 or 87% of the cellular dry weight in gluconate- or olive-oil-grown cells, respectively. The fatty acid composition
of the inclusions depended on the substrate used for cultivation. In cells cultivated on n-alkanes, the composition of the
fatty acids was related to the substrate, and intermediates of the β-oxidation pathway, such as hexadecanoic or pentadecanoic
acid, were among the acylglycerols. Hexadecanoic acid was also the major fatty acid (up 36% of total fatty acids) occurring
in the lipid inclusions of gluconate-grown cells. This indicated that strain PD630 utilized β-oxidation and de novo fatty
acid biosynthesis for the synthesis of storage lipids. Inclusions isolated from phenyldecane-grown cells contained mainly
the non-modified substrate and phenylalkanoic acids derived from the hydrocarbon oxidation, such as phenyldecanoic acid, phenyloctanoic
acid, and phenylhexanoic acid, and approximately 5% (w/w) of diacylglycerols. The lipid inclusions seemed to have definite
structures, probably with membranes at their surfaces, which allow them to maintain their shape, and with some associated
proteins, probably involved in the inclusion formation.
Received: 22 December 1995 / Accepted: 12 March 1996 相似文献
952.
Abstract The gonochoristic syllid Petitia amphophthalma is one of the truly interstitial polychaetes. P. amphophthalma does not show any epitokous modifications at maturity such as those that usually occur in syllids. The reproductive structures are unique: the male genital organs consist of a seminal vesicle in chaetigers 6–10, subdivided into a dorsal part tightly filled with spermatozoa and a ventral part with contents in different stages of spermatogenesis, one pair of sperm ducts and conspicuous gland cells situated in chaetigers 10 and 11. Their glandular secretions are discharged into the sperm duct together with those of other types of gland cells that form the duct. The oocytes develop freely within the body cavity of the females. Each of the fertile segments possesses a paired oviduct ending in a large ciliated funnel. Sperm ducts and oviducts are probably modifications of excretory organs; nephridia are absent in segments where gonoducts occur. A direct sperm transfer by lytic opening of the integument of the female and internal fertilization are inferred. Copyright © 1996 Published by Elsevier Science Ltd on behalf of the Royal Swedish Academy of Sciences 相似文献
953.
U Dammer M Hegner D Anselmetti P Wagner M Dreier W Huber H J Güntherodt 《Biophysical journal》1996,70(5):2437-2441
Molecular recognition between biotinylated bovine serum albumin and polyclonal, biotin-directed IG antibodies has been measured directly under various buffer conditions using an atomic force microscope (AFM). It was found that even highly structured molecules such as IgG antibodies preserve their specific affinity to their antigens when probed with an AFM in the force mode. We could measure the rupture force between individual antibody-antigen complexes. The potential and limitations of this new approach for the measurement of individual antigen/antibody interactions and some possible applications are discussed. 相似文献
954.
Mouse myeloma cells were transfected with pSV2-gpt and pSV2-neo based immunoglobulin expression vectors. Double transfectants were selected using the xanthine-guanine phosphoribosyl transferase (gpt) and the neomycin (neo) selection marker genes. A broad distribution in the level of mouse-human chimeric IgG expression was observed with series of independently isolated transfectoma clones. The relative amounts of secreted to membrane-bound antibodies correlated closely, which suggested, that fluorescence-activated cell sorting could be a valuable tool for the selection of high-yielding production cell lines. However, a single cycle of cell sorting did not steer the cloning process significantly toward cells that produce enhanced amounts of recombinant IgG. Only in cases in which the polyclonal transfectoma population contained a large percentage of nonproducing cells, these were successfully separated from the IgG-producing cell population. (c) 1996 John Wiley & Sons, Inc. 相似文献
955.
Detailed measurements of multiphase flows that prevail in bioreactors tell us that different transport mechanisms are dominating on different observation scales. The consequence in terms of reactor modeling is that the processes on different scales can be treated independently. A three-dimensional, dynamical model is presented that can be used to describe bubble column bioreactors on the reactor scale. It is based on the Navier-Stokes equation system. On the next smaller scale, the dynamics of the gas phase is described in a Lagrangian way, by tracking many bubble clusters or bubbles simultaneously on their way through the reactor. The model is capable of describing bubble columns of different size and can thus be used for scale-up. Its performance is demonstrated with a production-scale beer fermentor. (c) 1996 John Wiley & Sons, Inc. 相似文献
956.
Behavioral strategies of aphid hyperparasitoids to escape aggression by honeydew-collecting ants 总被引:2,自引:0,他引:2
We analyzed the behavioral interactions between two species of honeydew-collecting ants (Lasius niger, Myrmica laevinodis) and foraging females of four species of aphid hyperparasitoids (Aphidencyrtus aphidivorus, Dendrocerus carpenteri, Pachyneuron aphidis, Asaphes vulgaris) usingAphis fabae ssp.cirsiiacanthoidis andLysiphlebus cardui on thistles as aphid and primary parasitoid, respectively. The observed interaction patterns and foraging parameters varied within hyperparasitoid species and revealed different strategies based upon behavioral and morphological constraints.D. carpenteri generally tried to avoid ant encounters. This avoidance strategy was successful in interactions withM. laevinodis but failed when encountering the more aggressiveL. niger, which caused about 26% adult mortality. In contrast,A. aphidivorus, P. aphidis, andA. vulgaris possess jumping ability and were hardly exposed to mortality risks. The escape reaction jump off was used as soon as ants made physical contact with foraging females. While the flight strategy ofP. aphidis is connected with cryptic movement patterns without avoidance behavior,A. aphidivorus first avoided ants and jumped off only as a last resort. Similar patterns, but less expressive, are displayed byA. vulgaris. We suggest that these different strategies are responsible for different foraging success in ant-attended resources in field. 相似文献
957.
This note reports a microalgal test system which usesin-vivo chlorophyll a-fluorescence in combination with a solid phase extraction procedure to monitor photosystem II-herbicides in natural waters up to 0.05 g L–1. 相似文献
958.
959.
Qagatay Günes Detlev Staacke Bricitte von Wilcken-Beramann Benno Müller-Hill 《Molecular genetics and genomics : MGG》1995,246(2):180-195
We constructed mutants of the Trp repressor from Escherichia coli K-12 with all possible single amino acid exchanges at positions 79 and 80 (residues 1 and 2 of the recognition helix). We tested these mutants in vivo by measuring the repression of synthesis of β-galactosidase with symmetric variants of α- and β-centered trp operators, which replace the lac operator in a synthetic lac system. The Trp repressor carrying a substitution of isoleucine 79 by lysine, showed a marked specificity change with respect to base pair 7 of the α-centered trp operator. Gel retardation experiments confirmed this result. Trp repressor mutant IR79 specifically recognizes a trp operator variant with substitutions in positions 7 and 8. Another mutant, with glycine in position 79, exhibited loss of contact at base pair 7. We speculate that the side chain of Ile79 interacts with the AT base pairs 7 and 8 of the α-centered trp operator, possibly with the methyl groups of thymines. Replacement of thymine in position 7 or 8 by uracil confirms the involvement of the methyl group of thymine 8 in repressor binding. Several Trp repressor mutants in position 80 (i.e. AI80, AL80, AM80 and AP80) broaden the specificity of the Trp repressor for α-centered trp operator variants with exchanges in positions 3, 4 and 5. 相似文献