全文获取类型
收费全文 | 10485篇 |
免费 | 544篇 |
国内免费 | 2篇 |
出版年
2023年 | 49篇 |
2022年 | 35篇 |
2021年 | 103篇 |
2020年 | 115篇 |
2019年 | 147篇 |
2018年 | 343篇 |
2017年 | 318篇 |
2016年 | 414篇 |
2015年 | 351篇 |
2014年 | 491篇 |
2013年 | 707篇 |
2012年 | 619篇 |
2011年 | 780篇 |
2010年 | 460篇 |
2009年 | 307篇 |
2008年 | 519篇 |
2007年 | 556篇 |
2006年 | 582篇 |
2005年 | 510篇 |
2004年 | 438篇 |
2003年 | 488篇 |
2002年 | 387篇 |
2001年 | 284篇 |
2000年 | 250篇 |
1999年 | 190篇 |
1998年 | 94篇 |
1997年 | 36篇 |
1996年 | 43篇 |
1995年 | 46篇 |
1994年 | 35篇 |
1993年 | 36篇 |
1992年 | 65篇 |
1991年 | 64篇 |
1990年 | 68篇 |
1989年 | 50篇 |
1988年 | 57篇 |
1987年 | 55篇 |
1986年 | 49篇 |
1985年 | 54篇 |
1984年 | 54篇 |
1983年 | 38篇 |
1982年 | 43篇 |
1979年 | 47篇 |
1978年 | 41篇 |
1977年 | 34篇 |
1974年 | 31篇 |
1972年 | 28篇 |
1969年 | 30篇 |
1968年 | 32篇 |
1952年 | 41篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
31.
We measured reaction times (RTs) for identification of a target among distracters under stabilized image conditions in which the positions of the target and the distracters were constant within a single experimental session. Under these conditions, the observer need not search for the target because its position is known. We nevertheless found that the presence of even a single distracter could elevate RTs. The magnitude of this effect depended on the distance of the distracter from the target and, for some observers, the distance of the distracter from the fovea. When we added not one but six background elements in a ring around the target, RT increased even more. If, apart from these neighboring distracters, the target was surrounded by more distracters located beyond the nearest neighbors, RT was, in general, not increased further. These findings suggest that adding background elements in a search task can elevate RTs in ways that are not dependent on the positional uncertainty of the target. 相似文献
32.
Christiane Wloczyk Achim Kröger Thomas Göbel Gabriele Holdt Ralf Steudel 《Archives of microbiology》1989,152(6):600-605
Wolinella succinogenes grown on formate and elemental sulphur was found to use the polysulphide derivatives 2,2-tetrathiobispropionate (R2S4) or pentathionate (S5O
6
=
) as acceptors for formate oxidation. The specific activities of formate oxidation with these acceptors were similar to those with elemental sulphur. The main reaction products of R2S4 reduction were 2,2-dithiobispropionate (R2S2) and sulphide. Pentathionate was converted to thiosulphate and some elemental sulphur. The electrochemical proton potential
across the cytoplasmic membrane of the bacterium was measured in the steady state of electron transport from formate to R2S4. The electrical proportion () of the
determined through the distribution of labeled tetraphenylphosphonium cation was obtained as 0.17 Volt. The was zero, when a protonophore was present. The pH-difference across the membrane was negligible. Thus the
generated by sulphur respiration is close to that measured earlier with fumarate as the terminal acceptor of electron transport.Abbreviations DMO
5,5-dimethyloxazolidine-2,4-dione
- R2Sn (n=2–5)
2,2-polythiobispropionate
- TTFB
4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazol
- TPP
tetraphenylphosphonium cation 相似文献
33.
Recovery of rat mast cells after secretion: a morphometric study 总被引:1,自引:0,他引:1
Granule reconstitution in rat peritoneal mast cells following massive secretion was studied by morphometric techniques. Immediately following secretion, the earliest identifiable mast cells showed a substantial decrease in cell volume associated with granule loss. Cell volume then increased almost to the original level over a period of a month. The size of the Golgi apparatus increased markedly in the week following secretion and then returned to its original size. The total volume of granules increased slowly after the secretory depletion and by 34 days had not returned to the original value although the number of granules had recovered fully. The reconstitution of mast cells after secretion is a prolonged process with several phases resulting in mast cells of varying appearance and content. This heterogeneity generated by reconstitution post secretion must be considered in studies of populations of mast cells in vivo. 相似文献
34.
K Hubrich-Kühner H J Buhk H Wagner H Kr?ger D Simon 《Biochemical and biophysical research communications》1989,160(3):1175-1182
The eukaryotic DNA cytosine-5-methyltransferase (E.C.2.1.1.37) is known to methylate cytosine in DNA mainly, but not exclusively in C-G. In the present study the minor, non-C-G recognition sequences of a rat DNA methyltransferase were analyzed by Maxam-Gilbert sequencing of in vitro methylated SV40 DNA. The enzyme methylates C-A and C-T at a 50-fold lower initial rate than C-G. Methylation of C-C at the 5'C was not observed in the piece of DNA sequenced. The methylation of C-A is very low in the trinucleotides ACA and CAC, the other C-A containing trinucleotides in DNA are much better methylacceptors. C-T was found methylated predominantly in the sequences CCTAA, ACTAA, and ACTGT. A comparison of the activity with different substrates is in favour of the enzyme making its recognition in the major groove of the DNA. 相似文献
35.
Helmut Krüger Werner Schr?der Klaus Buchner Ferdinand Hucho 《Journal of Protein Chemistry》1990,9(4):467-473
Inhibition of protein kinase C (PKC) by calmodulin is investigated and we describe the localization of inhibitory sequences within the calmodulin molecule. We present evidence that calmodulin inhibits PKC through an inhibition of the activation of PKC associated with lipid membranes: Binding of PKC to lipid vesicles is not affected, but activation is abolished. The potent calmodulin antagonist R24571 (calmidazol) did not affect the inhibition of PKC by calmodulin at concentrations up to 10–5 M. Two tryptic fragments of calmodulin were isolated which inhibited PKC. They were only slightly less potent than intact calmodulin with an IC50 of 6 µ M compared to 1 µ M of intact calmodulin. They were identified as Ser38-Arg74 and His107-Lys148. Each of the inhibiting fragments contains an intact Ca2+-binding domain with complete helix-loop-helix structure (EF hand). Other calmodulin peptides showed only weak inhibitory activity. Both fragments did not stimulate cAMP phosphodiesterase even at concentrations 100-fold higher than the calmodulin concentration needed for maximal stimulation. None of the fragments acted as a calmodulin antagonist. 相似文献
36.
The intrinsic insensitivity of EcoRII recognition sites in RF DNAs of phage M13 and vector M13mp18 towards this restriction endonuclease can be overcome by adding site-specific oligodeoxyribonucleotide duplexes to the restriction sample. Since Dcm- DNA but not Dcm(+)-methylated DNA becomes susceptible under these conditions, this procedure constitutes an improvement of the Dcm methylation assay. 相似文献
37.
38.
In a 3:1 scaled model of the human aorta models of the Omniscience, Bj?rk-Shiley Convexo-Concave, Bj?rk-Shiley Monostrut, Medtronic-Hall, Duromedics (Hemex) and the Saint Jude Medical heart valve prostheses are studied in steady flow representing the systolic peak flow phase. Detailed flow visualization experiments show flow separations at all inner ring surfaces as well as at most of the occluders. The resulting stagnation areas increase the risk of thrombus accumulation. Flow separations also stimulate vortex formation and turbulent mixing at the downstream jet boundaries and thus may intensify blood damage by turbulent shear stresses. The different influences of struts and occluder guides on the flow around the occluders are discussed. The effects of the individual valve components on the flow fields are analyzed and correlated with the resulting pressure losses. 相似文献
39.
Piotr Borsuk Marek Gniadkowski Ewa Bartnik Piotr P. Stępień 《Journal of molecular evolution》1988,28(1-2):125-130
Summary AllAspergillus nidulans 5S rRNA pseudogenes known so far are the result of integration of an approx. 0.2-kbp-long DNA sequence into the 5S rRNA genes. This sequence, called block C, is present in at least five copies in theA. nidulans genome and seems to be associated either with 5S rRNA genes or pseudogenes. In contrast to the 78% sequence conservation of the C-block in pseudogenes, the truncated 5 halves of the pseudogenes are very highly conserved (96.9–100%). We postulate that the 5S rRNA pseudogenes are still a subject of concerted evolution. The C-block sequence shows similarity to the switch region of the mouse heavy chain immunoglobulin gene. A characteristic motif GGGTGAG is repeated several times in both sequences; the sequence conservation is 63%. 相似文献
40.
Characterization of granzymes A and B isolated from granules of cloned human cytotoxic T lymphocytes 总被引:14,自引:0,他引:14
O Kr?henbühl C Rey D Jenne A Lanzavecchia P Groscurth S Carrel J Tschopp 《Journal of immunology (Baltimore, Md. : 1950)》1988,141(10):3471-3477
A human CD8+ CTL clone with cytolytic potential was shown to express two serine proteases, a 50-kDa homodimer and a 27-kDa monomer, which were purified from cytoplasmic granules. N-terminal sequencing of the purified proteins revealed that the 50-kDa homodimer is the gene product of the human Hanukah factor cDNA clone and that it represents the human homologue to granzyme A. Similarly, the 27-kDa protein was shown to be the serine esterase encoded by the human lymphocyte protease cDNA clone and corresponds to granzyme B. There was no evidence for the presence of other granzymes, in particular for the human homologues to murine granzymes C, D, E, and F. The substrate best cleaved by granzyme A was Gly-Pro-Arg-amido-4-methyl-coumarin after the Arg residue and the pH optimum was 8 to 8.5. Upon triggering of the TCR-CD3 complex with an anti-CD3 mAb, granzyme A was released into the culture medium. Furthermore, a granule-associated hemolytic activity was detected after salt extraction and partial purification of granule proteins. This suggests that hemolytically active human perforin can be obtained from inactive granules. 相似文献