全文获取类型
收费全文 | 6836篇 |
免费 | 649篇 |
国内免费 | 5篇 |
出版年
2023年 | 27篇 |
2022年 | 55篇 |
2021年 | 124篇 |
2020年 | 73篇 |
2019年 | 117篇 |
2018年 | 100篇 |
2017年 | 110篇 |
2016年 | 174篇 |
2015年 | 333篇 |
2014年 | 342篇 |
2013年 | 365篇 |
2012年 | 505篇 |
2011年 | 470篇 |
2010年 | 289篇 |
2009年 | 290篇 |
2008年 | 400篇 |
2007年 | 418篇 |
2006年 | 391篇 |
2005年 | 399篇 |
2004年 | 360篇 |
2003年 | 325篇 |
2002年 | 321篇 |
2001年 | 89篇 |
2000年 | 59篇 |
1999年 | 86篇 |
1998年 | 82篇 |
1997年 | 65篇 |
1996年 | 48篇 |
1995年 | 65篇 |
1994年 | 52篇 |
1993年 | 50篇 |
1992年 | 51篇 |
1991年 | 41篇 |
1990年 | 56篇 |
1989年 | 40篇 |
1988年 | 34篇 |
1987年 | 55篇 |
1986年 | 29篇 |
1985年 | 42篇 |
1984年 | 42篇 |
1983年 | 34篇 |
1982年 | 44篇 |
1981年 | 39篇 |
1980年 | 36篇 |
1979年 | 33篇 |
1978年 | 31篇 |
1976年 | 24篇 |
1974年 | 29篇 |
1973年 | 33篇 |
1972年 | 23篇 |
排序方式: 共有7490条查询结果,搜索用时 62 毫秒
51.
52.
S E Hill J Buffey A J Thody I Oliver S S Bleehen S Mac Neil 《Pigment cell research / sponsored by the European Society for Pigment Cell Research and the International Pigment Cell Society》1989,2(3):161-166
In vitro melanocyte-stimulating hormone (MSH) stimulates melanogenesis in some, but not all, melanocytes and melanoma cells. In an attempt to explain this variation in response to alpha MSH, we examined cyclic adenosine monophosphate (cAMP) accumulation, tyrosinase activity, and melanin production in primary (1 degree) murine B16 melanoma cells and in two B16 cell lines (B16 F1 and B16 F10) that are known to respond to alpha MSH. In vivo all three B16 melanoma cell types produced pigmented tumours. In vitro alpha MSH increased tyrosinase activity and melanin content in the F1 and F10 cells but not in the B16 1 degree cells. alpha MSH, however, increased cAMP production in all three cell types, confirming that the inability of B16 1 degree cells to produce melanin in response to alpha MSH is not due to a lack of alpha MSH receptors or cAMP response to alpha MSH. Further, we present evidence for a separate pathway of melanogenesis that is independent of cAMP as calmodulin antagonists, which do not elevate cAMP, increased tyrosinase activity, and melanin production in both 1 degree and F1 cells. 相似文献
53.
Kyungsook C. Kim Anthony H. Caswell J. -P. Brunschwig Neil R. Brandt 《The Journal of membrane biology》1990,113(3):221-235
Summary It has been previously recognized that a number of protocols may cause breakage of the triad junction and separation of the constituent organelles of skeletal muscle. We now describe a fraction of triad junctions which is refractory to the known protocols for disruption. Triads were passed through a French press and the dissociated organelles were separated on a sucrose density gradient, which was assayed for PN200-110, ouabain and ryanodine binding. Ryanodine binding showed a single peak at the density of heavy terminal cisternae. On the other hand, the PN200-110 and ouabain, which are external membrane ligands, bound in two peaks: one at the free transverse tubule region and the other at the light terminal cisternae. Similarly, a two peak pattern of PN200-110 and ouabain binding was observed when triad junctions were broken by the Ca2+-dependent protease, calpain, which selectively hydrolyzes the junctional foot protein. The light terminal cisternae vesicles were subjected to three different procedures of junctional breakage: French press, hypertonic salt treatment, and protease digestion using calpain or trypsin. The treated membranes were then centrifuged on density gradients. Only extensive trypsin digestion caused a partial shift of ouabain activity into the free transverse tubule region. These observations suggest that the triads are a composite mixture of breakage susceptible, weak, and breakage resistant, strong, triads. Scatchard analysis of PN200-110 suggests that the transverse tubules of strong triads contain a relatively high number of dihydropyridine receptors compared to those of weak triads. Thin section electron microscopic images of the strong triads comparable to those of intact muscle are presented. 相似文献
54.
55.
56.
The effects of light-induced non-photochemical quenching on the minimal Fo, and variable Fv, fluorescence emissions at 690 and 730 nm in leaves were determined. Non-photochemical quenching of Fo, but not Fv, was found to be dependent upon the wavelength of emission, and was greater at 690 nm than at 730 nm. For emission at 730, compared to at 690 nm, approx. 30% of Fo was not affected by non-photochemical quenching processes in leaves of C3 plants; in maize leaves this was found to be approx. 50%. The data indicate that a substantial proportion of the pigments contributing to Fo emission at 730 nm are not quenched by light-induced, non-photochemical quenching processes and that there are large differences in the pigment matrices contributing to Fo and Fv emissions at 730 nm, compared to those at 690 nm. These findings have important implications for the accurate estimation and interpretation of non-photochemical quenching of fluorescence parameters and their use in the calculation of photochemical efficiencies in leaves. Measurements of fluorescence emissions at wavelengths above 700 nm are likely to give rise to significant errors when used for determinations of photochemical and non-photochemical quenching parameters. 相似文献
57.
Harry Adams Neil A. Bailey Philippe Bertrand Simon R. Collinson David E. Fenton Simon J. Kitchen 《Inorganica chimica acta》1996,250(1-2):139-148
The cyclocondensation of 2,5-diformylthiophene and the amines N,N-bis-(2-aminoethyl)-2-phenylethylamine, N,N-bis-(2aminoethyl)-t-butyl-amine and N,N-bis-(2-aminoethyl)-t-butyl-amine in the presence of silver(I) salts yields homodinuclear bibracchial tetraimine Schiff base macrocyclic complexes. The structures of two such complexes are also reported. The complex Ag2L4(NO3)(PF6) (2) crystallises in the triclinic space group
, No. 2) and has unit-cell dimensions a = 12.834(6), B = 13.183(6), C = 14.588(7) Å, = 64.86(4), β = 79.77(4), γ = 69.44(3)° with Z = 2; there is a monodentate and singly bridging nitrate anion present and the Ag---Ag separation is 4.161 Å. The complex Ag2L4(CH3CN)2(BF4)2·CH3CN (9) crystallises in the triclinic space group
, No. 2) and has unit-cell dimensions a = 9.297(4), B = 12.985(3), C = 21.770(5) Å, = 91.570(10), β = 92.33(3), γ = 97.92(3) ° with Z = 2; there is a strongly bonded acetonitrile molecule coordinated to each silver atom and the Ag---Ag separation is 4.920 Å. 相似文献
58.
An electrophoretic analysis using 17 enzyme loci was carried out on specimens of the gastric nematode of macropodid marsupials, Rugopharynx australis (Mönnig, 1926), collected from Macropus eugenii (Desmarest), M. fuliginosus (Desmarest), M. giganteus Shaw, M. robustus Gould, M. rufogriseus (Desmarest), M. rufus (Desmarest), Thylogale billardierii (Desmarest) and Wallabia bicolor (Desmarest) from south-eastern Australia. The extent of fixed genetic differences between nematodes from different host species ranged from 0–53%. The two distinct morphological forms of the parasite found in M. rufogriseus differed at 50% of loci. Specimens present in M. fuliginosus and M. giganteus were indistinguishable genetically, as were nematodes from M. rufus and M. robustus. Of the two morphologically distinct congeners included in the analysis as controls, Rugopharynx epsilon (Johnston & Mawson, 1939) was genetically distinct (46–69% fixed genetic differences) from all specimens of the R. australis complex while R. rufogrisea Magzoub, 1964 was closely related to one of the two species occuring in M. rufogriseus. It was concluded that R. australis is a species complex, with a genetically distinct species present in M. eugenii, M. fuliginosus/M. giganteus, M. robustus/M. rufus, W. bicolor and T. billardierii, and two species in M. rufogriseus. 相似文献
59.
Multiple phenotypes associated with Myc-induced transformation of chick embryo fibroblasts can be dissociated by a basic region mutation. 下载免费PDF全文
Chimaeric alleles were constructed to assay the biological functions of an N-terminal deletion and C-terminal mutations which were found in a naturally occurring mutant of feline vMyc, T17. The mutant alleles were assayed for their ability to transform chick embryo fibroblasts in vitro by a number of criteria, namely the ability to induce morphological transformation, an accelerated growth rate and growth in soft agar. Feline cMyc could transform the avian cells, whilst T17 vMyc could not, and the N-terminal deletion was responsible for conferring the primary transformation defect on the mutant protein. The C-terminal mutations which consist of a point mutation adjacent to the nuclear localisation signal and a point mutation/amino acid insertion within the basic region (BR) could, however, dissociate the Myc-induced parameters of transformation. This effect was a specific function of the BR mutation alone, and the mutation could be transferred into avian cMyc with comparable biological consequences. The BR mutation did not disrupt the sequence specific DNA binding activity of the protein in vivo, despite exerting a biological effect. These data suggest a novel phenotype where the mutation may affect a subset of Myc-regulated genes through altered DNA binding specificity or protein-protein interactions. 相似文献
60.