全文获取类型
收费全文 | 4560篇 |
免费 | 342篇 |
国内免费 | 1篇 |
出版年
2023年 | 24篇 |
2022年 | 41篇 |
2021年 | 106篇 |
2020年 | 84篇 |
2019年 | 82篇 |
2018年 | 122篇 |
2017年 | 66篇 |
2016年 | 146篇 |
2015年 | 240篇 |
2014年 | 268篇 |
2013年 | 326篇 |
2012年 | 433篇 |
2011年 | 339篇 |
2010年 | 226篇 |
2009年 | 238篇 |
2008年 | 255篇 |
2007年 | 228篇 |
2006年 | 252篇 |
2005年 | 221篇 |
2004年 | 220篇 |
2003年 | 198篇 |
2002年 | 194篇 |
2001年 | 63篇 |
2000年 | 48篇 |
1999年 | 44篇 |
1998年 | 58篇 |
1997年 | 51篇 |
1996年 | 43篇 |
1995年 | 39篇 |
1994年 | 24篇 |
1993年 | 28篇 |
1992年 | 29篇 |
1991年 | 31篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 13篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 12篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1972年 | 4篇 |
1971年 | 4篇 |
1970年 | 2篇 |
1967年 | 2篇 |
排序方式: 共有4903条查询结果,搜索用时 734 毫秒
61.
Bledius (Elbidus) bicornis (Germ.) and B. (Eucerotobledius) furcatus (Oliv.) (Coleoptera: Staphylinidae) are the most important burrowing species in the emergent areas and shores in the athalassic
lake of Fuente de Piedra (Málaga, S. of Spain).
A first estimate of the importance of these organisms in this system is presented. These insects kick out sediment during
their burrowing activity, which accumulates on the surface near the burrows as tumuli which can be easily eroded. The lake
perimeter (17 km) is densely colonized (usual densities from 1700 to 2500 ind m−2). The amount of granulated material that can be potentially kicked out was 46.22 g dry wt m−2 day−1. At the same time, the material that constitutes the tumuli shows different characteristics from the compact ground below
the surface. Thus, it is relatively enriched with organic matter (6.15 g per square meter), soluble phosphate (406.5 μg m−2) and ammonium (4856 μg m−2), whereas it lacks nitrate. Results of a transect from uninhabited areas to zones of maximum population density also show
a similarity between the higher ground level of ammonium and phosphate concentrations and population density. 相似文献
62.
I Neuman A R Solano C Paz P Mele F Cornejo Maciel J R Lemos H N Fernandez E J Podesta 《The Journal of steroid biochemistry and molecular biology》1991,40(1-3):441-451
Luteinizing hormone (LH) and human chorionic gonadotrophin (hCG) receptors are coupled to intracellular effector systems, most notably adenylate cyclase, through guanyl nucleotide-binding proteins or G-proteins. The molecular mechanism involved in the dynamic coupling of the LH/hCG receptor however, are not known. It has been postulated that receptor aggregation at the molecular level plays a critical role in this process. There have been attempts to understand the receptor association and dissociation phenomena at the molecular level. One of them involves the participation of the major histocompatibility complex (MHC) class I antigen in the mechanism of receptor activation and/or expression. One molecular basis for these mechanisms consists of a physical interaction between MHC proteins and receptors to form "compound receptors" able to transfer a hormonal signal to the cell. Using a photo-reactive probe we demonstrated that the LH/hCG receptors and the class I antigens are closely associated in the membrane. Thus, it is possible to form covalent complexes of hCG and class I antigens through the binding of the hormone to specific receptors. These findings imply that LH/hCG receptors and the MHC class I antigens may interact at the level of the plasma membrane in the mechanism of LH action. We also performed experiments using a single cell and limiting stimulation to a patch of membrane. The results stimulating the cell in a localized area suggested that even if all components are entirely free to float there is a constraint in the localization of the receptor, G-protein, and/or the effector, supporting the constraint dissociation model. Within a limited area subunits could dissociate, but they would not be free to diffuse throughout the membrane. Moreover the concept of compartmentalization that has been utilized to explain some inconsistencies in second-messenger action now can be proved by experimental design. 相似文献
63.
Cell carbon and nitrogen in D. viridis are strongly dependent on the culturing conditions. Both elements increase with increasing salinity. At 31°C cell carbon is maximum and cell nitrogen minimum. This temperature was described previously (Jiménez, C., Niell, F. X. & Fernandez, J. A. (1990). Hydrobiologia, 197, 165-72) as the optimal one for achieving the maximum oxygen evolution. These results point out a possible competence for the reducing power during carbon and nitrogen assimilation processes, and under conditions of high photosynthesis (carbon assimilation) there is a partial inhibition of nitrate reduction, making C:N ratio maximum under conditions of maximum net photosynthesis.The study of cell glycerol, nitrate, structural proteins and free amino acids indicates that all of these solutes accumulate in the cells as a result of the high salinity adaptation. 相似文献
64.
J Cornejo S I Beale M J Terry J C Lagarias 《The Journal of biological chemistry》1992,267(21):14790-14798
The unicellular rhodophyte, Porphyridium cruentum, and the filamentous cyanobacterium, Calothrix sp. PCC 7601, contain phycobiliproteins that have covalently bound phycobilin chromophores. Overnight incubation of solvent-extracted cells at 40 degrees C with methanol liberates free phycobilins that are derived from the protein-bound bilins by methanolytic cleavage of the thioether linkages between bilin and apoprotein. Two of the free bilins were identified as 3(E)-phycocyanobilin and 3(E)-phycoerythrombilin by comparative spectrophotometry and high pressure liquid chromatography. Methanolysis also yields a third bilin free acid whose absorption and 1H NMR spectra support the assignment of the 3(E)-phytochromobilin structure. This novel bilin is the major pigment isolated from cells that are pre-extracted with acetone-containing solvents. Since phytochrome- or phytochromobilin-containing proteins are not present in either organism, the 3(E)-phytochromobilin must arise by oxidation of phycobilin chromophores. This pigment is not obtained by similar treatment of a cyanobacterium and a rhodophyte that lack phycoerythrin. Therefore, 3(E)-phytochromobilin appears to be derived from phycoerythrobilin-containing proteins. Comparative CD spectroscopy of 3(E)-phytochrombilin and 3(E)-phycocyanobilin suggests that the two bilins share the R stereochemistry at the 2-position in the reduced pyrrole ring. Incubation of 2(R),3(E)-phytochromobilin with recombinant oat apophytochrome yields a covalent bilin adduct that is photoactive and spectrally indistinguishable from native oat phytochrome isolated from etiolated seedlings. These results establish that the phycobiliprotein-derived 2(R),3(E)-phytochromobilin is a biologically active phytochrome chromophore precursor. 相似文献
65.
1. Ferricytochrome c3 from D. gigas exhibits two low-spin ferric heme EPR resonances with gz-values at 2.959 and 2.853. Ferrocytochrome c3 is diamagnetic based on the absence of any EPR signals. 2. EPR potentiometric titrations result in the resolution of the two low-spin ferric heme resonances into two additional heme components representing in total the four hemes of the cytochrome, with EM values of -235 mV and -315 mV at heme resonance I and EM values of -235 mV and -306 mV at heme resonance II. 3. EPR spectroscopy has detected a significant diminution of intensity (approx. 60 p. 100) in the gx amplitude of ferricytochrome c3 in the presence of D. gigas ferredoxin II. The presence of ferredoxin II also causes a more negative shift in the EM of the second components of the signals at heme resonances I and II of cytochrome C3. Both observations suggest that an interaction has occurred between cytochrome C3 and ferredoxin II. 4. The results presented suggest that the heme ligand environment of ferricytochrome c3 from D. gigas is less perturbed and/or less asymmetric than environment for ferricytochrome c3 from D. vulgaris whose EPR behavior indicates the non-equivalence of all four hemes. 相似文献
66.
67.
68.
Mireille Bruschi E.Claude Hatchikian Jean Le Gall JoséJ.G. Moura António V. Xavier 《BBA》1976,449(2):275-284
Three forms of ferredoxin FdI, FdI′, and FdII have been isolated from Desulfovibrio gigas, a sulfate reducer. They are separated by a combination of DEAE-cellulose and gel filtration chromatographic procedures. FdI and FdI′ present a slight difference in isoelectric point which enables the separation of the two forms over DEAE-cellulose, while FdII is easily separated from the two other forms by gel filtration. The three forms have the same amino acid composition and are isolated in different aggregation states. Molecular weight determinations by gel filtration gave values of 18 000 for FdI and FdI′ and 24 000 for FdII, whereas a value of 6000 is determined when dissociation is accomplished with sodium dodecyl sulfate. The electronic spectra are different and their ultraviolet-visible absorbance rations are 0.77, 0.87 and 0.68 respectively for FdI, FdI′ and FdII. Despite these differences, the physiological activities of the three forms are similar as far as the reduction of sulfite by molecular hydrogen is concerned. 相似文献
69.
70.
Núria Cerdà‐Costa Francesc Xavier Gomis‐Rüth 《Protein science : a publication of the Protein Society》2014,23(2):123-144
The cleavage of peptide bonds by metallopeptidases (MPs) is essential for life. These ubiquitous enzymes participate in all major physiological processes, and so their deregulation leads to diseases ranging from cancer and metastasis, inflammation, and microbial infection to neurological insults and cardiovascular disorders. MPs cleave their substrates without a covalent intermediate in a single‐step reaction involving a solvent molecule, a general base/acid, and a mono‐ or dinuclear catalytic metal site. Most monometallic MPs comprise a short metal‐binding motif (HEXXH), which includes two metal‐binding histidines and a general base/acid glutamate, and they are grouped into the zincin tribe of MPs. The latter divides mainly into the gluzincin and metzincin clans. Metzincins consist of globular ~130–270‐residue catalytic domains, which are usually preceded by N‐terminal pro‐segments, typically required for folding and latency maintenance. The catalytic domains are often followed by C‐terminal domains for substrate recognition and other protein–protein interactions, anchoring to membranes, oligomerization, and compartmentalization. Metzincin catalytic domains consist of a structurally conserved N‐terminal subdomain spanning a five‐stranded β‐sheet, a backing helix, and an active‐site helix. The latter contains most of the metal‐binding motif, which is here characteristically extended to HEXXHXXGXX(H,D). Downstream C‐terminal subdomains are generally shorter, differ more among metzincins, and mainly share a conserved loop—the Met‐turn—and a C‐terminal helix. The accumulated structural data from more than 300 deposited structures of the 12 currently characterized metzincin families reviewed here provide detailed knowledge of the molecular features of their catalytic domains, help in our understanding of their working mechanisms, and form the basis for the design of novel drugs. 相似文献