全文获取类型
收费全文 | 5057篇 |
免费 | 18篇 |
国内免费 | 20篇 |
专业分类
系统科学 | 11篇 |
丛书文集 | 18篇 |
教育与普及 | 4篇 |
理论与方法论 | 6篇 |
现状及发展 | 2217篇 |
研究方法 | 262篇 |
综合类 | 2548篇 |
自然研究 | 29篇 |
出版年
2013年 | 52篇 |
2012年 | 102篇 |
2011年 | 118篇 |
2010年 | 26篇 |
2008年 | 87篇 |
2007年 | 112篇 |
2006年 | 99篇 |
2005年 | 98篇 |
2004年 | 119篇 |
2003年 | 80篇 |
2002年 | 91篇 |
2001年 | 181篇 |
2000年 | 149篇 |
1999年 | 118篇 |
1992年 | 112篇 |
1991年 | 72篇 |
1990年 | 75篇 |
1989年 | 66篇 |
1988年 | 70篇 |
1987年 | 78篇 |
1986年 | 56篇 |
1985年 | 97篇 |
1984年 | 92篇 |
1983年 | 78篇 |
1982年 | 62篇 |
1981年 | 54篇 |
1980年 | 69篇 |
1979年 | 174篇 |
1978年 | 141篇 |
1977年 | 111篇 |
1976年 | 116篇 |
1975年 | 123篇 |
1974年 | 148篇 |
1973年 | 122篇 |
1972年 | 123篇 |
1971年 | 149篇 |
1970年 | 198篇 |
1969年 | 142篇 |
1968年 | 150篇 |
1967年 | 127篇 |
1966年 | 137篇 |
1965年 | 104篇 |
1964年 | 39篇 |
1959年 | 39篇 |
1958年 | 68篇 |
1957年 | 47篇 |
1956年 | 39篇 |
1955年 | 37篇 |
1954年 | 46篇 |
1948年 | 28篇 |
排序方式: 共有5095条查询结果,搜索用时 15 毫秒
211.
Sarti P Silver RB Paroli L Nikonorov I Blanck TJ 《Cellular and molecular life sciences : CMLS》1999,56(11-12):1061-1069
Rat heart myocytes undergoing progressive damage demonstrate morphological changes of shortening and swelling followed by the formation of intracellular vacuoles and plasma membrane blebbing. The damaged myocytes displayed impaired N,N'-tetramethyl-p-phenyldiamine (TMPD) ascorbate-stimulated respiratory activity which was restored by the addition of reduced cytochrome c to the cell culture medium. To clarify the role played by cytochrome c in the impairment of cell respiration, polarographic, spectrophotometric and fluorescence as well as electron microscopy imaging experiments were performed. TMPD/ascorbate-stimulated respiratory activity returned to control levels, at approximately 20 microM cytochrome c, establishing the threshold below which the turnover rate by cytochrome c oxidase in the cell depends on cytochrome concentration. Mildly damaged cardiac myocytes, as indicated by cell shortening, retention of visible striations and free-fluorescein exclusion, together with the absence of lactate dehydrogenase leakage and exclusion of trypan blue, were able to oxidize exogenous cytochrome c and were permeable to fluorescein-conjugated cytochrome c. The results, while consistent with an early cytochrome c release observed at the beginning of cell death, elucidate the role played by cytochrome c in the kinetic control of mitochondrial electron transfer under pathological conditions, particularly those involving the terminal part of the respiratory chain. These data are the first to demonstrate that the sarcolemma of cardiac myocytes, damaged but still viable, is permeable to cytochrome c. 相似文献
212.
Glycine: a new anti-inflammatory immunonutrient 总被引:7,自引:0,他引:7
213.
CACP, encoding a secreted proteoglycan, is mutated in camptodactyly-arthropathy-coxa vara-pericarditis syndrome. 总被引:3,自引:0,他引:3
J Marcelino J D Carpten W M Suwairi O M Gutierrez S Schwartz C Robbins R Sood I Makalowska A Baxevanis B Johnstone R M Laxer L Zemel C A Kim J K Herd J Ihle C Williams M Johnson V Raman L G Alonso D Brunoni A Gerstein N Papadopoulos S A Bahabri J M Trent M L Warman 《Nature genetics》1999,23(3):319-322
Altered growth and function of synoviocytes, the intimal cells which line joint cavities and tendon sheaths, occur in a number of skeletal diseases. Hyperplasia of synoviocytes is found in both rheumatoid arthritis and osteoarthritis, despite differences in the underlying aetiologies of the two disorders. We have studied the autosomal recessive disorder camptodactyly-arthropathy-coxa vara-pericarditis syndrome (CACP; MIM 208250) to identify biological pathways that lead to synoviocyte hyperplasia, the principal pathological feature of this syndrome. Using a positional-candidate approach, we identified mutations in a gene (CACP) encoding a secreted proteoglycan as the cause of CACP. The CACP protein, which has previously been identified as both 'megakaryocyte stimulating factor precursor' and 'superficial zone protein', contains domains that have homology to somatomedin B, heparin-binding proteins, mucins and haemopexins. In addition to expression in joint synovium and cartilage, CACP is expressed in non-skeletal tissues including liver and pericardium. The similarity of CACP sequence to that of other protein families and the expression of CACP in non-skeletal tissues suggest it may have diverse biological activities. 相似文献
214.
A spelling device for the paralysed 总被引:29,自引:0,他引:29
Birbaumer N Ghanayim N Hinterberger T Iversen I Kotchoubey B Kübler A Perelmouter J Taub E Flor H 《Nature》1999,398(6725):297-298
215.
216.
Structure of pre-pro-von Willebrand factor and its expression in heterologous cells 总被引:30,自引:0,他引:30
D T Bonthron R I Handin R J Kaufman L C Wasley E C Orr L M Mitsock B Ewenstein J Loscalzo D Ginsburg S H Orkin 《Nature》1986,324(6094):270-273
Von Willebrand factor (vWF), a multifunctional haemostatic glycoprotein derived from endothelial cells and megakaryocytes, mediates platelet adhesion to injured subendothelium and binds coagulation factor VIII in the circulation. Native vWF is a disulphide-bonded homopolymer; the monomeric subunits, of apparent relative molecular mass (Mr) 220,000 (220K) are derived from an intracellular precursor estimated at 260-275K. Multimer assembly is preceded by the formation of dimers, linked near their C-termini, which then assemble into filamentous polymers. The importance of the removal of the large vWF pro-polypeptide during multimer assembly, and whether this or other stages of the complex post-translational processing require components specific to endothelial cells or megakaryocytes, is unknown. Here we report an analysis of the complete sequence of pre-pro-vWF and expression of the molecule in heterologous cells. The vWF precursor is composed of several repeated subdomains. When expressed in COS and CHO cells, it is cleaved and assembled into biologically active high relative molecular mass disulphide bonded multimers. This suggests that the information for assembly of this complex molecule resides largely within its primary structure. 相似文献
217.
I. W. Althaus J. J. Chou A. J. Gonzales R. J. LeMay M. R. Deibel K. -C. Chou F. J. Kezdy D. L. Romero R. C. Thomas P. A. Aristoff W. G. Tarpley F. Reusser 《Cellular and molecular life sciences : CMLS》1994,50(1):23-28
The tetramer of ethylenesulfonic acid (U-9843) is a potent inhibitor of HIV-1 RT* and possesses excellent antiviral activity at nontoxic doses in HIV-1 infected lymphocytes grown in tissue culture. Kinetic studies of the HIV-1 RT-catalyzed RNA-directed DNA polymerase activity were carried out in order to determine if the inhibitor interacts with the template: primer or the deoxyribonucleotide triphosphate (dNTP) binding sites of the polymerase. Michaelis-Menten kinetics, which are based on the establishment of a rapid equilibrium between the enzyme and its substrates, proved inadequate for the analysis of the experimental data. The data were thus analyzed using steady-state Briggs-Haldane kinetics assuming that the template:primer binds to the enzyme first, followed by the binding of the dNTP and that the polymerase is a processive enzyme. Based on these assumptions, a velocity equation was derived which allows the calculation of all the specific forward and backward rate constants for the reactions occurring between the enzyme, its substrates and the inhibitor. The calculated rate constants are in agreement with this model and the results indicated that U-9843 acts as a noncompetitive inhibitor with respect to both the template:primer and dNTP binding sites. Hence, U-9843 exhibits the same binding affinity for the free enzyme as for the enzyme-substrate complexes and must inhibit the RT polymerase by interacting with a site distinct from the substrate binding sites. Thus, U-9843 appears to impair an event occurring after the formation of the enzyme-substrate complexes, which involves either an event leading up to the formation of the phosphoester bond, the formation of the ester bond itself or translocation of the enzyme relative to its template:primer following the formation of the ester bond. 相似文献
218.
Role for microsomal Ca storage in mammalian neurones? 总被引:4,自引:0,他引:4
Alterations in the intracellular concentration of calcium ions [( Ca2+]i) are increasingly being found to be associated with regulatory functions in cells of all kinds. In muscle, an elevation of [Ca2+]i is the final link in excitation-contraction coupling while at nerve endings and in secretory cells, similar rises in [Ca2+]i are thought to mediate exocytosis. The discovery of calcium-activated ion channels indicated a role for intracellular calcium in the regulation of membrane excitability. Calcium transients associated with either intracellular release or the inward movement of Ca2+ across the membrane have been recorded in molluscan neurons and more recently in neurones of bullfrog sympathetic ganglia. Here, we report the first recordings of calcium transients in single mammalian neurones. In these experiments we have found that the methylxanthine, caffeine, causes the release of calcium from a labile intracellular store which can be refilled by Ca2+ entering the cell during action potentials. 相似文献
219.
Plasmodium falciparum strain-specific antibody blocks binding of infected erythrocytes to amelanotic melanoma cells 总被引:3,自引:0,他引:3
An important feature of Plasmodium falciparum malaria which differentiates it from other human malarias is that erythrocytes infected with trophozoites and schizonts are not present in the peripheral blood but are sequestered along capillary and venular endothelium. Infected erythrocytes attach via parasite-induced ultrastructural modifications on the surface of the infected cells, called 'knobs'. This sequestration may be important for parasite survival because it prevents infected erythrocytes from circulating through the spleen where they could be eliminated. We have established an in vitro correlate of sequestration and used it to demonstrate that immune sera from repeatedly infected Aotus monkeys inhibit binding of infected erythrocytes to endothelial cells. We have investigated whether antiserum that blocks binding of one isolate of P. falciparum to target cells can block or reverse binding of other isolates. We report here that sera which block or reverse binding are strain-specific, indicating that the corresponding antigens on the surface of the infected erythrocytes are strain (isolate)-specific. 相似文献
220.
Effector T lymphocyte line cells migrate to the thymus and persist there 总被引:15,自引:0,他引:15
Y Naparstek J Holoshitz S Eisenstein T Reshef S Rappaport J Chemke A Ben-Nun I R Cohen 《Nature》1982,300(5889):262-264