全文获取类型
收费全文 | 3648篇 |
免费 | 241篇 |
专业分类
3889篇 |
出版年
2024年 | 1篇 |
2023年 | 19篇 |
2022年 | 47篇 |
2021年 | 85篇 |
2020年 | 55篇 |
2019年 | 68篇 |
2018年 | 100篇 |
2017年 | 67篇 |
2016年 | 125篇 |
2015年 | 186篇 |
2014年 | 203篇 |
2013年 | 289篇 |
2012年 | 325篇 |
2011年 | 288篇 |
2010年 | 187篇 |
2009年 | 166篇 |
2008年 | 270篇 |
2007年 | 230篇 |
2006年 | 231篇 |
2005年 | 218篇 |
2004年 | 169篇 |
2003年 | 176篇 |
2002年 | 138篇 |
2001年 | 32篇 |
2000年 | 18篇 |
1999年 | 22篇 |
1998年 | 30篇 |
1997年 | 24篇 |
1996年 | 23篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 11篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1986年 | 3篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1973年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有3889条查询结果,搜索用时 15 毫秒
1.
Gian Maria Rossolini Patrizia Muscas Alessandra Chiesurin Giuseppe Satta 《FEMS microbiology letters》1994,119(3):321-328
Abstract Analysis of the Salmonella chromosomal region located upstream of the fimA gene (coding for the major type 1 fimbrial subunit) showed a close linkage of this gene to the folD gene (coding for the enzyme 5,10-methylenetetrahydrofolate dehydrogenase/5, 10-methenyltetrahydrofolate cyclohydrolase), indicating that the fim gene cluster of Salmonella , unlike that of Escherichia coli , has no regulatory genes located upstream of fimA and apparently terminates with this gene. The respective locations of the fim and folD genes in the E. coli and Salmonella genetic maps suggests that the fimA-folD intergenic region of Salmonella encompasses a junctional site of a genetic rearrangement that probably originated from the different chromosomal location of the fim genes in these species. 相似文献
2.
Monolayer black membranes from bipolar lipids of archaebacteria and their temperature-induced structural changes 总被引:1,自引:0,他引:1
Alessandra Gliozzi Ranieri Rolandi Mario De Rosa Agata Gambacorta 《The Journal of membrane biology》1983,75(1):45-56
The membrane of Caldariella acidophila, an extreme thermophilic archaebacterium, is characterized by unusual bipolar complex lipids. They consist of two nonequivalent polar heads, linked by a C40 alkylic component. The molecular organization of these lipids in the plasma membrane is still a matter of study. In this paper, we present current-voltage measurements on artificial bipolar lipid membranes, indicating that molecules are indeed organized as a covalently bound bilayer, in which each molecule is completely stretched and spans its entire thickness. Furthermore, conformational transitions of these artificial membranes (which could be formed only above 70 degrees C from a lipid/squalene dispersion) are analyzed in the 80 to 15 degrees C temperature range. Abrupt variations in capacitance and valinomycin-induced conductance seem to indicate the occurrence of at least two structural changes. Measurements are also extended to different solvent systems. Results are consistent with the picture of a monolayer bipolar lipid membrane in which few solvent molecules align themselves parallel to the lipophilic chains. The amount of solvent as well as the temperature at which conformational transitions occur, depend on the solvent system in which the lipid is dispersed. 相似文献
3.
Minirhizotron observations of roots of leeks and Brussels sprouts grown in the Wageningen Rhizolab were used to study the dynamics of root length. Day of appearance and the time of decay were assessed for individual root segments visible on the minirhizotron surface.A Brussels sprouts crop produced much more root length than leeks, but the average longevity of these roots was about half that of leek roots.To investigate whether root colour or UV fluorescence could be used as a quantitative index of root functionality or root age, changes in root colour (on a scale of greys) over time were measured with interactive image analysis. In both crops a gradual change towards black was found with ageing. Measurements of the intensity of the UV fluorescence showed that leek roots fluoresced more than Brussels sprouts roots. Over time, UV fluorescence decreased in Brussels sprouts roots but increased in leek roots. It is concluded that UV fluorescence cannot be used as a universal indicator of root age or root functionality, but in some plant species it may be used to separate (transparent) roots from the background with image analysis techniques. 相似文献
4.
Cecilia Del Casino Yi-Qin Li Alessandra Moscatelli Monica Scali Antonio Tiezzi Mauro Cresti 《Biology of the cell / under the auspices of the European Cell Biology Organization》1993,79(2):125-132
Summary— The distribution of microtubules was investigated in Nicotiana tabacum pollen tubes at different stages of tube growth by immunofluorescence microscopy. Using specific antibodies, the presence of microtubules consisting of different tubulin isoforms was tested. α-, β- and tyrosinated α-tubulin were present within the tube, whereas the acetylated form was lacking. The presence of tubulin subunits in pollen tube extracts was also investigated by immunoblotting analyses. The use of a confocal laser scanning microscope integrated with computer-assisted imaging, allowed a detailed visualization of the microtubule distribution and organization. Cytoplasmic microtubules organized as short bundles with various orientations were detected at the apex of long tubes. 相似文献
5.
Othman Al Musaimi Sophie V. Morse Lucia Lombardi Simona Serban Alessandra Basso Daryl R. Williams 《Journal of peptide science》2023,29(2):e3448
Successful manual synthesis of the TD2.2 peptide acting as a blood–brain barrier shuttle was achieved. TD2.2 was successfully synthesised by sequential condensation of four protected peptide fragments on solid-phase settings, after several unsuccessful attempts using the stepwise approach. These fragments were chosen to minimise the number of demanding amino acids (in terms of coupling, Fmoc removal) in each fragment that are expected to hamper the overall synthetic process. Thus, the hydrophobic amino acids as well as Arg(Pbf) were strategically spread over multiple fragments rather than having them congested in one fragment. This study shows how a peptide that shows big challenges in the synthesis using the common stepwise elongation methodology can be synthesised with an acceptable purity. It also emphasises that choosing the right fragment with certain amino acid constituents is key for a successful synthesis. It is worth highlighting that lower amounts of reagents were required to synthesise the final peptide with an identical purity to that obtained by the automatic synthesiser. 相似文献
6.
The bifurcations of a periodically forced predator-prey model (the chemostat model), with a prey feeding on a limiting nutrient,
are numerically detected with a continuation technique. Eight bifurcation diagrams are produced (one for each parameter in
the model) and shown to be topologically equivalent. These diagrams are also equivalent to those of the most commonly used
predator-prey model (the Rosenzweig-McArthur model). Thus, all basic modes of behavior of the two main predator-prey models
can be explained by means of a single bifurcation diagram. 相似文献
7.
Laura Chiarantini Luigia Rossi Alessandra Fraternale Mauro Magnani 《Molecular and cellular biochemistry》1995,144(1):53-59
Human and murine blood cells treated with ZnCl2 and bis(sulfosuccinimidyl)suberate (BS3) (a cross linking agent) undergo band 3 clustering and binding of hemoglobin to red blood cell membrane proteins. These clusters induce autologous IgG binding and complement fixation, thus favouring the phagocytosis of ZnCl2/BS3 treated cells by macrophages. The extension of red blood cell opsonization can be easily modulated by changing the ZnCl2 concentration in the 0.1–1.0 mM range thus providing an effective way to affect blood cell recognition by macrophages. In fact, murine erythrocytes treated with increasing ZnCl2 concentrations have proportionally reduced survivals when reinjected into the animal. Furthermore, the organ sequestration of ZnCl2/BS3 treated cells strongly resembles the typical distribution of the senescent cells. Since the ZnCl2/BS3 treatment can also be performed on red blood cells loaded with drugs or other substances, this procedure is an effective drug-targeting system to be used for the delivery of molecules to peritoneal, liver and spleen macrophages. 相似文献
8.
Dall'Olio Fabio; Mariani Erminia; Tarozzi Andrea; Meneghetti Alessandra; Chiricolo Mariella; Lau Joseph T. Y.; Facchini Andrea 《Glycobiology》1997,7(4):507-513
The extent of processing of N-linked oligosaccharides and thesialylation of the target cell membranes has been positivelycorrelated with resistance to lysis mediated by NK cells, buta conclusive evidence has never been reached. Colon cancer tissuesexpress an increased activity of ß-ga-lactoside 相似文献
9.
Alessandra Cesano Sophie Visonneau Livia Cioé Steven C. Clark Daniela Santoli 《Cancer immunology, immunotherapy : CII》1995,40(3):139-151
The TALL-104 cell line, originally derived from a patient with T cell leukemia, can be maintained indefinitely in culture in the presence of interleukin-2 (IL-2) and is endowed with a highly potent major-histocompatibilitycomplex (MHC)-non-restricted tumoricidal activity both in vitro and in animal models. The present study analyzes in detail the short- and long-term effects of irradiation and cyclosporin A (CsA) treatment on the growth and tumoricidal function of this T cell clone as compared to polyclonal lymphokine-activated killer (LAK) cell preparations from healthy donors. DNA and RNA syntheses by both TALL-104 and LAK cells were irreversibly arrested a few hours after irradiation with 40 Gy. However, 4-h51Cr-release assays, performed on different days (day 1 to day 7) after irradiation, showed that the cytotoxic efficiency of TALL-104 cells against hematopoietic and solid tumor targets was only modestly reduced, whereas that of LAK cells was severely inhibited. Moreover, the cytotoxic responses to recombinant human IL-2 and IL-12, measured 18 h after irradiation and cytokine addition, were normal in the case of TALL-104 cells but were abolished in the case of LAK cells. Co-culture of IL-2-or IL-12-preactivated TALL-104 cells with a tumor target for 5 days in the absence of cytokines resulted in a lower efficiency of lysis, as compared to the non-irradiated effectors, especially if the initial stimulus was IL-12. These findings suggest the requirement of multiple cytokine stimulation for optimal expression of tumoricidal activity by lethally irradiated TALL-104 cells. CsA, while abrogating TALL-104 cell proliferation at the low dose of 0.5 g/ml, inhibited their cytotoxic function marginally only at high doses (100 g/ml). By contrast, CsA reduced dose-dependently the cytotoxicity of LAK cells starting at very low doses (0.5 g/ml). CsA did not impair the ability of TALL-104 and LAK cells to produce interferon (IFN), tumor necrosis factor (TNF) , and granulocyte/macrophage-colony-stimulatory factor (GM-CSF) in response to IL-2, IL-12, or tumor targets. Irradiation reduced drastically IFN production by LAK, but not TALL-104 cells; release of TNF and GM-CSF by either type of effector was inhibited by 10%–50%, depending on the stimulus. The high resistance of the TALL-104 cells' tumoricidal function to irradiation and immunosuppressive drugs renders this immortal T cell clone a potentially safe and effective reagent for new adoptive-transfer approaches to cancer in MHC-incompatible recipients. 相似文献
10.
Giancarlo Bianchi Roberto Fiocchi Alessandra Tavani Luciano Manara 《Life sciences》1982,30(22):1875-1883
Single doses of naloxone (0.025 to 0.5 mg/kg) or of one of four quaternary narcotic antagonists (i.e. nalorphine allobromide, nalorphine methobromide, naloxone methobromide or naltrexone methobromide, 1 to 60 mg/kg) were given s.c. to rats before morphine, 5 mg/kg i.v. In the absence of antagonists morphine reduced G.I. transit of a charcoal meal to about 15% of drug-free controls and consistently delayed nociceptive reactions (55°C hot plate) in all animals. Doses of antagonists slightly reducing morphine antinociception (centrally effective = A) and restoring G.I. transit to about 50% of drug-free rats (peripherally effective = B) were estimated. The A:B ratio, indicating peripheral selectivity, was at least 8 for any of the quaternary antagonists given 10 min before morphine, but prolonging this interval may have resulted in a lower figure (i.e. less peripheral selectivity) because of reduced A and increased B. This was definitely so for naltrexone methobromide (A:B, > 60 at 10 min, about 1 at 80 min) and was not apparent for nalorphine methobromide according to available data, which for nalorphine allobromide and to a lesser extent for naloxone methobromide showed only an increase in B at intervals longer than 10 min. Both morphine-induced antinociception and inhibition of G.I. transit were reduced by naloxone at the lower doses tested and were fully prevented at the higher. These findings indicate that, unlike naloxone, the investigated quaternary narcotic antagonists are interesting prototype drugs for selective blockade of opiate receptors outside the CNS, although certain critical aspects, possibly biological N-dealkylation to the corresponding tertiary antagonists, condition peripheral selectivity. 相似文献