全文获取类型
收费全文 | 4119篇 |
免费 | 499篇 |
出版年
2021年 | 48篇 |
2019年 | 62篇 |
2018年 | 53篇 |
2017年 | 34篇 |
2016年 | 75篇 |
2015年 | 151篇 |
2014年 | 139篇 |
2013年 | 216篇 |
2012年 | 255篇 |
2011年 | 201篇 |
2010年 | 128篇 |
2009年 | 150篇 |
2008年 | 186篇 |
2007年 | 177篇 |
2006年 | 212篇 |
2005年 | 188篇 |
2004年 | 180篇 |
2003年 | 184篇 |
2002年 | 155篇 |
2001年 | 75篇 |
2000年 | 68篇 |
1999年 | 85篇 |
1998年 | 65篇 |
1997年 | 47篇 |
1996年 | 42篇 |
1995年 | 47篇 |
1994年 | 33篇 |
1993年 | 44篇 |
1992年 | 57篇 |
1991年 | 48篇 |
1990年 | 58篇 |
1989年 | 46篇 |
1988年 | 42篇 |
1987年 | 50篇 |
1986年 | 47篇 |
1985年 | 44篇 |
1984年 | 49篇 |
1983年 | 40篇 |
1982年 | 35篇 |
1981年 | 37篇 |
1980年 | 46篇 |
1979年 | 45篇 |
1978年 | 40篇 |
1977年 | 29篇 |
1976年 | 34篇 |
1975年 | 31篇 |
1974年 | 34篇 |
1972年 | 41篇 |
1971年 | 31篇 |
1968年 | 36篇 |
排序方式: 共有4618条查询结果,搜索用时 347 毫秒
11.
D. C. Coleman R. V. Anderson C. V. Cole E. T. Elliott L. Woods M. K. Campion 《Microbial ecology》1977,4(4):373-380
Flows of biomass and respiratory carbon were studied in a series of propylene-oxide sterilized soil microcosms. One-half of the microcosms received three pulsed additions of 200 ppm glucose-carbon to mimic rhizosphere carbon inputs. Biotic variables were: bacteria (Pseudomonas) alone, or amoebae (Acanthamoeba) and nematodes (Mesodiplogaster) singly, or both combined in the presence of bacteria.Over the 24-day experiment, respiration was significantly higher in the microcosms containing the bacterial grazers. Biomass accumulation by amoebae was significantly higher than that by nematodes. The nematodes respired up to 30-fold more CO2 per unit biomass than did amoebae. Similar amounts of carbon flowed into both respiratory and biomass carbon in microcosms with fauna, compared with the bacteria-alone microcosms. However, partitioning of available carbon by the microfauna varied considerably, with little biomass production and relatively more CO2-C produced in the nematode-containing microcosms. The amoebae, in contrast, allocated more carbon to tissue production (about 40% assimilation efficiency) and correspondingly less to CO2. 相似文献
12.
13.
Teresa K. Aman Bruce A. Maki Thomas J. Ruffino Eileen M. Kasperek Gabriela K. Popescu 《The Journal of biological chemistry》2014,289(27):18805-18817
Protein kinase A (PKA) enhances synaptic plasticity in the central nervous system by increasing NMDA receptor current amplitude and Ca2+ flux in an isoform-dependent yet poorly understood manner. PKA phosphorylates multiple residues on GluN1, GluN2A, and GluN2B subunits in vivo, but the functional significance of this multiplicity is unknown. We examined gating and permeation properties of recombinant NMDA receptor isoforms and of receptors with altered C-terminal domain (CTDs) prior to and after pharmacological inhibition of PKA. We found that PKA inhibition decreased GluN1/GluN2B but not GluN1/GluN2A gating; this effect was due to slower rates for receptor activation and resensitization and was mediated exclusively by the GluN2B CTD. In contrast, PKA inhibition reduced NMDA receptor-relative Ca2+ permeability (PCa/PNa) regardless of the GluN2 isoform and required the GluN1 CTD; this effect was due primarily to decreased unitary Ca2+ conductance, because neither Na+ conductance nor Ca2+-dependent block was altered substantially. Finally, we show that both the gating and permeation effects can be reproduced by changing the phosphorylation state of a single residue: GluN2B Ser-1166 and GluN1 Ser-897, respectively. We conclude that PKA effects on NMDA receptor gating and Ca2+ permeability rely on distinct phosphorylation sites located on the CTD of GluN2B and GluN1 subunits. This separate control of NMDA receptor properties by PKA may account for the specific effects of PKA on plasticity during synaptic development and may lead to drugs targeted to alter NMDA receptor gating or Ca2+ permeability. 相似文献
14.
Cytokinetic basis for the impaired activation of lymphocytes from patients with primary intracranial tumors 总被引:4,自引:0,他引:4
L H Elliott W H Brooks T L Roszman 《Journal of immunology (Baltimore, Md. : 1950)》1984,132(3):1208-1215
Patients with malignant brain tumors have a variety of immunologic abnormalities, including the impaired responsiveness of peripheral blood lymphocytes (PBL) to mitogens and alloantigens. We further investigated this impairment of lymphocyte reactivity by employing the techniques of limiting dilution analysis and cytokinetic analysis. PBL preparations from patients have approximately six times fewer phytohemagglutinin (PHA)-responsive cells than PBL from normal subjects. Similar results were obtained with purified T cell preparations. Cytokinetic analysis of PHA-induced [3H]thymidine incorporation employing colchicine blocking of mitosis demonstrated that the number of first generation cells entering the S-phase of mitosis for each 24-hr period was less for PBL from patients than for PBL from normal individuals. First generation responding cells from patients and normal subjects entered DNA synthesis at the same time (48 to 72 hr). Cytokinetic analysis over a period of 168 hr demonstrated that whereas PBL from normal individuals demonstrated second generation responding cells, PBL from the majority of patients did not, thus indicating a defect in their ability to undergo clonal expansion. Measurement of interleukin 2 (IL 2) activity in culture fluids from PHA-activated PBL from normal subjects and patients revealed significantly lower IL 2 levels in culture fluids from PBL from patients. The addition of various concentrations of lectin-free IL 2 to PBL from patients stimulated with PHA did not restore responsiveness to normal values. There was no difference between the levels of interleukin 1 (IL 1) produced by lipopolysaccharide-activated monocytes from normal subjects and patients. Overall, these results suggest that an intrinsic defect exists in T cells obtained from brain tumor patients that renders them unable to enter into normal mitogen-induced blastogenesis. 相似文献
15.
H.J. Elliott 《Journal of insect physiology》1976,22(9):1275-1279
The structure of the corpus allatum of Aphis craccivora was examined when the gland was in both the active and inactive state. Active glands contained large rounded nuclei around the periphery and a central nervous section with many neurosecretory axons. The cell membranes were very convoluted and often associated with vesicles, particularly in the axonal region and adjacent to the stromal sheath. Inactive glands exhibited smaller and irregular shaped nuclei and extensive vacuolation resulting in the loss of all or part of the central axonal region.The possible role of the gland components in relation to the endocrine function of the corpus allatum is discussed. 相似文献
16.
Preincubation of normal human skin fibroblasts with tunicamycin, which inhibits N-glycosylation of glycoproteins, resulted in a dose-dependent and reversible inhibition of binding and internalization of homologous low-density lipoproteins by the cells. The degradation of the internalized lipoproteins was not affected by the drug. Comparative studies with fibroblasts deficient in low-density-lipoprotein receptors indicated that tunicamycin exerts its inhibitory effect only via the receptor-mediated high-affinity binding and uptake of lipoproteins. These results suggest that expression of low-density-lipoprotein receptors on the cell surface of human skin fibroblasts depends on intact N-glycosylation. 相似文献
17.
18.
19.
Analysis of the interaction of antibodies with immunoglobulin idiotypes on neoplastic B lymphocytes: implications for immunotherapy 总被引:5,自引:0,他引:5
T J Elliott M J Glennie H M McBride G T Stevenson 《Journal of immunology (Baltimore, Md. : 1950)》1987,138(3):981-988
We have examined the reactions of a panel of nine monoclonal anti-idiotype antibodies with the surface immunoglobulin in situ on guinea pig L2C leukemic lymphocytes. Equilibrium binding constants were shown to range between 10(7) and 10(8) M-1 for univalent Fab' gamma fragments and between 10(8) and 10(9) M-1 for intact IgG. Saturation of the cell surface binding sites was achieved with 2.9 X 10(5) Fab' gamma molecules/cell and 1.2 X 10(5) IgG molecules/cell for each antibody, a result that is consistent with a bivalent mode of interaction for the IgG. Despite these overall similarities in binding characteristics antibodies showed striking differences in their ability to clear Ig from the cell surface by antigenic modulation in vitro. This suggested differences in the readiness with which the antibodies cross-linked neighboring surface Ig molecules. Such an interpretation was supported by differences in the times required to achieve bivalent binding at 0 degree C, and in the rates at which labeled antibody dissociated from the cell surface in the presence or absence of an excess of unlabeled antibody. The data are consistent with there being two functionally distinct types of anti-idiotype antibody: those that form predominantly intra-Ig bridges, with each antibody Fab being linked to an Fab on one target molecule ("monogamous" binding) and not favoring modulation; and those that form predominantly inter-Ig bridges ("bigamous" binding) and favor modulation. The nature of interaction is presumably dictated by the orientation of the particular idiotope concerned. This distinction could be of great importance in the therapeutic use of anti-idiotype to ablate B cell neoplasms. 相似文献
20.
Lactose malabsorption was studied by a breath hydrogen technique in 139 Samoan and 68 white schoolchildren. The Samoans were studied in four locations, two in Western Samoa and two in New Zealand, and the white children in both the Cook Islands and New Zealand. The prevalence of malabsorption varied with location: for Samoans it ranged from 41% to 60% in Western Samoa and 0% to 35% in New Zealand; white children had rates of 27% in the Cook Islands and 5% in New Zealand. Environmental factors rather than genetic factors are likely to play the main part in initiating if not perpetuating lactose malabsorption. In both races lactose malabsorption had no effect on the acceptance of, consumption of, and number of gastrointestinal symptoms caused by milk and milk biscuits. Children who had symptoms after consuming a particular dairy product were more likely to say they disliked it than those who reported no symptoms. 相似文献