全文获取类型
收费全文 | 1964篇 |
免费 | 239篇 |
出版年
2022年 | 19篇 |
2021年 | 42篇 |
2020年 | 22篇 |
2019年 | 41篇 |
2018年 | 45篇 |
2017年 | 42篇 |
2016年 | 55篇 |
2015年 | 95篇 |
2014年 | 92篇 |
2013年 | 102篇 |
2012年 | 127篇 |
2011年 | 145篇 |
2010年 | 80篇 |
2009年 | 82篇 |
2008年 | 105篇 |
2007年 | 89篇 |
2006年 | 69篇 |
2005年 | 70篇 |
2004年 | 77篇 |
2003年 | 68篇 |
2002年 | 67篇 |
2001年 | 60篇 |
2000年 | 47篇 |
1999年 | 57篇 |
1998年 | 27篇 |
1997年 | 22篇 |
1996年 | 31篇 |
1995年 | 23篇 |
1994年 | 17篇 |
1993年 | 13篇 |
1992年 | 38篇 |
1991年 | 18篇 |
1990年 | 35篇 |
1989年 | 39篇 |
1988年 | 30篇 |
1987年 | 16篇 |
1986年 | 14篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 15篇 |
1982年 | 8篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1979年 | 10篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1974年 | 8篇 |
1973年 | 9篇 |
1968年 | 9篇 |
排序方式: 共有2203条查询结果,搜索用时 312 毫秒
71.
De La Calzada CS Verdugo AL Perez JT Alcaine CC Fernandez AS 《Cardiovascular diseases》1981,8(1):73-83
A 50-year-old man presented with hypertrophic obstructive cardiomyopathy (HOC) associated with a left ventricular aneurysm and normal coronary arteries. His history revealed no evidence of myocardial infarction or atypical angina. Physical examination disclosed HOC but did not suggest the presence of an aneurysm. Although the patient was treated medically, heart failure ensued, and he died suddenly while working his farm. Subsequent investigation of the patient's family revealed that three of his five children were also affected by cardiomyopathy, which was especially pronounced in the eldest, a 22-year-old man. The possible hemodynamic relationship between HOC and left ventricular aneurysm is discussed, along with probable indications. The role of left ventricular aneurysm is also presented in relation to the natural history of the disease. 相似文献
72.
Aromatic amino acid auxotrophs of the methanol-utilizing yeast Hansenula polymorpha were effectively selected by the use of nystatin and a medium that inhibits the growth of tyrosine auxotrophs. The procedure resulted in a frequency of aromatic auxotrophs of 2% of survivors and an enrichment of 20-fold. The new procedure also takes less time than traditional procedures. Of the auxotrophic mutants isolated, two-thirds required tyrosine and the remainder were tyrosine-phenylalanine double auxotrophs. 相似文献
73.
The effect of lincocin (a plastid protein synthesis inhibitor) treatment on the greening process of bean (Phaseolus vulgaris L.) leaves have been studied. In comparison with control leaves treated ones had a decreased rate of chloroplast development. They had a marked chlorophyll deficiency and a decreased chlorophyll a/b ratio. Some long and short wavelength forms of chlorophyll a were lacking as evidenced from the absorption spectra at 25°C and the fluorescence spectra at 77°K. The –14CO2 fixation was inhibited by 80–90% in treated leaves. The fluorescence induced by the measuring light was greater in the treated leaves than in the control ones, and the kinetics of the decline of the relative fluorescence intensity were also different. Electron microscopic studies showed macrogranum-like structures and incomplete membrane vesicles in the treated plastids. After longer treatment a destruction of membranes was observed. The results indicate some structural and functional membrane deficiencies and instability of the membranes. 相似文献
74.
75.
A partially purified D-xylose isomerase has been isolated from cells of Streptomyces albus NRRL 5778 and some of its properties have been determined. D-Glucose, D-xylose, D-ribose, L-arabinose, and L-rhamnose served as substrates for the enzyme with respective Km values of 86, 93, 350, 153, and 312 mM and Vmax values measuring 1.23, 2.9, 2.63, 0.153, and 0.048 mumol min per mg of protein. The hexose D-allose was also isomerized. The enzyme was strongly activated by 1.0 mM Mg2+ but only partially activated by 1.0 mM Co2+. The respective Km values for Mg2+ and Co2+ were 0.3 and 0.003 mM. Mg2+ and Co2+ appear to have separate binding sites on the isomerase. These cations also protect the enzyme from thermal denaturation and from D-sorbitol inhibition. The optimum temperature for ketose formation was 70 to 80 C at pH values ranging from 7 to 9. D-Sorbitol acts as a competitive inhibitor with a Ki of 5.5 mM against D-glucose, D-xylose, and D-ribose. Induction experiments, Mg2+ activation, and D-sorbitol inhibition indicated that a single enzyme (D-xylose isomerase) was responsible for the isomerization of the pentoses, methyl pentose, and glucose. 相似文献
76.
Simple agarose gel electrophoretic method for the identification and characterization of plasmid deoxyribonucleic acid. 总被引:225,自引:28,他引:197 下载免费PDF全文
Agarose gel electrophoresis may be employed effectively for the detection and preliminary characterization of plasmid deoxyribonucleic acid (DNA) present in clinical isolates and laboratory strains of gram-negative microorganisms. The method is sensitive and does not require radioisotopes or ultracentrifugation. The estimation of plasmid mass from the extent of DNA migration in gels compares favorably with results obtained by electron microscopy of plasmid DNA purified by equilibrium density centrifugation. The method has proved to be a useful tool for survey work and the epidemiological investigation of plasmid dissemination, as well as an important adjunct to the genetic analysis of plasmids. 相似文献
77.
Summary It was found that the increase of the hydraulic retention time improved the organic matter removal and the conversion of organic to ammonia nitrogen. Methane concentration in the biogas was not affected by the hydraulic retention time.The reactor in operation during one year was not clogged. 相似文献
78.
79.
James V. McCann Steven R. Bischoff Yu Zhang Dale O. Cowley Veronica Sanchez‐Gonzalez George D. Daaboul Andrew C. Dudley 《Genesis (New York, N.Y. : 2000)》2020,58(7)
Extracellular vesicles (EVs) are abundant, lipid‐enclosed vectors that contain nucleic acids and proteins, they can be secreted from donor cells and freely circulate, and they can be engulfed by recipient cells thus enabling systemic communication between heterotypic cell types. However, genetic tools for labeling, isolating, and auditing cell type‐specific EVs in vivo, without prior in vitro manipulation, are lacking. We have used CRISPR‐Cas9‐mediated genome editing to generate mice bearing a CD63‐emGFPloxP/stop/loxP knock‐in cassette that enables the specific labeling of circulating CD63+ vesicles from any cell type when crossed with lineage‐specific Cre recombinase driver mice. As proof‐of‐principle, we have crossed these mice with Cdh5‐CreERT2 mice to generate CD63emGFP+ vasculature. Using these mice, we show that developing vasculature is marked with emerald GFP (emGFP) following tamoxifen administration to pregnant females. In adult mice, quiescent vasculature and angiogenic vasculature (in tumors) is also marked with emGFP. Moreover, whole plasma‐purified EVs contain a subpopulation of emGFP+ vesicles that are derived from the endothelium, co‐express additional EV (e.g., CD9 and CD81) and endothelial cell (e.g., CD105) markers, and they harbor specific miRNAs (e.g., miR‐126, miR‐30c, and miR‐125b). This new mouse strain should be a useful genetic tool for generating cell type‐specific, CD63+ EVs that freely circulate in serum and can subsequently be isolated and characterized using standard methodologies. 相似文献
80.
Susana Ruiz‐Ruiz Sergio Sanchez‐Carrillo Sergio Ciordia María C. Mena Celia Mndez‐García David Rojo Rafael Bargiela Elisa Zubeldia‐Varela Mnica Martínez‐Martínez Coral Barbas Manuel Ferrer Andrs Moya 《Aging cell》2020,19(1)
Composition of the gut microbiota changes during ageing, but questions remain about whether age is also associated with deficits in microbiome function and whether these changes occur sharply or progressively. The ability to define these deficits in populations of different ages may help determine a chronological age threshold at which deficits occur and subsequently identify innovative dietary strategies for active and healthy ageing. Here, active gut microbiota and associated metabolic functions were evaluated using shotgun proteomics in three well‐defined age groups consisting of 30 healthy volunteers, namely, ten infants, ten adults and ten elderly individuals. Samples from each volunteer at intervals of up to 6 months (n = 83 samples) were used for validation. Ageing gradually increases the diversity of gut bacteria that actively synthesize proteins, that is by 1.4‐fold from infants to elderly individuals. An analysis of functional deficits consistently identifies a relationship between tryptophan and indole metabolism and ageing (p < 2.8e?8). Indeed, the synthesis of proteins involved in tryptophan and indole production and the faecal concentrations of these metabolites are directly correlated (r2 > .987) and progressively decrease with age (r2 > .948). An age threshold for a 50% decrease is observed ca. 11–31 years old, and a greater than 90% reduction is observed from the ages of 34–54 years. Based on recent investigations linking tryptophan with abundance of indole and other “healthy” longevity molecules and on the results from this small cohort study, dietary interventions aimed at manipulating tryptophan deficits since a relatively “young” age of 34 and, particularly, in the elderly are recommended. 相似文献