全文获取类型
收费全文 | 3018篇 |
免费 | 305篇 |
国内免费 | 1篇 |
出版年
2023年 | 16篇 |
2021年 | 46篇 |
2020年 | 45篇 |
2019年 | 50篇 |
2018年 | 53篇 |
2017年 | 51篇 |
2016年 | 76篇 |
2015年 | 150篇 |
2014年 | 155篇 |
2013年 | 171篇 |
2012年 | 260篇 |
2011年 | 239篇 |
2010年 | 148篇 |
2009年 | 142篇 |
2008年 | 206篇 |
2007年 | 186篇 |
2006年 | 170篇 |
2005年 | 167篇 |
2004年 | 174篇 |
2003年 | 158篇 |
2002年 | 149篇 |
2001年 | 36篇 |
2000年 | 20篇 |
1999年 | 30篇 |
1998年 | 38篇 |
1997年 | 21篇 |
1996年 | 23篇 |
1995年 | 32篇 |
1994年 | 14篇 |
1993年 | 23篇 |
1992年 | 19篇 |
1991年 | 18篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 9篇 |
1975年 | 8篇 |
1974年 | 10篇 |
1973年 | 6篇 |
1972年 | 6篇 |
1971年 | 13篇 |
1969年 | 6篇 |
排序方式: 共有3324条查询结果,搜索用时 109 毫秒
41.
Microbial Activities in Undecompressed and Decompressed Deep-Seawater Samples 总被引:2,自引:1,他引:1 下载免费PDF全文
Microbial transformations of 14C-labeled substrates (sodium glutamate, Casamino Acids, glucose, and sodium acetate) were measured in undecompressed seawater samples collected from depths of 1,800 to 6,000 m, during 14- to 21-day incubation periods at in situ temperature (3°C). Each substrate was tested at two concentrations (ca. 0.5 and 5.0 μg/ml) and two in situ pressures. The data were compared to 1-atmosphere (ca. 1.013 × 102 kPa) controls. The rates of 14C incorporation and 14CO2 production as well as the amounts of total substrate utilization were generally lower at pressure than in the decompressed controls but were significantly different for each of the four substrates used. The utilization of acetate was the least affected by pressure; rates were similar to those measured at 1 atmosphere in two out of four experiments. In contrast, transformation rates of the amino acids at pressure averaged to only 38% of those in the controls. A single but reproducible “barophilic” response was observed with glucose as a substrate in samples collected from a depth of 4,500 m at a specific area in the northwestern Atlantic Ocean. Except for this latter set of experiments, the transformation of all substrates showed an increased lag period at pressure as compared to the 1-atmosphere controls. 相似文献
42.
Summary Cell recovery by means of continuous flotation of the Hansenula polymorpha cultivation medium without additives was investigated as a function of the cultivation conditions as well as of the flotation equipment construction and flotation operational parameters. The cell enrichment and separation is improved at high liquid residence times, high aeration rates, small bubble sizes, increasing height of the aerated column, and diameter of the foam column. Increasing cell age and cultivation with nitrogen limitation reduce the cell separation.Symbols CP
cell mass concentration in medium g·l–1
- CR
cell mass concentration in residue g·l–1
- CS
cell mass concentration in foam liquid g·l–1
-
V
equilibrium foam volume cm3
- V
gas flow rate through the aerated liquid column cm3·s–1
- VF
feed rate to the flotation column ml/min
-
1
V
S/V foaminess s
-
mean liquid residence time in the column s 相似文献
43.
Venous versus arterialised venous blood for assessment of blood glucose levels during glucose clamping: comparison in healthy men. 总被引:2,自引:0,他引:2
The aim of this study was to investigate the influence of the arteriovenous (A-V) gradient in blood glucose concentrations at low and high insulin levels on the determination of glucose requirements during glucose clamping in 9 healthy, insulin sensitive, male volunteers. In a random order two clamps were performed, once using arterialised venous blood (A Clamp, mean pO2 = 11.5 +/- 0.36 kPa, 86 +/- 2.7 mmHg), and once using venous blood (V clamp, mean pO2 = 7.9 +/- 0.21 kPa, 59 +/- 1.6 mmHg). Insulin levels were maintained at 48 +/- 2.4 mU/l from 0-180 min and at 1054 +/- 114 mU/l from 180-360 min. Elevation of insulin levels caused a significant rise of the A-V gradient: from 0.3 +/- 0.1 to 0.5 +/- 0.1 mmol/l (p < 0.05) and from 0.2 +/- 0.1 to 0.3 +/- 0.1 mmol/l (p < 0.05) during the A and V clamps, respectively. Despite these A-V glucose gradients no significant differences were found for the glucose requirements during the last 30 min of each period of insulin infusion between the A and V clamps: 43.70 +/- 3.4 vs 44.8 +/- 2.8 mumol.kg-1.min-1 during the low insulin level and 77.3 +/- 5.0 vs 76.2 +/- 3.4 mumol.kg-1.min-1 during the high insulin level. We conclude that the A-V glucose gradient, even at high insulin levels, does not influence the assessment of glucose requirements to a measurable extent, allowing the use of the simpler technique of taking venous rather than arterialised venous blood for the measurements of glucose levels during glucose clamping. 相似文献
44.
The reaction of methyl 4,6-O-benzylidene-3(2)-deoxy--
-erythro-hexopyranosid-2(3)-ulose with carbon disulfide, alkyl iodide, and sodium hydride gave methyl 4,6-O-benzylidene-3(2)-[bis(alkylthio)methylene]-3(2)-deoxy--
-erythro-hexopyranosid-2(3)-uloses. Methyl 4,6-O-benzylidene-2-[bis(methylthio)methylene]-2-deoxy--
-erythro-hexopyranosid-3-ulose (5) reacted with aromatic amines to give, in a rearrangement process, N-aryl-2-aryliminomethyl-4,6-O-benzylidene-2-deoxy--
-erythro-hex-1-enopyranosylamin-3-uloses. The reaction of 5 which hydrazine hydrate afforded 5-methylthio-(methyl-4,6-O-benzylidene-2,3-dideoxy--
-erythro-hexopyranosido)[3,2-c]pyrazole. 相似文献
45.
The gene for the purine salvage enzyme hypoxanthine phosphoribosyltransferase (HPRT) is expressed at a low level in many cells. As is the case with several other “housekeeping genes,” thorough studies of hprt gene regulation have been hampered by the low levels of its mRNA. We have used RNA/RNA hybridization in solution to determine the concentration of hprt-RNA in human cells. The sensitivity and specificity of the method have been validated, and it is shown that hprt-RNA can be accurately determined at a level of a few mRNA molecules per cell. As expected for a housekeeping gene, low and relatively constant hprt-RNA levels (0.3–0.8 pg/μg DNA) were found in primary cultures of normal amnion cells and fibroblasts, EBV-transformed lymphoblastoid cell lines, neuroblastoma, glioblastoma, and melanoma cell cultures. While resting lymphocytes were found to contain very low amounts of hprt-RNA, lymphocytes stimulated with phytohemagglutinin (PHA) showed a 10-fold increase to about 0.8–1.2 pg/μg DNA, which corresponds to 6–10 hprt-RNA molecules per cell. The level started to increase about 20 h after PHA stimulation, 5–10 h before the onset of DNA synthesis, and a steady-state level was reached after 2–3 days in culture. In PHA-stimulated lymphocytes from two brothers with inherited HPRT deficiency (LeschNyhans syndrome), the hprt-RNA level in PHA-stimulated lymphocytes was only about 25% of that in normal subjects. In T-cells selected for HPRT deficiency by growth in 6-thioguanine medium, the levels of hprt-RNA were either normal or very low, which probably reflects the different nature of the mutations involved. These results demonstrate the sensitivity of this method for determinations of low levels of RNA and clearly show induction of hprt-RNA after mitogenic stimulation of human lymphocytes. 相似文献
46.
47.
48.
Ferd Heine 《Journal of Ornithology》1859,7(5):329-337
Ohne Zusammenfassung 相似文献
49.
50.