全文获取类型
收费全文 | 7469篇 |
免费 | 814篇 |
国内免费 | 3篇 |
专业分类
8286篇 |
出版年
2022年 | 75篇 |
2021年 | 139篇 |
2020年 | 80篇 |
2019年 | 89篇 |
2018年 | 123篇 |
2017年 | 113篇 |
2016年 | 224篇 |
2015年 | 361篇 |
2014年 | 347篇 |
2013年 | 396篇 |
2012年 | 514篇 |
2011年 | 423篇 |
2010年 | 262篇 |
2009年 | 227篇 |
2008年 | 291篇 |
2007年 | 265篇 |
2006年 | 275篇 |
2005年 | 233篇 |
2004年 | 208篇 |
2003年 | 180篇 |
2002年 | 227篇 |
2001年 | 182篇 |
2000年 | 165篇 |
1999年 | 163篇 |
1998年 | 83篇 |
1997年 | 52篇 |
1996年 | 72篇 |
1995年 | 94篇 |
1994年 | 63篇 |
1993年 | 80篇 |
1992年 | 141篇 |
1991年 | 120篇 |
1990年 | 139篇 |
1989年 | 99篇 |
1988年 | 118篇 |
1987年 | 111篇 |
1986年 | 92篇 |
1985年 | 103篇 |
1984年 | 83篇 |
1983年 | 78篇 |
1982年 | 80篇 |
1981年 | 77篇 |
1980年 | 68篇 |
1979年 | 99篇 |
1978年 | 92篇 |
1977年 | 68篇 |
1976年 | 66篇 |
1975年 | 57篇 |
1972年 | 58篇 |
1971年 | 51篇 |
排序方式: 共有8286条查询结果,搜索用时 15 毫秒
41.
Nitrate reductase of primary roots of red spruce seedlings : effects of acidity and metal ions 总被引:1,自引:0,他引:1 下载免费PDF全文
Nitrate reductase activity (NRA) was found in primary roots, but not in foliage of red spruce (Picea rubens Sarg.) seedlings. Nitrate induced NRA:NH4+ did not induce and slightly depressed NRA in older seedlings. Induction required 8 hours and, once induced, NRA decreased slowly in the absence of exogenous NO3−. Seedlings were grown in perlite with a complete nutrient solution containing NH4+ to limit NR induction. Established seedlings were stressed with nutrient solutions at pH 3, 4, or 5 supplemented with Cl− salts of Al, Cd, Pb, or Zn each at two concentrations. NRA in primary root tips was measured at 2, 14, 28, and 42 days. NRA induction was greatest at pH 3, and remained high during the period of study. NRA induction at pH 4 was lower. Metal ions suppressed NRA at pH 3 and 5, but enhanced NRA at pH 4. It is concluded that acidity and soluble metals in the root environment of red spruce are unlikely to be important factors in nitrogen transformations in red spruce roots. 相似文献
42.
cAMP induces a rapid and reversible modification of the chemotactic receptor in Dictyostelium discoideum 总被引:8,自引:2,他引:6 下载免费PDF全文
Stimulation, within 1 min after cAMP stimulation, of aggregation-competent Dictyostelium discoideum amebae was found to cause a rapid (within 1 min) modification of the cell's surface cAMP receptor. The modified receptor migrated on SDS PAGE as a 47,000-mol-wt protein, as opposed to a 45,000-mol-wt protein labeled on unstimulated cells. The length of time this modified receptor could be detected depended upon the strength of the cAMP stimulus: 3-4 min after treatment with 10(-7) M cAMP, cells no longer possessed the 47,000-mol-wt form of the cAMP receptor. Instead, the 45,000-mol-wt form was present. Stimulation of cells with 10(-5) M cAMP, however, resulted in the persistent (over 15 min) expression of the modified receptor. The time course, concentration dependence, and specificity of stimulus for this cAMP-induced shift in the cAMP receptor were found to parallel the cAMP-stimulated phosphorylation of a 47,000-mol-wt protein. In addition, both phenomena were shown to occur in the absence of endogenous cAMP synthesis. The possibility that the cAMP receptor is phosphorylated in response to cAMP stimulation, and the role of this event in cell desensitization, are discussed. 相似文献
43.
Development of heart cells in culture: studies using an affinity purified antibody to a myosin light chain 总被引:1,自引:0,他引:1
Cultured neonatal rat heart cells can be used to study the factors that regulate cardiac contractility and myocyte development in vitro. An antibody to the 26,000 dalton light chain of myosin (MLC1), has been produced and purified on a Sepharose 4B affinity column prepared with rat heart myosin. When primary cultures of myocytes are studied by indirect immunofluorescence using this antibody a predictable pattern of myofibrillar structure is observed to develop over 72 h. This myosin cytoskeleton is highly organized and the myosin fibrils exhibit cross striations. The antibody does not stain non-muscle heart cells and there is no evidence for myocyte division in culture. The qualitative immunofluorescent pattern of myosin organization is the same in both spontaneously beating and in non-contracting cells. 相似文献
44.
Defective in vitro T cell colony formation in the acquired immunodeficiency syndrome 总被引:7,自引:0,他引:7
A Winkelstein R S Klein T L Evans B W Dixon W L Holder L D Weaver 《Journal of immunology (Baltimore, Md. : 1950)》1985,134(1):151-156
Depressed T cell immunity is a universal characteristic of the acquired immunodeficiency syndrome (AIDS). In the present study, 25 patients with AIDS and opportunistic infections, 22 individuals with AIDS-related complex (ARC, or chronic lymphadenopathy syndrome), and 20 healthy homosexuals were evaluated by means of the T cell colony assay. Forty-seven healthy heterosexual controls showed an average of 3964 +/- 319 colonies/7.5 X 10(5) cells, with a range of 880 to 9340. The mean in the 20 healthy homosexuals (3173 +/- 483) did not differ significantly from the controls; in this group, only three patients had values less than 1000 colonies/plate. By contrast, all AIDS patients and 14 ARC patients had colony counts less than 1000. The mean value for the AIDS patients was only 24 +/- 15 (p less than 0.0005 compared with either controls or healthy homosexuals); values in the ARC group were intermediate (1180 +/- 360). The addition of interleukin 2 to the plates promoted correction of the proliferative abnormality in ARC patients. This interleukin increased colony scores in the AIDS group, but the mean value was still significantly less than controls. Comparison indicated that the colony assay is a more sensitive indicator for detecting proliferative abnormalities than responses to PHA, Con A, or pokeweed mitogen in suspension cultures. 相似文献
45.
46.
F de Foresta J Deleys-Hertoghs J Cornelis B Klein J Rommelaere 《Comptes rendus des séances de la Société de biologie et de ses filiales》1985,179(2):276-282
Human skin fibroblasts which are naturally resistant to Parvovirus H-1 can be lysed by this virus after SV40 transformation. This observation raises the possibility that oncosuppression by Parvovirus involves a direct oncolytic effect. 相似文献
47.
48.
Serum melatonin and pineal indoleamine metabolism in a species with a small day/night N-acetyltransferase rhythm 总被引:5,自引:0,他引:5
M A Namboodiri D Sugden D C Klein L Tamarkin I N Mefford 《Comparative biochemistry and physiology. B, Comparative biochemistry》1985,80(4):731-736
Ovine serum and pineal melatonin levels are low during the day, increase five to ten-fold at night, decrease during a light pulse at night, and rapidly increase to night levels following the light-dark transition. N-Acetyltransferase activity increases three-fold at night, falls significantly in response to the light pulse, but does not increase following the light pulse. No significant change in N-acetylserotonin occurs under these conditions. These results suggest that the biochemical mechanisms controlling pineal melatonin synthesis in the sheep pineal gland may be different from those in the rat. 相似文献
49.
Lumazine has been demonstrated to be one of the two main compounds responsible for the extracellular fluorescence linked with the aggregation ability of Dictyostelium discoideum, strain Ax-2. The other compound, also having lumazine properties, is, however, different from the 7-hydroxylumazine proposed previously. The influence of pH on the fluorescence of lumazine was studied. The possible use of extracellular pteridines as pH markers was stressed and a method for the determination of the amount of "lumazine equivalents" in the extracellular medium of aggregating D. discoideum cells is elaborated. 相似文献
50.
The genome complexities of different methanogenic bacteria were investigated by using an optical method to study renaturation kinetics of single-stranded DNA. The observed genome sizes ranged from 1.0 X 10(9) to 1.8 X 10(9) daltons, which is a typical range for procaryotic cells. Melting profiles of the DNA of three methanogenic species from different families show fractions which have a higher A . T content than the average DNA of that species. 相似文献