全文获取类型
收费全文 | 45887篇 |
免费 | 3096篇 |
国内免费 | 20篇 |
专业分类
49003篇 |
出版年
2023年 | 247篇 |
2022年 | 576篇 |
2021年 | 1021篇 |
2020年 | 594篇 |
2019年 | 760篇 |
2018年 | 1078篇 |
2017年 | 897篇 |
2016年 | 1525篇 |
2015年 | 2301篇 |
2014年 | 2466篇 |
2013年 | 3378篇 |
2012年 | 3866篇 |
2011年 | 3755篇 |
2010年 | 2298篇 |
2009年 | 1993篇 |
2008年 | 2798篇 |
2007年 | 2735篇 |
2006年 | 2467篇 |
2005年 | 2224篇 |
2004年 | 2118篇 |
2003年 | 2036篇 |
2002年 | 1885篇 |
2001年 | 434篇 |
2000年 | 313篇 |
1999年 | 393篇 |
1998年 | 456篇 |
1997年 | 331篇 |
1996年 | 326篇 |
1995年 | 301篇 |
1994年 | 275篇 |
1993年 | 288篇 |
1992年 | 207篇 |
1991年 | 211篇 |
1990年 | 211篇 |
1989年 | 158篇 |
1988年 | 138篇 |
1987年 | 135篇 |
1986年 | 111篇 |
1985年 | 142篇 |
1984年 | 137篇 |
1983年 | 102篇 |
1982年 | 111篇 |
1981年 | 106篇 |
1980年 | 110篇 |
1979年 | 93篇 |
1978年 | 63篇 |
1977年 | 78篇 |
1975年 | 62篇 |
1974年 | 61篇 |
1973年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
Oxidative phosphorylation in an extract of Anabaena variabilis. 总被引:5,自引:0,他引:5
43.
Reduction of a Tetrazolium Salt in Determining Growth Activity of Yeast-Phase Histoplasma capsulatum 总被引:3,自引:0,他引:3 下载免费PDF全文
Maria E. Reca 《Applied microbiology》1968,16(2):236-238
A colorimetric method using a tetrazolium compound, 3,4,5-dimethylthiozalil-(1 or 2),2,5-diphenyltetrazolium bromide (TTBr), was developed for studying the growth activity of yeast-phase Histoplasma capsulatum. Materials extracted in phosphate buffer, pH 7.0, from cells at different stages of growth reduced TTBr. Colorimetric changes were correlated with enzymatic activity. Under standardized conditions specified herein, the optical density of the reduced tetrazole was an index of the growth activity of the organism. 相似文献
44.
Maria S. Cordova Ruben Lisker Alvar Loria 《American journal of physical anthropology》1967,26(1):55-65
Blood samples from 1090 Mexican Indians belonging to the Chol, Chontal, Totonac, Huastec, Mixe, Mazatec, Zapotec, Mixtec, Chinantec, Nahua, Cora and Huichol linguistic groups, were obtained and examined in regard to the following blood group antigens: A, B. M, N, P, C, c, D, E, e, Fy(a), K and Di(a). The gene frequencies were similar to what has been described for other Amerindians; high values for O, M, CDe, cDE and Duffy; low to absent Kell and presence of Diego in variable amounts. The frequency of chromosomes CDE and cDe/cde was somewhat higher than usual and some of the tribes had relatively high frequencies of the A and B antigens. It was felt that variable degrees of non-Indian admixture was at least partially responsible for this situation. A previous study dealing with the distribution of abnormal hemoglobins and glucose-6-phosphate dehydrogenase deficiency in these same tribes, had strongly suggested the possibility of some Negro admixture in the Chontal, Nahua and Cora tribes. However, this was not specifically reflected in their blood group distribution. This served to emphasize the need of investigating as many markers as possible when trying to characterize a population. 相似文献
45.
46.
Ian Carr 《Cell and tissue research》1967,80(1):140-144
Summary Membranous whorls have been seen in the nuclei of peritoneal and testicular cells which had been subjected to various experimental manoeuvres. It seems likely that this is an early manifestation of cell degeneration which is demonstrated readily only by glutaraldehyde fixation, and to that extent can be regarded as a glutaraldehyde artifact.
Acknowledgements. This work was supported by grants from the Medical Research Council, and the University of Sheffield Tuberculosis Research Fund, and by a grant to the Department from Unilever Ltd.I am grateful to Professor R. Barer for his advice and criticism, to Dr. G. A. Meek for guidance on electron microscopy, to Dr. E. J. Clegg for permission to use material from joint experiments. Technical and photographic assistance was provided by Messrs. P. GarLick and L. Murgatroyd and by Miss M. Tune. 相似文献
47.
Unexposed blanks prepared from Kodak AR 10 stripping film were dried by 3 methods: (1) fast, open drying with a fan for 25 min, (2) slow drying in a desiccator for 6 hr, and (3) very slow drying in a desiccator for 24 hr. The number of background grains depended on the mode of drying. Fast drying (method 1) gave 0.7 grain per 100 μ2, slow drying (method 2) gave 0.33 grain; very slow drying (method 3), only 0.17 grain. The increase of background after fast drying is assumed to be caused by the rapid shrinkage of wet emulsion. This causes an increase in the intraemulsion pressure which, in turn, sensitizes the silver bromide crystals to cause an increase in the number of developable grains. 相似文献
48.
Ectonucleotidase Activities Associated with the Olfactory Organ of the Spiny Lobster 总被引:2,自引:0,他引:2
Henry G. Trapido-Rosenthal William E. S. Carr Richard A. Gleeson 《Journal of neurochemistry》1990,55(1):88-96
The olfactory system of the Florida spiny lobster, Panulirus argus, has olfactory receptors that are excited by the purine nucleotides AMP, ADP, and ATP. These receptors reside on chemosensory neurons that are contained within aesthetasc sensilla on the lateral filaments of the antennules. Also associated with the lobster's olfactory system are ectonucleotidase activities that dephosphorylate excitatory nucleotides, resulting in the production of the nonstimulatory nucleoside adenosine. Our studies of the 5'-ectonucleotidase, ecto-ADPase, and ecto-ATPase activities of this olfactory system showed that each activity was characterized by Michaelis-Menten kinetics; Michaelis constants ranged from 6.9 to 33.5 microM, and maximum velocities ranged from 2.5 to 28.8 fmol/sensillum/s. Evidence that AMP dephosphorylation may serve as an inactivation process was shown by the close correlation between the kinetics of 5'-ectonucleotidase activity and the periodicity of olfactory sampling. Decreased magnesium ion concentration or increased calcium ion concentration resulted in increased ecto-ATPase activity; this activity was insensitive to vanadate ion. Ectonucleotidase activities may have multiple effects on the detection of exogenous nucleotides by a chemosensory system. These effects can be either direct, such as the conversion of an odorant to an inactive compound, or indirect, such as the conversion of an odorant to another compound that can activate or inhibit either receptors or enzymes associated with the system. 相似文献
49.
50.
Walter Malorni Pietro L. Indovina Giuseppe Arancia Stefania Meschini Maria T. Santini 《In vitro cellular & developmental biology. Plant》1990,26(4):399-410
Summary This paper describes the microscopic evidence supporting a cesium-induced delay in the fusion of chick embryo myoblast membranes
during in vitro myogenic differentiation. We have recently demonstrated that the sharp decrease in the conductivity and permittivity
of the membranes of these myogenic cells at the time of fusion is delayed 30 h by the addition of cesium to the culture medium
(Santini et al., Biochim. Biophys. Acta 945:56–64; 1988). We report here that this delay in fusion is substantiated by direct
microscopic observation and that cesium also induces ultrastructural changes in the myoblast cells themselves. Possible mechanisms
by which cesium may cause both the delay in fusion as well as the ultrastructural changes observed are discussed.
This investigation was partially supported by an Italian Consiglio Nazionale delle Ricerche grant 85.00.304.02 (to P. L. I.). 相似文献