全文获取类型
收费全文 | 6514篇 |
免费 | 775篇 |
国内免费 | 4篇 |
出版年
2021年 | 79篇 |
2019年 | 52篇 |
2018年 | 75篇 |
2017年 | 57篇 |
2016年 | 106篇 |
2015年 | 190篇 |
2014年 | 182篇 |
2013年 | 268篇 |
2012年 | 280篇 |
2011年 | 341篇 |
2010年 | 200篇 |
2009年 | 177篇 |
2008年 | 267篇 |
2007年 | 254篇 |
2006年 | 216篇 |
2005年 | 253篇 |
2004年 | 223篇 |
2003年 | 227篇 |
2002年 | 210篇 |
2001年 | 178篇 |
2000年 | 147篇 |
1999年 | 161篇 |
1998年 | 99篇 |
1997年 | 74篇 |
1996年 | 77篇 |
1995年 | 68篇 |
1994年 | 83篇 |
1993年 | 69篇 |
1992年 | 124篇 |
1991年 | 116篇 |
1990年 | 136篇 |
1989年 | 118篇 |
1988年 | 118篇 |
1987年 | 98篇 |
1986年 | 86篇 |
1985年 | 97篇 |
1984年 | 90篇 |
1983年 | 72篇 |
1982年 | 76篇 |
1981年 | 84篇 |
1980年 | 64篇 |
1979年 | 83篇 |
1978年 | 60篇 |
1977年 | 53篇 |
1976年 | 58篇 |
1975年 | 65篇 |
1974年 | 64篇 |
1973年 | 70篇 |
1972年 | 64篇 |
1968年 | 51篇 |
排序方式: 共有7293条查询结果,搜索用时 53 毫秒
101.
Action of Inhibitors of Ammonia Assimilation on Amino Acid Metabolism in Hordeum vulgare L. (cv Golden Promise) 总被引:6,自引:3,他引:3
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Barley (Hordeum vulgare L. cv Golden Promise) plants were grown in a continuous culture system in which the root and shoot ammonia and amino acid levels were constant over a 6-hour experimental period. Methionine sulfoximine (MSO), 1 millimolarity when added to the culture medium, caused a total inactivation of root glutamine synthetase with little effect on the shoot enzyme. Root ammonia levels increased and glutamine levels decreased, irrespective of whether the plants were grown in 1 millimolar nitrate or 1 millimolar ammonia. Levels of glutamate, aspartate, serine, threonine, and asparagine all increased. There was little alteration in the amino acid and ammonia levels in the shoot, suggesting that MSO is not rapidly transported.
The addition of azaserine (25 micrograms per milliliter) to nitrate-grown plants caused a rapid increase in root ammonia, glutamine, and serine levels with a corresponding decrease in glutamate, aspartate, and alanine. Glutamine levels also increased in the shoot.
The in vivo effect of MSO and azaserine was as would be predicted by their known in vitro inhibitory action if the glutamine synthetase/glutamate synthase pathway of ammonia assimilation was in operation.
相似文献102.
103.
104.
105.
106.
Cytokinesis essentially similar to that of vascular plants occurs in Ulothrix, an unbranched filamentous green alga. Plasmodesmata, similar to those of vascular plants, but different from those of many other algae, are also present. Cell plate formation and plasmodesmata also occur in Stigeoclonium, a branched green alga. 相似文献
107.
108.
William H. Petri James W. Fristrom Dan J. Stewart E. W. Hanly 《Molecular & general genetics : MGG》1971,110(3):245-262
Summary Late third instar imaginal discs of Drosophila melanogaster cultured in vitro in Robb's tissue culture medium synthesize 38S, 28S and 18S ribosomal RNAs which are qualitatively indistinguishable from their in vivo synthesized counterparts (Fig. 1). As found in other insect systems, the 38S molecule appears to be the precursor for both the 28S and 18S rRNAs (Figs. 2, 3 and 4). The 28S rRNA and a portion of the 38S pre-rRNA shift in sedimentation value upon exposure to heat or dimethylsulfoxide (Figs. 5 and 8). Studies of the thermal denaturations of these molecules (Figs. 6, 7 and 9) indicate the existence of a single class of 28S rRNA, but three classes of 38S pre-rRNAs. The addition of -ecdysone to the in vitro culture medium stimulates the net amount of rRNA synthesized, increases the rate of processing of the 38S precursor and increases the relative amount of 18S material produced (Figs. 10 and 12).This work was supported in part by grants from the National Science Foundation (GB-8176) and from the Atomic Energy Commission (AT-04-3-34).Predoctoral Trainees, PHS Training Grant No. 2-Tl-GM367 from Research Training Grants Branch, National Institute of General Medical Sciences.1 For purposes of simplification we shall refer to the rRNA molecules of D. melanogaster as being 38S, 30S, 28S and 18S; however, it should be noted that these values are approximate (see Hastings and Kirby, 1966; Greenberg, 1969; Tartof and Perry, 1970). 相似文献
109.
W. D. P. Stewart 《Plant and Soil》1971,35(1):377-391
Summary The various types of blue-green algae known to fix nitrogen are considered. Particular attention has been paid to the effects
of oxygen and other physiological parameters on nitrogenase activity and on the ecological distribution of the group. Data
on nitrogen fixation by cell-free extracts of blue-green algae are presented. 相似文献
110.
Androgen dynamics in vitro in the normal and hyperplastic human prostate gland 总被引:6,自引:6,他引:0
下载免费PDF全文
![点击此处可从《The Biochemical journal》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The dynamics of uptake and metabolism in vitro of androgens by normal and hyperplastic human prostate glands was studied by means of a new experimental design proposed by Gurpide & Welch (1969). Prostate slices were perfused with a medium containing [(3)H]testosterone and [(14)C]androstenedione, or 5alpha-dihydro-[(3)H]testosterone and [(14)C]testosterone. The entry into the slices, the irreversible metabolism, the conversion between the compounds and the tissue retention or ;uptake' of the steroids were measured at the steady state. A similar portion of the three androgens entered the tissue and was irreversibly metabolized. Conversion of testosterone into 5alpha-dihydrotestosterone was much greater than the interconversion of testosterone and androstenedione. The prostate slices retained 5alpha-dihydrotestosterone at a concentration three times that in the medium, whereas testosterone and androstenedione were retained to a smaller extent. At a steroid concentration of 0.11mumol/l in the medium, the various parameters did not differ significantly in experiments performed with slices from normal and hyperplastic glands. When the steroid concentration in the medium was increased tenfold, however, a difference between normal and hyperplastic glands was evident. The normal glands increased the uptake and metabolism proportionally to the elevation of the steroid concentration in the medium. In the hyperplastic glands the entry and metabolism lagged behind the increase in steroid supply, whereas the tissue uptake became disproportionately high. The possible causes of this finding are discussed. 相似文献