全文获取类型
收费全文 | 1963篇 |
免费 | 154篇 |
国内免费 | 1篇 |
出版年
2021年 | 18篇 |
2020年 | 10篇 |
2019年 | 9篇 |
2018年 | 27篇 |
2017年 | 15篇 |
2016年 | 29篇 |
2015年 | 63篇 |
2014年 | 59篇 |
2013年 | 89篇 |
2012年 | 105篇 |
2011年 | 92篇 |
2010年 | 84篇 |
2009年 | 77篇 |
2008年 | 126篇 |
2007年 | 117篇 |
2006年 | 84篇 |
2005年 | 91篇 |
2004年 | 97篇 |
2003年 | 84篇 |
2002年 | 89篇 |
2001年 | 22篇 |
2000年 | 16篇 |
1999年 | 23篇 |
1998年 | 31篇 |
1997年 | 22篇 |
1996年 | 17篇 |
1995年 | 17篇 |
1994年 | 13篇 |
1993年 | 26篇 |
1992年 | 14篇 |
1991年 | 11篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1987年 | 14篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 9篇 |
1982年 | 21篇 |
1981年 | 22篇 |
1980年 | 15篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 15篇 |
1976年 | 10篇 |
1975年 | 12篇 |
1973年 | 9篇 |
1970年 | 9篇 |
1967年 | 9篇 |
1956年 | 11篇 |
1954年 | 9篇 |
排序方式: 共有2118条查询结果,搜索用时 31 毫秒
41.
Two measures of group activity were assessed using 14 captive groups of pigtail monkeys (M. nemestrina). Absolute frequencies of movement of individuals (a direct measure of activity) was used as a criterion in the evaluation
of a sampling technique (an indirect measure of activity). The indirect measure involved monitoring the frequency of movement
of individuals past a fixed point in the environment. The indirect measure correlated highly with the criterion measure (r=.91). use of indirect measures of activity such as the one described here can increase the efficiency of data collection
by allowing observers sufficient time to monitor a greater range of behaviors. However, sampling techniques should be carefully
evaluated in an appropriate context before they are adopted for general use.
This project was supported in part by each of the following grants: an intramural grant from the University’s Innovative Fund
for under-graduate education to the Department of Psychology, and USPHS/NIH grants RR00166 to the Regional Primate Research
Center, and HD08633, toGene P. Sackett, Child Development and Mental Retardation Center, University of Washington. 相似文献
42.
Esther Senn Erwin Scharrer Siegfried Wolffram 《Biological trace element research》1992,33(1-3):103-108
The influence of glutathione (1 mmol/L) (GSH) on in vitro mucosal uptake and in vivo absorption of75Se-labeled selenite (10 μmol/L) was investigated in rat jejunum. For comparison, the effect ofl-cysteine (1 mmol/L) on in vivo absorption of75Se-labeled selenite was also studied. In the in vitro, uptake experiments, only the mucosal surface was exposed to the incubation
medium for 3 min. For the in vivo experiments, a luminal perfusion technique was employed. GSH inhibited in vitro mucosal
Se uptake, whereas absorption in vivo was stimulated by GSH.l-Cysteine also stimulated in vivo Se absorption, confirming former in vitro mucosal uptake experiments. Thus, unlikel-cysteine, GSH affected in vitro and in vivo absorption of Se from selenite differently. Enzymatic cleavage of products of
the reaction of selenite with GSH occuring more efficiently under in vivo than in vitro conditions may be a prerequisite for
the stimulatory effect of GSH on Se absorption. This apparently does not apply to the stimulatory effect of cysteine. Since,
GSH occurs in the intestinal lumen under physiological conditions, it may contribute to the high bioavailability of Se from
selenite. 相似文献
43.
Erwin Scharrer Esther Senn Siegfried Wolffram 《Biological trace element research》1992,33(1-3):109-120
The influence of several thiols (conc. 1 mmol/L) on mucosal uptake of75Se from75Se-labeled selenite (conc. 10 μmol/L) across the brush border of rat jejunum and cecum was investigated in vitro using a short-term
uptake technique.l-Cysteine (l-Cys) stimulated75Se uptake in the mid- and distal jejunum and cecum, but not in the proximal jejunum. The effect was maximal in the distal
jejunum.d-Cys was less effective in the jejunum and similarly effective in the cecum.l-Leucine (l-Leu) andl-glutamic acid significantly reduced the stimulatory effect ofl-Cys on Se uptake in the distal jejunum, whereas the respective effect ofd-Cys was not diminished byl-Leu. Cysteamine stimulated mucosal75Se uptake at all intestinal sites tested, whereas the effect of mercaptopyruvate was restricted to the distal jejunum. Thioglycolate
also enhanced75Se uptake in the distal jejunum. The stimulatory effects ofl-Cys, mercaptopyruvate, and thiologlycolate were Na+-dependent, whereas the effect of cysteamine also occurred in the absence of Na+. Mercaptosuccinate,d-penicillamine, ergothioneine, and thiosulfate did not enhance mucosa75Se uptake. It is concluded from these findings that the reaction of some thiols with selenite results in Se compounds that
are rapidly absorbed by the intestinal epithelium through various Na+-dependent and Na+-independent, mechanisms. The high bioavailability of Se from selenite found by others might thus be the result of the presence
of thiols in the gastrointestinal tract. 相似文献
44.
Three analogues of the peptidyl pheromone, pheromone of Saccharomyces kluyveri, synthesized based on the amino acid sequence proposed by Sato et al. (Agric Biol Chem 45:1531–1533, 1981) were tested for both shmoo-inducing and agglutinability-inducing actions. Purified natural pheromone of the yeast showed the highest activity among the peptides tested. When methionine in the peptides was oxidized, the activity decreased significatly. Pheromone of S. kluyveri induced sexual agglutinability in a cells of Saccharomyces cerevisiae, and shmoo in a cells of S. cerevisiae and S. kluyveri. a Pheromone of S. kluyveri had no agglutinability-inducing action on cells of S. cerevisiae. a Cells of S. kluyveri inactivated only pheromone of the same species, but a cells of S. cerevisiae inactivated pheromones of both S. cerevisiae and S. kluyveri. 相似文献
45.
We present here a procedure for the rapid measurement of both DNA and protein from the same aliquot of cell lysate. DNA estimates obtained by this method were compared to replicate determinations using the method of Kissane and Robins (1). The optimal range for the estimation of DNA (1 to 20 μg) is well suited for use with portions of extracts from individual cell cultures; the remainder of the extract remains available for enzyme assays or other parallel determinations. 相似文献
46.
Effects of inhibitors of ornithine and S-adenosylmethionine decarboxylases on L6 myoblast proliferation 总被引:2,自引:0,他引:2
C M Stoscheck B G Erwin J R Florini R A Richman A E Pegg 《Journal of cellular physiology》1982,110(2):161-168
The role of polyamines in myoblast proliferation was studied by treating cells of Yaffe's L6 line of rat myoblasts with inhibitors of polyamine synthesis. Both an irreversible inhibitor of ornithine decarboxylase--difluoromethyl-ornithine (DFMO)--and a competitive inhibitor of S-adenosyl-methionine decarboxylase--methylglyoxal-bis(guanylhydrazone) (MGBG)--depressed spermidine levels and inhibited myoblast proliferation. Spermine levels were not significantly depressed by either inhibitor and putrescine levels were decreased only by DFMO. Putrescine and spermidine, but not magnesium, prevented inhibition of myoblast proliferation by DFMO and MGBG; determination of 14C-DFMO uptake in the presence and absence of these compounds demonstrated that they did not reduce the rate or extent of inhibitor uptake and thus prevent its inhibition of ornithine decarboxylase. Thus it seems likely that these inhibitors reduce cell proliferation by inhibiting polyamine formation. Addition of spermidine to the cells led to a substantial reduction in the activity of S-adenosyl-methionine-decarboxylase, suggesting that the enzyme is subject to negative regulation by the products of the polyamine biosynthetic pathway. Unexpectedly, addition of spermidine also increased intracellular putrescine levels; this apparently resulted from conversion of spermidine to putrescine. Addition of putrescine or spermidine in the absence of serum did not increase the rate of myoblast proliferation although it did elevate intracellular polyamine levels as expected. We conclude that some threshold level of one or more polyamines (probably spermidine) is necessary but not sufficient for initiation and maintenance of myoblast proliferation in culture. 相似文献
47.
Bile acid amides and oxazolines were synthesized by a sequence of steps involving the reaction of the free bile acid with formic acid to yield the formyloxy derivative, preparation of the formyloxy acid chloride, condensation of the acid chloride with 2-amino-2-methyl-1-propanol to give the amide and, finally, cyclization of the amide with thionyl chloride to give the oxazoline. The oxazolines were characterized by physical constants, thin layer and gas-liquid chromatography and identified by elemental analysis and gas-liquid chromatography-mass spectrometry. Some of the bile acid oxazoline derivatives alter the activity of bacterial 7-dehydroxylases , and inhibit the growth of certain anaerobic bacteria in pure culture. 相似文献
48.
49.
The repetition frequency of a highly repetitive DNA sequence has been measured in the genomes of Ch. thummi thummi and Ch. th. piger. This sequence is known to be involved in the evolutionary duplication of defined chromosomal segments leading to a significant increase in the genome size of Ch. th. thummi. Reassociation of this highly repetitive DNA sequence which has a repeat length of 120 base-pairs, with total Ch. th. thummi and Ch. th. piger DNA has shown that the repetition frequency in the Ch. th. thummi DNA is 5.5 fold higher than in Ch. th. piger. In both genomes a 120 base-pair sequence is present as tandemly repeated sequence as shown by Southern analysis. 相似文献
50.