全文获取类型
收费全文 | 8898篇 |
免费 | 541篇 |
国内免费 | 296篇 |
出版年
2023年 | 96篇 |
2022年 | 87篇 |
2021年 | 217篇 |
2020年 | 245篇 |
2019年 | 363篇 |
2018年 | 363篇 |
2017年 | 204篇 |
2016年 | 183篇 |
2015年 | 189篇 |
2014年 | 541篇 |
2013年 | 637篇 |
2012年 | 361篇 |
2011年 | 529篇 |
2010年 | 343篇 |
2009年 | 429篇 |
2008年 | 432篇 |
2007年 | 464篇 |
2006年 | 388篇 |
2005年 | 321篇 |
2004年 | 255篇 |
2003年 | 223篇 |
2002年 | 206篇 |
2001年 | 129篇 |
2000年 | 119篇 |
1999年 | 117篇 |
1998年 | 130篇 |
1997年 | 133篇 |
1996年 | 116篇 |
1995年 | 147篇 |
1994年 | 93篇 |
1993年 | 94篇 |
1992年 | 100篇 |
1991年 | 87篇 |
1990年 | 79篇 |
1989年 | 79篇 |
1988年 | 62篇 |
1987年 | 87篇 |
1986年 | 58篇 |
1985年 | 102篇 |
1984年 | 140篇 |
1983年 | 118篇 |
1982年 | 135篇 |
1981年 | 95篇 |
1980年 | 88篇 |
1979年 | 69篇 |
1978年 | 51篇 |
1977年 | 62篇 |
1976年 | 39篇 |
1974年 | 41篇 |
1973年 | 35篇 |
排序方式: 共有9735条查询结果,搜索用时 140 毫秒
1.
The hypothesis that resource monopolization and defense increaseas the spatial clumping of resources increases was tested usinggroups of three convict cichlids competing for 120 Daphnia magnaprey. Spatial clumping was manipulated by varying the distance(3, 20, or 40 cm) between three tubes through which the preyappeared. As predicted, monopolization of prey (percentage eatenby the dominant fish) and frequency of aggression (chases perminute) by dominant fish increased significantly as the distancebetween the tubes decreased. However, there was no evidenceof individual flexibility in the aggressiveness (percentageof conspecifics chased) of dominant fish across treatments.Differences among dominant fish in aggressiveness were positivelycorrelated with their ability to monopolize prey, but the strengthof the correlation decreased as the distance between the tubesincreased. Aggression appears to be a more effective mechanismof interference competition when resources are clumped thanwhen resources are dispersed. 相似文献
2.
K. Yoshikawa 《Journal of biological physics》2002,28(4):701-712
The mechanism of the self-regulation of gene expression in living cells is generally explained by considering complicated networks of key-lock relationships, and in fact there is a large body of evidence on a hugenumber of key-lock relationships. However, in the present article we stress that with the network hypothesis alone it is impossible to fully explain the mechanism of self-regulation in life. Recently, it has been established that individual giant DNA molecules, larger than several tens of kilo base pairs, undergo a large discrete transition in their higher-order structure. It has become clear that nonspecific weak interactions with various chemicals, suchas polyamines, small salts, ATP and RNA, cause on/off switching in the higher-order structure of DNA. Thus, the field parameters of the cellular environment should play important roles in the mechanism of self-regulation, in addition to networks of key and locks. This conformational transition induced by field parameters may be related to rigid on/off regulation, whereas key-lock relationships may be involved in a more flexible control of gene expression. 相似文献
3.
Using cub growth as an index, I examine the influence of maternalnutrition, litter size, and cub sex on maternal care in cheetahs(Acinonyx jubatus) and compare cub and litter growth rates withthose of other large feilds. Seventy-nine free-living cheetahcubs in 21 litters from 15 mothers were weighed at least oncebetween 6 and 48 days of age. Eleven litters were weighed atthe beginning and end of a 5-day observation of their mothers.The mean cub growth rate varied significantly between litters,due primarily to differences in maternal food intake. Growthdeclined sharply when maternal food intake was less than 1.5kg/ day, but did not increase with greater levels of food intake.Lower limits of growth rates may therefore have been set bythe mother's food intake, whereas upper limits may be set bythe intrinsic physiological ability of cubs to grow. Althoughmale cubs were heavier than female cubs in the same litter whenfirst weighed, major differences in growth rate between thesexes were not apparent at this stage. Both cheetah cubs andlitters grow fast relative to other large felids, and I arguethat this may be an adaptation to the high rate of cheetah juvenilemortality from predation. 相似文献
4.
5.
6.
7.
8.
9.
Hideko Ohgusu 《Biochemical and biophysical research communications》2009,386(1):153-158
Ghrelin is a peptide hormone in which serine 3 is modified by n-octanoic acid through GOAT (ghrelin O-acyltransferase). However, the enzymological properties of GOAT remain to be elucidated. We analyzed the in vitro activity of GOAT using the recombinant enzyme. Unexpectedly, although the main active form of ghrelin is modified by n-octanoic acid, GOAT had a strong preference for n-hexanoyl-CoA over n-octanoyl-CoA as an acyl donor. Moreover, a four-amino acid peptide derived from the N-terminal sequence of ghrelin can be modified by GOAT, indicating that these four amino acids constitute the core motif for substrate recognition by the enzyme. 相似文献
10.
Zhaohua Zhang LiLi Ge Shanshan Zhang Jue Wang Wen Jiang Qian Xin Yun Luan 《Journal of cellular and molecular medicine》2020,24(23):13938
The aim of the study was to explore the mechanism of mesenchymal stem cell‐derived exosomes (MSC‐EXO) to protect against experimentally induced pulmonary hypertension (PH). Monocrotaline (MCT)‐induced rat model of PH was successfully established by a single intraperitoneal injection of 50 mg/kg MCT, 3 weeks later the animals were treated with MSC‐EXO via tail vein injection. Post‐operation, our results showed that MSC‐EXO could significantly reduce right ventricular systolic pressure (RVSP) and the right ventricular hypertrophy index, attenuate pulmonary vascular remodelling and lung fibrosis in vivo. In vitro experiment, the hypoxia models of pulmonary artery endothelial cell (PAEC) and pulmonary vascular smooth muscle cell (PASMC) were used. We found that the expression levels of Wnt5a, Wnt11, BMPR2, BMP4 and BMP9 were increased, but β‐catenin, cyclin D1 and TGF‐β1 were decreased in MSC‐EXO group as compared with MCT or hypoxia group in vivo or vitro. However, these increased could be blocked when cells were transfected with Wnt5a siRNA in vitro. Taken together, these results suggested that the mechanism of MSC‐EXO to prevent PH vascular remodelling may be via regulation of Wnt5a/BMP signalling pathway. 相似文献