宇宙中的暗物质

宇宙由什么组成？目前为止，我们还不知道。但是如果标准的引力理论成立，那么宇宙中的大部分物质是由一种无法被发现的形式存在，这些物质发出的光，当今的所有望远镜还不能观测到，这些看不见的物质就是暗物质。     

Changes of levels of glutamine synthetase isoforms in roots and leaves in response to nitrogen fertilizer application at different growth stages in irrigated rice

Nitrogen is a key element to control the growth and yield of crops. Fertilizer urea nitrogen (N) 60,45, and 30 kg/hm² was applied at three different stages, midtillering, panicle initiation, and flowering, of the growth and development of rice plants, respectively. At both midtillering and panicle initiation, the total activity of glutamine synthetase (GS) in rice roots and leaves was incrased remarkably as a result of a large amount of ammonia absorbed by roots. Native-PAGE and activity staining showed that the increase of total activity in rice roots and leaves was due to the synthesis of GSrb in roots and GS2 in leaves and that the activity of GSra in roots and GS1 in leaves remained constant. The results showed that the assimilation of external nitrogen was carried out by GSrb but not GSra in rice roots and that the activitry of GS2 was induced also by the external nitrogen, and that GSrb played main role in meeting the needs of the rapid tillering for nitrogen. At flowering, the activity of GS in rice roots and leaves did not change almost after topdressing. These rssults suggest that the change of GS activity in rice roots may use as a measure of the utilization efficiency of the fertilizer. Supported by the National Natural Science Foundation of China, Natural Science Foundation of Hubei Province, and International Rice Institute, P. O. Box 933 1099 Manila, Philippines Zhang Chufu: born in 1946, Professor, to whom Correspondence should be addressed     

Stimulated platelets use serotonin to enhance their retention of procoagulant proteins on the cell surface.

Activated platelets bind numerous adhesive and procoagulant proteins by receptor-mediated processes. Although there is little evidence to suggest that these processes are heterogeneous in platelets, we previously found that platelets co-stimulated with collagen and thrombin express functional alpha-granule factor V only on a subpopulation of cells. Here we show that these cells, referred to as 'COAT-platelets', bind additional alpha-granule proteins, including fibrinogen, von Willebrand factor, thrombospondin, fibronectin and alpha2-antiplasmin. These proteins are all transglutaminase substrates, and inhibitors of transglutaminase prevent the production of COAT-platelets. A synthetic transglutaminase substrate (CP15) also binds to COAT-platelets, and analysis by high performance liquid chromatography/mass spectrometry shows that a product is formed with a relative molecular mass (Mr) equal to CP15 plus 176. Serotonin, an abundant component of platelet-dense granules, has an Mr of 176, and fibrinogen isolated from COAT-platelets contains covalently linked serotonin. Synthetic bovine serum albumin-(serotonin)6 binds selectively to COAT-platelets and also inhibits the retention of procoagulant proteins on COAT-platelets. These data indicate that COAT-platelets use serotonin conjugation to augment the retention of procoagulant proteins on their cell surface through an as yet unidentified serotonin receptor.     

手中握有一根"大棒"

对爱德华·特勒强硬的政治态度和他在发展氢弹方面所起的作用几乎没人会提出疑义 ;然而 ,几乎没有哪一位科学家会像他那样招致意见相左的激烈反响。特勒于 1 90 8年出生在匈牙利一个富足的犹太人家庭。 1 8岁那年 ,他离开匈牙利前往德国 ,学了一段时间的化学以后 ,他最终在莱比锡攻读哲学博士学位 ,师从著名的物理学家维尔纳·海森伯。取得学位后 ,他与尼尔斯·玻尔和恩里科·费米共事 ,并在哥廷根大学谋得了一个初级教职。尽管当时出现了纳粹党人抬头的不祥之兆 ,特勒却出人意料地不问政治 ,可以理解的是 ,那时他正沉醉于量子物理这一新兴…     

技术创新、技术转移及其商品化——试论非线性研究方法的必要性

在过去的30年中，学者和决策者们已经形成了对技术创新、技术转移及其商品化的研究方法，以合理对待和加速全球的经济发展，然而，由于此类研究及相关政策采用线性组织方式，它们多数不能适应当今世界发展的需要，越来越多的证据表明，适用于过去工业经济系统的线性方法必须加以修正才能适应“新”经济中非线性的现实。     

卵形线轨道及其他置换难题

本书是美国数学协会（MAA）出版的数学通俗读物系列中的一本，通过一些与数字的排列有关的游戏和难题讲述置换群的基本理论，与文字材料同步配备了光盘（采用Windows95或其后版本等系统），读者可以在电脑上演绎本书的全部内容。