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This layer of carbohydrate helps protect the cell surface from mechanical damage. And because the oligosaccharides and polysaccharides attract water molecules, they also give the cell a slimy surface, which helps motile cells such as white blood cells squeeze through narrow spaces and prevents blood cells from sticking to one another or to the walls of blood vessels.

They have an important role in cell–cell recognition and adhesion. Transmembrane proteins called lectins are specialized to bind to particular oligosaccharide side chains. The oligosaccharide side chains of glycoproteins and glycolipids, although short (typically fewer than 15 sugar units), are enormously diverse. Unlike proteins, in which the amino acids are all joined together in a linear chain by identical peptide bonds, sugars can be joined together in many different arrangements, often forming elaborate branched structures. Using a variety of covalent linkages, even three different sugars can form hundreds of different trisaccharides.

The carbohydrate layer on the surface of cells in a multicellular organism serves as a kind of distinctive clothing, like a police officer’s uniform. It is characteristic of each cell type and is recognized by other cell types that interact with it. Specific oligosaccharides in the carbohydrate layer are involved, for example, in the recognition of an egg by sperm.

Similarly, in the early stages of a bacterial infection, carbohydrates on the surface of white blood cells called neutrophils are recognized by a lectin on the cells lining the blood vessels at the site of infection; this recognition causes the neutrophils to adhere to the blood vessel wall and then migrate from the bloodstream into the infected tissue, where they help destroy the invading bacteria.

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错误

《寂静的春天》是一篇科学小品,属于说明文的范畴。原作品于1962年问世,是世界上第一本将环保作为主题的书,被称为“生态运动”的起跑信号,作者是来自美国的雷切尔卡森。

《动物身上回进化出轮子来吗?》是一篇科普论文,既具有说明文的科学性,又具有议论文的严谨和思想性,文章事例丰富典型,数据科学精确,分析全面而清晰。

《昆虫记》也叫《昆虫物语》《昆虫世界》等,它既是一部科普著作,也是一部文学经典,作者是来自法国的法布尔,《我们周围的海洋》也是他的代表作。

《蝉》语言准确明晰、朴实生动,采用别具匠心的倒叙式说明顺序。

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经常被使用的变量可以定义成常量

常量分为整型常量、实型常量、字符常量和字符串常量

常量可分为数值型常量和非数值型常量

A)所谓常量，是指在程序运行过程中，其值不能被改变的量