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Proteoglycan
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◆ Glycosaminoglycans
● Each glycosaminoglycan chain is composed of a repeating disaccharide; that is, it has the structure A-B-A-B-A-, where A and B represent two different sugars. ● GAGs are highly acidic due to the presence of both sulfate and carboxyl groups attached to the sugar rings. ●糖胺聚糖的二糖单位之一是氨基己糖(氨基葡萄糖或 氨基半乳糖), 故称为糖胺聚糖。 ●糖胺聚糖包括∶ 硫酸软骨素(chondroitin sulfate) 透明质酸(hyaluronic acid)等。
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◆细胞外基质的组成可分为三大类∶
●蛋白聚糖(proteoglycan) ●结构蛋白 如胶原和弹性蛋白，它们赋予基质一定 的强度和韧性。 ●黏着蛋白(adhesive proteins) 如纤连蛋白和层粘蛋白，它们促使细胞 同基质结合。
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4.2.1 细胞外基质的基础物质
蛋白聚糖(Proteoglycan,PG)
■Structure Proteins: Structure Protein functions are not well understood. One class, the extensins, provide structural support and may form a barrier to invading microorganisms.
●Hemicellulose molecules bind to the surfaces of cellulose microfibrils, cross-linking them into a complex structural network.
■Pectins：
●Pectins are a heterogeneous class of negatively charged polysaccharides containing galacturonic acid（半乳糖醛酸）； ●Like the glycosaminoglycans of animal cell matrices, pectins hold water and thus form an extensive hydrated gel（水合胶）that fills in the spaces between the fibrous elements； ●When a plant is attacked by pathogens, fragments of pectins released from the wall trigger a defensive response by the plant cell； ●Purified pectin is used commercially to provide the gel-like consistency of jams and jellies.
◆基本特性： ●Collagen is a family of fibrous glycoproteins that are present only in extracellular matrices. ●Collagen is the single most abundant protein in the human body (constituting more than 25 percent of all protein), ● Collagens are found throughout the animal kingdom and are noted for their high tensile strength, that is, their resistance to pulling forces. It is estimated that a collagen fiber 1 mm in diameter is capable of suspending a weight of 10 kg without breaking. ● Collagen is produced primarily by fibroblasts, the cells found in various types of connective tissues, and 28 also by smooth muscle cells and epithelial cells.
CHAPTER 4
Interactions Between
Cells and Their Environment
OUTLINE
4.1 CELL SURFACE
4.2 CELL ENVIRONMENT
4.3 CELL RECOGNIZATION 4.4 CELL JUNCTION
An overview of cells interact with their environment
电镜下的细胞表面
细胞表面结构
4.1.2 PLANT CELL WALLS
植物细胞壁的作用
◆提供机械强度
◆保护细胞免遭渗透及机械损伤 ◆植物细胞壁是一个动态结构，能够进行很 多活动: ●与细胞壁有关的酶能够吸收细胞外的营养， 生成能够通过细胞膜进入细胞的小分子化 合物。 ●细胞壁也可以作为物质通透的障碍，在代 谢和分泌过程中起重要作用。
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构 成 糖 胺 聚 糖 的 二 糖
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◆透明质酸(hyaluronic acid)
细胞外基质中发现的大多数糖胺聚糖
都是作为蛋白聚糖的一个成分而存在，
惟一例外的是透明质酸，它在细胞外
基质中也可游离存在。
透明质酸在结缔组织中起强化、 弹性和润滑作用,具有抗压能力。
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4.2.2 结构蛋白
胶原(Collagen)
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For example:
●Enzymatic digestion of the ECM that
surrounds cultured cartilage(软骨） cells
or mammary gland epithelial cells causes a marked decrease in the synthetic and secretory activit extracellular matrix materials
back to the culture dish restores the
differentiated state of the cells and their ability to produce their usual cell products.
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Typical Extracellular matrix
◆结构与功能
● Protein-polysaccharide complex is called a proteoglycan ●A proteoglycan consists of a core protein molecule to which chains of glycosaminoglycans (GAGs,糖胺聚糖) are covalently attached. ●Because of the negative charges borne on the sulfated GAGs, proteoglycans bind huge numbers of cations, which in turn bind large numbers of water molecules. As a result, proteoglycans form a porous, hydrated gel that fills the extracellular space like packing material and resists crushing (compression) forces.
4.1 Cell Surface
◆在结构上，包括：
●细胞被(cell coat) ●细胞质膜
●表层胞质溶胶(corticol cytosol)
◆在功能上：
●保护细胞,使细胞有一个相对稳定的内环境； ●参与细胞内外的物质交换和能量交换； ●参与信号的识别和信息的传递； ●参与细胞运动； ●维护细胞的各种形态； ●与免疫、癌变等有十分密切关系。
PLANT CELL WALLS
4.2 Extracellular matrix
◆Many types of animal cells are surrounded by an extracellular matrix (ECM)--an organized network of extracellular materials that is present beyond the immediate vicinity of the plasma membrane. ◆The ECM is more than an inert packing material, it often plays a key regulatory role in determining the shape and activities of the cell . ◆能够分泌和形成细胞外基质的主要细胞类群是 成纤维细胞(fibroblast)和其他特化组织的细胞。
植物细胞壁的分泌合成
◆中间层形成： 主要是果胶。果胶是相邻两细胞壁所共有， 并且具有将两个细胞连接在一起的作用。 ◆初生壁形成： 是细胞正在生长时形成的。初生壁的厚度为 100-200nm,与动物细胞的基膜相当。初生壁 由纤维素、半纤维素、果胶和糖蛋白等松散 组成。 ◆次生壁形成： 是在细胞停止生长后分泌形成的。主要是增 加细胞壁的厚度和强度。次生壁主要在初生 壁的内层。