Cells160-page22

Cells160-page22 - linen, bamboo and hemp. It is also the...

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Cell - 22 The Cell Surface – External Structures Although the boundary of any cell is its cell or plasma membrane, the cells of many types of organisms, including plants, fungi, bacteria and many protists, have one or more rigid surface layers or a cell wall exterior to the plasma membrane. (Only animals and some protists and bacteria lack walls.) Cell walls are secreted by the cell they surround, and are composed of a number of different kinds of materials, depending on the Kingdom or Domain. Cell walls support, protect and provide shape to the cells they surround. Let's look a bit at the wall structure of plants. Plant Cell Wall Layers Primary Wall All plant cells secrete a primary cell wall exterior to the plasma membrane. The primary wall is composed extensively of cellulose, a polysaccharide of glucose discussed previously. Cellulose is non digestible for most organisms. It is the main constituent of wood, pulp and paper, and fabrics such as cotton,
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Unformatted text preview: linen, bamboo and hemp. It is also the bulk of the "fiber" of our diets. Secondary Walls Secondary walls consist of layers secreted interior to the primary wall and provide additional strength to those cells. Secondary walls will have lignin as well as cellulose and other polysaccharide materials and are important in woody tissues, and in the support and conducting tissues of plants Middle Lamella When plant cells divide, vesicles secrete pectins (Calcium pectate) and other "gummy" substances between the adjacent cell membranes of the divided cell. This material forms the middle lamella that "glues" plant cells together (Unlike membranes, walls are not sticky.) Plasmodesmata Plasmodesmata are membrane connections (little channels) between adjacent plant cells that pass through the wall layers. Plasmodesmata provide for intercellular cytoplasm communication....
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