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| Cell Wall |
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| A cell wall is a characteristic feature of plant cells, bacteria and fungi. The composition however varies in different groups. |
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| A cell wall surrounds the plasma membrane in all plant cells. It is a porous covering of about 0.1 milli microns to 10 milli microns in size. |
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| The cell wall is chiefly composed of insoluble polysaccharides. The most common constituent is cellulose. Certain other polysaccharides like pectin, hemicellulose, lignin and xylan may also be present. |
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| The cell wall is generally deposited in two layers a primary wall and a secondary wall. |
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| Primary wall is represented by the wall material deposited when the cell is young. It is thin and elastic and is composed of only cellulose. Meristematic cells and parenchyma cells in plants show only primary wall. |
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| Secondary wall is represented by wall material deposited in mature cells. Secondary wall is always internal to the primary walls. It is thick, rigid and non-elastic. It is usually composed of additional cellulose, hemicellulose or pectin. |
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| The deposition of secondary wall material is not uniform. At places it is discontinuous, to enclose areas called pits. The pits may be simple or bordered with overhanging wall material. |
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| Secondary walls are seen in specialised cells such as sclerenchyma, collenchyma, xylem vessels and tracheids. |
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| fig. 16.1 - Layers of Cell Wall in T.S. and L.S. |
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| The term middle lamella is used for describing a thin sticky, amorphous layer of cementing material that is found between two adjacent cells. It is deposited by the joint activity of the adjacent cells. |
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| fig. 16.2 - Cell Wall in Section, Showing Simple Pits
(Left) and Bordered Pits (Right) |
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| The cell wall is interrupted by narrow pores, which carry fine strands of cytoplasm interlinking the contents of the cell. These pores are called plasmodesmata. They allow movement of substances between adjacent cells. |
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| In the cells of fungi, the cellwall is generally composed of chitin, a polysaccharide. The chitin molecules also occur in bundles, like cellulose molecules. |
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| The cellwall of prokaryotes like bacteria is composed of peptidoglycans. Apart from maintaining the shape and size of plant cells, the cell wall is involved in some specific functions such as |
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 Protection against the attack of pathogens |
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 Providing mechanical support |
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 Allowing movement of materials in and out of the cell |
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| The cells of many protists are surrounded by a filamentous layer of oligosaccharides. This forms the cell coat. It is in fact a part of the cell membrane. In many cases the cell coat becomes deposited with salts of calcium and silica. The cell coat mainly provides protection to the cell from infection by pathogens. It also helps in the recognition of similar cells. |
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| fig. 16.3 - Components of a Cell |
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