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Plant cells have a thick, relatively rigid outer shell: the cellulosic wall, also called cellulosic skeletal membrane;
Primary and secondary cellulosic walls
Young plant cells have a thin and flexible cellulosic wall called primary wall.
The primary wall is elastic to allow cell growth. After the cell has grown to full size and shape, it forms secondary wallthicker and stiffer. The cellulose that forms the secondary wall is secreted through the plasma membrane, and deposits between it and the inner surface of the primary wall, on which it adheres strongly.
Constitution of the cellulosic wall
The plant cell wall is composed of long and resistant microfibrils of the cellulose polysaccharide. Cellulosic microfibrils are held together by a matrix formed of glycoproteins (sugar-bound proteins), hemicellulose and pectin (polysaccharides).
The molecular structure of the cellulosic wall applies the same principle as reinforced concrete, in which long, sturdy iron rods are dipped in cement and stone mortar.
In the cell wall, the cellulose microfibrils correspond to the iron rods of the concrete, while the glycoproteins and the matrix polysaccharides correspond to the mortar.