Cell Walls

The presence of a wall secreted by the cell is a characteristic of plant cells. Animal cells do not produce walls. Many, but not all, Protista have cell walls. Fungi and bacteria produce cell walls. The distinction between the cell membrane and cell wall is an important one. The cell membrane is a part of the cell and is a living structure. The cell wall is not part of the cell; rather, it is secreted by the cell, lies outside of it, and is not living. The cell wall may appear homogeneous when viewed through an ordinary light microscope, yet there are actually two forms of cell wall: the primary wall, which is produced when the cell is young and continuing to grow, and the secondary wall, which is produced after the cell has completed its growth. If a polarized light source and a polarizing microscope are used, a primary wall and a secondary wall can be distinguished. Both primary and secondary walls are composed largely of cellulose deposited in the form of microfibrils. Pentosans are also present in the wall. Whereas cellulose is composed of long chains of 6-carbon sugars (such as glucose) linked together, pentosans are composed of linkages of 5-carbon sugars. It is thought that the synthesis of these molecules is accomplished in the Golgi bodies. Pectin is another substance found in plant cell walls. Pectic substances also form a thin layer called the middle lamella and found between adjacent cells. Many cells, such as those of wood, contain lignin. The walls of cork cells and certain leaf cells possess a waxy material called suberin. Cell walls do not deter the passage of water and dissolved materials unless the walls are impregnated with suberin.

Soon after the completion of cell division, the primary cell wall is deposited by the daughter cells on each side of the middle lamella. As a result, the cell membrane rests against the primary cell wall rather than against the middle lamella. Because the secondary cell wall is secreted after the primary cell wall is formed, the secondary cell wall lies internal to the primary cell wall. Many perforations occur in the walls. Strands of cytoplasm commonly run through these perforations, producing linkages with the cytoplasm of adjacent cells. These strands are called the plasmodesmata.

The arrangement of cellulose fibers in the primary and secondary cell walls differs, being randomly oriented in the primary wall and spirally arranged in the secondary wall. Whereas the primary wall is flexible and can be stretched while the cell is growing, the secondary wall is more rigid. In many plants, the protoplast dies after secondary wall formation is complete. The constituents of the protoplast are then removed, leaving only the cell wall. This occurs in most wood cells.