Why Study the Structure of Cell Walls

The cell walls of plants are analogous to the skeletons of animals. The walls control the rate of growth of plant cells and thus of the plants. The walls are a structural barrier to some molecules and to invading pathogens. Cell walls are also a source of food, fiber and energy. Thus, knowledge of the mode of synthesis of cell walls, of the structure of cell walls, and of the function of cell walls is of great importance. 

Plant cell walls are of two general types primary cell walls and secondary cell walls. Primary cell walls are laid down by undifferentiated cells that are still growing and it is these primary walls that control cell growth. Secondary walls are derived from primary cell walls by cells which have stopped growing and are differentiating. This review is concerned with the structure of primary cell walls and will be limited to a consideration of the primary cell walls of dicotyledonous plants. The primary cell walls of dicots differ, at least to some degree, from the primary cell walls of monocots and from the cell walls of lower plants (3, 107). 

Success in structural studies of cell walls depends largely on the purity and homogeneity of the cell wall preparations examined. Some studies have been made of primary cell walls extracted from whole plant tissues. Since all tissues of intact plants contain a variety of cell types, the walls prepared from these tissues are not homogeneous. The desire for homogeneous wall preparations has been satisfied by use of suspension-cultured cells. Suspension-cultures, of at least some types of plant cells, can be maintained in a totally undifferentiated state. These types of suspension cultures provide a source of homogeneous primary cell walls. 

Suspension-cultured plant cells represent a somewhat artificial situation since one is interested in studying the cells of the intact plant. The structures of the walls of cells in culture and the walls of cells in the plant may conceivably differ and it is essential when studying suspension-cultured cells to keep this possibility in mind. However, the available data indicate that the walls of suspension-cultured cells are very similar in structure to the walls of intact plant tissues. Cell wall glycosyl and glycosyl-linkage compositions of pea (58) and red kidney bean (105) hypocotyl tissues show that these tissues contain walls which are very similar to the walls isolated from suspension-cultured sycamore (123) and suspension-cultured red kidney bean cells (105). In addition, it has been demonstrated that the primary walls of cambial cells which were prepared from the branches of sycamore trees are very similar to the walls of suspension-cultured sycamore cells (104). 

The hypothesis that the walls of suspension-cultured cells reflect the walls of intact plants is further substantiated by the fact the xyloglucan, a hemicellulosic component of the cell walls of bean and pea plants, is extremely similar to the xyloglucan of suspension-cultured sycamore cells (133). Indeed, amyloids, polysaccharides which can not be structurally 

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