FIGURE 20-41 Structure of the plant cell wall. (a) Overview of the organization of a typical plant cell, in which the organelle-filled cell with its plasma membrane is surrounded by a well-defined extracellular matrix called the cell wall. (b) Schematic representation of the cell wall of an onion. Cellulose and hemicellulose are arranged into at least three layers in a matrix of pectin. The sizes of the polymers and their separations are drawn to scale. To simplify the diagram, most of the hemicellulose cross-links and other matrix constituents (e.g., extensin, lignin) are not shown. See M. McCann and K. R. Roberts, 1991, in C. Lloyd, ed., The Cytoskeletal Basis of Plant Growth and Form, Academic Press, p. 126. (c) Quick-freeze deep-etch electron micrograph of the cell wall of a garden pea in which some of the pectin molecules were removed by chemical treatment. The abundant thicker fibers are cellulose microfibrils, and the thinner fibers are hemicellulose cross-links (red arrowheads).

[Part (b) courtesy Maureen C. McCann. Part (c) republished with permission of Oxford University Press, from Fujino, T., et al., “Characterization of cross-links between cellulose microfibrils, and their occurrence during elongation growth in pea epicotyl,” Plant Cell Physiol. 2000, 41(4):486–94; permission conveyed through the Copyright Clearance Center, Inc.]