Cellulose is synthesized by enzymes located in the plasma membrane
Cellulose, an important cell constituent , is a glucan in which the glucose residues are linked by ( β1→4)-glycosidic bonds forming a very long chain (Fig. 9.25). The synthesis of cellulose is catalyzed by cellulose synthase located in the plasma membrane. The required glucose molecules are delivered as UDP-glucose from the cytosol, and the newly synthesized cellulose chain is excreted into the extracellular compartment (Fig. 9.26). It has been shown in cotton-producing cells—a useful system for studying cellulose synthesis—that UDP-glucose is supplied from cytosolic sucrose by the action of a membrane-bound sucrose synthase. The UDP-glucose released is transferred directly to the cellulose synthase.
Alternatively UDP-glucose is synthesized from glucose 1-phosphate and UTP, catalyzed by UDP-glucose pyrophosphorylase. The synthesis of cellulose starts with the transfer of a glucose residue from UDP-glucose to sitosterol (Fig. 15.3), a plasma membrane lipid. The glu-cose residue is bound to the hydroxyl group of the membrane lipid via a glycosidic linkage and acts as a primer for the cellulose synthesis, thus anchoring the growing cellulose chain to the membrane. Cellulose never occurs in single chains but always in a crystalline array of many chains called a microfibril . It is assumed that, due to the many neigh-boring cellulose synthases in the membrane, allβ-1,4-glucan chains of a microfibril are synthesized simultaneously and spontaneously assemble to a microfibril.
Callose is a β-1,3-glucan (Fig. 9.25) with a long unbranched helical chain. Callose forms very compact structures and functions as a universal insula-tion material in the plant. In response to wounding of a cell, large amounts of callose can be synthesized very rapidly at the plasma membrane. According to present knowledge, its synthesis proceeds like the synthesis of cellulose (shown in Fig. 9.26). Membrane-bound sucrose synthase pro-vides UDP-glucose for callose synthesis. Callose synthesis is stimulated by an increase in the cytosolic Ca++ concentration. Wounding is accompanied by a Ca++ influx and an increase of the cytosolic Ca++ concentration, thus inducing the synthesis of callose for insulation. Plasmodesmata of injured cells are closed by callose formation in order to prevent damage to other cells of the symplast . Moreover, callose serves as a filling mate-rial to close defective sieve tubes.
In contrast to the synthesis of cellulose and callose localized outside of the plasma membrane, the synthesis of the cell wall polysaccharides hemicel-lulose and pectin takes place in the Golgi apparatus. In the synthesis GDP activated hexoses (e.g., GDP-mannose and GDP-fucose) are involved. The transfer of the polysaccharides synthesized in the Golgi apparatus to the cell wall proceeds via exocytotic vesicle transport.