Components of the plant cell wall (Baric et al., 1988; Carpita and Gibeaut, 1993; Carpita et al., 1996) are the middle lamella (intercellular substance), primary wall, and secondary wall. The middle lamella comprises the pectic layer between cells and holds adjoining cells together. The primary wall is thin (1-3 urn) and elastic, containing cellulose, hemicelluloses, pectins, and glycoproteins. This wall provides mechanical strength, maintains cell shape, regulates cell expansion (Cosgrove, 2001) and intercellular transport, protects against certain organisms and can function in cell signaling. Cellulose is a (3 1,4 D-glucan and the pectic substances consist of galacturonans, rhamnogalacturonans, arabinans, galactans, and arabinogalactans I. In contrast, hemicelluloses comprise xylans, glucomannans, galactoglucomannans, xyloglucans, jJ-D-glucans, and other polysaccharides. The latter include: p, 1-3-linked D-glucans


Plastids Plant cells abound with various types of plastids (Bogorad and Vasil, 1991), each delimited by an envelope consisting of two membranes. Plastids are often classified on the basis of developmental stage and the pigments that they contain. Chloroplasts (Edelman et al, 1982; Hoober, 1984) are the sites of photosynthesis and their ribosomes can perform protein synthesis (Halliwell, 1984). Chloroplasts, which contain chlorophylls and carotenoids, are diskshaped and 4-6 um in diameter (Fig. 6.7). These plastids comprise a ground substance (stroma) and traversed by thylakoids (flattened membranous sacs) which are usually stacked as grana. In addition,

the Chloroplasts of green algae and plants contain starch grains, small oil droplets and DNA.