Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Carbohydrates: Structure and Biological Functions
Cell walls contain large amounts of structural and protective polysaccharides

Most plant Cells are surrounded by a rigid, highly durable polysaccharide envelope, which can be compared to fiberglass-reinforced plastic. The framework of Plant Cell Walls consists of crisscrossing layers of long, elongated Cellulose fibers whose tensile strength exceeds that of steel wire of the same diameter (Fig. 11-19). This fibrous framework is reinforced by a cement-like matrix composed of other structural Polysaccharides and the polymer Lignin. The exceptionally thick cell walls in tree trunks enable them to withstand immense mechanical loads (Fig. 11-19). The Introduction/37.html">Bacterial Cell wall (Fig. 11-20) lies outside The Plasma Membrane, forming a rigid, porous shell around The Cell that physically protects the delicate Cell Membrane and Cytoplasm. The structural backbone of most bacterial cell walls is a cross-linked covalent framework that almost entirely encases the cell. It consists of long, parallel polysaccharide chains connected at regular intervals by cross-links of short polypeptide chains. The polysaccharide chains are composed of alternating monosaccharide residues of N-acetyl-D-glucosamine (Fig. 11-18) and N-acetylmuramic acid (a complex nine-carbon sugar) linked together by ß(1→4) glycosidic bonds (Fig. 11-20). Attached to each N-acetylmuramic acid residue is a side tetrapeptide chain. These parallel polysaccharide chains are cross-linked by short peptide chains whose composition varies among different bacterial species. In the pyogenic bacterium Staphylococcus aureus, which causes boils and wound suppuration, the acetylmuramic acid residues in adjacent polysaccharide chains are linked by peptide chains consisting of five Glycine residues. This entire cross-linked Structure surrounding the cell is called murein (from the Latin word murus, meaning wall) or peptidoglycan; the latter name emphasizes its hybrid nature as a combination of peptide and polysaccharide elements. Extending continuously across the entire surface of the bacterial cell, peptidoglycan can be viewed as a single giant sac-like molecule. In Gram-positive Bacteria (which retain the crystal violet stain upon Gram staining), peptidoglycan forms several concentric layers around the cell, interspersed with other macromolecular components. In Gram-negative bacteria, such as E. coli, the peptidoglycan framework is covered by a lipid-rich outer membrane containing hydrophobic Proteins (see Ch. 12). The integrity of cell walls is vital for the protection, growth, and division of bacteria. The action of penicillin—one of the most valuable Antibiotics used to combat bacterial infections—is based on its ability to inhibit The final stage of enzymatic peptidoglycan synthesis in susceptible microorganisms, leading to defective cell walls and arrested bacterial growth (Fig. 11-21).

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Fig. 11-19. Cellulose is the major component of plant cell walls. A. Transmission electron micrograph of The cell wall of the alga Chaetomorpha. The cell wall consists of crisscrossing layers of cellulose fibers impregnated with cementing polymer substances. B. Cross-section of a tree trunk (black locust) clearly showing annual growth rings. Spring wood contains large cells with thin walls, whereas wood formed later in the season has smaller cells and more layers of cellulose fibers. The lighter wood surrounding the heartwood is sapwood.

Fig. 11-20. A. Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF the peptidoglycan cell wall of the Gram-positive bacterium Staphylococcus aureus. B. STRUCTURE OF THE repeating disaccharide unit within the peptidoglycan backbone.

Fig. 11-21. Effect of penicillin on Staphylococcus aureus cells. A. Cells before penicillin Treatment. B. Penicillin treatment disrupts cell wall integrity, causing them to rupture.