Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Cellular Polysaccharides
Structure of Plant Cell Walls

Plant Cell Walls possess extraordinary mechanical strength, altering their Structure and composition as the plant grows. The primary components of plant cell walls are Polysaccharides, with Cellulose being the most abundant, which largely dictates the architecture of the wall.

Cellulose. This homopolysaccharide is the most widespread carbohydrate on Earth (plants produce up to 10¹¹ tons of cellulose annually). The monomers of cellulose are glucose residues linked into long chains (up to 10,000 glucose residues each) via β(1→4)-glycosidic bonds (Fig. 5.3). Such a molecule lacks complete rotational freedom around the ¹C—O and O—⁴C bonds, adopting a conformation favorable for The formation of intermolecular Hydrogen Bonds when the chains lie in an antiparallel arrangement. As a result, cellulose molecules assemble into microfibrils ranging from roughly 10 to 25 nm in thickness. Microfibrils intertwine to form fine threads, which in turn can wind around one another like strands in a rope, creating macrofibrils. Each macrofibril has a thickness of about 0.5 µm and can reach 6—8 µm in length. The tensile strength of macrofibrils is comparable to that of steel wire of equivalent thickness. Furthermore, distinct Regions of the microfibrils exhibit an ordered structure, imparting crystalline properties to The Cell wall. Thus, cellulose demonstrates a high degree of complexity and structural order within cell walls—a feature that is far from accidental, as this polymer performs protective and structural support Functions in plants. In this state, polysaccharides are inaccessible to the plant's own Enzymes, meaning cellulose cannot be utilized as a reserve nutrient. Only a few organisms (certain Bacteria, Fungi, Protozoa, and rare animals) possess enzyme systems capable of breaking down cellulose.

Cellulose micro- and macrofibrils within The plant cell wall are embedded in a matrix, which is also composed primarily of polysaccharides and evolves in structure during plant growth. At early Selection/3.html">Stages of development, the matrix consists of pectic substances, whereas xylans and various neutral polysaccharides ("hemicellulose") appear later. Pectic substances are polymers of a-galacturonic acid in which some hydrogen atoms are replaced by methyl groups (-CH₃) (Fig. 5.3). Xylans are polymers of xylose (Fig. 5.3).

At later stages of development, accompanying cell wall lignification, Lignin is deposited within the Cells—a chemically stable polymer containing A large number of aromatic rings. Additionally, plant Cell walls contain minor amounts of Glycoproteins, insoluble lipid polymers of diverse structures, and Waxes.

The Cell walls of certain plants contain rare polysaccharides with unusual structures. For instance, the walls and intercellular matrix of marine red Algae contain Agar, a complex heteropolysaccharide that is a mixture of sulfated polysaccharides—agarose and agaropectin. Agarose is composed of alternating residues of D-galactose and 3,6-anhydro-L-galactose linked alternately by β(1→4) and a(1→3) bonds. Agaropectin has a more complex structure, incorporating D-galactose, 3,6-anhydrogalactose, uronic acids, and sulfate. Agar is widely used as the most common gelling agent for solid media, which are indispensable in microbiology, as well as in the food industry for gelling products. Notably, the overwhelming majority of microorganisms are unable to degrade agar, which is one of its primary advantages over gelatin, another culture medium solidifying agent. Agarose finds extensive application in biochemical research: in aqueous environments, it forms a gel with large pores whose size is determined by its concentration. Agarose gels are used for the fractionation of Proteins and Nucleic Acids, as well as for cell immobilization.

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Fig. 5.3. Structure of polysaccharide fragments from plant cell walls. The dashed boxes enclose hydrogen atoms that can be replaced by methyl groups in Pectins.