Question

7. Polysaccharides play an important role in the cell walls of prokaryotes such bacteria and of certain eukaryotes like fungi and plants. Give 3 reasons why polysaccharides as opposed to other biomolecular polymers (fats, proteins, nucleic acids) were better suited to form these walls.

Answer 1

Ans. Polysaccharide polymers are better suited as cell wall constituents for following reasons-

I. Hydrophilicity: Most of the cell wall polysaccharides including peptidoglycan in bacteria, cellulose in plants and chitin in fungal cell wall are principally hydrophilic, i.e. they have affinity for water (though to a varying extent). The hydrophilic nature of cell wall components ensures that the cell remains solvated in the aqueous environment. Since the aqueous medium serves as the source of nutrients, the cell wall can also facilitate exchange of material into and out of the cell as needed.

Lipids in cell wall would make the cell wall hydrophobic and the cell would thus be isolated from the aqueous environment. Isolation from aqueous environment would further make exchange of matter across the cell wall infeasible, and the cell would be deprived of nutrients. Therefore, hydrophobic nature of cell wall in form of polysaccharides in better suited than lipids.

II. Relatively resistant to hydrolysis: Most of the cell wall polysaccharides are relatively stable against hydrolysis as they can be hydrolyzed by relatively few enzymes only. There are numerous hydrolytic enzymes that degrade nucleic acids, proteins and lipids. So, polysaccharides provide greater extent of stability of the cell wall against proteases, lipases, nucleases and most of other hydrolytic enzymes against the glycosidic bond. The relative stability of polysaccharide cell wall is crucial for overall stability and survival of the cell in diverse aqueous environments.

III. Economic and ease of synthesis: Both the synthesis of proteins and nucleic acids is regulated by the genetic material, consumes plenty of time (gene activation, transcription, translation, slow rate of translation as well as slow rate of Replication and Transcription) as well as required plenty of energy (synthesis of nucleic acids and proteins require energy). The synthesis of these biomolecules is localized to nucleus and cytoplasm. Moreover, there is always lesser availability (limited availability) of lipids, amino acids and nucleotides in the cell and aqueous environment than the monosaccharides – the most abundantly available biomolecules.

So, use of polysaccharides as cell wall component is economic in terms of energy required for its synthesis. There is no need of activating and expressing a gene every time cell wall synthesis is required, so its synthesis saves times. Being abundantly available, the monosaccharides can be used to synthesize cell wall without any limit of its availability.