Practice question

Hypertonic surroundings draw water out of bacterial cytoplasm through semipermeable inner membrane, generating inward osmotic pressure that tends to detach inner membrane from wall and shrink cytoplasm, a process termed plasmolysis. Survival depends on mechanical resistance supplied by peptidoglycan, a covalently crosslinked heteropolymer forming a single macromolecule sacculus surrounding entire cell, often described as exoskeleton. Its glycan strands crosslinked via peptide bonds possess high tensile strength, allowing it to withstand internal turgor of 2 to5 atmospheres in Gram-negatives and up to 20 atmospheres in Gram-positives, plus external osmotic shifts. Capsular polysaccharide, while important for immune evasion and desiccation resistance, is gel-like, highly hydrated, and lacks load-bearing stiffness. Outer membrane of Gram-negatives contributes some rigidity but remains fluid and present only in one group. Cytoplasmic proteins alone cannot counteract membrane deformation. Experimental evidence shows lysozyme digestion of wall in isotonic sucrose yields osmotically fragile protoplasts that burst upon dilution. Bacteria also accumulate compatible solutes like glycine betaine, trehalose, and ectoine to balance osmolarity, but primary immediate defense against plasmolysis remains peptidoglycan.