Practice question

Plant cell physiology depends heavily on water status because central vacuole can occupy 90 percent volume generating hydrostatic turgor pressure essential for cell expansion and mechanical support. Turgor results from water entry driven by intracellular solute accumulation, pressing protoplast against rigid cellulose cell wall. Membrane surrounding protoplast semipermeable allows water but restricts many solutes. If external soil solution or surrounding medium becomes hypertonic containing higher total effective concentration of non-penetrating solutes such as sodium chloride, mannitol or sucrose compared with vacuolar sap, extracellular water potential drops. Net water efflux occurs via aquaporins PIP and TIP, reducing vacuolar volume, decreasing pressure and causing plasma membrane to detach from cell wall. Cytological consequence visible under microscope as plasmolysis with protoplast rounding and chloroplasts clustering, wilting at whole plant level. Reversibility possible if protoplast returns to hypotonic solution restoring influx. Isotonic maintains volume, hypotonic increases turgor. Therefore hypertonic external conditions trigger plasmolysis in plant cells.