What happens when a cell is placed in different solutions? What mechanism is responsible for this?

When a cell is placed in a hypertonic solution (higher solute concentration outside), water moves out of the cell through osmosis, causing the cell to shrink. Animal cells become shriveled and crenated, while plant cells undergo plasmolysis—the cell membrane pulls away from the cell wall as the cytoplasm shrinks, causing the cell to become flaccid and potentially die if the condition persists. In contrast, when placed in a hypotonic solution (lower solute concentration outside), water enters the cell, causing it to swell. Animal cells may burst (lyse) due to excessive water intake since they lack a protective cell wall, while plant cells become turgid (swollen and firm) as the cell membrane presses against the rigid cell wall, which prevents bursting by providing counter-pressure.

When placed in an isotonic solution (equal solute concentration inside and outside), there is no net movement of water—water molecules move in both directions at equal rates, maintaining cellular volume and shape. Animal cells function optimally in isotonic conditions, which is why medical intravenous fluids are carefully formulated to match blood solute concentration. The mechanism responsible for these phenomena is osmosis—the passive movement of water molecules across the semi-permeable cell membrane from regions of higher water potential (lower solute concentration) to lower water potential (higher solute concentration). This movement continues until equilibrium is achieved or, in plant cells, until turgor pressure balances osmotic pressure. This osmotic behavior is critical for understanding cell biology, designing medical treatments, preserving food, and explaining how organisms maintain water balance in different environments.