Why do cells burst in a hypotonic solution

Osmosis is the passive movement of water across a semipermeable membrane from a region of higher water potential to a region of lower water potential. Water potential ($\Psi_w$) combines solute potential ($\Psi_s$) and pressure potential ($\Psi_p$). In a hypotonic solution the extracellular solute concentration is lower, making $\Psi_s$ less negative and $\Psi_w$ higher than inside the cell. Because $\Psi_w$ outside exceeds $\Psi_w$ inside, water flows inward. The influx expands the cytoplasm and stretches the plasma membrane.

Osmosis in simple terms

The increase in volume creates hydrostatic pressure inside the cell. When this pressure exceeds the tensile strength of the plasma membrane, the membrane ruptures and the cell contents spill out. Animal cells lack a rigid cell wall, so they are especially vulnerable to this type of lysis. Plant cells possess a cellulose wall that can resist higher pressures, often preventing bursting. The same principle explains why freshwater fish have gill adaptations to excrete excess water.

Key factors that determine whether a cell will lyse

• Magnitude of the external solute concentration
• Permeability of the membrane to water
• Presence of a cell wall or other rigid support
• Initial cell volume and surface‑to‑volume ratio

Procedure to predict bursting using water potential

1. 1Calculate the water potential of the external solution using $\Psi_w = \Psi_s + \Psi_p$
2. 2Compare it to the cell’s internal water potential
3. 3If $\Psi_{w,ext} > \Psi_{w,int}$, water will flow in
4. 4Estimate the resulting volume increase and check if it exceeds the membrane’s elastic limit

Comparison of solution types

Consider a red blood cell placed in a 0.5 % NaCl solution. Normal plasma is about 0.9 % NaCl, giving an intracellular solute potential of roughly –300 kPa. The 0.5 % solution has a solute potential of –167 kPa, so the water‑potential difference is +133 kPa, driving water into the cell. If the cell’s membrane can tolerate a 20 % increase in volume, the cell swells to about 1.2 × its original size; beyond roughly 1.5 ×, the membrane ruptures. The calculation shows why the cell bursts in the hypotonic environment.