14 5.5 Osmosis in living cells
The plasma membrane is semi-permeable. Three situations for a cell placed in a solution:
1Hypotonic (more dilute than the cell sap)
Water movement
- water enters the cell
Effect on an animal cell
- swells, may burst (lysis)
Effect on a plant cell
- becomes turgid (firm) — useful for support; the cell wall prevents bursting
2Isotonic (same concentration)
Water movement
- no net movement
Effect on an animal cell
- normal
Effect on a plant cell
- flaccid but normal
3Hypertonic (more concentrated)
Water movement
- water leaves the cell
Effect on an animal cell
- shrinks (crenation)
Effect on a plant cell
- membrane pulls away from the wall (plasmolysis), plant wilts
Everyday and ecological examples:
- Salting food (fish, meat, cheese, pickles) preserves it because bacteria placed in the very salty environment lose water by osmosis and die or cannot grow.
- Drinking sea water makes thirst worse: its osmotic pressure is higher than that of body fluids, so the body must use extra water to excrete the salt.
- Plant roots absorb water by osmosis because the cell sap in root hairs is more concentrated than the soil water. Over-fertilised soil (too much dissolved salt) or salty irrigation water can reverse the gradient so the roots lose water, and plants wilt.
- Freshwater and marine organisms face opposite osmotic problems; their adaptations (contractile vacuole, kidneys, urea in sharks) are the subject of Module 6.
- Desalination by reverse osmosis: if a pressure larger than the osmotic pressure of sea water (about 27–30 atm) is applied to the salt-water side, water is forced through the membrane against the natural osmotic direction, leaving the salts behind. This technology supplies drinking water to many coastal cities in Egypt.
✍️ Worked examples
Worked example 5.
- Problem. A red blood cell (osmotic equivalent about 0.30 mol/L of particles) is placed in (a) a 0.15 mol/L NaCl solution (0.30 mol/L particles), (b) distilled water, (c) a 0.5 mol/L NaCl solution (1.0 mol/L of particles).
- Describe the behaviour.
- Solution. (a) 0.15 mol/L NaCl gives about 0.30 mol/L of particles: isotonic; no net water movement; the cell keeps its shape (this is the "physiological saline").
- (b) Distilled water is hypotonic: water enters, the cell swells and may burst.
- (c) 1.0 mol/L particles is hypertonic: water leaves, the cell shrinks and becomes crinkled.