Osmosis is the net movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration. Aquaporins are channel proteins that allow water to cross cell membranes rapidly.
This content from B2.1 Membranes and membrane transport is required at both SL and HL.
How osmosis works
Water molecules move randomly in both directions across a membrane. If the membrane is permeable to water but impermeable to the solute, unequal solute concentrations produce unequal rates of water movement. The result is a net movement of water toward the more concentrated solution until equilibrium is reached.
Solutions are compared using the following terms:
| External solution | Solute concentration relative to the cell | Net water movement |
|---|---|---|
| Hypotonic | Lower | Into the cell |
| Hypertonic | Higher | Out of the cell |
| Isotonic | Equal | No net movement |
At isotonic equilibrium, water molecules continue crossing in both directions at equal rates. A common misconception is that water stops moving; the correct term is dynamic equilibrium.
What aquaporins do
The hydrophobic interior of the phospholipid bilayer restricts the movement of polar water molecules. Aquaporins form hydrophilic channels through the membrane, increasing its permeability to water. Water moves through them passively, so this transport does not require ATP and still follows the osmotic gradient.
Aquaporins do not pump water or determine its direction. The difference in solute concentration determines the direction of net movement; aquaporins increase the rate at which equilibrium is approached.
Exam technique
For an IB explain question, state the selectively permeable membrane, random movement in both directions, difference in solute concentration, and resulting net movement. Avoid defining osmosis using “water concentration”: the syllabus expects direction to be expressed in terms of solute concentration.
