Water potentialSpec 3.2.7, 3.2.9
In short
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. Pure water has the highest water potential. Root hair cells have a lower water potential than the soil water, so water enters them by osmosis.
Water potential describes how freely water molecules can move. A dilute solution has a high water potential. A concentrated solution has a low water potential. Pure water has the highest water potential.
- Osmosis
- The net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
When a dilute sugar solution and a concentrated sugar solution are separated by a partially permeable membrane, the dilute solution has the higher water potential and the concentrated solution has the lower water potential. Water moves through the partially permeable membrane down this water potential gradient.
Use the phrase 'water potential' and the word 'net'. Say 'from a higher water potential to a lower water potential', not 'from a weak solution to a strong solution' alone.
Importance in organisms
- Plants: root hair cells have a lower water potential than the soil water, so water enters them by osmosis. The water then moves from cell to cell along a water potential gradient into the xylem.
- Plants: loss of water from leaf cells lowers their water potential, so water is drawn in from neighbouring cells.
- Animals: if the blood has a lower water potential than the cells, water leaves the cells. If the blood has a higher water potential, water enters the cells. Animal cells have no cell wall, so they can burst or shrink.
- Organisms must keep the water potential of their body fluids in a narrow range so that cells neither swell too much nor lose too much water.
Do not describe water potential as the 'amount of water'. Explain what happens by comparing the water potential of the solution with that of the cell.
Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026