Module 2: Water Potential and Osmosis
These free OCR A Level Biology Water Potential and Osmosis revision notes have been written for specification points 2.1.5(e.i) and 3.1.3(d).
Osmosis and Water Potential
Osmosis is the net movement of water across a partially permeable membrane, from an area of higher water potential to an area of lower water potential.
Water potential (Ψ) measures how likely water is to move from one area to another. It’s measured in kilopascals (kPa) because water molecules exert pressure.
Water potential is determined by:
- Solute potential (Ψs): Solute molecules attract water, reducing how freely water can move.
- Pressure potential (Ψp): The pressure from the cell’s contents pressing on the membrane or wall.
Giving us: Ψ = Ψs + Ψp
- Pure water has the highest water potential: 0 kPa.
- Adding solutes lowers water potential, making the value more negative.
- In animal cells, Ψp is usually very small.
When comparing two places with different water potentials, the difference is called the water potential gradient.
The diagram below shows water potential gradient between cells in animal tissue:
Osmotic Effects on Cells
Osmosis can affect the physical structure and metabolic function of a cell.
Cells can shrink, swell, and even burst if the pressure exerted by the solution contained within the cell surface membrane is too high.
Effects of Osmosis on Animal Cells
| External Solution | Direction of Water Movement | Effect on Cell |
|---|---|---|
| Higher Ψ (hypotonic) | Into cell | Swells and may burst (cytolysis) |
| Equal Ψ (isotonic) | No net movement | No change |
| Lower Ψ (hypertonic) | Out of cell | Shrinks (crenation) |
Effect of Osmosis on Plant Cells
| External Solution | Direction of Water Movement | Effect on Cell |
|---|---|---|
| Higher Ψ (hypotonic) | Into cell | Becomes turgid |
| Equal Ψ (isotonic) | No net movement | No change |
| Lower Ψ (hypertonic) | Out of cell | Plasmolysed (the plant tissue as a whole becomes flaccid) |
Water transport pathways in plants
Water potential and osmosis are important in plants as they drive the movement of many other substances (dissolved in water-based solutions).
The table below outlines the different pathways that water can take through plant tissue:
| Pathway | Route | How it works | Features |
|---|---|---|---|
| Apoplast | Through cell walls and intercellular spaces | Water moves by mass flow, no membranes involved | Fastest route; blocked by Casparian strip in endodermis |
| Symplast | Through the cytoplasm, via plasmodesmata | Water moves by osmosis from cell to cell through the cytoplasm | Slower than apoplast; allows selective control of substances |
| Vacuolar | Through the cytoplasm and vacuoles | Water crosses the tonoplasts and cell membranes between cells | Even slower, less common than the other two |
The diagram below shows the pathways that water can take through plant tissue from a root hair cell to the xylem:
