3.2 Osmosis
- Syllabus
- 0610–2026–2027
- Topic
- 3.2
- Level
- —
Learning objectives
3.2.1Water as a solvent• Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport3.2.2Water diffuses through partially• State that water diffuses through partially permeable membranes by osmosis3.2.3Osmosis through cell membranes• State that water moves into and out of cells by osmosis through the cell membrane3.2.4Osmosis using materials such• Investigate osmosis using materials such as dialysis tubing3.2.5Effects on plant tissues of immersing• Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations3.2.6Plants are supported by the pressure• State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall3.2.7Osmosis as the net movement of water• Describe osmosis as 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 membrane3.2.8Effects on plant cells of immersing• Explain the effects on plant cells of immersing them in solutions of different concentrations by using the terms: turgid, turgor pressure, plasmolysis, flaccid3.2.9Importance of water potential• Explain the importance of water potential and osmosis in the uptake and loss of water by organisms
Explain water's role as a solvent
A solvent is a liquid in which solutes dissolve. Water is the solvent for many substances in organisms, allowing them to mix, react and be carried in solution.
| Process | Role of water as a solvent |
|---|---|
| digestion | soluble products of digestion dissolve so they can be absorbed |
| transport | dissolved nutrients, ions, gases and wastes can be carried in blood, tissue fluid or plant transport fluids |
| excretion | soluble wastes such as urea dissolve in water and can leave the body in urine |
The useful sequence is: a substance dissolves in water → it can move in a fluid or contact reacting molecules → it can be absorbed, transported or excreted.
Do not merely say that organisms contain water. Link its solvent property to a named dissolved substance and to digestion, transport or excretion.
Recognise osmosis across a membrane
Water diffuses through a partially permeable membrane by osmosis.
| Required feature | Meaning |
|---|---|
| water molecules | osmosis concerns water, not every solute |
| partially permeable membrane | water can pass, while some larger dissolved particles cannot |
| unequal water concentration on the two sides | produces unequal opposing movements and therefore net movement |
A cell surface membrane is partially permeable. A cell wall is freely permeable and is not the membrane that controls osmosis.
Osmosis is passive: it can continue without energy supplied by respiration.
Diffusion of a solute across a membrane is not osmosis. Osmosis specifically describes water moving through a partially permeable membrane.
Predict water movement into and out of cells
Water moves into and out of cells by osmosis through the cell membrane.
| Surrounding solution | Net water movement | Typical result |
|---|---|---|
| more dilute than the cell contents | into the cell | cell gains water |
| same effective concentration as the cell contents | no net movement | no overall change |
| more concentrated than the cell contents | out of the cell | cell loses water |
Compare the cell contents with the surrounding solution, identify which is more dilute, then trace the net movement of water through the cell membrane from the more dilute side towards the more concentrated side.
Animal cells have no cell wall: excessive water entry can make them swell and burst, while water loss makes them shrink.
Water molecules move both ways. The prediction concerns net movement, and the cell membrane—not the cell wall—is the partially permeable barrier.
Investigate osmosis with dialysis tubing
Dialysis tubing models a partially permeable membrane. Fill a tied length with concentrated sucrose solution, blot and measure its starting mass, then place it in distilled water for a fixed time.
| Variable | Example |
|---|---|
| independent | concentration of solution inside or outside the tubing |
| dependent | change in mass, volume or liquid level |
| controls | tubing surface area, solution volume, temperature and time |
Water moves through the tubing into the more concentrated solution, so the bag gains mass or the liquid level rises. Solute movement must be considered separately according to whether that solute can cross the membrane.
Rinse and blot each bag consistently before weighing, check for leaks, repeat each concentration and compare mean percentage change: (final − initial) ÷ initial × 100.
A wet outer surface can create a false mass gain. The experiment supports osmosis only when membrane integrity, time and starting dimensions are controlled.
Investigate osmosis in plant tissue
Place equal-sized plant-tissue pieces in a range of solution concentrations for the same time, then measure how their mass or length changes.
| External solution | Tissue result | Interpretation |
|---|---|---|
| dilute | mass or length increases; tissue becomes firmer | net water entry |
| concentration giving zero percentage change | no overall mass or length change | no net water movement |
| concentrated | mass or length decreases; tissue becomes softer | net water loss |
Cut pieces from the same tissue, use equal initial dimensions and solution volumes, control temperature and time, blot identically, repeat, calculate means and express change as a percentage of the initial value.
Plot mean percentage change against solution concentration. The concentration where the curve crosses 0% estimates the concentration producing no net water movement.
Comparing raw final masses can be misleading when starting sizes differ. Use change or percentage change, and do not claim that dissolved sucrose itself moved into the tissue unless that was tested.
Relate cell water pressure to plant support
Plants are supported by the pressure of water inside their cells pressing outwards on the cell wall.
When water enters a plant cell, the vacuole and cell contents press the cell membrane against the strong cell wall. The wall resists further expansion, so the firm cell contributes to support of the tissue.
| Water status | Pressure against wall | Tissue effect |
|---|---|---|
| cell has gained water | high | firm; stem or leaf is supported |
| cell has lost water | low | soft; tissue may wilt |
Many firm, water-filled cells together support non-woody stems and leaves without bones. Loss of water lowers this pressure across the tissue and the plant droops.
The pressure acts outwards on the cell wall, not inwards on the membrane. The rigid wall limits expansion and prevents the plant cell from bursting under normal conditions.
Define osmosis using water potential
Osmosis is the net movement of water molecules from a region of higher water potential (a more dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane.
| Description | Water potential |
|---|---|
| pure water or more dilute solution | higher |
| more concentrated solution | lower |
| direction of net osmosis | higher → lower |
When water potentials are negative, the value closer to zero is higher. For example, water moves from −200 kPa to −450 kPa, provided a partially permeable membrane separates the regions.
Water molecules move randomly both ways. A water-potential difference makes more move from higher to lower water potential than in the reverse direction; at equal water potential there is no net movement.
Do not reverse the direction because a concentrated solution contains 'more solute'. Osmosis tracks water: net water movement is from higher to lower water potential.
Explain plant-cell states after osmosis
The water potential outside a plant cell determines the direction of osmosis and therefore its turgor state.
| External condition | Water movement and cell state |
|---|---|
| higher water potential outside | water enters; vacuole expands; turgor pressure rises; cell becomes turgid |
| equal water potential | no net movement; cell may be flaccid with little turgor pressure |
| lower water potential outside | water leaves; vacuole shrinks; cell becomes flaccid; continued loss causes plasmolysis |
Turgor pressure is the pressure of the cell contents pressing outwards on the cell wall. Plasmolysis occurs when water loss causes the cell membrane and cytoplasm to pull away from the cell wall.
Turgid cells make plant tissue firm. Flaccid or plasmolysed cells provide little support, so leaves and non-woody stems wilt.
A plasmolysed cell has not lost its cell wall: the living contents have contracted away from it. Flaccid means low turgor; it does not always mean plasmolysed.
Explain water uptake and loss in organisms
Water-potential gradients determine whether organisms gain or lose water by osmosis across cell membranes.
| Context | Water-potential gradient and consequence |
|---|---|
| plant root hair | soil water potential higher than cell sap → water enters the root by osmosis |
| plant during water shortage | dry soil and continued evaporation reduce replacement of water → cells lose turgor and the plant wilts |
| freshwater single-celled organism | surroundings have higher water potential → water continually enters; a contractile vacuole expels excess water |
| mammalian kidney tubule | water moves by osmosis across membranes during reabsorption when a water-potential gradient is maintained |
Uptake continues only while a gradient exists. Solute uptake can lower water potential inside root cells, while water transport away from the root helps maintain the gradient from soil to root.
A complete explanation names the two regions, compares their water potentials, states that water crosses a partially permeable cell membrane by osmosis, and gives the organism-level consequence.
Water does not always enter organisms. Its net direction depends on the water-potential gradient, so a concentrated external solution can cause water loss instead.