Osmosis is a special case of diffusion: the movement of water through a partially permeable membrane. It explains why dialysis tubing swells, why potato cylinders gain or lose mass, and why a watered plant stands upright. The language of water potential and turgor pressure is Extended content.
Water diffuses through partially permeable membranes by osmosis, and moves into and out of cells this way through the cell membrane. Extended: 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. Plants are supported by the pressure of water inside their cells pressing outwards on the cell wall.
Fig. 1: water molecules cross the partially permeable membrane both ways; the large sucrose molecules cannot, so the net flow of water is towards the concentrated side
Section 01
Water, the solvent, crosses the membrane.
Water is the solvent in which the chemistry of life happens. A partially permeable membrane lets small molecules such as water through but stops larger ones such as sucrose. Dialysis (Visking) tubing is an artificial partially permeable membrane, so it is used to model a cell.
Water as a solvent: digestion (enzymes work in solution and digested food dissolves), excretion (urea is removed dissolved in water as urine) and transport (blood plasma and xylem sap carry dissolved substances).
Sucrose inside, water outside: water moves into the tubing by osmosis, the tubing swells and the liquid rises up the capillary tube.
Water inside, sucrose outside: water moves out of the tubing and the level in the capillary falls.
Same liquid on both sides: water still crosses in both directions, but there is no net movement, so the level stays the same.
Examiner trap
In osmosis it is the water that moves, not the solute. The sucrose molecules are too large to pass through the membrane. Never write that "sucrose moves out by osmosis".
Section 02
Plant tissue in solutions.
Cut equal cylinders from one potato, blot and weigh them, then leave them in water and in sucrose solutions of different concentrations. Their cells gain or lose water by osmosis.
Solution
Mass change
Cylinder afterwards
Water movement
water
gain (about +8%)
longer, firm, stays straight
into the cells
0.3 mol/dm3 sucrose
almost none
almost unchanged
no net movement
1.0 mol/dm3 sucrose
loss (about −9%)
shorter, soft, bends
out of the cells
Use percentage change: the cylinders start with slightly different masses, so (change ÷ starting mass) × 100 allows a fair comparison.
Support: plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. Switch to the wilting plant in the sim: without water the cells go soft and the plant droops.
Examiner trap
"No change in mass" does not mean no water moved. Water still crosses both ways at the same rate; there is no net movement because the solution matches the concentration of the cell sap.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the dialysis tubing tab, sucrose solution is inside the tubing and water is in the beaker. What happens to the level in the capillary tube?
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Why can sucrose molecules not cross dialysis tubing?
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A potato cylinder left in water for 30 minutes becomes:
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In the potato sim, which solution produced almost no change in mass?
Section 03
Turgid, flaccid, plasmolysed.
Extended. Use water potential to predict the direction of osmosis. A dilute solution has a high water potential and a concentrated solution has a low water potential. Water always moves from higher to lower water potential.
Turgid: in a solution with a higher water potential than the cell sap, water enters the vacuole by osmosis. The cell contents push outwards on the wall; this is turgor pressure. The strong cell wall stops the cell bursting.
Flaccid: when the water potential is about equal inside and out, there is no net movement; the cell is soft, not pressed against its wall.
Plasmolysis: in a solution with a lower water potential, water leaves; the vacuole and cytoplasm shrink and the cell membrane pulls away from the cell wall. The gap fills with the outside solution because the wall is fully permeable.
Animal cells: a red blood cell has no cell wall, so in pure water it swells and bursts; in a concentrated solution it shrinks.
Importance: differences in water potential drive water uptake by root hairs from the soil and water loss from leaves, and turgor keeps stems and leaves firm and upright.
Examiner trap
Plant cells do not burst in pure water: the cell wall resists the turgor pressure. Only cells without a wall, such as red blood cells, burst. A plasmolysed cell has lost so much water that its membrane has pulled away from the wall; a flaccid cell has only lost its turgor.
Stage 2 · Exam
Exam-style questions
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Extended: which statement correctly describes osmosis?
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Extended: plant cells are placed in a concentrated salt solution. Which term describes the cells after an hour?
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A potato cylinder of mass 2.50 g has a mass of 2.30 g after soaking in a sugar solution. What is the percentage change in mass?
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Extended: why does a red blood cell burst in pure water but a plant cell does not?
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Osmosis, mastered.
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