Lesson overview
Osmosis: Why does a limp lettuce leaf go crisp in water?
Cut potato cylinders, weigh them, soak them in water and five sucrose solutions, and weigh them again. Work out the percentage changes, find the concentration with no net movement of water, then bend the cylinders and look at their cells.
Suggested sequence
- Lesson 1 · Water and membranes. Water as a solvent, partially permeable membranes and osmosis, with dialysis-tubing bags. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Potato in sugar solutions. Plant tissue in solutions of different concentrations: the potato investigation, turgor and plasmolysis. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Water for whole organisms. Water potential in organisms: root hairs, wilting, red blood cells and food that keeps. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 7 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 8 questions, 15 min, lesson closed.
Learning goals
Part 1: Water and membranes
- Describe the role of water as a solvent in organisms with reference to digestion, excretion and transport (3.2.1)
- State that water diffuses through partially permeable membranes by osmosis, and 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) (3.2.2, 3.2.7)
- Investigate osmosis using materials such as dialysis tubing (3.2.4)
Part 2: Potato in sugar solutions
- State that water moves into and out of cells by osmosis through the cell membrane (3.2.3)
- Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations (3.2.5)
- State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall, and explain the effects on plant cells of solutions of different concentrations using turgid, turgor pressure, plasmolysis and flaccid (3.2.6, 3.2.8)
Part 3: Water for whole organisms
- Explain the importance of water potential and osmosis in the uptake and loss of water by organisms (3.2.9)
- State that water moves into and out of cells by osmosis, including animal cells without a wall, and that plants are supported by water pressure in their cells (3.2.3, 3.2.6)
Misconceptions to listen for
- “Osmosis is the movement of sugar through a membrane.” — Osmosis is the net movement of water. In these examples the membrane lets water through but holds the sugar back.
- “Water moves towards where there is more water.” — Water moves from the dilute solution, where there is more water, into the concentrated solution.
- “Water is only a filler in cells.” — Water dissolves the substances that are digested, excreted and transported; without it none of these could happen.
- “Water only moves one way in osmosis.” — Water molecules cross both ways; more cross from the dilute side, so the net movement is towards the concentrated solution.
- “The sugar molecules move across to even out the concentrations.” — This membrane holds the sugar back; only water crosses.
- “Sugar moved out of bag A.” — The bag gained mass: water moved in. Sugar molecules cross dialysis tubing only very slowly.
- “Bag B lost mass because water evaporated.” — Bag A in the same conditions gained mass; the difference is the direction of the water potential gradient.
- “The change for bag C is 1.5 %.” — That is the change in grams. Divide it by the starting mass (12.0 g), then multiply by 100.
- “A loss can be written as a positive percentage.” — Show a loss with a minus sign: −9 %.
- “The ions are carried up on their own, like a gas.” — The ions travel dissolved in water; water is the solvent that transports them.
Scope
Water as a solvent (3.2.1); osmosis through partially permeable membranes, in terms of water potential (3.2.2, Supplement 3.2.7); dialysis tubing (3.2.4) as a data investigation in the reading and practice; water entering and leaving cells (3.2.3) and plant tissue in sucrose solutions (3.2.5) through the potato-cylinder practical; support by turgor (3.2.6) with turgid, flaccid and plasmolysis (Supplement 3.2.8); water uptake and loss by organisms (Supplement 3.2.9). Masses in the experiment come from a teaching model.