The shape of the lesson
By the end of the lesson, learners can
- Water, the solvent, crosses the membrane: Water is the solvent in which the chemistry of life happens.
- 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.
- Turgid, flaccid, plasmolysed: Extended.
- Osmosis (Extended): net movement of water from higher water potential (dilute) to lower water potential (concentrated) through a partially permeable membrane.
- Percentage change = (final − initial) ÷ initial × 100; divide by the initial mass.
- No change in mass means the solution matches the concentration of the cell sap.
- Plant cells: turgid, flaccid, plasmolysed; the wall stops bursting. Animal cells can burst.
One picture every idea returns to
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.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Water follows the solute.. Osmosis is a special case of diffusion: the movement of water through a partially permeable membrane. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Water, the solvent, crosses the membrane | Water is the solvent in which the chemistry of life happens. Project tab 4 of the lab (Osmosis with dialysis tubing) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: 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. Project tab 5 of the lab (Potato cylinders in sucrose solutions) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Turgid, flaccid, plasmolysed | Extended. Project tab 6 of the lab (Plant cell and red blood cell in different solutions) and let a student drive it while the class predicts. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 20 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Turgid, flaccid, plasmolysed, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| 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". |
| "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. |
| 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. |
Osmosis: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the role of water as a solvent in organisms, with reference to digestion, excretion and transport.
A length of dialysis tubing is filled with concentrated sucrose solution, attached to a capillary tube and placed in a beaker of water. Predict and explain what happens to the liquid level in the capillary tube over 30 minutes.
Potato cylinders were weighed, left in solutions for 40 minutes, then blotted and reweighed.
| Sucrose / mol dm−3 | Initial mass / g | Final mass / g |
|---|---|---|
| 0.0 | 2.40 | 2.64 |
| 0.4 | 2.50 | 2.45 |
| 0.8 | 2.45 | 2.21 |
Calculate the percentage change in mass for each cylinder, to one decimal place.
Using your answers to the previous question, estimate the concentration of sucrose that would cause no change in mass, and explain what this tells you about the potato cells. Give one reason the cylinders were blotted before reweighing.
Extended. A plant has not been watered for several days and has wilted. Use the terms water potential, turgor pressure and flaccid to explain why it wilted and how it recovers after watering.
Extended. Red blood cells and onion epidermis cells are each placed in (a) pure water and (b) a concentrated salt solution. Describe and explain the appearance of each type of cell in each liquid.
Answer key and marking notes
- Digestion: enzymes act on food in solution, and the small soluble products dissolve in water so they can be absorbed.
- Excretion: urea and excess salts are removed dissolved in water, as urine.
- Transport: substances such as glucose, ions and hormones are carried dissolved in blood plasma (and in xylem and phloem in plants).
- The level rises.
- Dialysis tubing is partially permeable: water molecules can pass through but sucrose molecules are too large.
- The water in the beaker is more dilute than the solution in the tubing (Extended: it has a higher water potential).
- So there is a net movement of water into the tubing by osmosis, increasing the volume inside and pushing liquid up the capillary.
- 0.0: (2.64 − 2.40) ÷ 2.40 × 100 = +10.0%.
- 0.4: (2.45 − 2.50) ÷ 2.50 × 100 = −2.0%.
- 0.8: (2.21 − 2.45) ÷ 2.45 × 100 = −9.8%.
- The change crosses zero between 0.0 and 0.4 mol/dm3, close to 0.4: about 0.3 to 0.35 mol/dm3 is a reasonable estimate (best read from a graph).
- At this concentration there is no net movement of water, so the cell sap has about the same concentration (water potential) as the solution.
- Blotting removes surface solution, which would otherwise add mass that is not due to osmosis.
- As the soil dries its water potential falls, so the root cells cannot take in enough water to replace what the leaves lose.
- With less water in the vacuoles, the cells no longer press outwards on their walls: turgor pressure falls and the cells become flaccid.
- Flaccid cells cannot support the stem and leaves, so the plant wilts.
- After watering, the soil water has a higher water potential than the root cells, water enters by osmosis, the cells become turgid again and the plant is supported.
- (a) Pure water has a higher water potential than the cells, so both take in water by osmosis. The red blood cell swells and bursts because it has no cell wall; the onion cell becomes turgid because its cell wall resists the turgor pressure.
- (b) The salt solution has a lower water potential, so both lose water by osmosis. The red blood cell shrinks with a crinkled outline.
- The onion cell becomes plasmolysed: the vacuole and cytoplasm shrink and the cell membrane pulls away from the cell wall.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which is an example of water acting as a solvent in the human body?
- AUrea is removed from the body dissolved in the urine
- BWater evaporates from sweat on the skin to cool the body
- CWater in cells presses outwards to keep them firm
- DWater is broken down to release oxygen in the lungs
A bag of dialysis tubing is filled with pure water, sealed and placed in a beaker of concentrated sucrose solution. What happens to the mass of the bag after 30 minutes?
- AIt decreases, because water moves out of the bag by osmosis
- BIt increases, because sucrose moves into the bag by diffusion
- CIt increases, because water moves into the bag by osmosis
- DIt stays the same, because the tubing is fully permeable
A strip of potato is 50 mm long. After 1 hour in water it is 54 mm long. What is the percentage change in length?
- A+8.0%
- B+7.4%
- C+4.0%
- D−8.0%
Potato cylinders were left in sucrose solutions for 1 hour. The graph shows the percentage change in mass. What is the best estimate of the concentration of the solution inside the potato cells?
- A0.25 mol/dm3
- B0.40 mol/dm3
- C0.20 mol/dm3
- D0.80 mol/dm3
How are the stems and leaves of non-woody plants kept upright?
- AWater inside the cells presses outwards on the cell walls
- BThe cell walls pull inwards on the vacuoles to stiffen them
- CStarch grains stored in the cells make them rigid
- DAir in the spaces between cells is kept at high pressure
Solution X is 0.1 mol/dm3 sucrose and solution Y is 0.5 mol/dm3 sucrose. They are separated by a partially permeable membrane. Which statement is correct?
- AX has the higher water potential, so water moves from X to Y
- BY has the higher water potential, so water moves from X to Y
- CX has the higher water potential, so sucrose moves from Y to X
- DY has the higher water potential, so water moves from Y to X
Which statement describes a plasmolysed plant cell?
- AThe cell membrane has pulled away from the cell wall
- BThe cell wall has burst because of high turgor pressure
- CThe vacuole is full and presses hard against the cell wall
- DThe cell is soft but the membrane still touches the wall
A gardener puts far too much fertiliser around some plants and they wilt. What is the best explanation?
- AThe soil water potential falls below that of the root cells, so water leaves the roots
- BThe soil water potential rises above that of the root cells, so too much water enters
- CThe fertiliser ions enter the root cells by osmosis and make them burst
- DThe fertiliser blocks the stomata so the leaves cannot take in water
Quiz key
Urea dissolves in water, so it can be carried in the blood and removed in urine; that is water acting as a solvent in excretion. Evaporation of sweat is water acting as a coolant, not a solvent. Water pressing outwards gives support (in plants, against the cell wall), which is not dissolving anything. Water is not split to release oxygen in the lungs; that happens only in photosynthesis in plants.
The water inside the bag is more dilute than the sucrose solution, so there is a net movement of water out of the bag through the partially permeable tubing and the bag gets lighter. Sucrose molecules are too large to pass through the tubing, so they cannot enter. Water moving in would happen only if the more concentrated solution were inside the bag. The tubing is partially permeable, not fully permeable.
Change = 54 − 50 = +4 mm. Percentage change = 4 ÷ 50 × 100 = +8.0%. +7.4% divides by the final length (4 ÷ 54) instead of the starting length. +4.0% is the change in millimetres, not a percentage. −8.0% has the wrong sign: the strip got longer because its cells took in water.
Where the line crosses 0% there is no net movement of water, so the solution outside matches the concentration inside the cells. The line crosses zero at 0.25 mol/dm3. 0.40 is the first concentration at which the cylinders lost mass, so the crossing point lies below it. 0.20 is the last point that still gained mass, so the crossing lies above it. 0.80 is where the cylinders lost the most mass, the most concentrated solution.
When a plant cell takes in water by osmosis, the vacuole swells and the cell contents press outwards on the strong cell wall. Many firm cells pressed together support the stem and leaves. The wall resists the push; it does not pull inwards. Starch is a store of energy, not a means of support. Air spaces are not under pressure; when cells lose water the plant wilts even though the air spaces are still there.
X is the more dilute solution, so it has the higher water potential. Water moves by osmosis from higher water potential (X) to lower water potential (Y). Y is more concentrated, so it has the lower water potential. Sucrose molecules cannot cross a partially permeable membrane, and osmosis is the movement of water, not solute. Water moving from Y to X would be from lower to higher water potential.
In a plasmolysed cell so much water has been lost by osmosis that the vacuole and cytoplasm shrink and the cell membrane pulls away from the cell wall. Plant cell walls do not burst, because they resist the turgor pressure. A full vacuole pressing on the wall describes a turgid cell. A soft cell with the membrane still against the wall is flaccid, the stage before plasmolysis.
Dissolved fertiliser makes the soil water much more concentrated, lowering its water potential below that of the root cell sap. Water then leaves the roots by osmosis, the cells lose turgor pressure and become flaccid, and the plant wilts. A concentrated solution has a lower water potential, not higher. Osmosis is the movement of water, not ions, and plant cells do not burst because of their walls. Leaves do not take in water through the stomata; water enters through the roots.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.