75-minute lesson
Year 12 Science: Cell membranes and osmosis
Lesson objective
- Describe the structure and role of the cell membrane.
- Distinguish between diffusion, osmosis and active transport.
- Investigate osmosis and calculate percentage change in mass.
Success criteria:
- I can describe the fluid mosaic model of the membrane in simple terms.
- I can predict the direction of water movement between solutions of different concentration.
- I can calculate percentage change in mass and interpret the results.
Materials
- One large potato
- A sharp knife and chopping board (adult to cut, or supervise closely)
- Table salt and tap water
- Three cups or jars, labelled
- Kitchen scales accurate to 0.1 g (or 1 g with larger pieces)
- Paper towel and a ruler
Introduction
8 minIntroduce today's words:
- selectively permeable: Allowing some substances to pass through but not others.
- diffusion: Net movement of particles from a region of higher concentration to lower concentration, without energy input from the cell.
- osmosis: Net movement of water across a selectively permeable membrane from a region of lower solute concentration to higher solute concentration.
- active transport: Movement of substances against a concentration gradient using energy (ATP) and carrier proteins.
- hypertonic: Describes a solution with a higher solute concentration than the cell contents.
- hypotonic: Describes a solution with a lower solute concentration than the cell contents.
Ask your child what they already know about cell membranes and osmosis.
Explanation
15 minEvery cell is surrounded by a cell membrane made mainly of a double layer of phospholipids with proteins embedded in it. This is often called the fluid mosaic model. The membrane is selectively permeable: small, non-polar molecules such as oxygen and carbon dioxide pass through easily, while ions and large molecules need protein channels or carriers.
Diffusion is the net movement of particles down a concentration gradient. It is passive: it does not need energy from the cell. Oxygen diffuses from the air sacs in your lungs into your blood this way.
Osmosis is a special case of diffusion involving water. Water moves across a selectively permeable membrane from where there is less dissolved solute (more 'free' water) to where there is more solute.
If a cell is placed in a hypotonic solution (like pure water), water moves in. Plant cells become firm (turgid) because the cell wall stops them bursting. Animal cells, without a wall, can swell and burst.
If a cell is placed in a hypertonic solution (like strong salt water), water moves out. Plant cells become soft and flaccid, and the membrane may pull away from the cell wall (plasmolysis).
Active transport moves substances against their concentration gradient using energy from ATP. Root hair cells use it to take in mineral ions from soil water that is more dilute than the cell contents.
Osmosis matters in Australian agriculture. Rising salinity in some soils, such as parts of the Murray–Darling Basin and the WA wheatbelt, makes it harder for crops to draw water in by osmosis.
Worked examples
11 minPotato osmosis investigation
How does salt concentration affect the mass of potato strips?
Step 1: Cut six potato strips the same size (e.g. 5 cm × 1 cm × 1 cm). Remove the skin.
Step 2: Prepare three solutions of 200 mL each: tap water (0 g salt), weak salt water (2 g salt) and strong salt water (10 g salt).
Step 3: Blot each strip, weigh it and record the starting mass. Place two strips in each cup.
Step 4: After 1–2 hours (or overnight in the fridge), blot dry and reweigh.
Step 5: Calculate percentage change in mass for each strip and average for each solution.
Answer: Strips in water gain mass; strips in strong salt water lose mass.
Percentage change in mass
A strip had a mass of 5.0 g at the start and 4.4 g after soaking in strong salt water. Calculate the percentage change.
Step 1: Change = final − initial = 4.4 − 5.0 = −0.6 g.
Step 2: Percentage change = (change ÷ initial) × 100 = (−0.6 ÷ 5.0) × 100.
Step 3: = −12%.
Answer: −12% (a loss of mass because water left the cells by osmosis).
Guided practice (do together)
15 min1. Which best defines osmosis?
(a) Movement of any particle from low to high concentration (b) Net movement of water across a selectively permeable membrane towards higher solute concentration (c) Movement of glucose using ATP (d) Movement of salt across a membrane towards lower water concentration
2. A plant cell is placed in pure water. What happens?
(a) It bursts (b) It becomes flaccid (c) It becomes turgid (d) Nothing happens
3. Which process requires energy from ATP?
(a) Active transport (b) Diffusion (c) Osmosis (d) Evaporation
4. Red blood cells are placed in a strongly hypertonic salt solution. They will…
(a) Swell and burst (b) Stay the same (c) Become turgid (d) Shrink as water leaves
Independent practice
19 min5. Why must the potato strips be blotted before weighing?
(a) To remove surface water that would add to the mass (b) To make them cooler (c) To stop osmosis (d) To add salt
6. A strip went from 4.0 g to 4.4 g. What is the percentage change in mass? (number, e.g. 5 or -5)
7. A strip went from 5.0 g to 4.5 g. What is the percentage change in mass? (number, e.g. 5 or -5)
8. What is the term for a plant cell membrane pulling away from the cell wall in a hypertonic solution?
Questions to check understanding
- Can you describe the fluid mosaic model of the membrane in simple terms?
- Can you predict the direction of water movement between solutions of different concentration?
- Can you calculate percentage change in mass and interpret the results?
- What was the trickiest part today?
Answer guide
1. Net movement of water across a selectively permeable membrane towards higher solute concentration — Osmosis is specifically water moving towards the side with more dissolved solute.
2. It becomes turgid — Water enters, and the cell wall resists bursting, so the cell becomes turgid.
3. Active transport — Active transport moves substances against a gradient and requires ATP.
4. Shrink as water leaves — Water moves out towards the higher solute concentration, so cells shrink.
5. To remove surface water that would add to the mass — Surface liquid is not part of the cells and would distort the mass.
6. 10 — (4.4 − 4.0) ÷ 4.0 × 100 = 0.4 ÷ 4.0 × 100 = 10%.
7. -10 — (4.5 − 5.0) ÷ 5.0 × 100 = −0.5 ÷ 5.0 × 100 = −10%.
8. plasmolysis — Loss of water causes plasmolysis.
Review
7 minRecap the success criteria together. Watch for these common misconceptions:
- Students often say salt 'moves into' the potato in osmosis. Osmosis describes water movement; the membrane limits salt movement.
- Some think diffusion stops at equilibrium. Particles keep moving; there is just no net movement.
- Some think 'concentration' always refers to water. Be explicit: solute concentration and water concentration change in opposite directions.
Extension activities
- Extend the investigation to five or six salt concentrations to produce a more precise graph.
- Research how mangroves cope with living in salty water along Australia's northern coasts.
- Find out how oral rehydration solutions use the movement of sodium and glucose to help the body absorb water.
Suggested follow-up
- Revisit cell membranes and osmosis tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
Tips for parents
- Weigh carefully and record to the same precision each time; a table set up in advance helps.
- Ask your student to plot average percentage change against salt concentration and estimate where the line crosses zero: that is roughly the concentration of the potato's cell contents.
- Safety: an adult should supervise or do the cutting; cut on a stable board, away from the body.