• Quick retrieval from earlier learning (Osmosis and percentage change in mass). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. A potato cylinder had a mass of 4 g. After 30 minutes in a sugar solution its mass was 3.2 g. What is the percentage change in mass? (Use a minus sign for a loss.) → -20 %Prompt: percentage change = (final − initial) ÷ initial × 100
2. Potato pieces were left in sugar solutions. Percentage change in mass: 0 mol/dm³: +12 % 0.2 mol/dm³: +4 % 0.4 mol/dm³: −4 % 0.6 mol/dm³: −12 % 0.8 mol/dm³: −20 % Between which concentrations is the solution that matches the concentration inside the potato cells? (0–0.2 mol/dm³ / 0.2–0.4 mol/dm³ / 0.4–0.6 mol/dm³ / 0.6–0.8 mol/dm³) → 0.2–0.4 mol/dm³Prompt: Where there is no net movement of water, the mass does not change.
3. Cucumber slices sprinkled with salt become limp and watery. Why? (Water leaves the cells by osmosis into the more concentrated salty surface / The cucumber starts to photosynthesise / Salt enters and the cells burst / Salt particles push water out by force) → Water leaves the cells by osmosis into the more concentrated salty surfacePrompt: Water moves from a dilute solution to a more concentrated one.

• Show a microscope image of cells and ask what students think they are looking at.
• Share the objective: Diffusion, osmosis or active transport?. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Which process is this: urea passing from cells into the blood?
• Step 1: Diffusion: particles move down a concentration gradient with no energy needed.
• Step 2: So the answer is: diffusion
• Answer: diffusion
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Which process is this: oxygen entering the blood in the lungs? (diffusion / osmosis / active transport) → diffusionPrompt: Is it water specifically? Is it going against the gradient?
2. Glucose is absorbed from the small intestine even when the blood already holds more glucose than the gut. How? (by active transport, using energy from respiration / it cannot be absorbed / by osmosis / by diffusion down the gradient) → by active transport, using energy from respirationPrompt: The glucose is moving against its gradient.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Root hair cells contain many mitochondria. What does this suggest? (They absorb water only by diffusion / They use active transport, which needs energy / They do not respire / They photosynthesise) → They use active transport, which needs energyPrompt: What do mitochondria provide?
2. Transport in cells: which statement is true? (Active transport only happens in plants. / Water moves by osmosis from a concentrated to a dilute solution. / Osmosis is the movement of water molecules. / Osmosis needs energy from respiration.) → Osmosis is the movement of water molecules.Prompt: Which process needs energy?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-8.