• Quick retrieval from earlier learning (Diffusion: how particles spread out). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Oxygen moves from the alveoli into the blood. Why? (There is more oxygen in the alveoli than in the blood / The heart pushes air in / Blood pulls oxygen like a magnet / Oxygen is heavier than blood) → There is more oxygen in the alveoli than in the bloodPrompt: Compare the concentration on each side.
2. Purple dye spread 8 mm through a block of jelly in 2 hours. What was the mean rate of diffusion, in mm per hour? → 4 mm/hPrompt: rate = distance ÷ time
3. Diffusion: which statement is NOT true? (A steeper concentration gradient gives faster diffusion. / Diffusion stops completely when the room is cold. / A larger surface area speeds up diffusion. / Diffusion happens in gases and liquids.) → Diffusion stops completely when the room is cold.Prompt: Which direction is the net movement?

• Show a microscope image of cells and ask what students think they are looking at.
• Share the objective: Surface area to volume ratio and exchange. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A cube-shaped cell model has sides of 3 cm. What is its surface area to volume ratio, written as ? : 1?
• Step 1: surface area = 6 × 3 × 3 = 54 cm²
• Step 2: volume = 3³ = 27 cm³
• Step 3: 54 ÷ 27 = 2
• Step 4: So the answer is: 2
• Answer: 2
• 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. A cube-shaped cell model has sides of 5 cm. What is its surface area to volume ratio, written as ? : 1? → 1.2Prompt: Surface area of a cube = 6 × side × side.
2. An elephant has a much smaller surface area to volume ratio than a mouse. What problem does this cause? (Moving too fast / Losing heat quickly enough / Getting too little food / Losing heat too quickly) → Losing heat quickly enoughPrompt: Heat is lost through the surface.

• 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 8 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. Exchange surfaces: which statement is NOT true? (Thick walls make exchange surfaces work better. / Fish gills have many thin filaments to increase surface area. / Villi increase the surface area of the small intestine. / Large animals need transport systems to reach all their cells.) → Thick walls make exchange surfaces work better.Prompt: What happens to SA:V as an object gets bigger?
2. Why does a large multicellular animal need lungs and a heart, but an amoeba does not? (Diffusion is faster in big animals / Amoebae are not alive / Its small SA:V ratio and long distances mean diffusion alone is too slow / Large animals do not use oxygen) → Its small SA:V ratio and long distances mean diffusion alone is too slowPrompt: How far is the middle of your body from your skin?

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