• 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. Carbon dioxide made by respiring cells moves into the blood. Which way is the gradient? (higher in the blood, lower in the cells / the same on both sides / there is no carbon dioxide in cells / higher in the cells, lower in the blood) → higher in the cells, lower in the bloodPrompt: Where is carbon dioxide being made?
2. Purple dye spread 15 mm through a block of jelly in 3 hours. What was the mean rate of diffusion, in mm per hour? → 5 mm/hPrompt: rate = distance ÷ time
3. Diffusion: which statement is NOT true? (Diffusion happens in gases and liquids. / A larger surface area speeds up diffusion. / Diffusion does not need energy from respiration. / Diffusion stops completely when the room is cold.) → 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 3 cm. What is its surface area to volume ratio, written as ? : 1? → 2Prompt: Surface area of a cube = 6 × side × side.
2. Why does a large multicellular animal need lungs and a heart, but an amoeba does not? (Diffusion is faster in big animals / Large animals do not use oxygen / Amoebae are not alive / Its small SA:V ratio and long distances mean diffusion alone is too slow) → 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?

• 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. Why does a large multicellular animal need lungs and a heart, but an amoeba does not? (Amoebae are not alive / Diffusion is faster in big animals / Large animals do not use oxygen / Its small SA:V ratio and long distances mean diffusion alone is too slow) → 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?
2. Exchange surfaces: which statement is NOT true? (Single-celled organisms need lungs. / Villi increase the surface area of the small intestine. / Smaller objects have a larger surface area to volume ratio. / Fish gills have many thin filaments to increase surface area.) → Single-celled organisms need lungs.Prompt: What happens to SA:V as an object gets bigger?

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