• 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 cells, lower in the blood / there is no carbon dioxide in cells / the same on both sides / higher in the blood, lower in the cells) → higher in the cells, lower in the bloodPrompt: Where is carbon dioxide being made?
2. Agar cubes containing indicator were put into acid. Time for the whole cube to change colour: 1 cm cube 6 min, 2 cm cube 12 min, 3 cm cube 18 min. Which cube's centre was reached fastest by the acid? (the 1 cm cube / the 2 cm cube / the 3 cm cube) → the 1 cm cubePrompt: Fastest means the shortest time.
3. Diffusion: which statement is NOT true? (Diffusion moves particles from low to high concentration. / A steeper concentration gradient gives faster diffusion. / A larger surface area speeds up diffusion. / Diffusion does not need energy from respiration.) → Diffusion moves particles from low to high concentration.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? (Its small SA:V ratio and long distances mean diffusion alone is too slow / Large animals do not use oxygen / Diffusion is faster in big animals / Amoebae are not alive) → 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 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. An elephant has a much smaller surface area to volume ratio than a mouse. What problem does this cause? (Losing heat quickly enough / Losing heat too quickly / Moving too fast / Getting too little food) → Losing heat quickly enoughPrompt: Heat is lost through the surface.
2. Exchange surfaces: which statement is NOT true? (Thick walls make exchange surfaces work better. / Smaller objects have a larger surface area to volume ratio. / Large animals need transport systems to reach all their cells. / Fish gills have many thin filaments to increase surface area.) → Thick walls make exchange surfaces work better.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.