• 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 / Oxygen is heavier than blood / Blood pulls oxygen like a magnet / The heart pushes air in) → There is more oxygen in the alveoli than in the bloodPrompt: Compare the concentration on each side.
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 24 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? (A larger surface area speeds up diffusion. / Diffusion happens in gases and liquids. / A steeper concentration gradient gives faster diffusion. / Diffusion moves particles from low to high concentration.) → 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 are the alveoli in the lungs so numerous and tiny? (to filter out food / to make the lungs heavier / to give a huge surface area for gas exchange / to store air for later) → to give a huge surface area for gas exchangePrompt: Many small bubbles have more surface than one big bag.

• 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? (Large animals need transport systems to reach all their cells. / Villi increase the surface area of the small intestine. / Fish gills have many thin filaments to increase surface area. / Thick walls make exchange surfaces work better.) → Thick walls make exchange surfaces work better.Prompt: What happens to SA:V as an object gets bigger?
2. Which feature does NOT increase the effectiveness of an exchange surface? (a good blood supply / a large surface area / being ventilated / a thick wall) → a thick wallPrompt: Diffusion needs a short path.

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