• 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. Which change would make diffusion faster? (a smaller surface area / a lower temperature / a thicker membrane / a higher temperature) → a higher temperaturePrompt: Think about how fast the particles move.
2. Agar cubes containing indicator were put into acid. Time for the whole cube to change colour: 1 cm cube 8 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? (Diffusion stops completely when the room is cold. / A larger surface area speeds up diffusion. / Diffusion happens in gases and liquids. / Diffusion does not need energy from respiration.) → 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 2 cm. What is its surface area to volume ratio, written as ? : 1? → 3Prompt: Surface area of a cube = 6 × side × side.
2. Why are the alveoli in the lungs so numerous and tiny? (to make the lungs heavier / to filter out food / to store air for later / to give a huge surface area for gas exchange) → 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. / Thick walls make exchange surfaces work better. / Villi increase the surface area of the small intestine. / Smaller objects have a larger surface area to volume ratio.) → Thick walls make exchange surfaces work better.Prompt: What happens to SA:V as an object gets bigger?
2. Why are the alveoli in the lungs so numerous and tiny? (to filter out food / to make the lungs heavier / to store air for later / to give a huge surface area for gas exchange) → to give a huge surface area for gas exchangePrompt: Many small bubbles have more surface than one big bag.

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