• 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. Purple dye spread 6 mm through a block of jelly in 2 hours. What was the mean rate of diffusion, in mm per hour? → 3 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. / Particles stop moving once they are evenly spread. / Diffusion does not need energy from respiration.) → Particles stop moving once they are evenly spread.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 1 cm. What is its surface area to volume ratio, written as ? : 1? → 6Prompt: 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? (Losing heat quickly enough / Moving too fast / 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 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. A cube-shaped cell model has sides of 6 cm. What is its surface area to volume ratio, written as ? : 1? → 1Prompt: Surface area of a cube = 6 × side × side.
2. Why are the alveoli in the lungs so numerous and tiny? (to store air for later / to make the lungs heavier / to filter out food / 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.