Science lesson plan · 8th grade (Year 9 in England) · 45 minutes
Surface area to volume ratio and exchange
Cells, genetics and evolution
CCSS NGSS.HS-LS1-2CCSS NGSS.SEP 5England: GCSE Combined Science: Cell biology (exchange surfaces)
Objective and success criteria
Students will: Surface area to volume ratio and exchange.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Diffusion: how particles spread out
Leads to: the next skill in this area.
Materials: Microscope images or slides, Cell diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- 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.
- 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 / The heart pushes air in / Blood pulls oxygen like a magnet) → There is more oxygen in the alveoli than in the bloodPrompt: Compare the concentration on each side.
- Purple dye spread 12 mm through a block of jelly in 3 hours. What was the mean rate of diffusion, in mm per hour? → 4 mm/hPrompt: rate = distance ÷ time
- Diffusion: which statement is NOT true? (Diffusion does not need energy from respiration. / A larger surface area speeds up diffusion. / Diffusion happens in gases and liquids. / Diffusion moves particles from low to high concentration.) → Diffusion moves particles from low to high concentration.Prompt: Which direction is the net movement?
Hook · 5 min
- 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.
I do: model it · 10 min
- 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.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- 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.
- 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.
You do: independent practice · 10 min
- 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.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Why are the alveoli in the lungs so numerous and tiny? (to store air for later / to make the lungs heavier / to give a huge surface area for gas exchange / to filter out food) → to give a huge surface area for gas exchangePrompt: Many small bubbles have more surface than one big bag.
- A cube-shaped cell model has sides of 4 cm. What is its surface area to volume ratio, written as ? : 1? → 1.5Prompt: Surface area of a cube = 6 × side × side.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Diffusion: how particles spread out.
- Use concrete materials: microscope images or slides.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-8.
Free lesson plan from eurekami.com/lesson-plan-builder