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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.
  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 8 min, 2 cm cube 16 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 moves particles from low to high concentration. / A larger surface area speeds up diffusion. / A steeper concentration gradient gives faster diffusion. / Diffusion happens in gases and liquids.) → 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.
  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 too quickly / Losing heat quickly enough / Moving too fast / Getting too little food) → Losing heat quickly enoughPrompt: Heat is lost through the surface.

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.
  1. An elephant has a much smaller surface area to volume ratio than a mouse. What problem does this cause? (Losing heat too quickly / Moving too fast / Losing heat quickly enough / Getting too little food) → Losing heat quickly enoughPrompt: Heat is lost through the surface.
  2. 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.

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

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Surface area to volume ratio and exchange

Name

Date

Score/ 8

  1. Question 1

    A cube-shaped cell model has sides of 1 cm. What is its surface area to volume ratio, written as ? : 1?

    Answer

  2. Question 2

    Why does a large multicellular animal need lungs and a heart, but an amoeba does not?

    • A. Its small SA:V ratio and long distances mean diffusion alone is too slow
    • B. Amoebae are not alive
    • C. Diffusion is faster in big animals
    • D. Large animals do not use oxygen

  3. Question 3

    Exchange surfaces: which statement is NOT true?

    • A. Thick walls make exchange surfaces work better.
    • B. Villi increase the surface area of the small intestine.
    • C. Smaller objects have a larger surface area to volume ratio.
    • D. Fish gills have many thin filaments to increase surface area.

  4. Question 4

    Why are the alveoli in the lungs so numerous and tiny?

    • A. to filter out food
    • B. to give a huge surface area for gas exchange
    • C. to make the lungs heavier
    • D. to store air for later

  5. Question 5

    A cube-shaped cell model has sides of 2 cm. What is its surface area to volume ratio, written as ? : 1?

    Answer

  6. Question 6

    Which feature does NOT increase the effectiveness of an exchange surface?

    • A. a thick wall
    • B. a good blood supply
    • C. being ventilated
    • D. a large surface area

  7. Question 7

    A cube-shaped cell model has sides of 5 cm. What is its surface area to volume ratio, written as ? : 1?

    Answer

  8. Question 8

    A cube-shaped cell model has sides of 6 cm. What is its surface area to volume ratio, written as ? : 1?

    Answer

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Answer key: Surface area to volume ratio and exchange

  1. 1. 6

  2. 2. A. Its small SA:V ratio and long distances mean diffusion alone is too slow

  3. 3. A. Thick walls make exchange surfaces work better.

  4. 4. B. to give a huge surface area for gas exchange

  5. 5. 3

  6. 6. A. a thick wall

  7. 7. 1.2

  8. 8. 1