Lesson Plan

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Lesson Plan
Grade: Date: 25/02/2026
Subject: Biology
Lesson Topic: investigate the effect of changing surface area to volume ratio on diffusion using agar blocks of different sizes
Learning Objective/s:
  • Describe how surface area‑to‑volume ratio influences diffusion rate.
  • Explain the mathematical relationship between cube size and SA:V ratio.
  • Conduct a controlled experiment to measure diffusion into agar blocks.
  • Analyse data to determine the correlation between SA:V ratio and diffusion depth.
  • Evaluate sources of experimental error and suggest improvements.
Materials Needed:
  • Agar powder and distilled water for a 1 % agar solution.
  • Food‑colouring or potassium permanganate solution as diffusible solute.
  • Moulds to cast agar cubes of 1 cm, 2 cm, 3 cm and 4 cm.
  • Ruler or caliper, stopwatch, and clear cuvettes or beakers.
  • Digital camera (optional) for recording colour change.
  • Heat‑resistant gloves, safety goggles, and lab coat.
 Introduction:
Begin with a quick demonstration of a coloured dye spreading in water to capture interest. Review prior knowledge that diffusion moves particles from high to low concentration and that cells rely on high SA:V ratios for efficient exchange. State that by the end of the lesson students will be able to design and carry out an experiment investigating how changing SA:V ratio affects diffusion rate.
Lesson Structure:
  1. Do‑now (5'): Students answer a short question on diffusion and SA:V ratio on a worksheet.
  2. Mini‑lecture (10'): Recap diffusion theory, present the SA:V equations, and explain the hypothesis.
  3. Practical set‑up (10'): Prepare 1 % agar, cast cubes, measure side lengths, and label cuvettes.
  4. Data collection (15'): Immerse cubes, start stopwatch, record colour penetration every 5 minutes for 30 minutes.
  5. Data analysis (10'): Calculate SA, V, SA:V ratio; plot graph of ratio vs depth of colour penetration.
  6. Group discussion (5'): Interpret results, relate findings to the hypothesis and biological examples.
  7. Exit ticket (5'): Write one sentence summarising how SA:V ratio influences diffusion.
Conclusion:
Summarise that smaller cubes with higher SA:V ratios showed faster colour penetration, confirming the hypothesis. Students complete an exit ticket stating the key relationship. For homework, ask them to research another biological system where a high SA:V ratio is critical, such as alveoli or plant root hairs.