Diffusion and Adaptations

National Curriculum Links

• Year 7 Science: Biology – Organisation and the digestive system
• Pupils should be taught to:
  • Describe the role of diffusion in the movement of substances.
  • Describe the structure and function of the small intestine, including adaptations for absorption.
  • Explain how partial permeability of membranes affects diffusion.
• Key stage 3 Programme of Study (National Curriculum for England, Science, Biology 3.3)

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Lesson Duration

60 minutes
Class Size: 25 students

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Learning Objectives

By the end of this lesson, students will be able to:

1. Define diffusion and explain how it occurs across a partially permeable membrane.
2. Describe the purpose of a partially permeable membrane in controlling diffusion.
3. Identify factors affecting the rate of diffusion.
4. Explain the primary adaptations of the small intestine and villi for absorption.
5. Interpret simple diagrams showing diffusion and the structure of villi.

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Resources Needed

• Whiteboard and markers
• Projector or large display screen
• Printed worksheet with diagrams and checkpoint questions (25 copies)
• Clear plastic bags (to simulate cell membranes)
• Food colouring or potassium permanganate solution
• Petri dishes or shallow clear dishes
• Simple hand lens or magnifying glasses
• A3 paper and coloured pencils for drawing tasks
• Interactive quiz or mini-whiteboards for student responses
• Pre-drawn diagram posters: diffusion across membranes and villi structure

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Lesson Breakdown

1. Introduction and Engagement (10 minutes)

Objective: Activate prior knowledge and introduce diffusion and membranes.

• Starter Discussion (5 mins):
  Ask students: "Have you ever noticed how a smell spreads in a room? How do you think that happens?"
  Briefly discuss their ideas and steer towards the concept of diffusion—movement of particles from high to low concentration.

• Definition and Explanation (5 mins):
  Write the definition on the board:

  Diffusion is the movement of particles from an area of high concentration to an area of low concentration.
  Introduce partially permeable membrane as a barrier allowing only certain substances to pass through.

• Visual Aid:
  Show a simple diagram: particles moving across a membrane that only allows some particles through.
  Tip: Use arrows to indicate movement and a dotted line to represent the membrane.

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2. Chunk 1: Diffusion Through Partially Permeable Membranes (15 minutes)

Objective: Understand diffusion and role of membranes.

• Explanation (7 mins):
  Use a real-life analogy: a tea bag in hot water (tea particles moving into water by diffusion).
  Explain how in biological systems, membranes are like filters. Only small molecules (e.g., oxygen, glucose) pass through; larger ones don’t.

• Practical Demonstration (8 mins):
  Demonstration set-up:

  • Use clear plastic bags to simulate cell membranes filled with starch solution.
  • Place bags in iodine solution. Iodine will diffuse through the bag if partially permeable, turning the starch solution blue/black if it enters.
    Ask students to observe colour change inside the bag and discuss what is happening.

• Checkpoint 1 (Interactive Q&A):

  • What direction do particles move in diffusion?
  • What does “partially permeable” mean?
  • Why can some molecules pass through the membrane but not others?
    Use mini-whiteboards for students to write short answers.

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3. Chunk 2: Factors Affecting Diffusion (10 minutes)

Objective: Identify and explain factors influencing diffusion rate.

• Explanation (6 mins):
  Introduce factors:

  • Concentration gradient: bigger difference = faster diffusion
  • Temperature: higher temperature = faster movement
  • Surface area: more area = quicker diffusion through membranes (link to villi later)
  • Distance (thickness of membrane): thinner membrane = faster diffusion

• Visual Aid:
  Show a chart or diagram illustrating each factor.

• Activity (4 mins):
  In pairs, students rank the factors from most to least important for diffusion speed, then explain their reasoning verbally to a partner.

• Checkpoint 2 (Quick Quiz):
  Teacher asks rapid-fire Qs: "If the temperature increases, what happens to diffusion speed?" Students signal thumbs up/down.

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4. Chunk 3: Adaptations of the Small Intestine and Villi (15 minutes)

Objective: Understand how the small intestine is adapted for absorption via diffusion.

• Explanation (7 mins):
  Explain:

  • Small intestine absorbs nutrients after digestion.
  • Villi (tiny finger-like projections) increase surface area massively.
  • Villi have a single layer of thin cells (thin membrane) for faster diffusion.
  • Rich blood supply maintains a concentration gradient by transporting absorbed nutrients away.

• Visual Aid:
  Display a large diagram of the small intestine with magnified villi. Label: surface area, blood vessels, thin membrane.

• Group Task (8 mins):
  Students get A3 paper and coloured pencils. They draw labelled diagrams of a villus showing:

  • Thin membrane
  • Blood capillaries
  • Increased surface area shape
    After drawing, each group explains aloud one adaptation and how it helps diffusion.

• Checkpoint 3 (Peer Teaching):
  Students quiz each other using their drawings. E.g., "Why is the villus shape important?"

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5. Plenary and Assessment (10 minutes)

Objective: Review learning and assess understanding.

• Recap discussion:
  Review key points orally: diffusion definition, role of membranes, factors affecting diffusion, adaptations of villi.

• Quiz (5 mins):
  Short multiple choice or true/false questions based on the lesson content.
  Example questions:

  • Diffusion moves particles from low to high concentration (T/F)
  • The membrane lets all substances pass through (T/F)
  • Villi increase surface area for absorption (T/F)

• Exit Ticket:
  On a small slip, students write one thing they learned about diffusion and one question they still have. Collect for teacher review.

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Differentiation

• Support: Provide sentence starters and labelled diagrams for less confident learners.
• Challenge: Ask more able students to explain how diseases or damage to villi might reduce absorption efficiency.

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Homework Suggestion

• Write a short paragraph explaining why diffusion through a partially permeable membrane is essential for the body, using examples from the lesson.

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Teacher Notes

• Emphasise key vocabulary frequently: diffusion, membrane, partially permeable, concentration gradient, villi, absorption.
• Use everyday examples to make abstract concepts concrete.
• Encourage peer discussion to embed understanding.
• The practical bag demonstration is simple but highly effective for visualising diffusion.

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This lesson ensures full alignment with the National Curriculum for Year 7 Science, focusing on biology content foundational for further study, using scaffolded and chunked teaching for optimal comprehension and engagement.