Scaling up

    OCR
    GCSE

    As an organism increases in size, its surface area to volume ratio (SA:V) decreases, rendering simple diffusion insufficient for metabolic exchange. Multicellular organisms therefore require specialized exchange surfaces and mass transport systems to minimize diffusion distances and maintain concentration gradients. This topic quantifies the relationship between geometric dimensions and physiological requirements, linking mathematical scaling to biological adaptation.

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    Objectives
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    Exam Tips
    4
    Pitfalls
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    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for defining osmosis as the net movement of water molecules from a dilute to a concentrated solution across a partially permeable membrane
    • Credit responses that explicitly link a small SA:V ratio in multicellular organisms to the requirement for a mass transport system
    • Award 1 mark for identifying that the left ventricle wall is thicker to generate higher pressure for pumping blood around the body
    • Candidates must state that active transport requires energy from respiration to move substances against a concentration gradient
    • Award 1 mark for describing the role of xylem in transporting water and mineral ions and phloem in transporting dissolved sugars

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly identified the movement of water, but you must mention the 'partially permeable membrane' to secure the mark."
    • "Your calculation of the surface area is correct, but you didn't express it as a ratio to the volume."
    • "Good description of the heart's structure; now explain *why* the left ventricle wall is thicker in terms of pressure and distance."
    • "You've confused diffusion with active transport—remember active transport requires energy and goes against the gradient."

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for defining osmosis as the net movement of water molecules from a dilute to a concentrated solution across a partially permeable membrane
    • Credit responses that explicitly link a small SA:V ratio in multicellular organisms to the requirement for a mass transport system
    • Award 1 mark for identifying that the left ventricle wall is thicker to generate higher pressure for pumping blood around the body
    • Candidates must state that active transport requires energy from respiration to move substances against a concentration gradient
    • Award 1 mark for describing the role of xylem in transporting water and mineral ions and phloem in transporting dissolved sugars

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When explaining the need for transport systems, always reference the 'diffusion distance' and 'SA:V ratio' rather than just size
    • 💡In osmosis questions, use the phrase 'partially permeable membrane'—'semi-permeable' is accepted but 'selective' is less precise for OCR
    • 💡For 6-mark questions on the heart, trace the path of blood: Vena Cava → Right Atrium → Right Ventricle → Pulmonary Artery → Lungs → Pulmonary Vein → Left Atrium → Left Ventricle → Aorta

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Describing osmosis as the movement of 'solution' or 'particles' rather than specifically 'water molecules'
    • Stating that capillaries have 'thin cell walls' (plant feature) instead of 'walls one cell thick' (animal vessel structure)
    • Confusing the direction of movement in active transport, often incorrectly stating it moves down a concentration gradient
    • Failing to simplify Surface Area to Volume ratios to the form n:1, losing calculation marks

    Key Terminology

    Essential terms to know

    Surface Area to Volume Ratio (SA:V)
    Limitations of Diffusion in Multicellular Organisms
    Adaptations of Exchange Surfaces (Alveoli, Villi, Gills)
    Mass Transport Systems

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Explain
    Describe
    Compare
    Suggest

    Practical Links

    Related required practicals

    • •{"code":"PAG B2","title":"Investigation of osmosis in plant tissues","relevance":"Determining solute concentration of cells via mass change"}

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