Diffusion and surface area to volume ratio Learning
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Diffusion and surface area to volume ratio Learning objectives: • Recall a definition for diffusion • Understand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of the cell • Describe an experiment to investigate diffusion using non-living systems. • Explain how surface area to volume ratio affects the rate of diffusion
Why can’t we live like an amoeba? For a unicellular organism such as an Amoeba, nutrients and oxygen can diffuse directly into the cell from its external environment, and waste substances can diffuse directly out. Task 1: Why can’t larger organisms do the same? What do they do instead?
Why can’t we live like an amoeba? Unlike unicellular organisms, larger organisms have a small surface area to volume ratio, and therefore diffusion does not occur quickly enough to sustain them. Larger organisms therefore require organ systems with specialised adaptations to supply all of their cells with nutrients and oxygen (through diffusion) and collect waste (through diffusion) to be excreted from the body.
Surface area to volume ratio Task 2: Calculate the surface area to volume ratio of two different cubes (created using multilink cubes), and complete the conclusion. Number of multilink cubes 1 8 x Surface area (cm 2) 6 x 2 Volume (cm 3) x 3 SA: V (surface area ÷ volume) Conclusion: • A large cube (representing a large organism) has a _____ surface area to volume ratio. • A small cube (representing a small organism) has a _____ surface area to volume ratio.
Surface area to volume ratio Task 3: Follow instructions on ‘Effect of size on uptake by diffusion’ to complete practical EXTENSION: Answer the questions on the back of ‘Effect of size on uptake by diffusion’
Thumbs up or down Learning objectives: • Recall a definition for diffusion • Understand why simple, unicellular organisms can rely on diffusion for movement of substances in and out of the cell • Describe an experiment to investigate diffusion using non-living systems. • Explain how surface area to volume ratio affects the rate of diffusion