National 5BiologySQARevision NotesUnit 1: Cell BiologyTransport Across MembranesDiffusion

Diffusion (SQA National 5 Biology): Revision Note

Exam code: X807 75

Cara Head

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Cara Head

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Passive transport

  • All cells are surrounded by a cell membrane, which separates the inside of a cell from its outside environment

  • Cells must move substances across their membranes in order to:

    • take in nutrients and gases

    • remove waste

  • Membrane transport can be either passive or active

  • Passive transport can be defined as follows:

membrane transport that occurs down a concentration gradient and does not require energy

  • Examples of passive transport include:

    • diffusion

    • osmosis

The process of diffusion

  • Diffusion can be defined as:

the movement of molecules down a concentration gradient from a higher to a lower concentration

  • This movement continues until the concentration of substances is equal

  • Note that diffusion occurs as a result of the random movement of molecules

  • Substances can diffuse through the cell membrane to enter or exit a cell

    • E.g. oxygen and carbon dioxide can diffuse across cell membranes during gas exchange

Diagram illustrating diffusion across a cell membrane, showing molecules moving from higher to lower concentration through a lipid bilayer over time.
Diffusion is the movement of molecules down a concentration gradient from a higher to a lower concentration

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    Cara Head

    Author: Cara Head

    Expertise: Biology & Psychology Content Creator

    Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding