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1.2Unit 1 — Cell Biology

Transport Across Cell Membranes — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 3 parts Lesson 2 of 6

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Section 1 of 3Structure of the cell membrane

  • Fluid Mosaic Model:
    • Composed of a phospholipid bilayer with protein molecules embedded throughout.
    • Phospholipids are fluid and move laterally, giving flexibility to the membrane.
    • The cell membrane is selectively permeable: small soluble molecules (oxygen, carbon dioxide, water, glucose) pass through easily; large insoluble molecules (starch, proteins) cannot pass without transport mechanisms.

Section 2 of 3Passive transport: Diffusion and Osmosis

  • Passive transport: Movement of substances down a concentration gradient (from higher to lower concentration) without requiring cellular energy (ATP).
    • Diffusion: Movement of solute molecules (e.g. O2 entering cells, CO2 leaving cells) down a concentration gradient until equilibrium is reached.
    • Osmosis: Movement of water molecules from an area of higher water concentration to an area of lower water concentration across a selectively permeable membrane.
  • Effects of osmosis on cells:
    Solution TypeAnimal Cell (Red Blood Cell)Plant Cell
    Higher water concentration (pure water / hypotonic)Water enters cell by osmosis; cell swells and bursts (lysis) (no cell wall).Water enters cell; vacuole swells; cytoplasm pushes against cell wall; cell becomes turgid (cell wall prevents bursting).
    Equal water concentration (isotonic)No net water movement; cell shape and volume remain unchanged.No net water movement; cell volume remains unchanged.
    Lower water concentration (concentrated salt/sugar solution / hypertonic)Water leaves cell by osmosis; cell shrinks / shrivels.Water leaves vacuole; cytoplasm pulls away from cell wall; cell becomes plasmolysed.

Section 3 of 3Active transport

  • Active transport:
    • Movement of molecules or ions against the concentration gradient (from an area of lower concentration to higher concentration).
    • Requires membrane carrier proteins that actively pump molecules across.
    • Requires cellular energy in the form of ATP generated during cellular respiration.
    • Example: Sodium-potassium pump maintaining electrical nerve impulses; plant root hair cells accumulating mineral ions from dilute soil water.

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