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GCSE Biology 02 — Transport Systems

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Topics include Principles of Diffusion & Exchange Surfaces, Osmosis & Active Transport, The Human Heart & Double Circulation, Blood Vessels & Blood Composition, Plant Transport: Xylem & Transpiration, Regulation of Transpiration & Stomata, and Translocation in Phloem.

Biology EN
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Principles of Diffusion & Exchange Surfaces

How substances move down concentration gradients and how large organisms are adapted to maximize this exchange.

Key points

  • Diffusion is the net movement of particles from an area of higher concentration to lower concentration.
  • As organisms increase in size, their surface area to volume ratio (SA:V) decreases, making diffusion alone insufficient for transport.
  • Specialised exchange surfaces (e.g., alveoli, villi) have adaptations to maximise the rate of diffusion according to Fick's Law principles.
  • Key adaptations include a large surface area, thin membranes (short diffusion distance), and steep concentration gradients (maintained by blood supply or ventilation).
  • The rate of diffusion increases with higher temperature, steeper concentration gradients, and larger surface area.

Worked example

Question

Explain how the structure of the alveoli maximises the rate of gas exchange.

Solution

1. Large Surface Area: Millions of alveoli provide a huge area for diffusion to occur.
2. Short Diffusion Distance: The alveolar walls and capillary walls are only one cell thick, allowing gases to cross quickly.
3. Steep Concentration Gradient: Continuous blood flow maintains low O₂ in the blood, and ventilation maintains high O₂ in the alveoli, ensuring rapid diffusion.

Common pitfalls

  • Confusing surface area with SA:V ratio (large organisms have a large surface area but a small ratio).
  • Thinking individual particles stop moving when equilibrium is reached (they still move, but net movement is zero).

Prerequisites

  • Particle theory (random movement)
  • States of matter (liquids and gases)
  • Cell membrane structure
Further resources