Specialised Exchange Surfaces

• What do multicellular organisms require for efficient exchange of materials?

  Specialised surfaces
• Why can't substances diffuse directly across the cell membrane in multicellular organisms?

  Cells are not in direct contact with the environment
• What is a reason for the large diffusion distances in multicellular organisms?

  Cells are far from their environment
• Why do larger organisms have higher metabolic rates?

  They require more oxygen and glucose
• What have multicellular organisms evolved to solve diffusion problems?

  Specialised exchange surfaces
• What are the key features of specialised exchange surfaces?

  • Large surface area for substance exchange
  • Thin walls to minimise diffusion distance
  • Extensive blood supply and/or ventilation for steep concentration gradients
  • Surrounded by selectively permeable plasma membranes to control exchanges
• What is the purpose of a large surface area in exchange surfaces?

  To provide a larger area for exchange
• How do thin walls benefit exchange surfaces?

  They minimise the diffusion distance
• Why is an extensive blood supply important for exchange surfaces?

  It maintains steep concentration gradients
• What role do selectively permeable plasma membranes play in exchange surfaces?

  They control what substances are exchanged