The need for exchange surfaces

• Exchange Surfaces


  Specialised surfaces that facilitate the exchange of substances between an organism and its environment
• Single-celled organisms


  • Have a high surface area to volume (SAV) ratio
  • Allows for exchange of substances via simple diffusion
  • Large surface area allows for maximum absorption of nutrients and gases, and secretion of waste products
  • Small volume means short diffusion distance to all organelles
• Multicellular organisms


  • As they increase in size, their SA:V ratio decreases
  • Less surface area for absorption of nutrients/gases and secretion of waste
  • Greater volume results in longer diffusion distances to cells and tissues
• Specialised systems in multicellular organisms

  • Gas exchange system
  • Circulatory system
  • Lymphatic system
  • Urinary system
  • Xylem and phloem
• Need for specialised gas exchange system

  • Supply oxygen for aerobic respiration to produce ATP
  • Remove carbon dioxide, a toxic waste product of aerobic respiration
• Diffusion for single-celled vs multicellular organisms

  • Viable transport mechanism for single-celled organisms
  • Not viable for larger multicellular organisms due to long diffusion times
• Metabolic rate


  • Amount of energy expended by an organism within a given period of time
  • Basal metabolic rate (BMR) is the metabolic rate at rest
• Organism body mass


  Higher metabolic rate
• Smaller animals


  • Higher BMR per unit of body mass
  • Greater SAV ratio so they lose more heat, requiring more energy to maintain body temperature