Adaptations of organisms

• Cell membranes


  • Gases and non-polar substances move across by simple diffusion
  • Polar and charged substances move across through carrier or channel proteins
  • Water moves by osmosis
  • Substances move from low to high concentration by active transport using ATP
• Exchange of materials needs to take place over a large enough surface area to supply every cell in the volume with the materials to fulfill its metabolic needs
• As organism size increases, their surface area to volume ratio gets smaller
• Factors affecting diffusion rate

  • Larger surface area
  • Shorter diffusion path
  • Higher concentration gradient
• Ventilation systems bring oxygen supplies and remove CO2
• Circulation systems carry oxygen away from cells and bring fresh oxygen to them
• Use of oxygen in respiration and production of CO2 in cells maintain concentration gradients
• Movement of aquatic organisms and turbulence in water increases oxygenation and concentration gradients
• Unicellular organisms


  • Have a high surface area to volume ratio
  • Have short diffusion paths
  • Can get sufficient oxygen and glucose by diffusion alone
  • Don't require specialised gas exchange surfaces
• Flatworms


  • Multicellular with smaller surface area to volume ratio than unicellular
  • Flattened body shortens diffusion path
  • High surface area to volume ratio
  • No specialised gas exchange surface, rely on diffusion
• Earthworms


  • Cylindrical shape gives high surface area to volume ratio
  • Diffusion distance too large for diffusion alone to supply oxygen
  • Have a circulatory system to maintain diffusion gradients
  • Blood vessels close to external surface for short diffusion path
  • Blood contains haemoglobin to transport oxygen
• Amphibians


  • Aquatic as tadpoles using gills
  • Adult gas exchange surface is thin moist skin
  • Have a closed circulatory system with haemoglobin-containing blood
  • Use primitive lungs during activity to supplement skin diffusion
• Multicellular animals


  • Surface area to volume ratio decreases, so diffusion across body surface is insufficient
  • Adaptations evolve to increase surface area and maintain concentration gradients, e.g. specialised gas exchange surfaces and ventilation mechanisms
  • Internal gas exchange surfaces (lungs) minimise water loss in terrestrial animals