Exchange between organisms & their environment

• Cells need oxygen, glucose, fatty acids, amino acids, vitamins and minerals in order to function effectively
• Cells excrete carbon dioxide, urea and water
• Small organisms have a large SA:V ratio so efficient exchange of substances can occur across the body surface
• Large organisms have a smaller SA:V ration so it is harder to exchange substances between the environment and all cells
• Larger organisms have adapted to make exchange more efficient:

  • flattened shape (brings cells closer to the surface)
  • Specialised exchange systems (lungs and gills)
  • Mass transport systems (carry substances to and from individual cells)
• Single-celled organisms can diffuse gases, nutrients and waste directly in and out of the cell
• Single-celled organisms have a quick diffusion rate as substances only have to travel a short distance
• Multi-cellular organisms need specialised exchange surfaces because:

  • the distance of the cells from the surface is too large
  • low SA:V ratio
  • high metabolic activity required a constant, rapid supply of glucose and oxygen
• Multicellular organisms need to transport hormones, enzymes, food and waste products around the body
• Mammals are warm blooded (endothermic) which requires energy
• Features of specialised exchange surfaces:

  • Large SA:V ratio
  • Short diffusion distance
  • Partially permeable
  • Movement of external materials maintains diffusion gradient
  • Movement of internal materials maintains diffusion gradient
• Fick's law states that the rate of diffusion is directly proportional to surface area multiplied by difference in concentration divided by the length of diffusion path