specialised exchange surfaces

• organisms need to exchange surfaces within their environment

  • take in - oxygen and glucose for aerobic respiration and metabolic reactions
  • excrete - carbon dioxide and urea
• • in single celled organisms substances can diffuse directly into the cell across the cell surface membrane, diffusion rate is quick due too the small distances
  • multicellular - diffusion across outer membrane is too slow:
  • - some cells are deep in the body
  • - larger animals have a low surface area to volume ratio, difficult to exchange enough
  • - they have a higher metabolic rate so use oxygen and glucose faster
• rather using straightforward diffusion to absorb and excrete substances multicellular animals need specialised exchange surfaces e.g. alveoli
• most exchange surfaces have a large surface area

  • root hair cells - cells grow into hairs, gives root large surface area and increases rate of absorption of water and mineral ions from the soil
• exchange surfaces are thin

  • alveoli, each alveolus is made from a single layer of thin, flat cells ( alveolar epithelium )
  • O2 diffuses out CO2 diffuses the opposite direction
  • thin alveolar epithelium helps to decrease the distance which O2 an CO2 diffusion takes place so increases the rate
• good blood supply/ ventilation

  • alveoli - surrounded by large capillary network so each has its own blood supply so blood constantly takes O2 away and brings more CO2

  lungs - ventilated so air is constantly replaces

  these features help maintain a concentration gradient

  • fish gills - large network of capillaries so are well supplied with blood and are also well ventilated - fresh water constantly passes over them, helps maintain a concentration gradient increasing rate of CO2 diffusion into blood