Organisms exchange substances with the environment

• Smaller animals 


  Higher surface area to volume ratio
• larger animals


  low surface area to volume ratio
• factors affecting heat exchange

  • Size- smaller organisms need a high metabolic rate to generate enough heat to stay warm
  • shape- animals with a compact shape=smaller surface area to volume= minimise heat loss
• Gas exchange adaptations

  1. Larger surface area
  2. thin= short diffusion pathway
• single-celled organisms 


  absorb and release gasses by diffusion through their outer surface
• fish: counter-current system

  1. Water containing oxygen, enters fish through its mouth and passes out through the gills
  2. Each gill made of thin plates= gill filaments
  3. Gills covered in lamallae (tiny structures)
  4. lamallae have lots of capillaries
  5. Blood flows in one way, water in opposite= maintains conc gradient
• insects: trachea

  • Spiracles- air moves into tracheae through pores
  • O2 travels down gradient towards cells
  • tracheoles= further extension of tracheae—> thin, permeable walls
  • CO2 moves down conc gradient to be released
• plants: mesophyll cells

  • Large surface area
  • stomata= gases move in and out through pores
• xerophytic adaptations

  • Stomata- trap moist air= reduce conc gradient of water between leaf and air
  • hairs on epidermis=- trap moist air
  • Curled leaves- protect from wind
  • less stomata- less water can escape