Gas exchange

• explain how the body of a single celled organism is adapted for gas exchange.

  Thin flat shape and larger surface area to volume ratio
• Describe the tracheal system of an insect

  spiracles- pores on the surface that can open and close to allow for diffusion

  trachea-large tubes full of air that allow for diffusion

  tracheoles- smaller branches off trachea and is permeable to allow gas exchange with cells
• Explain how an insect's tracheal system is adapted for gas exchange


  1. Tracheoles have thin walls=short diffusion distance to cells

  2. High number of highly branched tracheoles=so short diffusion distance to cells and large surface area

  3. Trachea provides tubes full of air= fast diffusion

  4. Contraction of abdominal muscles changes pressure in body causing air to move in and out and it maintains a concentration gradient diffusion

  5. Fluid in end of tracheoles is drawn to tissue by osmosis during exercise=diffusion is faster through air rather than fluid to gas exchange surfaces.
• explain structural and functional compromises in insects that allow efficient gas exchange whilst reducing water loss


  1. Thick waxy cuticle- increases the diffusion distance so less water loss

  2. Spiracles can open to allow for gas exchange and close to limit water loss

  3. Hairs around spiracles-trap moist air reducing the water potential so water loss
• explain how the gills of fish are adapted for gas exchange.

  1. gills are made of many filaments covered with many lamellae= this increases the surface area for diffusion

  2. thin lamellae wall- so shirt diffusion distance between water/blood

  3. lamellae have large number of cappilaries so it maintains a concentration gradient
• What is the counter current flow?

  1. blood and water flow in opposite directions over lamellae

  2. so oxygen concentration is always higher in water than blood

  3. so this maintains a concentration gradient of oxygen between blood and water

  4. for diffusion across the whole lamellae
• explain how the leaves of dicotyledonous plants are adapted for gas exchange

  1. Many stomata-large surface area for gas exchange

  2. Spongy mesophyll contains air spaces- large surface area for gasses to diffuse through

  3. thin-short diffusion distance
• explain structural and functional compromises in xerophytic plants that allow efficient gas exchange

  1. Thicker waxy cuticle- increases diffusion distance so less evaporation

  2.Sunken stomata in pits/rolled leaves/hair-traps water vapour so reduced water potential between leaf and air so theres less evaporation

  3. Spines- Reduces surface area to volume ratio
• Explain the essential features of the alveolar epithelium that makes it adapted as a surface for gas exchange


  1. Flattened cells-short diffusion distance

  2. Folded-Larger surface area

  3.Permeable-allow diffusion of O2/CO2

  4.Moist-gases can dissolve for diffusion

  5. Good blood supply from large network of capillaries-maintains concentration gradient
• Describe how GE occurs in the lungs

  Oxygen diffuses from alveolar air space into blood down its concentration gradient across alveolar epithelium then across capillary endothelium
• Explain the importance of ventilation

  Brings in air containing higher concentration of oxygen and removes air with lower concentration of oxygen, maintaining concentration gradients
• What is inspiration?


  1.Diaphragm muscles contract- flattens

  2.External intercostal muscles contract and internal intercostal muscles relax-the ribcage is pulled up/out

  3.increasing volume and decreasing pressure in thoraic cavity

  4.Air moves into lungs down the pressure gradient
• What is expiration? 


  1.Diaphragm relaxes-moves upwards

  2.External intercostal muscles relax,internal intercostal muscles may contract and ribcage is moves down/in.

  3.decreasing volume and increasing pressure in thoraic cavity

  4.Air movees out of lungs down pressure gradient
• Suggest why expiration is passive at rest


  Internal intercostal muscles so not normally need to contract and expiration is aided by elastic recoil in alveoli