Gas exchange

• Features of gas exchange surfaces

  • Large surface area
  • Thin to provide short diffusion pathway
  • Steep diffusion gradient for faster diffusion of gasses
• How does gas exchange occur in single celled organisms?

  1. They absorb and release gases by diffusion through their outer membrane

  2. Large surface area, thin surface and a short diffusion pathway

  3. Oxygen can take part in biochemical reactions as soon as it diffuses into the cell
• Insect gas exchange system

  Tracheal system
• Insect anatomy


  • Have an exoskeleton
  • Made from hard fibrous material for protection
  • Also made up of a lipid layer to precent water loss
• Tracheal system made up


  • Trachea
  • Tracheoles
  • Spiracles
• Structure and function of spiracles


  • Valve like structures
  • Run along the length of the abdomen of a insect
  • Oxygen and carbon dioxide enter and leave through these openings
• Structures and function of the trachea


  • Network of internal tubes
  • Tubes contain rings of cartilage (tough, connective tissue)
  • Rings allow them to be kept open
  • They attach to the spiracles and branch off into smaller tubes
• Structure and function of the tracheoles

  • Smaller tubes branched off from tracheoles
  • Extent throughout all the tissue of an insect
  • Deliver oxygen straight to respiring cells
• Adaptations for gas exchange: large surface area

  • Large number of tracheoles
• Adaptations for gas exchange: concentration gradient

  • Use up oxygen during respirations
  • Producing carbon dioxide
  • Steep concentration gradient from the tracheoles tot the atmosphere
• Adaptations for gas exchange: short diffusion pathway

  • Walls of tracheoles are thin and permeable
  • Short distance between trachea and spiracles
• Rhythmic abdominal movements


  Pushes gasses in and out of spiracles
• Movement of gasses when an insect is in flight

  1. During flight, muscle begin to respire anaerobically

  2. Results in the production of lactate

  3. Lowers the water potential of cells and more water moves in to the tracheoles

  4. This draws in more air from the atmospheres
• Reason why fish need specialised exchange surfaces

  • They are waterproof
  • Have small surface area to volume ratio
  • There is a lower concentration of oxygen in water compared to air
  • Thirty times less oxygen
• Fish gill anatomy


  • Four layers of gills on both sides of head
  • Gills made up of stacks of gill filaments
  • Each gill filament is covered in gill lamellae
• Adaptations for gas exchange: Large surface area to volume to ratio

  • Many gill filaments covered in many gill lamellae
  • Faster/ rapid diffusion
• Adaptations for gas exchange: short diffusion distance

  • Gill lamellae very thin
  • Each surrounded by capillary network
• Adaptations for gas exchange: concentration gradient

  • Counter-current flow principle
• Counter current flow principle

  • Water flows in the opposite direction tot he flow of blood in the capillaries
  • Ensures equilum is not reach ensures that a diffusion graitnet is maintained across the entrie length of the gill lamellae
• Key structures involved in gas exchange within plants

  • Palisade and spongy mesophyll
  • Stomata created by guard cells
• Palisade mesophyll


  • Site of photosynthesis
  • Where oxygen is produced and carbon dioxide of used up
• Concentration gradient within mesophyll layers

  1. Co2 used in photosynthesis

  2. Moves down its concentration gradient into the spongy mesophyll through the stomata

  3. O2 produced in respiration

  4. Moves down its concentration to the atmosphere through the stomata
• Adaptation of plants to reduce water loss

  • The stomata will close at night when photosynthesis is not occurring
• Why do they require adaptations to limit water loss?

  • Water evaporates off the surface of insects
  • The adaptions for gas exchange proc=vide ideal conditions for evaporation
• Insect adaptations to prevent water loss


  • Waterproof lipid exoskeleton
  • Have a small surface area to volume ration to minimise water lost by evaporation
  • When too much water is beginning to be lost they can close their spiracles to reduce water loss
• Xerophytic plants or xerophytic plants


  • Plants adapted to survive in environments with little water
  • In warm, dry and windy conditions
• Adaptations of xerophytic plants

  • Thicker cuticle to reduce evaporation
  • Longer root network to reach more water
  • Sunken stomata to trap moisture and increase local humidity, reducing water potential gradient
  • Hairs to trap moisture and increase local humidity
  • Curled leaves to trap water and increase local humidity
• Human gas exchange system made up of

  • Trachea
  • Lungs
  • Bronchi
  • Bronchioles
  • Alveoli
• Ventilation


  • The process of inhaling and exhaling in humans
  • Results in the exchange of gasses
• Intercostal muscles


  • Work as antagonistic pairs
  • When one contracts the other relaxes
• Alveoli


  • Tiny air sacs
  • Site of gas exchange
  • 300 million in each lu g
• Gas exchange occurs between the

  The alveolar epithelium and the blood (capillary endothelium)
• Large surface area so

  • Faster/ rapid diffusion
• Thin walls/ one cell thick so

  • Minimal diffusion distance/ short diffusion pathway
  • Faster/ rapid diffusion
• Surrounded by network of capillaries so


  • Provides steep and maintained concentration gradient
  • Faster/ rapid diffusion