Surface area:volume and gas exchange in plants and animals

• As surface area and volume increases the SA:V decreases
• Multicellular organisms have too bigger volume for their surface area.
• ROD = SAD / TOES


  Rate of diffusion = surface area x concentration difference / thickness of exchange surface
• Single celled organisms have a high surface area:volume so oxygen can diffuse across cell membrane. Sufficient for needs.

  
• Stomata allow for gas exchange in and out of a leaf.

  
• Stomata close to prevent water loss.
• Xerophytes - plants adapted to living in dry environments, or where their water loss due to transpiration exceed their water uptake.
• Adaptations of xerophytes

  1. Waxy cuticle - reduce transpiration
  2. Rolling of leaves - Traps moist air in leaf reducing water potential gradient, therefore transpiration
  3. Hairy leaves - traps moist air next to leaf, reducing water potential gradient, therefore transpiration.
  4. Stomata in pits and grooves - traps moist air next to leaf, reducing water potential gradient, therefore transpiration.
  5. Reduced SA:V - thin needles reduce SA and therefore water loss, increases photosynthesis
  6. Number and distribution of stomata - less stomata = less water lost, stomata at bottom of leaf = shaded from sun
• Gas exchange in insects:

  • Takes place in tracheae
  • Openings of tracheae are called spiracles
  • Tracheae divide into spiracles

  
• How is a diffusion gradient maintained?

  Oxygen is used up in respiration and carbon dioxide is produced creating a diffusion gradient in both directions.

  
• How are tracheae supported and kept open?

  Chitin rings
• How are spiracles opened and closed?

  Muscles
• What structures are present to help reduce evaporation at the spiracles?

  Hairs - trap water which increases the water potential gradient and therefore reduces water loss.
• Fish have a specialised internal gas exchange surface called gills.
• Concentration of dissolved oxygen in water decreases as temperature rises.
• Fish move water continually over their gas exchange surface, in one direction, as the oxygen consumption when the fish is at rest is really low in water (compared to air) and water density is really high.
• Structure of the gills

  A) Gill fillaments

  B) Gill lamella or gill plates

  C) Gill bar

  D) Gill rakers

  E) Gill filaments

  F) Gill bar

  
• Gas exchange in the gills takes place at the gill plates
• Thin gill plates means there is a short diffusion pathway
• Counter current flow - blood and water move in opposite directions to maintain the concentration gradient across the whole gill lamellae. Allows for extremely efficient exchange of oxygen and carbon dioxide between water and blood.

  
• Parallel flow mechanism

  • Sharks don't use counter current mechanism
  • Drawback - equilibrium is reached half way along gill
  • Blood only gets 50% oxygenated

  
• Ventilation in fish

  Inspiration

  • Mouth opens and operculum moves out
  • Floor of mouth lowers, increases volume and decreases pressure
  • Outside water pressure closes opercular valve
  • Water flows in over gills from high to low pressure

  Expiration

  • Mouth closes and operculum moves in
  • Floor of mouth rises, decreases volume and increases pressure
  • Inside pressure opens opercular valve
  • Water flows out of opercular valve from high to low pressure.

  
• Abdominal pumping increases the efficiency of gas exchange, why?

  Abdominal pumping maintains a concentration gradient so air enters quicker through the spiracles.
• Describe and explain how fish maintain water flow over their gills

  Ventilation in fish

  1. Mouth opens
  2. Floor of mouth lowers decreasing pressure and water enters increasing volume
  3. Mouth closes
  4. Floor of mouth raises increasing pressure and opercular valves open
  5. Increased pressure pushes water over gills
• Describe and explain how the structure of the mammalian breathing system enables efficient uptake of oxygen into the blood.

  1. alveoli provide a large surface area
  2. walls of alveoli thin to provide a short diffusion pathway
  3. walls of capillary thin / close to alveoli provide
  4. walls (of capillaries / alveoli) have flattened cells
  5. cell membrane permeable to gases
  6. many blood capillaries provide a large surface area
  7. Intercostal muscles move to ventilate lungs and maintain a concentration gradient
  8. Branching bronchioles for efficient flow of air
  9. Cartilage rings keep airways open