Key Questions 4

• Things that need to be exchanged between organisms and their environment

  • Respiratory gases
  • Nutrients
  • Excretory products
  • Heat
• Surface area to volume ratio

  Calculated for a 2 cm cube with 10 mm sides = 0.6
• Factors affecting rate of diffusion into cells

  • Surface area
  • Thickness of exchange surface
  • Permeability of cell-surface membrane to substance
  • Concentration gradient of substance between inside and outside of cell
• Erythrocyte


  Red blood cell
• RBC adaptations


  • Biconcave shape for greater surface area for oxygen diffusion
  • No organelles so maximum volume to carry haemoglobin
• Specialised exchange surfaces


  • Large surface area to volume ratio
  • Very thin for shorter diffusion distance
  • Movement of environmental medium to maintain concentration gradient
  • Effective internal transport system to ensure diffusion gradient is maintained
• Diffusion equation


  (Surface area x Difference in concentration) / Length of diffusion path
• Mass transport of oxygen in insects


  Contraction of muscles through abdominal pumping enabling mass movements of air in and out
• Oxygen reaching working muscles in insects

  Through the tracheoles / tracheal tubes
• Anaerobic respiration in insects

  Creates lactate which reduces water potential of respiring cells, causing them to take up water by osmosis, pulling gas into the tracheoles
• Advantage of water at end of tracheoles

  Loss of water from the tracheoles will pull gas into them
• Purpose of insect spiracles opening and closing

  To balance the need for gaseous exchange with the need to conserve water
• Insect adaptations to reduce water loss

  • Small surface area to volume ratio
  • Waterproof coverings
  • Spiracles that can be opened and closed
• Tracheal system limits the size of insects because it relies on diffusion to bring oxygen to tissues, and if too large it would take too long for oxygen to reach respiring tissue rapidly enough
• Gills


  Made up of gill filaments
• Increasing gill surface area

  • Many gill filaments with many gill lamellae
• Countercurrent flow in gills

  Ensures there is always a diffusion gradient between water and the full length of the capillaries, allowing maximum uptake of oxygen
• Features of leaves that increase gaseous exchange

  • Spongy mesophyll contains air spaces for short diffusion distance
  • Many stomata to allow diffusion throughout leaf, reducing diffusion distance
• Stomata


  Mainly found on the underside of leaves to limit evaporation and transpiration
• Cells controlling stomata


  Guard cells
• Disadvantage of open stomata

  Excessive water loss
• Adaptations of xerophytes to limit water loss

  • Thick waxy cuticle
  • Rolling of leaves
  • Hairy leaves
  • Stomata sunken in pits or grooves
  • Reduced surface area to volume ratio of leaves
• How xerophyte adaptations reduce water loss

  Reduce water potential gradient and therefore slower diffusion of water vapour from air spaces, hence reduced evaporation
• How thick cuticle reduces transpiration

  Increases diffusion distance
• Humans need a high rate of gaseous exchange because they have a large volume of cells and have to maintain a high body temperature
• Structures of human gas exchange system

  • Trachea
  • Lungs
  • Bronchi
  • Bronchioles
  • Alveoli
• Purpose of cartilage rings in trachea

  To prevent it collapsing when air pressure inside is low
• Role of goblet cells in trachea

  Produce mucus
• Why alveoli can stretch and recoil

  They have elastic fibres due to the protein elastin
• Muscles relaxing during inspiration


  Internal intercostal muscles
• Tidal volume


  The volume of air that enters and leaves the lungs during one normal breath
• Muscles relaxing during expiration

  External intercostal muscles and the diaphragm
• Calculating pulmonary ventilation


  Tidal volume x Breathing rate
• Thoracic volume during expiration

  Decreases
• Thoracic volume during inspiration

  Increases
• Pressure in thoracic cavity during inspiration

  Below atmospheric pressure so air moves in down a pressure gradient
• Changes to red blood cells in capillaries during gas exchange

  They flatten themselves against the side of the capillary that is undergoing gas exchange, resulting in a shorter diffusion distance
• Digestion


  The breakdown of large, insoluble molecules into smaller, soluble molecules
• Advantage of physical breakdown before chemical breakdown

  Increases surface area for chemical digestion
• Purpose of villi


  Increase surface area