3.2 gas exchange

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

  thin, flat shape and a large surface area to volume ratio

  short diffusion distance to all parts of cell so rapid diffusion

  e.g of oxygen/carbon dioxide
• describe the tracheal system of an insect

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

  tracheae - large tubes full of air that allow diffusion

  tracheoles - smaller branches from tracheae and are permeable to allow gas exchange with cells
• explain how an insects tracheal system is adapted for gas exchange (1)

  tracheoles have thin walls so short diffusion distance to cells

  high numbers of highly branched tracheoles so short diffusion to cells so large surface area

  tracheae provides tubes full of air so fast diffusion
• explain how an insects tracheal system is adapted for gas exchange (2)

  contraction of abdominal muscles (abdominal pumping) changes pressure in body causing air to move in/out so maintains concentration gradient for diffusion

  fluid in end of tracheoles drawn in to tissues by osmosis during exercise (lactate produced in anaerobic respiration lowers water potential of cells) so diffusion is faster through air (rather than fluid) to gas exchange surface
• explain structural and functional compromises in terrestrial insects that allow efficient gas exchange while limiting water lost

  thick waxy cuticle/exoskeleton so increases diffusion distance so less water lost (evaporation)

  spiracles can open to allow gas exchange and close to reduce water loss (evaporation)

  hairs around spiracles so trap moist air reducing water potential gradient so less water lost (evaporation)
• explain how the gills of fish are adapted for gas exchange

  gills made of many filaments covered with many lamellae so increase surface area for diffusion

  thin lamellae wall/epithelium so short diffusion distance between water/blood

  lamellae have a large number of capillaries so remove oxygen and bring carbon dioxide quickly so maintains concentration gradient
• explain how the gills of fish are adapted for gas exchange (counter current flow)


  blood and water flow in opposite directions through/over lamellae

  so oxygen concentration always higher in water (than blood near)

  so maintains a concentration gradient of oxygen between water and blood

  for diffusion along whole length of lamellae

  (if it was a parallel flow equilibrium would be reached so oxygen wouldn't diffuse into blood along the whole gill plate)
• explain how the leaves of dicotyledonous plants are adapted for gas exchange

  many stomata (high density) so large surface area for gas exchange (when opened by guard cells)

  spongy mesophyll contains air spaces so larger surface area for gases to diffuse through

  thin so short diffusion distance
• explain structural and functional compromises in xerophytic plants that allow efficient gas exchange while limiting water loss

  thicker waxy cuticle so increases diffusion distance so less evaporation

  sunken stomata in pits/rolled leaves/hairs so 'trap' water vapour/protect stomata from wind so reduced water potential gradient between leaf/air so less evaporation

  spines/needles so reduces surface area to volume ratio
• xerophyte

  plant adapted to live in very dry conditions

  e.g cacti and marram grass
• structure of the human gas exchange system

  lungs

  trachea

  bronchi

  bronchioles

  alveoli (air sacs)

  capillary network (surrounding alveoli)
• explain the essential features of the alveolar epithelium that make it adapted as a surface for gas exchange

  flattened cells/one cell thick so short diffusion distance

  folded so larger surface area

  permeable so allows diffusion of oxygen/carbon dioxide

  moist so gases can dissolve for diffusion

  good blood supply from large network of capillaries so maintains concentration gradient
• describe how gas exchange occurs in the lungs (oxygen)

  oxygen diffuses from alveolar space into blood down its concentration gradient

  across alveolar epithelium then across capillary endothelium
• describe how gas exchange occurs in the lungs (carbon dioxide)

  carbon dioxide diffuses from blood into alveolar air space down its concentration gradient

  across capillary endothelium then across alveolar epithelium
• explain the importance of ventilation

  brings air containing higher concentration of oxygen and removes air with lower concentration of oxygen

  maintaining concentration gradients
• explain how humans breath in (inspiration)

  diaphragm muscles contract so flattens

  external intercostal muscles contract and internal intercostal muscles relax (antagonistic) so ribcage pulled up/out

  increasing volume and decreasing pressure (below atmospheric) in thoracic cavity

  air moves into lungs down a pressure gradient
• explain how humans breath out (expiration)

  diaphragm relaxes so moves upwards

  external intercostal muscles relax and internal intercostal muscles may contract so ribcage moves down/in

  decreasing volume and increasing pressure (above atmospheric) in thoracic cavity

  air moves out of lungs down pressure gradient
• suggest why expiration is normally passive at rest

  internal intercostal muscles do not normally need to contract

  expiration aided by elastic recoil in alveoli
• suggest how different lung diseases reduce the rate of gas exchange

  thickened alveolar tissue (e.g fibrosis) so increases diffusion distance

  alveolar wall breakdown so reduces surface area

  reduce lung elasticity so lungs expand/recoil less so reduces concentration gradients of oxygen and carbon dioxide
• suggest how different lung diseases affect ventilation

  reduce lung elasticity (e.g fibrosis - build up of scar tissue) so lungs expand/recoil less so reducing volume of air in each breath (tidal volume) so reducing maximum volume of air breathed out in one breath (forced vital capacity)

  narrow airways/reduce airflow in/out of lungs (e.g asthma - inflamed bronchi) so reducing maximum volume of air breathed out in 1 second (forced expiratory volume)

  reducing rate of gas exchange so increased ventilation rate to compensate for reduced oxygen in blood
• suggest why people with lung disease experience fatigue

  cells receive less oxygen so rate of aerobic respiration reduced so less ATP made
• correlation coefficient

  examining an association between 2 sets of data
• t test

  comparing means of 2 sets of data
• chi-squared

  for categorial data
• explain the difference between correlations and casual relationships

  correlation - change in one variable reflected by a change in another, identified on a scatter graph

  causation - change in one variable causes a change in another variable

  correlation does not mean causation - may be other factors involved