Gaseous Exchange


Exchange Surfaces

  • respiratory surfaces for rapid efficient diffusion of gases in and out of cells
  • gills alveoli tracheae spongy mesophyll
  • to achieve the maximum rate of diffusion:
    • large surface area to volume ratio
    • thin for short dffusion paths
    • permeable for easy diffusion

unicellular organisms

  • amoeba- cell membrane is gas exchange surface
  • diffusion occurs through whole of body surface
  • large surface area to volume ratio
  • membrane is thin and moist
  • diffusion paths are short
  • theres a limit to cell size and point where length of diffusion path limits efficiency of diffusion
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Multicellular Animals

  • small surface area to volume ratio
  • more oxygen needed
  • body surface tough and impermeable 
  • need to maintain diffusion gradient
  • internalised respiratory systems and methods of ventilation


  • flat shape to increase surface area to volume ratio


  • tubular elongated shape increases surface area to volume ratio
  • lives in damp soil to keep skin moist and gas diffuses through skin directly into blood capillaries
  • oxygen transported in opposite direction to carbon dioxide
  • closed blood system and respiratory pigment
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  • the gill allows for a one way current of water via a specialised pumping mechanism
  • many folds on the gill form a large surface area for g.e.
  • cartilaginous fish have 5 gills and the blood travelling in the same direction as the water flow which is relatively inefficient
  • bony fish have an operculum covering the gills and have a counter current bloodflow 
  • there are four pairs of gills with thin gill filament plates providing a large surface area 
  • gills are a specialised area rather than a whole body surface extended by the gill filaments with an extensive network of blood capillaries for efficient diffusion
  • water is forced over the gills filaments by pressure differences to maintain a continous undirectional flow of water 
  • lower pressure in the opercular cavity than the bucco pharynx
  • operculum is a valve and pump
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Ventilation of Gills & Counter Current Flow

  •  mouth opens operculum closes 
  • mouth floor lowered increasing volume decreasing pressure
  • water flows in as external pressure is higher than internal pressure 
  • this is reversed to force water over gills

counter current flow 

  • water flows in opposite direction to blood
  • diffusion gradient is maintained across whole gill plate
  • blood meets water with a relatively higher oxygen content
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Terrestrial Organisms

  • water evaporates from the body surface meaning dehydration
  • g.e.s need to be thin permeable and with a large surface area which conflicts with the need to preserve water


  • frogs toads newts
  • gaseous exchange takes place through the skin and lungs- through the skin mainly when inactive with water or air.
  • skin is moist and permeable with a well developed capillary network
  • lungs are simple sacs with good airsupply, ventilated by movements of the mouth


  • crocodiles lizards snakes
  • ribs provide ventilation mechanism for lungs 
  • lungs are more complex internal structure with ingrowth of tissues for larger surface area
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Birds & Insects

  • large volumes of air are needed to provide energy for flight
  • ventilation is a system of air sacs ventilated by movement of ribs stimulated by the flight muscles


  • rigid exoskeleton covered by a cuticle 
  • small surface area to volume ratio cannt use their body as a exchange surface
  • paired holes called spiracles running along the side of the body
  • the spiracles lead to a system of branched chitin lined air tubes called tracheae
  • spiracles open and close like valves
  • this allows gaseous exchange and limits water loss
  • diffusion removes co2 and takes in o2 during resting, and in movement or flight movements of the abdomen ventilate the tracheae
  • ends of the tracheae has branches called tracheoles where oxygen passes directly into cells
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Humans System

  • lungs supply a large surface area increased by alveoli lined with moisture in which gases dissolve, thin walls to shorten the diffusion path and an extensive capillary network for rapid diffusion and transport to maintain diffusion gradients
  • lungs are enclosed in an airtight compartment with the diaphragm below and enclosed with ribcage
  • ribs move by intercostal muscles enabling ventilation
  • air is drawn in down the trachea branching off into bronchi and then bronchioles


  • air sacs provide large surface area
  • thin walls= short diffusion path
  • extensive capillary network
  • deoxygenated blood enters capillaries around alveoli
  • oxygen diffuses in 
  • co2 diffuses out
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Ventilation Of The Lungs

  • negative pressure breathing
  • forcing air down
  • pressure inside the lungs is lower than atmospheric pressure


  • active process as muscles require energy
  • external intercostal muscles contract, ribs pulled up and out
  • diaphragm muscles contract, moves down
  • increased volume of thorax, reduction of pressure
  • atmospheric air pressure is greater than that of the lungs, air moves in


  • passive process 
  • external intercostal muscles relax, ribs move down and in
  • diaphragm relaxes, moves up
  • decreased volume of thorax, increase in pressure air moves out
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Extra Info

  • surrounding each lung and lining the thorax are pleural membranes 
  • between which are cavities containing pleural fluid
  • fluid acts as lubricant for friction free movement
  • anti sticking chemical surfactant stops alveoli sticking togther by reducing surface tension
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  • leaf blade is thin and flat with large surface area
  • spongy mesophyll allows for diffusion of gases
  • tissues separated by air spaces
  • stomatal pres permit gas exchange
  • diffuse along a concentration gradient through stomata
  • gases diffuse through air spaces between spongy mesophyll and into the cells

adaptations for photosynthesis of leaf

  • large surface area to capture as much sunlight as possible
  • thin so light can penetrate lower cells
  • cuticle and epidermis are transparent for light to penetrate mesophyll
  • palisade cells are elongated and densly arranged and packed with chloroplasts
  • chloroplasts can rotate and move within the paisades to get nto the best position for absorption
  • air spaces allow co2 to diffuse in and o2 out
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  • small pores found on lower surface of leaf
  • each pore has 2 guard cells
  • guard cells have chloroplasts and uneven walls, inner= thick outer= thin
  • stomata control gas exchange between atmosphere and inner leaf

water loss

  • water evaporates through stomata in transpiration
  • sunlight increases evaporation on upper surface so stomata are hidden under
  • waxy cuticle reduces water loss on upper surface
  • stomata close at night when there isnt a valid reason to keep them open (cant photosynthesise)
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Opening & Closing Of Stomata

  • water enters guard cells, they swell and pore opens
  • water leaves guard cells, they become flaccid and pore closes
  • chloroplasts in the guard cells make ATP
  • ATP provides energy for active transport to take up k+ from epidermal cells
  • starch converted to malate
  • WP lowers, water enters by osmosis
  • guard cells are turgid and pore opens

reverse for closing

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