Gas and Solute Exchange

Very brief summary of how surfaces are adapted to maximise effectiveness

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  • Created by: jenny100
  • Created on: 15-05-12 11:03

Structure of Leaves

  • CO2 diffuses into the air spaces within the leaf then it diffuses into the cells where photosynthesis occurs
  • The leafs structure is adapted so this can happen easily
  • The underneath of the leaf is the exchange surface and is covered in tiny holes called stomata which the CO2 diffuses through
  • Water vapour and oxygen diffuse out through the stomata
  • The size of the stomata are controlled by guard cells, they close if the plant is losing too much water, without the guard cells the plant would wilt
  • The flattened shape of the leaf increases the area of this exchange surface so it is more effective
  • The walls are thin so the air spaces inside the leaf increase and more CO2 can enter the leaf

Water vapour escapes by diffusion because there's a lot inside the leaf and less in the air outside <-- this diffusion is called transpiration and occurs more when the air around the leaf is dry. Hot dry wind helps carry the water vapour away

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The Breathing System Background

  • The thorax is the top part of your body
  • It is separated from the lower part of the body by the diaphragm
  • Trachea (windpipe) splits into two tubes called the 'bronchi', one into each lung
  • The bronchi split into smaller tubes called bronchioles
  • Bronchioles end at small bags called alveoli

Breathing In

  • Intercostal muscles & diaphragm contract
  • Thorax volume increases
  • This decreases pressure drawing air in

Breathing Out

  • Intercostal muscles & diaphragm relax
  • Thorax volume decreases
  • Air is forced out
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Diffusion Through Cell Membranes


  • Gas exchange occurs in the lungs
  • Lungs remove waste CO2 and transfer oxygen to the blood
  • To do this the lungs contain alveoli (little air sacks)

To maximise the diffusion of oxygen and CO2 they have:

  • A huge surface area
  • Moist lining for dissolving gas
  • Very thin walls
  • Copious (abundant) blood supply 

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Diffusion Through Cell Membranes 2


  • Villi get digested food into the blood
  • The inside of the small intestine is covered in millions and millions of little projections called villi
  • They increase surface greatly so the digested food is absorbed quicker into the blood
  • They only have a single layer of surface cell (easier diffusion)
  • And a very good blood supply to provide quick absorption

A larger surface area helps in both cases because as molecules can only diffuse across a close membrane so the bigger the surface area the more molecules can be diffusing.

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