Gas and Solute Exchange

• 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

• 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

Alveoli

• 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 

Villi

• 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.