Exchange systems in plants

Stomata are another very important feature for gaseous exchange and transport in plants. They’re found on the underside of the leaf and basically act like pores allowing carbon dioxide to enter to be used for photosynthesis and allowing oxygen, a waste product of photosynthesis, to leave.

The size of stomata is controlled by guard cells. One issue with having pores is that not only can oxygen leave and carbon dioxide enter but water can also escape through transpiration. If the plant has enough water the guard cells swell and the stomata open. However, if the plant is losing water faster than it can replace it then the guard cells shrink and the stomata close to prevent the plant from wilting.


Water is mainly lost through the stomata in the leaves. The rate at which water is lost (transpiration) depends on the rate of evaporation. The level of transpiration can be increased if conditions are:

  • Hot: if the weather’s hot then the air around the leaf will be drier. Water will then leave the plant due to transpiration.
  • Dry: if the air outside the plant is dry (low humidity) then water will leave the leaf faster due to diffusion. However, if there’s more water in the air (an increase in humidity) then the leaf will transpire at a slower rate.
  • Windy: when a leaf loses water the air surrounding it becomes damper. If there’s a wind then this will blow the surrounding water vapour away making the air around the leaf drier. Therefore, in strong winds, a plant loses a lot more water and may have to close of its stomata.