Gas Exchange in Animals and Plants


Leaf epidermis

A single-cell layer that covers the leaves. It forms a divide between the plant and its external environment.


The leaf epidermis is covered with tiny spores called stomata. The stomata allow gases, including water vapour, to move in and out of the leaf. Each stomata has a guard cell on each side.

  • Carbon dioxide enters through open stomata
  • Water loss is prevented with closed stomata
  • These objectives must be balanced
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Stomata open and close with the help of guard cells, which control the diameter of the pore by changing shape. When guard cells take up water, they swell and become tight, opening the pore. Inversely, when the guard cells lose water, they shrink and reduce the pore diameter.

  • Gains water = wider pores
  • Loses water = smaller pores
  • Stomata are open in the light and close in the dark


Gaseous exchange occurs in all animals and involves the movement of gases between the internal and external environments by diffusion across cell membranes.

Oxygen is essential for all cells to carry out cellular respiration to release energy from the nutrients they have consumed. The respiratory system enables the exchange of gases between an organism and its environment.


Fish must obtain oxygen in order to remove carbon dioxide. Water flowing over them ensures maximum oxygen uptake. As the water is only flowing in one direction, the water can enter and flow over the gills and then leave via the gill slit.


Insects do not have lungs or capillaries for gas exchange. This occurs in the spiracles, which are breathing pores.