Gas Exchange

Gas Exchange takes place at a respiratory surface

This surface must have:

  • large surface area
  • thin permeable surface
  • moist exchange surface

Gas Exchange in Plants

Gases enter leaf through stomata under leaf

Guard cells enclose stomata and swell to reduce water loss

Gases diffuse through and into mesophyll cells

Diffusion rate increases during day due to photosynthesis:

  • oxygen concentration increases
  • carbon dioxide concentration decreases
  • relative concentration gradients increase… diffusion rate increases

Palisade mesophyll cells are packed with chloroplasts

Palisade cells packed together for maximum absorption

Gas Exchange in Fish


The water surrounding a fish contains a small percentage of dissolved oxygen.

The fish uses a special gas exchange organ (gills).

 It has a large surface area and a short distance for gas exchange to the blood capillaries.

Water flows over the filaments, and oxygen can diffuse down its concentration gradient the short distance between water and blood.

Carbon dioxide diffuses the opposite way down its concentration gradient

Approximately 80% of the dissolved oxygen in water is extracted by the gills

Gas Exchange in Humans

Humans have approximately 600 million alveoli

Walls are two cells thick (easy for oxygen and carbon dioxide to diffuse)

They are constantly moistened by water

They contain phagocyte cells to kill bacteria


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