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Diffusion in biology

In biology, diffusion explains how useful materials (like oxygen) and waste products (such as CO2) enter and leave cells. Without diffusion, respiration and photosynthesis couldn’t take place.

Diffusion is essential for life


In humans, respiration takes place in the lungs and in all living cells.

Gaseous exchange in the lungs

Gaseous exchange in the lungs takes place in tiny air sacs called alveoli. (NOTE: alveoli is plural, alveolus is singular). Carbon dioxide diffuses out of the red blood cells and into the alveolus, while oxygen diffuses out of the alveolus and into the bloodstream. The gases do this by moving through the very thin walls of tiny blood vessels called capillaries.

Diagram shows how CO2 diffuses out of blood flow and diffusion of O2 into blood flow

Cellular respiration

After the blood has left the lungs, it reaches the rest of the body where cellular respiration occurs. Oxygen, carbon dioxide and glucose move in and out of and the blood stream by diffusion.


From blood flow O2 and glucose are diffused into the body while CO2 is drawn from tissue to the blood diagram


Plants also exchange gases through diffusion during photosynthesis and respiration.



When photosynthesis occurs, carbon dioxide diffuses into the leaf through the stomata (pores, or tiny holes, in the leaf) while oxygen moves out of the leaf in the same way. NOTE: stomata is plural, stoma is singular.

Cross section of a leaf showing how CO2 enters and O2 exits through diffusion


6CO2 + 6H2O C6H12O8 + 6O2
Carbon dioxide + water glucose + oxygen




When plants respire, the opposite is true: oxygen diffuses into the leaf and carbon dioxide is given out through the stomata.

Cross section of a leaf showing how O2 enters and CO2 exits through diffusion

C6H12O8 + 6O2                                            →        E + 6CO2 + 6H2O

Glucose + oxygen                           →        energy (ATP) + carbon dioxide + water

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