Liquid and gas particles move around continuously, which causes them to spread out. Diffusion is the spreading out of particles of a substance in solution, or particles of a gas.

This results in a net movement of particles from an area of high concentration to an area of low concentration.

If possible, the liquid or gas particles will eventually spread evenly throughout the medium. As diffusion does not require any energy, it is a passive process.

Diffusion Across Membranes

Cells are surrounded by a cell membrane, which allows substances to move in and out through diffusion.

Substances such as carbon dioxide, oxygen, water, food and waste can be transported across the cell membrane by diffusion. Cells must take in useful substances and remove waste substances for the cell to function.


Cells need oxygen for respiration, which takes place in the mitochondria. When we inhale oxygen, it enters our lungs and reaches the alveoli.

Oxygen diffuses across the barrier between the alveoli and the surrounding capillaries. Carbon dioxide diffuses from the blood to the alveoli, where it is exhaled.

Cells are exposed to a high concentration of oxygen, as it is constantly transported by the bloodstream from the lungs. Since there’s a higher concentration of oxygen outside the cell than inside, a concentration gradient is established.

Therefore, oxygen passes across the membrane and into the cell by diffusion.

Oxygen is used for respiration in the cell, producing carbon dioxide (CO2) as a waste product, which then diffuses out of the cell.

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