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 be evenly 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. Substances can cross this membrane by diffusion to move in and out of cells.

Substances that are transported across the cell membrane by diffusion include carbon dioxide, oxygen, water, food substances and waste. 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 surrounded by a large amount of oxygen because it is constantly being transported in the bloodstream from the lungs. This means that there is a higher concentration of oxygen outside the cell than inside the cell, so there is a concentration gradient.

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

Oxygen is used for respiration in the cell, which produces carbon dioxide (CO2) as a waste product. Then the carbon dioxide moves out of the cell by diffusion.