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Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration. In cells, this process usually occurs across the cell membrane, and it requires no cellular energy.

The rate at which molecules cross the membrane depends on several factors, including hydrophobicity, size, and electrical charge.

Small nonpolar molecules, like oxygen and carbon dioxide, can easily slip through the lipid bilayer. Also, many large fat-soluble molecules, like steroids and fatty acids, readily pass through the membrane by simple diffusion.

Small, uncharged but polar molecules, such as water, can cross the membrane, but at a slower rate.

Larger uncharged molecules, such as glucose, cannot pass directly through the membrane. Instead, they move by facilitated diffusion through membrane transport proteins such as channels or carriers.

On the other hand, charged ions, no matter their size, and non-lipid-soluble molecules cannot cross the hydrophobic core of the lipid bilayer and need specialized membrane proteins, like ion channels, to move across.

Even after the concentration stabilizes, particles continue to move randomly between the two sides. In living cells, this bidirectional exchange reaches a dynamic steady state, where there is no net movement, but molecules continue to move in both directions at equal rates.