Diffusion affects cells by allowing molecules to pass through or around a cell's selectively permeable outer membrane. Diffusion is powered by kinetic movement and occurs without any energy output.
Diffusion is a process that causes high concentrations of molecules to be dispersed into a less concentrated area. When molecules are transported by diffusion, they may evenly arrange themselves in the new space, reaching a point known as equilibrium. Diffusion allows for the rapid movement of substances such as oxygen and water around a cell's membrane, as well as the intake of molecules from a cell's surrounding area. Molecules that are too large to be diffused across a membrane may be transported by special proteins that bind to the molecules through a process known as facilitated diffusion.
Certain solutions may be detrimental to a cell. Hypertonic solutions contain high concentrations of salt molecules, which are drawn to the interior of cells that have fewer salt molecules than the surrounding solution. This causes the cells to shrink, due to raised levels of salt. Hypotonic solutions contain fewer salt molecules than the interior of the cell, causing dangerous levels of water to fill the cell, which puts the cell at risk of bursting. Plant cells are protected from bursting by a rigid outer cell layer, while some animal cells have mechanisms that allow them to remove excess water from their interiors.