Diffusion and osmosis in cells

Cells must constantly exchange materials with their surroundings, taking in nutrients and oxygen and removing waste. Much of this exchange happens without the cell spending energy, through the passive movement of particles. The two most important passive processes are diffusion and osmosis, both of which depend on differences in concentration on either side of the cell membrane.

Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, until they are evenly spread. It results simply from the random motion of molecules and requires no energy from the cell. Oxygen entering a cell and carbon dioxide leaving it are familiar examples. Osmosis is a special case of diffusion that concerns water: it is the movement of water molecules across a partially permeable membrane, from a solution with more free water (a dilute solution) toward one with less (a more concentrated solution). The cell membrane lets water pass but restricts many dissolved substances, so osmosis tends to balance water concentrations on the two sides.

Osmosis has direct consequences for living cells. An animal cell placed in pure water may swell and burst as water flows in, while one in a concentrated solution shrinks as water leaves. Plant cells, supported by their rigid cell wall, instead become firm when full of water, which helps keep a plant upright. Reverse osmosis applies pressure to push water the opposite way through a membrane and is used to purify water rather than as a natural cell process. When a cell must move substances against a concentration gradient, it uses active transport, which does require energy.

Frequently asked questions

What is the difference between diffusion and osmosis?

Diffusion is the movement of any particles from high to low concentration. Osmosis is specifically the movement of water across a partially permeable membrane.

Why do cells rely on diffusion?

Diffusion lets cells exchange substances such as oxygen and carbon dioxide passively, following concentration gradients without using cellular energy.

What happens to a cell placed in pure water?

Water enters by osmosis. An animal cell may swell and burst, while a plant cell becomes firm because its rigid wall resists the pressure.