Osmosis

Living cells may be thought of as microscopically small bags of solutions contained within semipermeable Figure 2. Osmosis equalizes concentration. Illustration by Hans & Cassidy. Courtesy of Gale Group.
membranes. For the cell to survive, the concentration of solutes within the cell must stay within a safe range.

A cell placed in a solution more concentrated than itself (a hypertonic solution) will shrink due to loss of water, and may die of dehydration. A familiar example is a carrot placed in salty water. Within a few hours the carrot will become limp and soft because its cells have shrivelled. A cell placed in a solution more dilute than itself (a hypotonic solution) will expand as water enters it. Under such conditions the cell may burst.

Organisms have various methods for keeping their cell solute concentrations within safe levels. Some live only in surroundings that are isotonic (have the same solute concentration as their own cells). For example, jellyfish which live in saltwater have much higher cell solute concentrations than do fresh water creatures. Other animals continually replace lost water and solutes by drinking and eating, and remove the excess water and solutes through excretion of urine. Plant cells are protected from bursting by the rigid cell wall which surrounds the cell membrane. As water enters, the cell expands until it pushes up tight against its cell wall. The cell wall pushes back with an equal pressure, so no more water can enter.

Osmosis contributes to the movement of water through plants. Solute concentrations increase going from soil to root cells to leaf cells, and the resulting differences of osmotic pressure help to draw water upward. Osmosis also controls the evaporation of water from leaves by regulating the size of the openings (stomata) in the leaves' surfaces.