Osmosis and Diffusion

in Cells


The tendency of a fluid, usually water, to pass through a semipermeable membrane into a solution where the solvent concentration is higher, thus equalizing the concentrations of materials on either side of the membrane.


The net passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout.

Hypo, Hyper, and Iso

Hypotonic - when a cell is surrounded by a solution containing more water than the cell itself, so the cell absorbs more water and bloats.

Hypertonic - when a cell is surrounded by a solution containing less water than the cell itself, so the cell releases water and shrinks.

Isotonic - when the outside and inside of a cell contain the same amount of water.

Osmosis and Diffusion in Cells

Animal cells do not have cell walls. In hypotonic solutions, animal cells swell up and explode as they cannot become turgid because there is no cell wall to prevent the cell from bursting. In hypertonic solutions, water diffuses out of the cell due to osmosis and the cell shrinks. Thus, the animal cell has always to be surrounded by an isotonic solution.

Osmosis is a vital function to the growth and stability of plant life. Without osmosis, photosynthesis would never occur and plants would wilt and die. A wilted plant looks wilted because the vacuoles of the cells do not have proper amounts of water. Although osmosis is still occurring, the lack of volume of water in the plant causes all of the cell walls to lose their turgor, and thus the plant loses its upright and healthy state. Osmosis distributes water through selectively permeable membranes to maintain this proper volume and pressure of all plant cells.

For diffusion, cells distribute a solute equally through the cell. For example, when cells use oxygen and glucose, carbon dioxide and waste chemicals build up so the cells diffuse it out.

Water Potential

The measure of the relative tendency of water to move from one area to another, and is commonly represented by the Greek letter Ψ (Psi). Water potential is caused by osmosis, gravity, mechanical pressure, or matrix effects including surface tension.