Osmosis (Passive Transport) — Definition & Importance

The Diffusion of Water

Osmosis is the diffusion of water across a membrane.

Look at Picture A, shown above. The right-hand side of the beaker has a higher concentration of salt molecules than the left side does. This means that the concentration of water on the right-hand side is lower. Salt can't cross the membrane, but water can freely. In Picture B, the water from the left side crosses the membrane to the right side in order to make the concentration of water and salt equal on both sides. Just like in diffusion, the water moves from a region of higher concentration to one of lower concentration.

Basically, we just want the salt and water ratios to match on both sides (like three water molecules for each salt molecule on both sides). Once it matches, osmotic pressure from the salt solution side pushes back on the water, causing it to stop rushing in.

The concentration of water can be described in three ways: hypertonic, hypotonic, or isotonic.

Isotonic is a fancy way to say equilibrium; the solutions on both sides of the membrane are the same concentration.

Hypertonic means the solution outside the membrane has a higher solute concentration than the solution inside the cell. This causes animal cells to shrink and the vacuoles of plant cells to collapse.

Hypotonic means the concentration of solute inside the cell is higher than it is in the solution outside the cell. This causes animal cells to swell up and burst. For plant cells, the vacuoles swell and push the organelles into the cell walls.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Plasma Membrane}}$ : A semi-permeable phospholipid bilayer

$\color{#8925AE}{\text{Diffusion}}$ : The movement of particles from areas of high concentration to areas of low concentration

$\color{#8925AE}{\text{Osmosis}}$ : The movement of water molecules from areas of high water concentration to areas of low water concentration

$\color{#8925AE}{\text{Facilitated Diffusion}}$ : The movement of particles from areas of high concentration to areas of low concentration with the help of an enzyme

$\color{#8925AE}{\text{Passive Transport}}$ : The movement of particles across the membrane with no help from the cell

(Video) Osmosis and Water Potential

by Amoeba Sisters

In this video, the Amoeba Sisters explain osmosis, which is the diffusion of water or other solvents through a semipermeable membrane. They use this concept to explain how salt water can kill plants. They also explain the ideas and math behind water potential, which is closely related to osmosis.

(Video) Passive Transport

by Teacher's Pet

The diffusion of water has a special name. It is called osmosis. We need to look at something special in this type of diffusion: the amount of solute versus pure solvent in the solutions inside and outside of the cell. Watch the video to find out why this is such a big deal.