Osmosis Explained Simply (GCSE Biology)

Definition of osmosis:

Osmosis is the diffusion of water molecules from a region, where the water molecules are at a higher concentration, to a region where they are at a lower concentration, through a partially permeable membrane.

Illustration of osmosis demonstrating a semi-permeable membrane separating two solutions with different concentrations of particles.

Diffusion vs osmosis:

Osmosis and diffusion are similar in some ways and different in other ways.

Osmosis	Diffusion
Osmosis requires a partially permeable membrane	Diffusion doesn’t require a partially permeable membrane
Involves water molecules moving from a region of higher water concentration to a region of lower water concentration through a partially permeable membrane	Involves ions, atoms and molecules moving from an area of higher concentration to an area of lower concentration
Occurs in liquids (water)	Occurs in solids, liquids and gases
Is a passive process (doesn’t require energy)	Is a passive process (doesn’t require energy)
The concentration of the solvent doesn’t become equal on both sides of the partially permeable membrane.	The concentration of the diffused molecules becomes distributed equally in a given space.

Plant cells vs animal cells in osmosis:

Diagram illustrating the effects of hypotonic, isotonic, and hypertonic solutions on animal and plant cells. Hypotonic solution shows a lysed animal cell and a turgid plant cell. Isotonic solution shows a normal animal cell and a flaccid plant cell. Hypertonic solution shows a shriveled animal cell and a plasmolyzed plant cell.

When animal cells are placed into solutions with varying solute concentrations, the animal cells will either:

expand in size, gain water and eventually burst in distilled water or more dilute (hypotonic) solutions

lose water and shrink in more concentrated (hypertonic) solutions

With plant cells, they have a cell wall which is strong and prevents the plant cell from bursting.

When plant cells are placed into solutions with varying solute concentrations, the plant cells will either:

expand in size, gain water and the vacuole and the cytoplasm will push against the cell wall when the plant cell is placed in distilled water or more dilute (hypotonic) solutions; the plant cell becomes turgid