Osmosis (Cambridge International AS Biology)

The Process of Osmosis

Spec point — describe and explain the processes of osmosis

• All cells are surrounded by a cell membrane which is selectively permeable

• Water can move in and out of cells by osmosis

• Osmosis is the diffusion or net movement of water molecules from a dilute solution (high water potential) to a more concentrated solution (low water potential) across a selectively permeable membrane

  • The cell membrane is selectively permeable which means it allows certain molecules (like water) through the bilayer

Osmosis and the selectively permeable membrane

How osmosis works

Exam Hint Water can pass through the phospholipid bilayer because water molecules are small molecules that can pass between phospholipids in the cell membrane.

Although water molecules are polar, they can still pass through the bilayer because of their small size.