Osmosis (AQA A Level Biology): Revision Note

The process of osmosis

• Water can move in and out of cells by osmosis

• Osmosis is defined as:

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

• A dilute solution has a high water potential (the left-hand side of the diagram below) and a concentrated solution has a low water potential (the right-hand side of the diagram below)

Osmosis in plant cells

• Plant and animal cells are affected differently by osmosis, as plant cells have a cell wall

• In a hypertonic solution (lower water potential outside), water leaves the plant cell, the protoplast shrinks and pulls away from the cell wall — this is called plasmolysis

  • Without enough water, cells lose turgor and the plant wilts

• In a hypotonic solution (higher water potential outside), water enters the plant cell, the vacuole expands, and the cell becomes turgid — the cell wall prevents bursting

  • Turgidity supports the plant, helping it stay upright and catch sunlight

• In an isotonic solution, water moves in and out equally, so there is no net change, and the cell is neither turgid or plasmolysed

Osmosis in animal cells

• Animal cells, like plant cells, gain or lose water by osmosis, but the effects are more severe as they lack a cell wall

  • In a hypertonic solution (lower water potential outside), water leaves the cell, causing it to shrink and shrivel

  • In a hypotonic solution (higher water potential outside), water enters the cell, which may swell and burst (cytolysis)

  • In an isotonic solution, water moves in and out equally, so there is no net change to the cell

• Maintaining a stable water potential in animal tissue fluid is essential to prevent cell damage