Osmosis: Water may move across cell membranes via osmosis. Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

The idea

Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution to a concentrated solution. A dilute solution has a higher water potential, meaning more free water molecules per unit volume, while a concentrated solution has a lower water potential. The membrane allows small water molecules through but restricts larger solute molecules. Water therefore moves down its water potential gradient. In animal cells, water entering by osmosis can cause the cell to swell and possibly burst, while water leaving can cause shrinkage. In plant cells, water entering the vacuole increases turgor pressure, making the cell firm; water leaving causes plasmolysis. Osmosis is passive and requires no energy from respiration.

What earns the marks

• Osmosis is the diffusion of water through a partially permeable membrane.
• Water moves from a dilute solution to a concentrated solution, or down a water potential gradient.
• A dilute solution has a higher water potential than a concentrated solution.
• The partially permeable membrane allows water molecules through but not larger solute molecules.
• In plant cells, water entering by osmosis increases turgor pressure and makes the cell firm; water leaving causes plasmolysis.
• In animal cells, water entering by osmosis may cause swelling or bursting, and water leaving may cause shrinkage.
• Osmosis is a passive process and does not require energy from respiration.

Common mistakes and how to put them right

• Saying that osmosis is the movement of water from a high concentration of water to a low concentration of water without mentioning the membrane.

  Put it right: Osmosis must involve a partially permeable membrane.

• Confusing osmosis with diffusion of solute particles.

  Put it right: In osmosis, it is water molecules that move through the partially permeable membrane.

• Thinking that osmosis requires energy.

  Put it right: Osmosis is passive and relies on the water potential gradient.

Exam tips

• Always state the direction of water movement using the terms dilute and concentrated, or higher and lower water potential.
• When describing cells, link water movement to observable effects such as turgidity, plasmolysis, swelling or shrinkage.
• Use the phrase partially permeable membrane to show that you understand the role of the membrane in osmosis.