Osmosis: Water may move across cell membranes via osmosis.

The idea

Cell membranes are partially permeable, so water can cross them by osmosis while most dissolved solutes cannot. Osmosis is passive: it needs no energy from respiration and happens because water molecules move randomly, with a net movement from the side with a higher water concentration (dilute solution) to the side with a lower water concentration (concentrated solution). In a plant root hair cell, soil water is usually more dilute than the cytoplasm and vacuole sap, so water enters and travels to the xylem. In an animal cell placed in pure water, water enters, the cell may swell and burst; in a concentrated salt solution, water leaves and the cell shrinks. Plant cells change less because the cellulose cell wall resists expansion, making the cell turgid, or pulls away from the wall when water is lost, causing plasmolysis.

What earns the marks

• States that cell membranes are partially permeable, allowing water through but restricting most solutes.
• Defines osmosis as the net diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.
• Explains that osmosis is passive and results from the random movement of water molecules, not from active transport.
• Uses the idea of water concentration or water potential gradient to predict the direction of water movement.
• Applies the process to named cells, such as a root hair cell absorbing soil water or a red blood cell in distilled water.
• Describes plant cell outcomes, including turgidity when water enters and plasmolysis when water leaves.

Common mistakes and how to put them right

• Saying that osmosis moves solutes rather than water.

  Put it right by stating that osmosis is the movement of water molecules only.

• Describing osmosis as requiring energy from respiration.

  Put it right by stating that osmosis is passive and driven by random molecular motion down a water concentration gradient.

• Claiming that water moves from a concentrated solution to a dilute solution.

  Put it right by stating that water moves from the dilute solution to the concentrated solution.

Exam tips

• Define osmosis using all three required ideas: water, partially permeable membrane, and movement from dilute to concentrated solution.
• When predicting direction, compare water concentrations rather than solute concentrations, then state which side gains water.
• Link each answer to the specific cell named in the question, such as a root hair cell or a plant leaf cell, instead of describing osmosis only in general terms.