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How does water move into and out of plant and animal cells?

Define osmosis and explain its effects on plant and animal cells in solutions of different concentration

A focused answer to the O-Level Biology outcome on osmosis. The definition, the meaning of a partially permeable membrane, and the effects on plant cells (turgid, flaccid, plasmolysed) and animal cells (swell or shrink).

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

SEAB wants a clear definition of osmosis and an explanation of what happens to plant cells and animal cells when they are placed in solutions of different concentration. You should understand the term partially permeable membrane and be able to use osmosis to predict whether water enters or leaves a cell and what effect that has.

The answer

What osmosis is

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

A partially permeable membrane lets small water molecules through but not larger dissolved solute molecules such as sugar. The cell surface membrane is partially permeable, so water can cross it but most solutes cannot.

Osmosis is really a special case of diffusion that applies to water across a partially permeable membrane, and like diffusion it is passive.

Comparing water concentrations

  • A dilute solution has a lot of water and little solute, so a high water concentration.
  • A concentrated solution has little water and much solute, so a low water concentration.

Water moves by osmosis from the dilute side (more water) to the concentrated side (less water).

Effect on animal cells

An animal cell, such as a red blood cell, has no cell wall:

  • In a dilute solution (more water outside), water enters; the cell swells and may burst.
  • In a concentrated solution (less water outside), water leaves; the cell shrinks and crinkles.
  • In a solution of the same water concentration, there is no net movement and the cell stays the same.

Effect on plant cells

A plant cell has a strong cellulose cell wall:

  • In a dilute solution, water enters; the cell swells until the wall resists, and the cell becomes firm, or turgid. The wall stops it bursting.
  • In a concentrated solution, water leaves; the cell becomes soft, or flaccid, and if a lot of water leaves the membrane pulls away from the wall, called plasmolysis.

Turgid cells help support soft plant tissues, which is why a wilting plant recovers when watered.

Examples in context

Example 1. Wilting and recovery. On a hot day a plant loses water and its cells become flaccid, so the plant wilts. Watering raises the water concentration in the soil, water enters the cells by osmosis, they become turgid again, and the plant stands up.

Example 2. Root hair cells. Soil water has a higher water concentration than the cell sap of a root hair cell, so water moves into the root by osmosis. This is how plants take up most of their water from the soil.

Try this

Q1. Define osmosis. [2 marks]

  • Cue. The net movement of water molecules from a region of higher water concentration to one of lower water concentration through a partially permeable membrane.

Q2. State what happens to a red blood cell placed in pure water and explain why. [2 marks]

  • Cue. It swells and may burst, because pure water has a higher water concentration, so water enters by osmosis and the cell has no wall to resist.

Q3. Explain why a plant becomes turgid after watering. [2 marks]

  • Cue. Water enters the cells by osmosis; the contents push against the cell walls, which resist, making the cells firm (turgid) and supporting the plant.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksPieces of potato of equal mass are placed in three solutions: pure water, a dilute sugar solution, and a concentrated sugar solution. After one hour the pieces are reweighed. Predict and explain the change in mass of the piece in pure water and the piece in concentrated sugar solution.
Show worked answer →

In pure water: the water outside has a higher water concentration than the cell contents, so water moves into the potato cells by osmosis across the partially permeable cell membranes. The piece gains mass and becomes firm.

In concentrated sugar solution: the solution outside has a lower water concentration than the cell contents, so water moves out of the cells by osmosis. The piece loses mass and becomes soft and flaccid.

Markers reward the correct direction of water movement in each case (linked to the relative water concentrations), the word osmosis, and the matching change in mass (gain in water, loss in concentrated solution).

Original4 marksExplain why a plant cell placed in pure water becomes turgid but does not burst, while a red blood cell placed in pure water may burst.
Show worked answer →

In both cases water enters the cell by osmosis because the cell contents have a lower water concentration than pure water.

A plant cell has a strong cellulose cell wall. As water enters, the cell swells and the contents push against the wall, but the wall resists and stops the cell bursting; the cell becomes turgid (firm).

A red blood cell has no cell wall, only a cell membrane. As water enters it keeps swelling with nothing to resist, so the membrane stretches and the cell may burst (haemolysis).

Markers reward osmosis into both cells, the cell wall resisting the swelling in the plant cell (turgid), and the absence of a wall in the red blood cell allowing it to burst.

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