Lesson 3 of 4 / Movement of substances
Osmosis in plant and animal cells
Why can a plant cell become firm while an animal cell bursts?
In this lesson: Link water movement to changes in plant and animal cells.
About 5 min
The key ideaBoth cell types gain or lose water by osmosis. A plant cell has a rigid cell wall that limits expansion; an animal cell does not.
Explore the idea
Why can a plant cell become firm while an animal cell may burst?
A schematic outcome after the cell is placed in the solution.
Turgid.Water enters by osmosis and the vacuole expands. The cell contents push against the cell wall, which resists further expansion. The cell becomes turgid; pressure builds until net water entry stops.
Initial net water movementInto the cell at first
- Cell membrane
- Cell wall
- Vacuole
- Nucleus
Structures and size changes are simplified. The cell membrane is partly permeable; a plant cell wall is freely permeable to water. The plant examples begin without turgor pressure. Water potential and pressure can change as water moves.
Explanation
A cell membrane is partially permeable. If the surrounding solution has a higher water potential than the cell contents, water enters the cell by osmosis. An animal cell swells and may burst if enough water enters because it has no cell wall.
In a plant cell, water entering enlarges the vacuole and pushes the cell contents against the cell wall. The wall resists further expansion. The cell becomes turgid, or firm. This firmness helps support non-woody plant parts.
If the surroundings have a lower water potential, water leaves the cell by osmosis. An animal cell shrinks. A plant cell loses turgor and becomes flaccid. With sufficient water loss, the cell membrane pulls away from the cell wall: the cell is plasmolysed. The wall keeps its shape.
If water potentials are equal, there is no net movement of water. In the simple plant-cell comparison here, the cell is flaccid rather than turgid because its contents are not pressing firmly against the wall. Water molecules still move in both directions.
Step by step
- 1
State where the water goes
Compare the water potential outside with the water potential of the cell contents.
- 2
Describe the cell change
Use precise terms such as swell, shrink, turgid, flaccid or plasmolysed.
- 3
Use the relevant structure
The partially permeable membrane controls which substances cross. The cell wall resists expansion but does not stop water reaching the membrane.
Worked example
Salt on cucumber slices
Salt dissolves in water around living cucumber cells, producing a solution with a lower water potential than the cell contents. Explain why the slices become less firm.
One way to explain it
Water leaves the cucumber cells by osmosis through their partially permeable cell membranes, from higher water potential inside to lower water potential outside. The cells lose turgor, so the tissue becomes less firm.
Why this answer works
- Specify water loss by osmosis.
- Give the direction and the water potential difference.
- Connect loss of turgor to reduced firmness; do not claim the cell walls dissolve.
Is this true? "The plant cell wall stops water from entering."
Water can pass through the freely permeable cell wall. The partially permeable cell membrane is the relevant barrier for osmosis; the wall provides mechanical support.