Lesson 3 of 5 / Movement of substances
Osmosis in plant and animal cells
Why can a plant cell become firm while an animal cell bursts?
I can link water movement to changes in plant and animal cells.
I can define osmosis and investigate and describe its effects on plant and animal tissues.Syllabus
Syllabus K224 / K225, Topic 2(b). Define osmosis and investigate and describe its effects on plant and animal tissues.
About 5 min
Make a guess. You are not marked.
A red blood cell and a leaf cell are both put in pure water. What happens to each?
Show the answer
The red blood cell bursts; the leaf cell becomes firm.
Water enters both by osmosis. The leaf cell wall resists swelling; the red blood cell has no wall to stop it bursting.
The key idea
Both 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 solution around a cell has a higher water potential than the cell contents, water enters the cell by osmosis. An animal cell has no cell wall, so it swells and may burst if enough water enters.
In a plant cell, the water that enters fills the vacuole. The vacuole pushes the cell contents against the cell wall. The wall is strong and stops the cell getting bigger. The cell becomes turgid, which means firm. Turgid cells help hold up soft plant parts such as leaves.
If the solution outside has a lower water potential, water leaves the cell by osmosis. An animal cell shrinks. A plant cell loses its firmness and becomes flaccid. If it loses a lot of water, the cell membrane pulls away from the cell wall. The cell is then plasmolysed. The wall keeps its shape.
If the water potentials inside and outside are equal, there is no net movement of water. A plant cell in this case is flaccid, not turgid, because its contents are not pressing hard against the wall. Water molecules are still moving 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.
Watch out for this
A student says: "The plant cell wall stops water from entering." What is wrong with this?
Show the answer
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.
Check your understanding
A plant cell is placed in a solution with much lower water potential. Which observation indicates plasmolysis?
Show the answer
The cell membrane pulls away from the cell wall.
Sufficient water loss makes the cell contents shrink and the membrane pull away from the wall.
Quick recall
Why can a plant cell become firm while an animal cell bursts?