Osmosis & Plant Cells
Biology: Cambridge International A Level, topic 4.2.4
Osmotic Balance
Blood must be kept at the same water potential as blood cells (and other body cells) so that cells do not burst or shrink. External environments can be hypertonic, hypotonic, or isotonic.
Low water potential
- A low water potential solution (hypertonic) has a lower water concentration and a higher ion concentration than cells.
- If the blood becomes too concentrated, cells lose water by osmosis and shrink.
- The red blood cells become crenated.
High water potential
- A high water potential solution (hypotonic) has a higher water concentration and a lower ion concentration than cells.
- If the blood becomes too dilute, cells gain water by osmosis and burst.
Equal water potential
- Blood normally has the same water and ion concentration as cells (isotonic).
- Homeostasis works to regulate the blood's water and ion concentrations so that water does not move into or out of cells.
Osmosis and Plant Cells
Placing plant cells in solutions with different water potentials can cause the cells to change shape.
Flaccid
- When water moves out of cells by osmosis, cell volume decreases, so the cell becomes flaccid (floppy).
- Unlike animal cells, the cells do not shrink very much in size because the cell wall maintains their shape, but the cell now exerts less turgor pressure on the cell wall.
- This causes the plant to wilt.
Plasmolysis
- If a plant cell loses a large amount of its water by osmosis, the cell volume may decrease so much that the cell membrane is pulled away from the cell wall.
- This process is called plasmolysis.
- This causes cell damage.
Turgid
- When water moves into cells by osmosis, cell volume increases, and the cell is said to be turgid (swollen).
- Unlike animal cells, which would burst, plant cells do not burst.
- This is because they have inelastic cell walls. The swollen cell exerts turgor pressure on the cell wall.
- This turgor pressure is very important to support the plant. Water is drawn up through the plant from the roots to maintain turgor pressure.
Test yourself on Osmosis & Plant Cells
Which of these is not true of a solution with a high water potential?
Which cells would not burst if left in distilled (pure) water?