62 WJEC Biology AS Level - Marianne Izen - 2nd Edition (1.3 Cell Membranes And Transport) Flashcards
Here are two cells, with their solute and pressure potentials indicated. Will water move from cell A to B or from cell B to A?
When is the external solution hypotonic to the cell?
If the water potential of the external solution is less negative (higher) than the solution inside the cell, the external solution is hypotonic to the cell and water flows into the cell.
When is the external solution hypertonic to the cell?
If the water potential of the external solution is more negative (lower) than the solution inside the cell, the external solution is hypertonic to the cell and water flows out of the cell.
When is the external solution isotonic to the cell?
If the cell has the same water potential as the surrounding solution, the external solution and cell are isotonic and there will be no net water movement.
Explain what happens to plant cells in a hypertonic solution?
Plant cells in a hypertonic solution lose water by osmosis.
The vacuole shrinks and the cytoplasm draws away from the cell wall.
This process is called plasmolysis and, when complete, the cell is flaccid.
Flaccid means ‘floppy’ and such cells cannot provide support so when a plant loses too much water and its cells become flaccid, the plant wilts.
Describe incipient plasmolysis.
If the external concentration is high enough that the cell has lost just enough water that its membrane begins to be pulled away from the cell wall, the cell is at incipient plasmolysis.
The cell wall does not exert any pressure on the cytoplasm and so there is no pressure potential
Water potential = Pressure potential + Solute potential
Water potential = 0 + solute potential = solute potential
This means the cell’s water potential is equal to the solute potential of the external solution.
Explain what happens to plant cells in a hypotonic solution.
A plant cell in a hypotonic solution takes in water until prevented by the opposing pressure from the cell wall.
As water enters the cells, the contents expand and push out more on the cell wall, increasing the pressure potential.
The pressure potential rises until it is equal and opposite to the pull inwards of the solute potential.
No more water can enter, and as there is no tendency for the cell to absorb water, its water potential is zero.
Plasmolysis (Key-Term)
The retraction of the cytoplasm and the cell membrane
from the cell wall as a cell loses water by osmosis.
Incipient plasmolysis (Key-Term)
Cell membrane and cytoplasm are partially detached from the cell wall due to insufficient water to make cell turgid.
What is the water potential, pressure potential and solute potential in incipient plasmolysis?
What is the water potential, pressure potential and solute potential in a turgid cell?
Draw a diagram to explain mass change in solutions of different concentrations.
