Osmosis and plant cells
What is the water potential equation?
The overall water potential of the cell is the pressure potential added to the solute potential. Plants are under the influence of two opposing forces; * solute potential - due to the solutes in the cytoplasm and vacuole pulling water in.
Isotonic...
If the cell has the same water potential as the surrounding external solution then the cell and external solutions are isotonic and so there will be no net movement of water.
Hypotonic...
If the water potential of the external solution is less negative (higher) than that of the solution inside the cell, it is hypotonic to the cell and water will flow into the cell.
Hypertonic...
If the water potential of the solution outside the cell is more negative (lower) than that of the solution inside the cell then it is hypertonic to the cell and water will flow out of the cell.
What does a cell wall mean for plants?
In a plant cell the presence of a cell wall introduces an extra factor concerning water movement in and out of cells.
When does a plant stop taking in water?
A plant in a hypotonic solution will take in water until opposing pressure from the cell wall prevents it. As water is taken in the contents of the cell expand and push out the cell wall, increasing the pressure potential. The pressure potential will continue to increase until it is equal and opposite to the pull inwards of solute potential. No more water can enter after this point and as there is no more tendency for the cell to absorb water, the water potential is 0.
What is the definition of solute potential?
It is a measure of the osmotic strength of a solution. It is the reduction in water potential due to the presence of solute molecules
What is the definition of turgid?
This is used to describe a plant cell that is holding as much water as possible. Further entry of water into the cell is resisted as the cell cannot expand any further
What is incipient plasmolysis?
This is when the concentration of the external solution is high enough that the cell has lost just enough water that it's membrane begins to be pulled away from the cell wall. The cell wall does not exert any pressure on the cytoplasm and so the pressure potential is 0. The cell's water potential will be equal to the solute potential of the external solution
What is pressure potential?
Water entering a plant cell by osmosis expands the vacuole and causes the cytoplasm to push against the cell wall. The cell will only expand a little and so pressure outwards build up resisting the entry of anymore water and causing the cell to become turgid. This pressure that the cell experiences is the pressure potential and since it is a push outwards it has a positive sign.
When is a cell known as 'turgid'?
When a cell has reached the point where it can no longer take in any more water it is described as turgid. Turgor in plants is extremely important, especially in young seedlings as it provides support, maintains shape and keeps them upright.
What is plasmolysis?
When in a hypotonic solution plant cells lose water through osmosis. The vacuole shrinks and the cytoplasm draws away from the cell wall. This is the process of plasmolysis and when complete the cell becomes flaccid. When flaccid the cell can no longer provide the necessary structural support to the plant causing it to wilt when the plant cells have lost too much water.
