Water Potential. Flashcards
What is another way of thinking about water potential?
The tendency of water to leave a system. (E.g a cell)
Define water potential.
The potential (free) energy of water molecules in a system (e.g a cell) compared with the potential (free) energy of water molecules in pure water at s.t.p
What symbol is water potential given?
Ѱ
What will affect water potential?
Anything that slows down or interferes with the free movement of of water. Including temperature, pressure and solute concentration.
What is the Ѱ of pure water at s.t.p?
0kPa.
In what direction will water move down the water potential gradient?
From higher to lower water potential.
What is an isotonic solution?
A solution with the same water potential as another solution.
What is a hypertonic solution?
A solution with a lower water potential than another (more solute is present).
What is a hypotonic solution?
A solution with a higher water potential (less solute) than another.
What is Ѱcell and its equation?
ѰCell is an expression for the water potential of a cell.
ѰCell = Ѱs + Ѱp
What is Ѱs?
This is solute potential I.e the contribution made to Ѱcell by dissolved substances.
What is Ѱp?
Is pressure potential I.e the contribution made to Ѱcell by internal pressure.
What will happen to a plant cell in pure water or a weak solution?
The cell will become turgid.
Explain the process by which a cell becomes turgid.
1 - Water enters the cell by osmosis as Ѱoutside is greater than Ѱinside.
2 - As a result the vacuole expands, pushing on the cell wall and causing Ѱp to increase.
3 - as Ѱp increases Ѱs decreases (as internal solution becomes more dilute)
4 - Ѱp and Ѱs reach equal but opposite values and cancel out.
5 - Ѱcell = 0 meaning the net tendency of water to enter the cell is zero. The cell is now in equilibrium with its solution.
Describe an experiment to determine Ѱcell.
A solution causing no mass or size change Must be isotonic with the cell. This can be estimated by placing plant tissue in a range of concentrations for about an hour. Afterwards each piece is remeasured or reweighed and change in mass is calculated. A curve may then be produced by plotting this against solution molarity. The Ѱcell value may then be extrapolated.
What occurs when Ѱoutside is less than Ѱcell e.g in strong sucrose solution.
Water leaves the cell by osmosis.
What happens as water leaves the cell by osmosis?
Ѱs starts to go up and Ѱp starts to go down.
What happens when Ѱp=0 (I.e 1atm like outside)?
At this point Ѱcell=Ѱs and the cell reaches incipient plasmolysis.
What is incipient plasmolysis?
This is the point at which Ѱp=0 and the membrane is just about to pull away from the cell wall.
Why is incipient plasmolysis important?
Because Ѱcell=Ѱs as Ѱp=0 so you can find a value for Ѱs.
What is the problem with using a cell at incipient plasmolysis to estimate Ѱcell?
As this event cannot be seen (as it is just about to happen), Ѱs must be estimated by percentage plasmolysis itself. By convention a solution which causes 50% plasmolysis is said to be one that causes incipient plasmolysis and is used to calculate Ѱs.
Describe an experiment to estimate Ѱs.
Place uniform onion epidermis samples in a range of sucrose (or other solute) solutions. After one our the sample is viewed and the number of cells plasmolised in a random field of view is expressed as a percentage plasmolysis. This produces a series of results which can be plotted against concentration and the molarity required to give 50% plasmolysis can be extrapolated.
