Water Potential

Tonicity: the ability of an extracellular solution to make water move into or out of a cell by osmosis
A solution’s tonicity is related to its osmolarity
Osmolarity: the total concentration of all solutes in the solution
Water potential: the force responsible for movement of water in a system; a measure of the free energy of water which is less when it is has to surround solutes
- Has the symbol psi
- Is measured in bars or megapascals

Solute potential (also called osmotic potential) џs: determined by solute concentration
- Solute potential: 0 or negative
Pressure potential џp: results from exertion of pressure on membranes/walls as water moves in or out; can be positive or negative
- Negative pressure: water being pulled

Positive pressure: seen when plant cell is in hypotonic solution

Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid.
When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall.
Due to the cell wall, pressure starts to build up inside the cell.
This has the tendency to stop more water entering the cell and also stops the cell from bursting.
When a plant cell is fully inflated with water, it is called turgid.
Pressure potential is called turgor pressure in plants

The water potential of pure water is given the value ZERO
Because pure water has the highest concentration of water molecules, and thus the highest water potential, the water potential of all other solutions must be lower than zero i.e. negative.
Adding solute decreases water potential!
The more solute there is present in a solution the more negative it becomes.
- So, solute potential will be a negative number if not pure water.
Hypertonic solutions have negative solute potentials.

Water moves from areas of higher water potential to areas of lower water potential (i.e. towards the more negative, concentrated region). \n