D2.3 Water Potential
This module covers the core concepts of water potential, osmosis, and turgor pressure in plant and animal cells according to the 2025 IB Biology syllabus. We explore the thermodynamic principles governing water movement and the mathematical quantification of these processes.
Osmosis and Water Potential
Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential. It is a passive process, relying entirely on the kinetic energy of the molecules. The partially permeable membrane allows the passage of water molecules (solvent) but restricts the movement of solute molecules.
Water Potential ($\Psi$) is a measure of the potential energy in water, driving its movement. Pure water at standard temperature and pressure has a water potential of exactly $0 \text{ kPa}$ (or MPa). When solutes are dissolved in water, the water potential becomes negative. Water always moves from a less negative (higher) water potential to a more negative (lower) water potential.
Application: Intravenous Fluids
In medical settings, intravenous (IV) fluids must have the same osmolarity as blood plasma to prevent red blood cells from undergoing lysis (bursting) or crenation (shrinking). Normal saline ($0.9\% \text{ NaCl}$) is isotonic to human tissues.
Components of Water Potential
The total water potential of a plant cell is determined by two main factors: solute potential ($\Psi_s$) and pressure potential ($\Psi_p$).
The equation is: $\Psi = \Psi_s + \Psi_p$
- Solute Potential ($\Psi_s$): Also known as osmotic potential. The addition of solutes (like glucose, C₆H₁₂O₆, or sodium chloride, NaCl) lowers the water potential, making $\Psi_s$ always negative in a solution.
- Pressure Potential ($\Psi_p$): Also known as turgor pressure in plant cells. As water enters a plant cell, the protoplast pushes against the rigid cellulose cell wall, generating a positive hydrostatic pressure. $\Psi_p$ is usually positive, though it can be zero in a flaccid cell or negative in xylem vessels (tension).
Skill: Calculating Water Potential
If a plant cell has a solute potential ($\Psi_s$) of $-800 \text{ kPa}$ and a pressure potential ($\Psi_p$) of $300 \text{ kPa}$, its overall water potential ($\Psi$) is:
$\Psi = -800 \text{ kPa} + 300 \text{ kPa} = -500 \text{ kPa}$
If this cell is placed in a solution with $\Psi = -400 \text{ kPa}$, water will enter the cell since $-400 \text{ kPa}$ is higher (less negative) than $-500 \text{ kPa}$.
Turgor Pressure and Plasmolysis
In a hypotonic environment (higher water potential outside), water enters the plant cell, increasing turgor pressure. The cell becomes turgid, which is essential for maintaining the structural integrity of non-woody plant tissues.
In a hypertonic environment (lower water potential outside), water leaves the cell. The protoplast shrinks and pulls away from the cell wall, a process called plasmolysis. The cell is then completely flaccid, and turgor pressure ($\Psi_p$) is zero.
Interactive Vocabulary
Hover over the cards to reveal the definitions.
Isotonic
A solution with the same water potential/osmolarity as the cell or tissue being compared.
Hypertonic
A solution with a lower (more negative) water potential than the cell, causing water to exit.
Hypotonic
A solution with a higher (less negative) water potential than the cell, causing water to enter.
Check Your Understanding
Question 1: What is the water potential of pure water at standard temperature and pressure?
The water potential of pure water is $0 \text{ kPa}$. It is the highest possible water potential; any addition of solutes will result in a negative water potential.
Question 2: A plant cell with $\Psi_s = -600 \text{ kPa}$ and $\Psi_p = 200 \text{ kPa}$ is placed in a beaker of pure water. What will happen?
The cell's initial water potential is $\Psi = -600 + 200 = -400 \text{ kPa}$. Since the pure water has $\Psi = 0 \text{ kPa}$, water will move into the cell from the beaker via osmosis (from higher to lower water potential). The cell's turgor pressure ($\Psi_p$) will increase until its $\Psi$ reaches $0 \text{ kPa}$.
Question 3: Why don't plant cells burst when placed in a hypotonic solution, unlike animal cells?
Plant cells have a rigid cellulose cell wall. As water enters and the protoplast expands, the cell wall exerts an inward pressure (turgor pressure, $\Psi_p$) that opposes further water influx, preventing the cell from bursting. Animal cells lack this cell wall and will undergo lysis.
