Preview
IB DP Biology — D: Continuity and Change | SL | D2.3 Water Potential
📋 What You Will Learn
- Identify the components of water potential and explain how each component affects the direction of water movement
- Explain osmosis in terms of water potential gradients across selectively permeable membranes
- Describe the processes of plasmolysis, turgor, and flaccidity in plant cells and relate them to water potential
- Analyse experimental data from osmosis investigations (e.g. change in mass or length of plant tissue) to determine the water potential of cells
- Apply water potential concepts to explain water movement through the plant — from soil to root to leaf to atmosphere
🔍 Guiding Question How does the movement of water through a plant depend on differences in water potential between cells, tissues, and the environment?
🌍 Why This Matters A wilting plant on a hot afternoon is not dying of thirst — it is losing the battle of water potential. Every litre of water that moves from soil to leaf tip, defying gravity, does so because of a precisely maintained gradient of water potential across hundreds of millions of cells. Understanding this gradient is the key to understanding how plants survive drought, how crops are irrigated, and how a single mechanism connects soil chemistry to the opening of a stomatal pore.
What Is Water Potential?
Here is the problem every plant cell has to solve: water must move in a precise direction — into roots, up stems, out through leaves — without any pumps or motors. The solution is elegantly physical. Water moves down a gradient of free energy, and the measure of that free energy in water is called water potential.
Before exploring how water moves, we need the language to describe it. Water potential is not an intuitive concept — it is a thermodynamic quantity — but its biological consequences are entirely predictable once you understand its two components.
📖 Key Definition — Water Potential (Ψ) Water potential is the free energy per unit volume of water in a system, measured in kilopascals (kPa). Water always moves by osmosis from a region of higher water potential to a region of lower (more negative) water potential across a selectively permeable membrane. Pure water at atmospheric pressure has a water potential of 0 kPa — the maximum possible value. All solutions have a water potential below zero (negative values).
📖 Key Definition — Solute Potential (Ψ s ) Solute potential (also called osmotic potential) is the component of water potential due to dissolved solutes. Adding solutes to water reduces the free energy of water molecules — solute particles attract water molecules, reducing their tendency to move. Solute potential is always zero or negative: the more solutes dissolved, the more negative the solute potential.
📖 Key Definition — Pressure Potential (Ψ p ) Pressure potential is the component of water potential due to physical pressure acting on water. In a turgid plant cell, the cell wall pushes inward on the cell contents, generating a positive pressure potential (turgor pressure) that raises water potential. In xylem vessels under tension, pressure potential can be negative, lowering water potential further.
Water Potential Equation
\[ \Psi = \Psi_s + \Psi_p \]
Where \(\Psi\) is total water potential, \(\Psi_s\) is solute potential (always ≤ 0), and \(\Psi_p\) is pressure potential (positive in turgid cells, negative in xylem under tension, zero in a flaccid cell or open solution).