IB Biology HLTopic 1 — Life’s Building Blocks & VarietyPaper 1 & 2Core idea~9 min read
Water Potential
Water potential is osmosis with a number attached. Instead of arguing about which solution is “more concentrated”, you give each one a value in kPa and let the numbers tell you which way the water goes. The only thing that trips people up is that almost every value is negative.
📚 What you need to know
Water potential (Ψ) is the potential energy of water, per unit volume, relative to pure water.
The unit is the kilopascal (kPa).
Pure water at atmospheric pressure and 20 °C is given a value of 0 kPa. Everything is measured against it.
Adding solutes lowers water potential into negative values. A concentrated solution has a lower (more negative) water potential.
Energy is stored in the hydrogen bonds between water and solute molecules, so less energy is left as potential energy for movement.
Water moves from high water potential to low water potential — that is, from less negative to more negative.
This is the same rule as “water moves from dilute to concentrated”, just written in energy terms.
What water potential actually measures
Potential energy is energy stored in something because of its position relative to other things. Here it is the energy stored in water molecules because of their position relative to other molecules, especially dissolved solutes.
Water potential is a measure of the tendency of water molecules to move. High water potential means the molecules have plenty of potential energy for movement and a strong tendency to move away. Low water potential means the opposite.
The definition, word for word
Water potential is the potential energy of water, per unit volume, relative to pure water
Why pure water is zero
Water molecules in pure water do have potential energy — it is just impossible to measure in absolute terms. So pure water is defined as 0 kPa and everything else is compared with it. It is a reference point, chosen for convenience, not a claim that pure water has no energy.
Why solutes make it negative
Here is the chain of reasoning, and it is worth learning as a chain. Solute molecules dissolve and bind to water molecules through hydrogen bonds. Energy is stored in those bonds. That energy is therefore no longer available to the water molecules as potential energy for movement. So the more solute you add, the less potential energy the water has, and the lower the water potential becomes — dropping below zero into negative values.
Negative numbers confuse people here, so keep it concrete. −200 kPa is a higher water potential than −900 kPa, in exactly the same way that −2 °C is warmer than −9 °C. Water would move from the −200 solution towards the −900 one.
Left is low water potential, right is high. Water always travels leftwards along this line.
Which way does water move?
Water potential describes the tendency of water molecules to move from a dilute solution to a solution with a high solute concentration. Look at it from both ends:
A solution with a high water potential contains water molecules with greater potential energy for movement, so they have a greater tendency to move.
A solution with a low water potential has many hydrogen bonds between water and solute molecules, which reduces that potential energy and therefore the tendency to move.
So the net movement is always from high water potential to low water potential. The same sentence can be written three ways, and all three are correct:
Way of saying it
Water moves from
Water moves to
Water potential
Higher water potential
Lower water potential
Potential energy
Higher potential energy
Lower potential energy
Solute concentration
Lower solute concentration
Higher solute concentration
Check yourself with a sanity test. Water should always end up where the solute is. If your answer has water leaving a concentrated solution for a dilute one, you have flipped a sign somewhere.
Worked examples
WORKED EXAMPLE
A cell has a water potential of −450 kPa. It is placed in a solution with a water potential of −300 kPa. State and explain the direction of net water movement. [3]
Step 1 — compare the values−300 kPa is higher (less negative) than −450 kPa
So the solution has the higher water potential.
Step 2 — apply the rule
Water moves from high water potential to low water potential.
Step 3 — state the direction
There is a net movement of water from the solution into the cell, so the cell gains water by osmosis.
Water moves into the cellThe solution is hypotonic to the cell — the two ways of describing it agree.
WORKED EXAMPLE
Explain why adding solute to pure water lowers its water potential. [3]
Step 1 — what forms
Solute molecules form hydrogen bonds with the water molecules as they dissolve.
Step 2 — where the energy goes
Energy is stored in these bonds, so less energy remains available to the water molecules as potential energy for movement.
Step 3 — the consequence
The water potential therefore falls below the 0 kPa of pure water and becomes negative; the more solute added, the more negative it becomes.
3 marks“There is less water” is not the explanation and scores nothing. The marks are for the energy argument.
💡 Exam tip
Write the unit (kPa) and the sign on every value. A missing minus sign changes the answer completely.
Learn the definition in full: potential energy of water, per unit volume, relative to pure water. All three parts are marking points.
Pure water is 0 kPa, the highest value any water can have.
Use “higher” and “lower” rather than “bigger” and “smaller” when comparing negatives — it avoids confusing yourself.
If you are unsure of a direction, translate the numbers into “dilute” and “concentrated” and check the answer still makes sense.
Water potential is about the tendency of water to move, so link every explanation back to potential energy.
⚠ Common mix-up
−900 kPa is not higher than −200 kPa. More negative means lower.
Water does not move towards high water potential. It moves away from it.
A solution cannot have a positive water potential from solutes alone. Only pressure can push a value above zero.
Water potential is not the same as solute concentration. They are inversely related, not identical.
Pure water being 0 kPa does not mean it has no energy. It is a defined reference point.
Do not use mol dm-3 for water potential. The unit is kPa.
Up next: Water Movement in Plant Tissue — splitting water potential into its two parts, solute potential and pressure potential, and using them to explain turgor.
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