IB Biology HLTopic 1 — Life’s Building Blocks & VarietyPaper 1 & 2Core skill~11 min read
Water Movement in Plant Tissue
A plant cell has something an animal cell does not: a wall that pushes back. That means its water potential is made of two competing parts — one pulling water in, one pushing it out. Add them together and you can predict exactly what the cell will do.
📚 What you need to know
Water potential is written Ψ or Ψw and has two components in a plant cell.
Solute potential (Ψs), also called osmotic potential, is the effect of dissolved solutes. Pure water has a solute potential of zero; adding solutes makes it more negative.
Pressure potential (Ψp) is the hydrostatic pressure on the water. Inside a plant cell it is usually positive — this is turgor pressure.
Pressure potential can be negative in xylem vessels, where water is pulled up under tension.
The two add together: Ψw = Ψs + Ψp
In a hypotonic solution water enters, Ψp rises, and the cell becomes turgid. Inflow stops when the water potentials inside and outside are equal.
In a hypertonic solution water leaves, Ψp falls, the cell loses turgor and the plant wilts.
The two parts of water potential
Solute potential
Solute potential is simply the effect that dissolved solutes have on water potential. Pure water, with nothing dissolved in it, has a solute potential of zero. As solutes are added, solute potential decreases and becomes more negative.
The mechanism is the one from the previous page. Solute molecules bind to water molecules through hydrogen bonds as they dissolve, the potential energy available in the water is transferred into those bonds, and that reduction in potential energy lowers the water potential. Provided pressure potential stays constant, a fall in solute potential means a fall in water potential.
Pressure potential
Pressure potential is the hydrostatic pressure that the water is under. In a plant cell it is normally positive, because the expanding cytoplasm and vacuole press outwards on the inside of the cell wall. That is turgor pressure, and it is what gives plant tissue its support.
It is not always positive, though. In xylem vessels, water and dissolved minerals are pulled upwards under tension, and pressure potential there is negative.
The equation to memorise
Ψw = Ψs + Ψp
Notice it is a sum, not a subtraction. Because Ψs is negative and Ψp is usually positive, the two work against each other — the solutes pull water in, the pressure pushes it back out.
Following one cell through
Take a plant cell and drop it into pure water. Work through the three snapshots below and the whole idea of turgor falls out of the arithmetic.
The solute potential never changed. All that happened was that pressure built up until it balanced it exactly.
Read across the three panels. In pure water both components are zero, so the total water potential is 0 kPa. The cell placed in that water has solutes in its cytoplasm, giving a solute potential of −600 kPa, and no pressure yet, so its water potential is −600 kPa — lower than the surroundings, so water moves in.
As water enters, the protoplast presses on the wall and pressure potential climbs. When it reaches +600 kPa, the sum is −600 + 600 = 0 kPa. The cell now has the same water potential as the pure water outside, there is no gradient, and the net inflow stops. The cell is turgid.
Plant tissue in hypotonic and hypertonic solutions
In a hypotonic solution
The cytoplasm contains dissolved substances that lower the solute potential of the cell, which lowers the water potential inside it. Water therefore moves from the surrounding solution into the cell, down the water potential gradient.
The inward movement increases the volume of the cytoplasm, so pressure potential increases as the cytoplasm presses on the cell wall. Eventually pressure potential rises to the point where the water potential is equal inside and outside, and the inward movement stops. Cells in this state are turgid and provide structural support to the plant.
In a hypertonic solution
Now the surrounding solution has the lower solute potential, which gives it the lower water potential. Water moves out of the cell down the water potential gradient.
Losing water reduces the volume of the cytoplasm, so pressure potential inside the cell decreases. The cells lose turgor pressure and the plant begins to wilt. Push it far enough and the protoplast pulls away from the wall — plasmolysis.
Surrounding solution
What happens to Ψp
State of the tissue
Hypotonic (dilute)
Increases as water enters
Turgid and supportive
Isotonic
Stays as it is
No change in volume
Hypertonic (concentrated)
Decreases as water leaves
Flaccid, then plasmolysed
Why plants wilt on a hot day. Water is lost from the leaves faster than the roots can replace it, so cells lose turgor. Without turgor pressure the tissue has nothing holding it rigid, and the whole plant droops.
Worked examples
WORKED EXAMPLE
A plant cell has a solute potential of −950 kPa and a pressure potential of +420 kPa. Calculate its water potential. [2]
Step 1 — write the equationΨw = Ψs + ΨpStep 2 — substitute and addΨw = (−950) + (+420) = −530Ψw = −530 kPaThe cell is not yet fully turgid — if it were, the two values would cancel to 0 kPa.
WORKED EXAMPLE
Explain, in terms of water potential, what happens when plant tissue is placed in a concentrated sucrose solution. [4]
Step 1 — compare the two water potentials
The concentrated solution has a lower (more negative) solute potential, and therefore a lower water potential than the cell cytoplasm.
Step 2 — direction of movement
Water moves out of the cell, down the water potential gradient, through the partially permeable cell surface membrane.
Step 3 — effect inside the cell
The volume of the cytoplasm falls, so the pressure potential decreases and the cell loses turgor pressure.
Step 4 — the visible result
The tissue becomes flaccid; if enough water is lost the protoplast pulls away from the cell wall and the cell is plasmolysed.
4 marksAnswer in the language the question uses. It asked for water potential, so lead with potentials, not with “concentration”.
💡 Exam tip
Write the equation Ψw = Ψs + Ψp before substituting. It is often worth a method mark on its own.
Put brackets round negative values when you substitute, so you do not lose a sign.
Ψs is never positive. Ψp is usually positive inside cells but negative in xylem.
A fully turgid cell has a water potential of 0 kPa in pure water, because the two components cancel out.
Match your vocabulary to the question. If it says water potential, answer in kPa and potentials, not in tonicity.
Always finish plant answers with the observable state: turgid, flaccid, plasmolysed, wilting.
⚠ Common mix-up
Do not subtract the solute potential. The formula is a sum; Ψs is already negative.
Solute potential does not change when a cell becomes turgid. Pressure potential is what rises.
Pressure potential is not always positive. Xylem under tension has a negative value.
Water potential is a property of the solution, not of the membrane. The membrane only decides what can cross.
Turgid is not the same as plasmolysed. They are opposite ends of the same process.
Equal water potential does not mean water has stopped moving. It means there is no net movement.
Up next: Membrane Transport — the other ways substances cross a membrane, once water has had its turn.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
