Biology HL Paper 2 (May 2025, TZ2) · Question 4
Mixed-topicMatching part: 4(b)
Also tests: 8.2 Plant Transport
Practice this question →6.3 Water Potential and Osmotic Systems
Calculate water potential and predict animal- and plant-cell responses.
Estimated time: 76 minutes
IB syllabus: D2.3 · SL and HL
Potential Predicts Net Water Movement
HL extensionD2.3
Osmosis is the net movement of water from higher water potential to lower water potential through a selectively permeable membrane. Pure water at standard reference conditions has ψ = 0; dissolved solute lowers free energy and makes solute potential ψs negative. Pressure potential ψp is commonly positive inside a turgid plant cell. The components combine as ψ = ψs + ψp, usually expressed in kPa or MPa.
Tonicity compares a solution with a particular cell and depends on effectively non-penetrating solutes. A hypotonic environment has higher water potential and produces net water entry; a hypertonic environment produces net loss; isotonic conditions give no net volume change. Individual water molecules still cross at isotonic equilibrium.
Walls Transform Entry into Pressure
An animal cell in a strongly hypotonic solution swells and may lyse because the plasma membrane cannot resist unlimited expansion. In a hypertonic solution it loses water and crenates. Medical replacement fluids are formulated near physiological osmotic conditions so cells do not undergo dangerous volume changes, though real clinical solutions must also match appropriate ions and chemistry.
A plant cell in a hypotonic environment gains water. The vacuole expands, the protoplast presses against the wall and positive pressure potential rises until internal and external water potentials match. The cell becomes turgid. In a hypertonic solution, water loss reduces pressure, the cell becomes flaccid and the plasma membrane may pull away from the wall in plasmolysis. The wall prevents bursting but does not prevent water movement.
Experiments Locate an Isotonic Point
Potato cylinders can be placed in a concentration series, blotted consistently and reweighed. Percentage mass change controls for small differences in starting mass. Replicates reveal random variation and support a mean. The x-intercept of percentage mass change against concentration estimates the external solution producing no net water movement; it does not prove that every cell has identical solute concentration.
HL extensionD2.3
At equilibrium, equal total water potentials—not necessarily equal solute concentrations—stop net movement. A walled cell may have a more negative solute potential than its surroundings but an offsetting positive pressure potential. This is why comparing concentration alone can fail when hydrostatic pressure differs.
Water-potential chamber
Set solute and pressure potentials and compare plant-cell turgor with an unsupported animal cell.
Exchange · gradients · inheritance
Cell function laboratory
Test Yourself
A plant cell has ψs = −950 kPa and ψp = +420 kPa. What is its total water potential?
Test Yourself
A turgid plant cell has ψs = −1.1 MPa and ψp = +1.1 MPa and is placed in pure water at 0 MPa. What happens initially?
Exam questions on this topic
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