IB Biology · Theme D: Continuity and change · SL and HL
D2.3 Water potential
A one-page summary of D2.3 Water potential, the key terms to know, and sample exam questions with answers. For the full lesson, open the illustrated revision slides or practise in the app.
Guiding questions
What factors affect the movement of water into or out of cells?
How do plant and animal cells differ in their regulation of water movement?
What D2.3 covers
Water as a solvent, and the direction of osmosis
- D2.3.1Water dissolves by surrounding its solutes
- D2.3.2Water moves toward the more concentrated solution
- D2.3.3Predicting net water movement for a cell
Osmosis in tissues, cells and medicine
- D2.3.4Plant tissue gains or loses water in solutions
- D2.3.4Reading the data to find the isotonic concentration
- D2.3.5Cells without a wall swell, burst or shrink
- D2.3.5Contractile vacuoles bail the water out
- D2.3.5Multicellular animals keep their tissue fluid isotonic
- D2.3.6Cell walls turn water uptake into turgor
- D2.3.7Isotonic solutions protect cells in medicine
HL — water potential
- D2.3.8 · HLWater potential: potential energy per unit volume
- D2.3.9 · HLWater moves from higher to lower water potential
- D2.3.10 · HLSolute and pressure potentials add up in walled cells
- D2.3.11 · HLPotentials explain how plant tissue responds
D2.3 Water potential: summary
Water as the solvent
- Polar water surrounds ions (δ⁻ oxygen to cations, δ⁺ hydrogen to anions) and forms hydrogen bonds with polar solutes.
Osmosis and tonicity
- Water moves to the higher solute concentration. Hypertonic higher, hypotonic lower, isotonic equal, always comparative.
Cells without a wall
- Hypotonic: swell and may burst (haemolysis). Hypertonic: shrink and crenate. Freshwater cells use a contractile vacuole; animals keep isotonic tissue fluid.
Cells with a wall
- Hypotonic: turgor pressure builds. Hypertonic: plasmolysis. Tissue data give the isotonic concentration where change is zero; error bars show standard error.
Medicine
- 0.9% saline IV fluids and organ-bathing solutions are isotonic, so cells keep their normal volume.
HL · Water potential
- Ψw in kPa, pure water 0. Water moves from higher to lower Ψw. Ψw = Ψs + Ψp; Ψs ≤ 0, Ψp positive in cells, negative in xylem.
Key terms
- Solvation
- The dissolving of a solute in water, as water molecules gather around it by hydrogen bonding and attraction between charges.
- Osmosis
- The net movement of water across a selectively permeable membrane from lower to higher solute concentration.
- Hypertonic / hypotonic / isotonic
- Comparative terms for a solution with a higher, a lower, or an equal solute concentration than another solution.
- Dynamic equilibrium
- The state in an isotonic solution where water crosses the membrane both ways at equal rates, so there is no net movement.
- Haemolysis
- The bursting of red blood cells after they take up too much water in a hypotonic solution.
- Crenation
- The shrivelled, spiky appearance of an animal cell that has lost water in a hypertonic solution.
- Contractile vacuole
- An organelle in freshwater single-celled organisms that collects excess water and expels it from the cell.
- Turgor pressure
- The pressure that builds up when the protoplast of a walled cell presses against the cell wall in a hypotonic solution.
- Plasmolysis
- The shrinking of a walled cell’s protoplast away from its cell wall in a hypertonic solution.
- Water potential (Ψw)
- The potential energy of water per unit volume, relative to pure water at atmospheric pressure and 20 °C, which is 0 kPa; water moves from higher to lower Ψw.
- Solute potential (Ψs)
- The part of water potential due to dissolved solutes; always zero or negative.
- Pressure potential (Ψp)
- The part of water potential due to pressure; generally positive in walled cells, negative in xylem vessels under tension.
- Turgid cell
- A walled cell with a positive Ψp, pressed firmly against its wall; its Ψw is closer to zero than its Ψs.
- Flaccid cell
- A walled cell with no turgor pressure: Ψp is zero, so Ψw equals Ψs.
- Incipient plasmolysis
- The point at which the protoplast just begins to pull away from the wall: Ψp is zero, so Ψw equals Ψs.
Sample exam questions
Three of the 45 multiple-choice questions for D2.3. Try each one before opening the answer.
Question 1. Why can water dissolve both ionic solutes, such as NaCl, and polar solutes, such as glucose?
- Water molecules fit between all solute particles.
- Polar water attracts ions and forms hydrogen bonds.
- Water has a net positive charge.
- Water reacts chemically with all solutes.
Show the answer
Answer: B. The polar water molecule is attracted to both positive and negative ions and forms hydrogen bonds with polar groups.
Question 2. In the hydration shell around a dissolved Cl⁻ ion, which part of each water molecule faces the ion?
- The δ⁻ oxygen atom
- The non-polar bonds
- No preferred orientation
- The δ⁺ hydrogen atoms
Show the answer
Answer: D. The negative ion attracts the slightly positive hydrogen atoms.
Question 3. A strongly hypotonic fluid is given intravenously by mistake. What is the risk to the patient’s red blood cells?
- Shrinkage and crenation
- No effect on blood cells
- Swelling and bursting
- Immediate clotting
Show the answer
Answer: C. Water enters the red cells by osmosis; they swell and may burst (haemolysis).
Linking questions
Questions that connect D2.3 to other parts of the course, the kind that come up in Paper 2.
- Water is a polar molecule that forms hydrogen bonds (A1.1.2) and is an effective solvent (A1.1.5). Explain why a polar solute such as glucose or an ion such as Na⁺ dissolves in water, whereas a non-polar hydrocarbon does not, and outline one consequence of this difference in solubility for living organisms. (see A1.1)
- Osmosis (B2.1.5) is the movement of water across a selectively permeable membrane, and the direction of net movement depends on solute concentration. Explain how the membrane’s permeability to water but not to solutes, and the presence of aquaporins, determine the direction and rate of osmosis. (see B2.1)
- Tissue fluid is formed by pressure filtration of plasma and exchanges substances with cells (B3.2.11, B3.2.12). Explain why the concentration of solutes in tissue fluid must stay close to that inside the cells it bathes, and predict what would happen to the cells if it did not. (see B3.2)
- What variables influence the direction of movement of materials in tissues? Using osmosis in plant tissue as an example, discuss how the solute concentration of the surrounding solution, and the presence of a cell wall, affect whether water enters or leaves a cell and what happens to the cell. (see B2.1, B3.2)
Practise D2.3
Study notes, every question and full markschemes for D2.3 are in the app with Pro. Two lessons are completely free to try: A1.1 Water and B1.1 Carbohydrates and lipids.