Osmosis and Water Potential

Spec Mapping: This lesson is mapped to OCR H420 Module 2.1.5 — Biological membranes (refer to the official OCR H420 specification document for exact wording). It develops osmosis as the movement of water down a water potential gradient, the formal water-potential equation $\Psi = \Psi_s + \Psi_p$ , and the responses of animal and plant cells to changes in external water potential.

Water is the most abundant molecule in living cells, and the movement of water across membranes is precisely controlled. In A-Level biology this movement is described using the formal language of water potential ( $\Psi$ ). This lesson develops the OCR H420 Module 2.1.5 content on osmosis as the movement of water down a water potential gradient.

1. What Is Osmosis?

Key Definition — Osmosis: The net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane.

Osmosis is a special case of diffusion — but because water is such a small polar molecule, and membranes contain aquaporins, it has its own distinct vocabulary.

Three features define osmosis:

It involves water molecules only.
It requires a partially permeable membrane (water passes, most solutes do not).
The direction is from higher to lower water potential.

Water movement in osmosis is a result of random thermal motion — no ATP is used. It is a passive process.

2. Water Potential — The Formal Language

In earlier years of secondary school you learned osmosis in terms of "concentrated" and "dilute" solutions. At A-Level we use water potential (symbol ψ, the Greek letter psi), measured in kilopascals (kPa). This is a more rigorous approach that accounts for both dissolved solutes and physical pressure.

Key Definition — Water Potential: The potential energy of water molecules in a solution compared with pure water at the same temperature and atmospheric pressure. Symbol ψ, units kPa.

Pure water at atmospheric pressure has a water potential of 0 kPa — this is the reference value.
Adding any solute lowers water potential because solute particles attract water and restrict its movement. Solutions always have a negative water potential (e.g. −400 kPa, −1000 kPa).
More negative = lower water potential = more concentrated solute.
Less negative (closer to zero) = higher water potential = more dilute solute.

Water always moves from higher (less negative) ψ to lower (more negative) ψ.

Exam Tip: When comparing, always say "higher (less negative) water potential" and "lower (more negative) water potential" — not "higher in number".

3. Components of Water Potential

Water potential is the sum of two components:

ψ = ψ_s + ψ_p

ψ_s (solute potential, sometimes osmotic potential) — always negative or zero, caused by solutes. The more solutes, the more negative.
ψ_p (pressure potential) — usually positive (pressure exerted by a cell wall on the cytoplasm) or zero, occasionally negative (e.g. xylem under tension).

3.1 Animal cells

Animal cells have no cell wall, so ψ_p ≈ 0 and water potential is driven almost entirely by solute content.

3.2 Plant cells

Plant cells have a strong cellulose cell wall. As water enters, the cell contents push against the wall, creating pressure potential (ψ_p). Pressure potential contributes to turgor, essential for plant support.

4. Osmosis in Animal Cells

Animal cell membranes cannot withstand much osmotic stress. If placed in solutions of different water potential, red blood cells (erythrocytes) behave as follows:

Solution type	Water potential	Net water movement	Result
Hypotonic (lower solute, higher ψ)	Higher ψ outside	Water moves into cell	Cell swells and bursts — haemolysis
Isotonic (same solute, same ψ)	Equal ψ	No net movement	Cell unchanged
Hypertonic (higher solute, lower ψ)	Lower ψ outside	Water moves out of cell	Cell shrinks — crenation

graph TD
  A[Red Blood Cell] --> B["Hypotonic solution<br/>psi outside higher<br/>water enters<br/>HAEMOLYSIS"]
  A --> C["Isotonic solution<br/>psi equal<br/>no change"]
  A --> D["Hypertonic solution<br/>psi outside lower<br/>water leaves<br/>CRENATION"]

Haemolysis is why intravenous fluids must be isotonic (0.9% saline, "physiological saline"). Pure water would cause red blood cells to burst within seconds.

Crenation gives the cell a wrinkled, shrivelled appearance because the cytoplasm pulls the membrane inward.

5. Osmosis in Plant Cells

Plant cells behave differently because of the cell wall. Four key terms apply:

5.1 Turgid

In a solution with higher water potential than the cell (e.g. pure water), water enters the cell by osmosis. The vacuole expands and the cytoplasm pushes against the cell wall. The cell becomes turgid — rigid and firm. The cell wall prevents bursting because it is strong and slightly elastic. Turgor pressure is essential for plant support, especially in herbaceous plants.

5.2 Flaccid

If water leaves until there is no net pressure on the wall, the cell becomes flaccid — limp but not yet damaged.

5.3 Plasmolysed (incipient plasmolysis and full plasmolysis)

If the cell continues to lose water (e.g. in a strongly hypertonic solution), the cytoplasm shrinks away from the cell wall as the vacuole contracts. The point at which the plasma membrane just begins to pull away from the wall is called incipient plasmolysis; when the cytoplasm is entirely pulled away it is fully plasmolysed. The cell wall does not collapse — the gap between wall and membrane fills with the external solution.

Plasmolysis is usually reversible if the cell is returned promptly to water; prolonged plasmolysis damages the cell.

5.4 Wilting vs plasmolysis

Wilting is the macroscopic drooping of a whole plant when large numbers of cells become flaccid. It does not necessarily involve plasmolysis — it can occur whenever water loss exceeds uptake.

Plant cell state	Condition	Appearance
Turgid	Higher ψ outside; water in	Firm, wall under tension
Flaccid	ψ equal; no net water	Limp; wall not pushed
Incipient plasmolysis	Lower ψ outside	Membrane starts to pull away
Plasmolysed	Lower ψ outside; water out	Cytoplasm pulls away from wall

6. Required Practical — Determining the Water Potential of Plant Tissue

A standard practical investigates the water potential of potato (or similar) tissue by placing chips in solutions of different known sucrose molarities and finding the one in which no mass change occurs.

6.1 Method

Cut potato chips of equal length, width and thickness using a cork borer and scalpel.
Blot dry each chip and record its initial mass.
Prepare sucrose solutions of 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 mol dm⁻³ in test tubes.
Place one chip (or a few of known mass) in each tube and leave for a fixed time (e.g. 30 minutes or 1 hour).
Remove, blot dry, and record the final mass.
Calculate % change in mass: ((final − initial) / initial) × 100.

6.2 Analysis

Plot % change in mass (y-axis) against sucrose concentration (x-axis). The line crosses zero at the sucrose concentration whose water potential matches that of the potato tissue. Use a reference table to convert this sucrose concentration to a water potential in kPa.

Positive % change = water entered (sucrose solution had higher ψ than the tissue). Negative % change = water left (sucrose solution had lower ψ than the tissue).

6.3 Sources of Error

Uneven blotting — some chips dried more than others.
Evaporation from tubes during incubation — seal with foil.
Surface area variation — use the same cork borer.
Short incubation — water may not reach equilibrium.
Temperature drift — use a water bath for consistency.

7. Common Exam Mistakes

Saying water moves from "high concentration to low concentration". Water moves from higher water potential to lower water potential — these are the opposite when discussing solutes.
Forgetting the negative sign — −100 kPa is higher than −400 kPa.
Using "isotonic/hypertonic/hypotonic" for plant cells. These terms are used for animal cells; for plants use turgid, flaccid, plasmolysed.
Saying plant cells "burst" in water. The cell wall prevents bursting — they become turgid.
Writing that "osmosis is the movement of solute particles". It is the movement of water.
Calling osmosis "the diffusion of water" without mentioning the partially permeable membrane.
Saying crenation means "the cell explodes". It means the cell shrivels.

8. Exam-Style Questions

Define water potential. Explain what happens to water potential when a solute is added to pure water. (3)
Describe what happens to a red blood cell placed in pure water. Explain your answer using water potential. (4)
A plant cell has a water potential of −500 kPa. It is placed in a solution of water potential −200 kPa. In which direction does water move, and what will happen to the cell? (3)
Explain why potato chips gain mass in pure water but lose mass in a concentrated sucrose solution. (4)

Model answer for (3): "The cell (−500 kPa) has a lower (more negative) water potential than the solution (−200 kPa). Water therefore moves from the solution into the cell by osmosis across the partially permeable plasma membrane. The cell will gain water, the vacuole will expand, and the cytoplasm will press against the cell wall, making the cell turgid."

Summary

Osmosis is the passive movement of water across a partially permeable membrane from higher to lower water potential.
Water potential (ψ) is measured in kPa, with pure water at 0 and solutions always negative.
ψ = ψ_s + ψ_p — the sum of solute potential (negative) and pressure potential (usually positive in plant cells).
Animal cells in hypotonic solutions undergo haemolysis; in hypertonic solutions they undergo crenation. They need isotonic surroundings.
Plant cells become turgid, flaccid or plasmolysed depending on external water potential. Turgor is essential for support.
The potato chip practical determines the water potential of plant tissue by finding the solution in which no mass change occurs.

9. The Water-Potential Equation in Full

The OCR H420 specification expects mastery of the water-potential equation and its sign conventions:

$\Psi = \Psi_s + \Psi_p$

where:

$\Psi$ is total water potential — what determines the direction of net water movement.
$\Psi_s$ is solute (osmotic) potential — always $\leq 0$ . The more solute, the more negative.
$\Psi_p$ is pressure potential — usually $\geq 0$ in turgid plant cells; can be slightly negative in xylem under tension during transpiration.

For pure water at atmospheric pressure: $\Psi_s = 0$ , $\Psi_p = 0$ , so $\Psi = 0$ . This is the reference state — every aqueous solution has $\Psi < 0$ .

9.1 Sign convention worked example

A plant cell with $\Psi_s = -800$ kPa and $\Psi_p = +300$ kPa has:

$\Psi = -800 + 300 = -500 \text{ kPa}$

Placed in a bathing solution of $\Psi = -200$ kPa, water moves from the solution into the cell (from higher $\Psi$ , $-200$ , to lower $\Psi$ , $-500$ ). As the cell takes up water, $\Psi_p$ rises and $\Psi_s$ becomes slightly less negative (dilution) until $\Psi_{cell} = \Psi_{solution} = -200$ kPa.

A-Level depth: Note that $\Psi$ rises (becomes less negative) as the cell takes up water — both because $\Psi_s$ dilutes and because $\Psi_p$ rises. The reverse is true if a cell loses water: $\Psi_s$ becomes more negative and $\Psi_p$ falls (eventually to zero at incipient plasmolysis, beyond which $\Psi_p = 0$ and further loss is driven by $\Psi_s$ alone).

Lesson 4: Osmosis and Water Potential