Osmosis and 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 (ψ). This lesson covers OCR A-Level Biology A specification point 2.1.5 (e)(ii) — 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.

Lesson 4: Osmosis and Water Potential