Osmosis, Tonicity & Water Potential

You’ll be able to

Define osmosis and predict water movement across a selectively permeable membrane
Classify solutions as hypertonic, hypotonic, or isotonic and predict effects on cells
Use the water-potential equation to determine the direction of water movement

Osmosis: the diffusion of water

Osmosis is the passive movement of water across a selectively permeable membrane, from a region of higher water concentration (lower solute concentration) to a region of lower water concentration (higher solute concentration). Water is small and polar; some crosses the bilayer directly and much more passes through channel proteins called aquaporins. No ATP is needed — water simply moves down its own concentration gradient until the solute concentrations on both sides are balanced (or until pressure stops it).

Tonicity: comparing two solutions

Tonicity describes how a surrounding solution affects a cell’s water balance, always by comparing solute concentration outside versus inside. A hypertonic solution has more solute than the cell, so water leaves and the cell shrinks (an animal cell shrivels/crenates; a plant cell plasmolyzes). A hypotonic solution has less solute, so water rushes in and the cell swells (an animal cell may burst/lyse; a plant cell becomes firm and turgid — the healthy state, held by its wall). An isotonic solution has equal solute, so water moves in and out equally and the cell holds steady.

Water potential

Ψ = Ψp + Ψs

Ψ is water potential; Ψp is pressure potential and Ψs is solute potential. Water always moves from higher Ψ to lower Ψ. Pure water at atmospheric pressure has Ψ = 0; adding solute makes Ψs (and thus Ψ) negative.

Water potential: predicting the direction

Water potential (Ψ), measured in units of pressure (bars or MPa), predicts which way water will move: water always flows from higher Ψ to lower Ψ. It has two parts. Solute potential (Ψs) is always zero or negative — adding solute lowers Ψ, because solute "ties up" water. Pressure potential (Ψp) is the physical pressure on the solution; it is positive when a cell wall pushes back (turgor) and can be negative under tension. Comparing the total Ψ of a cell and its surroundings tells you exactly where water goes.

Worked example

A plant cell has a water potential of Ψ = −0.8 MPa. It is placed in a solution with Ψ = −0.3 MPa. Which way does water move, and what happens to the cell?

1.Water always moves from higher (less negative) water potential toward lower (more negative) water potential.
2.Compare the values: the solution is at −0.3 MPa and the cell is at −0.8 MPa. Since −0.3 is higher than −0.8, the solution has the higher Ψ.
3.Therefore water moves from the solution (−0.3) into the cell (−0.8).
4.Water entering the cell makes it gain water and swell; against its wall it becomes turgid.

Answer: Water moves from the solution (Ψ = −0.3 MPa) into the cell (Ψ = −0.8 MPa), because water flows toward lower water potential. The cell takes up water and becomes turgid.

Checkpoint

A red blood cell is placed in a hypotonic solution (lower solute concentration than the cell). What happens?

Watch out

Remember that "hyper-" and "hypo-" refer to solute, and water goes the opposite way — toward the higher solute. In a hypertonic solution water moves out; in a hypotonic solution water moves in. Track the water, not just the label.

Checkpoint

Cell A has a water potential of Ψ = −0.5 MPa and neighboring cell B has Ψ = −0.9 MPa. In which direction will water flow between them?

On the exam

On free response, state the rule explicitly — "water moves from higher Ψ to lower Ψ" — then plug in the numbers, remembering that a less negative value is the higher potential. Adding solute makes Ψs more negative and lowers Ψ, pulling water in.

Answer the 2 checkpoints as you read.