Osmosis and Diffusion: Examples, Tonicity, and Practice

Cells and Transport Teaching Resources · High school biology; AP extensions where noted.

Diffusion describes net movement of particles down a concentration gradient due to random molecular motion. Osmosis concerns water movement across a selectively permeable membrane. Osmosis is not the movement of food or wastes into a cell.

Objectives and prerequisites

Students will distinguish diffusion from osmosis, predict initial water movement in a stated situation, and explain dynamic equilibrium. First review solute, solvent, concentration, and selective permeability. Materials: paper and pencil; no lab materials are required for the examples.

Diffusion: separate motion from net movement

Imagine two connected compartments containing a gas at different concentrations. Molecules move in both directions. Initially, more cross from the concentrated side than in the reverse direction, producing net movement down the gradient. At equilibrium the rates balance, but molecular motion continues. Diffusion can occur without a membrane; a membrane adds a permeability constraint.

Osmosis: state the conditions

For a simple classroom prediction, assume equal initial pressure, a membrane permeable to water, and solutes that cannot cross. Water then moves net toward the side with more dissolved nonpenetrating particles. In a fuller explanation, net water movement follows a water-potential difference, and pressure can oppose the solute effect. A cell wall is not the selectively permeable plasma membrane.

Tonicity is a comparison

• Hypotonic outside solution: lower effective nonpenetrating solute concentration than the cell; net water entry tends to increase cell volume.
• Hypertonic outside solution: higher effective concentration; water loss tends to reduce cell volume.
• Isotonic outside solution: no sustained change in cell volume from net water movement under the stated conditions. Water still crosses in both directions.

An animal cell may swell or lyse in a sufficiently hypotonic environment. A plant wall resists expansion, allowing turgor pressure to develop. In a sufficiently hypertonic environment, a plant cell can lose water and its membrane can pull away from the wall: plasmolysis. Equal total solute concentration is not always isotonic when solutes differ in permeability.

Three original practice problems

1. A cell has 0.15 M nonpenetrating solute; the surrounding solution has 0.30 M. Assume water permeability and equal initial pressure. Predict initial net water movement and classify the surroundings.
2. A permeable divider separates regions with different oxygen concentrations. What happens to oxygen at equilibrium: do the molecules stop?
3. Two solutions have equal total solute concentration, but one contains a solute that readily enters the cell. Why might their effects on cell volume differ?

Answer explanations

1: water initially moves net out; the surroundings are hypertonic relative to this cell. 2: molecules continue crossing; opposing rates balance at equilibrium. 3: a penetrating solute can redistribute, changing the effective osmotic difference. Tonicity depends on membrane permeability as well as concentration.

Common misconceptions

Facilitated diffusion is not a backup used only when osmosis stops; different substances can move by different mechanisms simultaneously. Reverse osmosis requires applied pressure and should not be presented as normal spontaneous cellular water movement. Mixing cream into hot coffee also involves bulk fluid motion, so it is not a clean demonstration of diffusion alone.

Teaching note: Have students draw equal arrows in both directions at equilibrium. For AP, ask how increasing pressure on one side changes a water-potential prediction before introducing numerical calculations.

Continue the unit

• Connect gradients to membrane transport
• Investigate water movement using shell-less eggs
• Check predictions across membranes

Sources and further reading

• OpenStax Biology 2e: passive transport