Diffusion & Osmosis

Plan a concentration comparison

Predict whether potato tissue will gain or lose mass in two sucrose solutions. Keep core size, temperature, and soaking time constant. Record concentration, starting mass, final mass, and percentage mass change; plot the last quantity against concentration. The ideal 22°C, 24-hour cases below change sign between 0.2 M and 0.4 M. Explain how that sign change helps locate a no-net-change condition without claiming that water molecules stop moving.

Then connect the scales: use dialysis tubing to distinguish solute passage from water movement, and use onion-cell observations to relate a membrane response to the surrounding cell wall. These are three different evidence sources, not three interchangeable measurements of mass.

If the immediate task is simply predicting a before-and-after mass change, LabXchange's free Egg Lab is another option: its official page describes egg weighing, prediction feedback, and an analysis worksheet (checked October 15, 2026). This course instead uses dialysis tubing, potato tissue, and onion cells for the comparisons described here. Choose the setup that matches your question; free resources can also include guidance and feedback. Continue with the AP Biology collection.

What you will investigate

This diffusion and osmosis virtual lab connects molecular movement across a membrane with indicator evidence, tissue mass changes, and cell appearance. Compare which substances can cross the modeled dialysis membrane, investigate potato tissue in solutions of different concentration, and examine how a cell wall changes the response of a plant cell. The comparisons distinguish movement of water from movement of a particular solute.

Diffusion results from random motion even when there is a net movement down a concentration difference. At equilibrium, molecules do not stop crossing in both directions. A stable concentration or mass reading therefore needs an explanation of balanced processes, rather than a claim that all molecular motion has ended.

Water potential and a controlled tissue comparison

The course uses Ψ = ΨP + ΨS and ΨS = −iCRT. The pressure and solute contributions must be considered together when predicting the direction of net water movement. The dissociation factor matters: equal molar concentrations of sucrose and the course's sodium-chloride model do not produce the same solute-potential calculation.

Ideal potato-core setup at 22°C, after 24 h	Percentage mass change
Sucrose 0 M	+17.88%
Sucrose 0.2 M	+5.37%
Sucrose 0.4 M	−5.90%
Sucrose 1.0 M	−23.96%

These are the defined ideal model values, not measured results from any potato selected by a student. The sign indicates gain or loss relative to starting mass. Percentage change makes comparisons between samples with different starting masses more interpretable than comparing their final masses alone. Keep exposure time attached to the result; a shorter run need not have reached the equilibrium comparison in the table.

Find the isotonic comparison from evidence

Plot percentage mass change against solution concentration and identify the crossing near no net mass change. The model's isotonic sucrose concentration is 0.3002 M for the specified tissue. Treat this as a property of the defined tissue model and conditions, not a universal concentration for every plant tissue.

A zero crossing and a single sample with an apparently unchanged balance reading are not equally strong evidence. Instrument resolution, handling, or measurement error can hide a small change. Repeated concentration comparisons and a graph can support a more informative interpretation. The course also investigates how sample size changes the approach to equilibrium through the surface-area-to-volume relationship.

Membrane evidence and cell-wall effects

Indicator comparisons distinguish the membrane's modeled permeability to small substances from its exclusion of larger substances. A color change identifies the relevant chemical evidence; it does not imply that every dissolved molecule must have crossed in the same direction. Track water and solute separately when interpreting mass. The course even includes a comparison where gaining water and losing total mass can occur together because material moves in more than one way.

For plant cells, the cell wall and pressure contribution constrain swelling. A hypertonic comparison can produce plasmolysis, while an animal-cell comparison has a different structural boundary. Do not describe the plant cell wall as a barrier that prevents all water movement. Its role in the pressure balance differs from the membrane's permeability.

Standards and classroom discussion

The main catalog alignment is AP Biology Unit 2 and its diffusion/osmosis investigation. Ask students to support a water-movement prediction with a potential comparison, a percentage-mass graph, and a cell explanation. Evaluate handling and measurement limitations before treating a small difference as a biological conclusion. Continue with photosynthesis leaf disks or enzyme activity.

Build an explanation from your records

Make a prediction before changing the conditions, then record a comparison that actually tests it. Keep the conditions attached to each row: a number without its setup can be misleading even when the arithmetic is correct. Explain what changed, what stayed fixed, and which observation supports your conclusion. If the result differs from your prediction, revise the explanation using the evidence rather than choosing a new answer without a reason.

The reference examples on this page describe the course model or its stated source data. They are not measurements collected from a physical laboratory by the reader. Compare like conditions, keep units visible, and distinguish rounded instrument displays from underlying model quantities. Use the investigation's stated limits when deciding whether a result can be generalized beyond the simulated setup. This guide supports preparation and interpretation; practical laboratory technique and safety training remain separate learning activities.