Passive Transport: Simple Diffusion, Facilitated Diffusion, and Osmosis

Active learning works for this topic because students often confuse the three types of passive transport and their conditions. Hands-on labs and graphing tasks let them see how concentration gradients, molecule size, and protein channels shape real movement across membranes.

• JC 1
• Biology
• 4 activities
• 20 min–50 min

Learning Objectives

1. Compare and contrast the mechanisms of simple diffusion and facilitated diffusion, including their reliance on concentration gradients and protein carriers.
2. Calculate the direction and magnitude of water movement between plant cells and their external environment using water potential equations.
3. Analyze experimental data from plasmolysis experiments to determine the solute potential of plant cells at incipient plasmolysis.
4. Evaluate the assumptions and limitations of using plasmolysis experiments to determine solute potential.
5. Explain the role of specific membrane transport proteins, such as GLUT transporters, in the facilitated diffusion of glucose.

Problem-Based Learning

⏱ 45 min·Pairs

Lab Demo: Plasmolysis in Onion Cells

Peel onion epidermis, mount on slides, add distilled water then hypertonic sucrose solutions (0.2M to 1M). Observe under microscope, sketch stages, measure % plasmolysis at incipient point. Calculate ψs using ψw = 0.

Prepare & details

Differentiate between simple diffusion and facilitated diffusion in terms of mechanism, energy requirement, and saturation kinetics, using the contrast between oxygen diffusion and glucose transport via GLUT transporters as examples.

Facilitation Tip: During the Plasmolysis in Onion Cells lab, remind students to focus on the cell wall’s role in limiting expansion while predicting water loss.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Stations Rotation

⏱ 50 min·Small Groups

Stations Rotation: Diffusion Types

Stations include: simple diffusion (ink in agar), facilitated (model with beads on slotted board), osmosis (potato cylinders in salt), control (no movement). Groups rotate, record rates, discuss saturation.

Prepare & details

Apply water potential equations to predict the direction and magnitude of osmotic water movement between plant cell compartments with defined solute and pressure potentials.

Facilitation Tip: For Station Rotation, assign each group one diffusion type and have them present a 1-minute summary of key features before rotating.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 30 min·Pairs

Graphing: Saturation Kinetics

Provide datasets for O2 (simple) vs glucose (facilitated) uptake rates at varying concentrations. Pairs plot graphs, identify Vmax/Km differences, compare to real GLUT data.

Prepare & details

Analyse data from a plasmolysis experiment to determine the solute potential of a plant cell at incipient plasmolysis and evaluate the assumptions underlying this method.

Facilitation Tip: While Graphing Saturation Kinetics, circulate to check that students plot initial rates rather than equilibrium values to highlight saturation.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Problem-Based Learning

⏱ 20 min·Whole Class

Whole Class: Water Potential Prediction

Present scenarios with ψs and ψp values for vacuole, cytoplasm, wall. Students predict water flow direction in think-pair-share, then verify with class vote and teacher animation.

Prepare & details

Differentiate between simple diffusion and facilitated diffusion in terms of mechanism, energy requirement, and saturation kinetics, using the contrast between oxygen diffusion and glucose transport via GLUT transporters as examples.

Facilitation Tip: In the Whole Class Water Potential Prediction, ask students to sketch a quick ψ diagram on the board before revealing the class consensus.

Setup: Groups at tables with access to research materials

Materials: Problem scenario document, KWL chart or inquiry framework, Resource library, Solution presentation template

• Analyze
• Evaluate
• Create
• Decision-Making
• Self-Management
• Relationship Skills

Teaching This Topic

Teach passive transport by starting with simple diffusion, then layering in facilitated diffusion and osmosis so students see each as a special case. Use analogies they can test, like comparing a screen door to a protein channel, but always circle back to particle movement and gradients. Avoid rushing to definitions; let the lab data guide the vocabulary.

What to Expect

Students will correctly match molecules to transport modes, explain saturation in facilitated diffusion, and predict water movement using water potential terms. They will also connect lab observations to particle movement across membranes.

Watch Out for These Misconceptions

Common Misconception

During Station Rotation: Diffusion Types, watch for students assuming all polar molecules use carrier proteins.

What to Teach Instead

Use the glucose station’s saturation graph to show that as glucose concentration rises, transport slows once carriers are saturated, while oxygen’s diffusion rate keeps climbing.

Common Misconception

During Plasmolysis in Onion Cells, watch for students thinking solutes move out of the cell during plasmolysis.

What to Teach Instead

Have students re-examine the microscope slide and note that the cell membrane detaches from the wall as water leaves, but visible solute stays inside the cytoplasm.

Common Misconception

During Graphing Saturation Kinetics, watch for students believing passive transport always reaches equilibrium instantly.

What to Teach Instead

Point to the leveling-off shape of the facilitated diffusion curve to emphasize that rate depends on available carriers, not just gradient steepness.

Assessment Ideas

Quick Check

After Station Rotation: Diffusion Types, present students with a diagram showing a cell membrane with oxygen, glucose, and water molecules and ask them to label each molecule’s primary mode of transport and justify their choice based on molecular properties and the presence or absence of transport proteins.

Discussion Prompt

During Whole Class: Water Potential Prediction, pose the scenario: 'Imagine a plant cell placed in a solution with a very negative solute potential. Describe the expected movement of water, the resulting change in turgor pressure, and the final state of the cell using water potential terms.' Listen for correct use of ψs and ψp in explanations.

Exit Ticket

After Plasmolysis in Onion Cells, provide a table of solute potentials for two plant cells and their surrounding solutions and ask students to calculate the net water movement direction and predict whether plasmolysis will occur for each cell. Include a question asking them to identify one assumption made in their calculations.

Extensions & Scaffolding

• Challenge: Ask students to design a dialysis bag experiment that distinguishes between facilitated diffusion and osmosis in the same setup.
• Scaffolding: Provide a partially completed water potential calculation table with missing values for solute potential or pressure potential.
• Deeper exploration: Have students research how aquaporins change water potential predictions in human kidney cells.

Key Vocabulary

Simple Diffusion	The net movement of molecules across a membrane from an area of high concentration to an area of low concentration, without the assistance of transport proteins or energy input.
Facilitated Diffusion	The net movement of molecules across a membrane down their concentration gradient, requiring the assistance of specific membrane transport proteins like channels or carriers.
Water Potential (ψ)	A measure of the potential energy of water per unit volume relative to pure water, indicating the likelihood of water movement; it is the sum of solute potential and pressure potential.
Solute Potential (ψs)	The component of water potential that is inversely proportional to solute concentration; it is always negative or zero, becoming more negative as solute concentration increases.
Pressure Potential (ψp)	The component of water potential due to hydrostatic pressure, typically positive in plant cells due to the cell wall resisting turgor pressure.
Incipient Plasmolysis	The stage in plasmolysis where the plasma membrane begins to pull away from the cell wall, occurring when the water potential of the cell is equal to the water potential of the external solution.