Biology · Year 12 · Molecular Foundations and Cell Architecture · Autumn Term

Passive Transport: Diffusion and Osmosis

Investigate the mechanisms of simple diffusion, facilitated diffusion, and osmosis across selectively permeable membranes.

• National Curriculum Attainment Targets
• A-Level: Biology - Transport Across Cell Membranes

About This Topic

Passive transport includes simple diffusion, facilitated diffusion, and osmosis, all driven by concentration gradients across selectively permeable cell membranes without energy expenditure. Simple diffusion moves small non-polar molecules like oxygen directly through the phospholipid bilayer. Facilitated diffusion relies on channel proteins for ions or carrier proteins that change shape to transport polar molecules such as glucose. Osmosis governs water movement from regions of high water potential to low, crucial for cell turgor and volume regulation.

This topic aligns with A-Level Biology standards on transport across cell membranes, linking to molecular foundations and cell architecture. Students explain gradient-driven movement, compare protein roles, and predict effects on animal and plant cells in hypotonic, isotonic, or hypertonic solutions: lysis or crenation in animals, turgidity or plasmolysis in plants. These concepts underpin homeostasis and prepare for active transport studies.

Active learning excels with this topic through hands-on experiments like potato osmometers or dialysis tubing setups. Students measure quantifiable changes in mass or length, predict outcomes, then compare with data. Group discussions refine predictions, turning abstract gradients into observable phenomena that students retain long-term.

Key Questions

1. Explain how the concentration gradient drives the net movement of substances in passive transport.
2. Compare the roles of channel proteins and carrier proteins in facilitated diffusion.
3. Predict the osmotic effects on animal and plant cells when placed in hypotonic, isotonic, and hypertonic solutions.

Learning Objectives

• Explain the role of the concentration gradient in driving net movement during simple diffusion and facilitated diffusion.
• Compare and contrast the mechanisms by which channel proteins and carrier proteins facilitate the transport of specific solutes across membranes.
• Predict the effect of placing animal and plant cells into hypotonic, isotonic, and hypertonic solutions on cell volume and integrity.
• Calculate the change in mass or length of a biological sample due to osmosis under specified conditions.

Before You Start

Cell Structure and Function

Why: Students need to understand the basic components of a cell, including the plasma membrane and its role as a barrier, before studying transport across it.

Molecular Movement and States of Matter

Why: Understanding that molecules are in constant random motion and how this relates to concentration is fundamental to grasping diffusion.

Key Vocabulary

Concentration gradient	The gradual difference in the concentration of a substance between two areas. Movement occurs from an area of high concentration to an area of low concentration.
Selectively permeable membrane	A barrier that allows certain molecules or ions to pass through by diffusion, and occasionally specialized facilitated processes.
Water potential	A measure of the potential energy of water per unit volume relative to pure water. Water moves from an area of higher water potential to an area of lower water potential.
Turgor pressure	The pressure exerted by the cell contents against the cell wall in plant cells. It is crucial for maintaining plant rigidity.
Plasmolysis	The process in plant cells where the plasma membrane pulls away from the cell wall due to the loss of water by osmosis.

Watch Out for These Misconceptions

Common Misconception

Diffusion and osmosis require cellular energy.

What to Teach Instead

Both are passive processes driven solely by concentration gradients, needing no ATP. Practical labs measuring diffusion rates without metabolic inhibitors help students see movement occurs spontaneously. Group analysis of results reinforces energy independence.

Common Misconception

Osmosis moves solute particles across membranes.

What to Teach Instead

Osmosis specifically involves net water movement down its potential gradient. Dialysis tubing experiments visually separate solute diffusion from water effects via mass changes. Peer teaching during lab rotations clarifies solvent versus solute paths.

Common Misconception

All cells respond identically to hypotonic solutions.

What to Teach Instead

Animal cells lyse while plant cells become turgid due to cell walls. Comparative potato and red blood cell experiments reveal structural differences. Student-led comparisons in small groups build nuanced predictions.

Active Learning Ideas

Lab Practical: Potato Osmosis Cylinders

Prepare uniform potato cylinders and weigh them. Place in distilled water, 0.9% saline, and 2M sucrose for 45 minutes. Reweigh, calculate percentage mass change, and plot against solution concentration to determine isotonic point. Groups present findings.

60 min·Small Groups

Demonstration: Visking Tubing Model

Fill dialysis tubing with starch and glucose solution, tie securely, and submerge in iodine and Benedict's solution. Observe colour changes over 20 minutes indicating diffusion rates. Students note selective permeability and discuss protein roles.

30 min·Whole Class

Simulation Game: Facilitated Diffusion Relay

Assign students roles as molecules, channels, or carriers. Use props like coloured beads for polar substances. Time relay races with and without 'proteins' to show rate differences. Debrief on saturation effects.

25 min·Pairs

Prediction Cards: Cell Scenarios

Distribute cards describing solution types and cell types. Pairs predict and sketch changes, then test with microscope slides of onion cells in solutions. Compare drawings to observations.

40 min·Pairs

Real-World Connections

• Medical professionals, such as nurses and doctors, monitor patient hydration levels by observing how cells respond to intravenous fluids, understanding osmosis to prevent cell lysis or crenation.
• Food scientists use osmosis principles when preserving foods through methods like salting or sugaring, which draws water out of microbial cells, inhibiting their growth and spoilage.

Assessment Ideas

Quick Check

Present students with diagrams of cells in different solutions. Ask them to label each solution as hypotonic, isotonic, or hypertonic and describe the predicted movement of water and the resulting cell appearance.

Discussion Prompt

Pose the question: 'Imagine a plant cell and an animal cell are placed in the same beaker of pure water. Explain the different outcomes for each cell and why these differences occur, referencing water potential and cell wall presence.'

Exit Ticket

Students are given a scenario involving a potato strip placed in a concentrated salt solution. Ask them to write two sentences explaining what will happen to the potato strip's mass and why, using terms like osmosis and water potential.

Frequently Asked Questions

How does concentration gradient drive passive transport?

Concentration gradient provides the driving force: substances move from high to low concentration until equilibrium. In diffusion, random molecular motion nets this flow; osmosis follows water potential differences. Classroom models with coloured inks spreading in water make gradients visible, helping students quantify rates with Fick's law basics.

What is the difference between channel and carrier proteins?

Channel proteins form pores for rapid ion flow down gradients, always open or gated. Carrier proteins bind specific molecules, undergo conformational change for transport, and saturate at high concentrations. Relay simulations distinguish constant flow from binding-limited rates, aiding student recall.

How can active learning help students understand passive transport?

Hands-on labs like potato osmosis or visking tubing let students measure real mass changes, linking predictions to data. Collaborative relays model protein functions, while microscope observations of cell responses visualise effects. These approaches shift passive concepts from rote memory to experiential understanding, boosting retention and application.

What happens to cells in hypotonic, isotonic, and hypertonic solutions?

Hypotonic: water enters, animal cells swell and may lyse, plant cells become turgid. Isotonic: no net movement, cells stable. Hypertonic: water exits, animal cells crenate, plant cells plasmolyse. Practical tests with eggs or onion epidermis confirm predictions, with graphing reinforcing solution impacts.