Osmosis Practical Investigation

Overview

In this lesson, students investigate how different solution concentrations affect the mass of potato tissue. They apply osmosis as the movement of water down a water-potential gradient, while developing skills in controlling variables, collecting repeat measurements, processing data and evaluating reliability.

Learning intentions

Students will:

• Explain osmosis as the net movement of water down a water-potential gradient through a selectively permeable membrane.
• Predict how potato tissue will respond to solutions of different concentrations.
• Collect repeated, accurate measurements while controlling relevant variables.
• Present results in a suitable table and graph, then evaluate the reliability of the investigation.

Success criteria

• I can explain the direction of water movement using water potential.
• I can identify the independent, dependent and controlled variables.
• I can calculate mass change and display results appropriately.
• I can use evidence to evaluate reliability and explain possible improvements.

Curriculum links

• Cells as the basis of life: students explain how cell structures enable biological processes needed for life, including movement of substances across cell membranes.
• Cells and their environments: students relate changes in plant tissue to differences between the cell contents and surrounding solution.
• Biochemical processes and scientific investigation skills: students collect, process and evaluate biological data.
• Living systems: students identify cell components and functions, with a focus on the cell membrane and maintenance of living cells.

Lesson structure (60 minutes)

1. 0–6 min · Hook and prediction. Teacher displays two images of potato strips before and after soaking and asks, “Why might one strip gain mass while another loses mass?” using the osmosis hook slides. Students make an individual prediction and briefly justify it using the terms water, concentration or membrane.

2. 6–16 min · Explicit teaching and planning. Teacher explains osmosis as the net movement of water from higher water potential to lower water potential across a selectively permeable membrane; model the relationship between solution concentration, water movement and cell turgor using the osmosis explanation and method slides. Students annotate the osmosis investigation worksheet with the research question, prediction, independent variable, dependent variable and controlled variables.

3. 16–22 min · Safety and method briefing. Teacher demonstrates measuring, blotting and weighing potato pieces, then reviews goggles, careful handling of equipment, spills and safe disposal; assign groups of four or five and distribute labelled cups containing different sucrose concentrations. Students identify how to keep potato size, soaking time, solution volume, temperature and blotting procedure consistent.

4. 22–38 min · Practical investigation. Teacher circulates, checks that groups record initial mass before placing tissue into solutions, and prompts students to complete at least three repeat measurements per concentration. Students measure and record initial mass, place potato pieces into the assigned solutions, then remove, blot consistently and record final mass; they calculate mass change and percentage mass change where time permits.

5. 38–51 min · Data processing. Teacher uses the results-processing slides to model a results table, mean mass change and a graph with solution concentration on the x-axis and mean percentage mass change on the y-axis. Students complete their table on the results and graph sections, calculate a mean, plot data and identify the concentration at which there is little or no net mass change.

6. 51–57 min · Evaluate reliability. Teacher facilitates a class comparison of results and asks, “Which evidence makes your conclusion more or less reliable?” Students write a conclusion linking mass change to water-potential differences and evaluate reliability using repeat measurements, consistency between groups and possible sources of error.

7. 57–60 min · Exit check. Teacher displays the plenary questions using the plenary and exit-question slide. Students answer: “If potato tissue loses mass, where did water move, and why?” and “Name one improvement that would increase reliability,” then submit their completed worksheet.

Resources

• the osmosis investigation slide deck
• the osmosis investigation worksheet
• Potato or similar plant tissue, prepared into equal-sized pieces
• Sucrose solutions of several known concentrations, including water
• Small labelled cups or test tubes
• Electronic balances, rulers and forceps
• Paper towel, goggles and trays
• Timer and waste container

Assessment

• Check predictions, variable identification and safety decisions during planning.
• Observe whether groups use consistent measuring, blotting and recording procedures; check that results include repeat measurements and appropriate units.
• Assess the worksheet for a correctly structured table, graph, evidence-based conclusion and evaluation of reliability. Use the exit responses to identify misconceptions about the direction of water movement.

Differentiation

• Provide a partially completed method and a variable bank for students requiring support; use sentence starters such as “Water moved from ___ to ___ because ___.”
• Pre-label equipment and assign group roles: equipment manager, measurer, recorder and checker. Allow students to use a calculator or spreadsheet for means and percentage change.
• Support EAL/D learners with a visual diagram of the membrane and paired discussion before written responses; explicitly teach water potential, selectively permeable and turgor.
• Extend capable students by asking them to compare group means, comment on anomalous results and explain why the zero-mass-change concentration estimates the tissue’s water potential relative to the solution.