Diffusion in Cells

Overview

In this tenth lesson of the unit Cells: The Basis of Life, students investigate diffusion using agar cubes as a model for cells. They collect and process evidence to explain how surface-area-to-volume ratio affects the rate and extent of diffusion, then connect this relationship to the survival of cells and the organisation of multicellular organisms.

Learning intentions

Students will:

• plan and conduct a fair investigation using an agar diffusion model
• identify independent, dependent and controlled variables
• calculate or compare surface-area-to-volume ratios for different-sized cubes
• analyse results using tables, calculations and evidence-based conclusions
• explain why a high surface-area-to-volume ratio supports cell survival.

Success criteria

• I can describe diffusion as the net movement of particles from a region of higher concentration to lower concentration.
• I can identify variables and collect reliable measurements during an investigation.
• I can use evidence to compare diffusion in agar cubes of different sizes.
• I can explain why cells remain small or require specialised structures to exchange materials efficiently.

Curriculum links

• Cells as the basis of life — cell structures and functions, cells and their environments, and biochemical processes.
• Students explain how cell structures enable biological processes needed for life, including the exchange of substances across cell membranes.
• Cells to systems — students connect cell size and organisation with the functioning of complex multicellular organisms.
• Scientific investigation skills: planning, conducting, processing data, evaluating reliability and communicating a conclusion.

Lesson structure (60 minutes)

1. 0–5 min · Hook and retrieval. Teacher displays a diagram of two agar cubes and asks, “Which cube will change colour more quickly throughout its interior, and why?” using the diffusion hook and retrieval slides. Students make an individual prediction, then recall the meaning of diffusion and share their reasoning with a partner.

2. 5–13 min · Model and method. Teacher explains that agar represents cell material and the coloured solution represents a substance diffusing into the model; explicitly model how to calculate cube surface area, volume and surface-area-to-volume ratio, using the model and method slides. Students annotate the diffusion investigation worksheet with the research question, prediction, variables, safety controls and proposed data table.

3. 13–20 min · Group planning and safety. Teacher places students in groups of five or six, checks each plan, and clarifies that cube size is the independent variable while diffusion distance or the percentage of colour change is the dependent measure. Students assign roles—equipment manager, timer, measurer, recorder and safety monitor—and revise their method to keep solution concentration, time, temperature and cube shape consistent.

4. 20–35 min · Practical investigation. Teacher distributes pre-cut agar cubes of at least two sizes, coloured solution, trays, rulers and forceps, then monitors safe handling and prompts groups to start timing immediately. Students measure each cube, place it in the solution, record observations at agreed intervals, and calculate the depth or proportion of coloured agar; they wear safety glasses, avoid tasting materials and follow local laboratory procedures.

5. 35–45 min · Process and represent data. Teacher pauses the practical, models one sample calculation and directs students to complete their results table and graph on the data processing and graphing pages. Students calculate surface area, volume and surface-area-to-volume ratio, compare diffusion measurements, identify patterns and note anomalies or limitations.

6. 45–55 min · Explain and apply. Teacher uses the analysis and cell-survival discussion slides to facilitate questions: “Why did the smaller cube receive substances throughout its volume more effectively?” and “Why is diffusion alone insufficient for a large multicellular organism?” Students write a CER response—claim, evidence and reasoning—linking high surface-area-to-volume ratio with efficient exchange across cell membranes, cell survival and the need for tissues or transport systems in larger organisms.

7. 55–60 min · Plenary and exit check. Teacher displays the plenary and exit-ticket prompt and collects responses before students pack away equipment. Students answer: “A cell becomes larger but keeps the same shape. Predict how its surface-area-to-volume ratio and diffusion efficiency will change. Explain why.”

Resources

• the complete diffusion investigation slide deck
• the diffusion investigation worksheet
• Pre-cut agar cubes in at least two sizes
• Coloured solution, such as dilute sodium hydroxide with a suitable indicator
• Clear trays or beakers, forceps and paper towel
• Rulers, stopwatches or classroom timers
• Safety glasses, gloves if required, and laboratory waste containers
• Calculator access and graphing materials or spreadsheet software

Assessment

• Check group plans before the practical, focusing on a valid question, correctly identified variables, repeatable steps and controlled conditions.
• During the investigation, question students about why cube size, diffusion distance and surface-area-to-volume ratio matter; check calculations and graph conventions as students work.
• Use the CER response and exit ticket to assess whether students can interpret evidence and explain the relationship between cell size, diffusion and survival.

Differentiation

• Provide a worked example for surface area, volume and ratio calculations, a labelled data table, a variable bank and sentence starters such as “The smaller cube diffused faster because…”.
• Allow students to use calculators, a formula card or a spreadsheet; provide pre-cut cubes and assigned roles for students requiring fine-motor, organisational or processing support.
• Pair EAL/D students with supportive peers and include a visual sequence of the method; explicitly teach and display terms including diffusion, concentration gradient, surface area, volume and ratio.
• Extend capable students by asking them to calculate percentage diffusion, compare the reliability of repeated trials, evaluate whether agar is a valid cell model and propose a follow-up investigation.