Slice Lab

Slice Lab: the factors that affect the rate of diffusion

This free game is part of our Pearson Edexcel iGCSE Biology (4BI1) course for exams in EXAM YEARS 2026-2029.

The four factors that affect diffusion

Substances move into and out of cells by diffusion, osmosis and active transport. For the Pearson Edexcel International GCSE you need to know how four factors change the rate of movement:

• Surface area to volume ratio: the bigger the ratio, the faster substances can get in and out. Small cells and thin, flat structures have big ratios.
• Distance: the shorter the distance substances have to travel, the faster they arrive. Double the distance and diffusion takes about four times as long.
• Temperature: at a higher temperature the particles have more kinetic energy, so they move faster and diffuse more quickly.
• Concentration gradient: the bigger the difference in concentration (the steeper the gradient), the faster the net movement. When the concentrations are equal there is no net movement.

The agar cube practical

Slice Lab is based on a classic practical for spec point 2.17 (investigate diffusion using non-living systems). Agar jelly is made with an alkali (sodium hydroxide) and phenolphthalein indicator, which is pink in alkali. Cubes of the pink agar are placed in dilute hydrochloric acid. The acid diffuses in from every face and neutralises the alkali, so the agar turns colourless from the outside in.

You time how long each cube takes to go completely clear. Smaller cubes go clear first, because they have a bigger surface area to volume ratio and a shorter distance from the surface to the middle.

• Independent variable: the size of the cube (and so its surface area to volume ratio).
• Dependent variable: the time taken to turn colourless.
• Control variables: the concentration, volume and temperature of the acid.

Working out surface area to volume ratio

For a cube with sides of length L: surface area = 6 × L² and volume = L³.

• 1 cm cube: 6 cm² : 1 cm³ = 6 : 1
• 2 cm cube: 24 cm² : 8 cm³ = 3 : 1
• 3 cm cube: 54 cm² : 27 cm³ = 2 : 1

As an object gets bigger, its volume grows faster than its surface area, so the ratio falls. Cutting a 4 cm cube into eight 2 cm cubes keeps the volume the same (64 cm³) but doubles the surface area (96 cm² to 192 cm²). That is exactly what happens every time you slice the jelly in the game.

How to play Slice Lab

Two launchers fire blocks of pink agar over a tank of acid. Hold and swipe across a block to slice it: each slice cuts every piece into eight smaller cubes. Once in the acid, every piece has to turn clear within 6 seconds.

• Each jelly has a label: a cross means it is too big to go clear in time, a tick means it will make it. Stop slicing at the tick for PERFECT double points.
• A guide shows how long each size takes at the current temperature and acid strength, so you can see the time drop to a quarter every time the size halves.
• Thin sheets and rods clear without slicing: big, but nowhere far from the surface, just like a leaf.
• Cold snaps (5°C) slow diffusion down, heatwaves (45°C) speed it up, and later waves use up the acid so the concentration gradient gets less steep. Press PUMP to add fresh acid, the way blood flow and breathing keep gradients steep in your lungs.
• Don't slice the glass flasks. Answer a lab check after every wave, build combos and climb from Lab Trainee to Head of the Lab.

Why this matters for living organisms

Single-celled organisms such as Amoeba have a large surface area to volume ratio and short diffusion distances, so diffusion across their surface is enough. Large multicellular organisms have a small ratio and long distances to their inner cells, so they need exchange surfaces and a transport system.

• Alveoli give a huge surface area with walls one cell thick, and blood flow keeps the concentration gradient steep.
• Villi in the small intestine massively increase the surface area for absorption.
• Leaves are thin and flat, so gases only have a short distance to diffuse.
• Elephants' ears add surface area so a huge animal can lose heat.

Exam tips

• Always say surface area to volume ratio, not just "surface area".
• For temperature, mention kinetic energy: the particles move faster.
• Say the concentration gradient is steeper or less steep, and that diffusion is net movement from high to low concentration.
• Show your working in surface area to volume calculations, and simplify the ratio to x : 1.