Why Are Cells So Small? The Gelatin Cube Surface-Area Experiment
By Ahmed, founder
Your body has trillions of cells, and most are far too small to see. Why aren’t there fewer, bigger ones? One answer comes from geometry, and you can test it with gelatin.
The experiment
You need: firm gelatin (set in a tray), a knife (adult), food colouring in water, a ruler and spoons.
- A grown-up cuts gelatin into cubes: 1 cm, 2 cm and 3 cm on each side.
- Place the cubes in a bowl of coloured water and leave them for 10 to 15 minutes.
- Remove the cubes and pat dry. A grown-up cuts each in half.
- Measure how far the colour has soaked in from the surface. It should be about the same depth in each.
- Estimate what fraction of each cube’s volume is coloured.
What you see
In the 1 cm cube, the colour reaches nearly the centre. In the 3 cm cube, a coloured shell surrounds a large uncoloured middle. Colour only travels a short distance in the same time.
The numbers
| Cube side | Surface area (cm²) | Volume (cm³) | Surface area ÷ volume |
|---|---|---|---|
| 1 cm | 6 | 1 | 6 |
| 2 cm | 24 | 8 | 3 |
| 3 cm | 54 | 27 | 2 |
As the cube grows, volume grows faster than surface area. The bigger the cube, the less surface it has per unit of volume.
Why it matters for cells
A cell takes in food and oxygen and gets rid of waste through its surface (the cell membrane). The inside needs to be supplied: the amount of cell inside is its volume. A big cell would have too little surface to feed all that volume, and materials would take too long to travel across it. So cells stay small, and big organisms are made of many cells, not a few giant ones.
This is a model, not real cells, but it makes the idea measurable. Real cells also have other tricks, such as folded membranes, to increase surface area.
Common mix-ups
- “Bigger animals have bigger cells.” Mostly they have more cells.
- “Cells are small because we haven’t found big ones.” Some exist, such as an egg yolk, but they work differently and have special supply arrangements.
Safety
Adults use the knife, and gelatin is for experiments, not eating, once it has been handled or coloured.
Related lessons
The full lesson is Why are you made of many small cells, not one big one? in Cells: The Smallest Living Things. Start with How big is a cell? Sign up for three lessons free.