Why Cells Are Small

Lab Brief #3

🎯 Your mission

Set it up. Test it. Be amazed.

⚡ The twist

Every species solves the same problem differently.

What you'll learn

A cell must move materials in and waste out across its surface. As a cell grows, its volume rises faster than its surface area, so the surface eventually cannot serve the inside. That limit is why organisms grow by making more cells rather than bigger ones.

Key words

surface area to volume ratio
The ratio that falls as an object grows, limiting how well its surface serves its inside.
diffusion
The spreading of particles from where they are crowded to where they are not.

Make a hypothesis first

Before you watch: what do you think it'll do?

Say your guess out loud — then find out if you were right.

Get ready

🧰 What you need

potato
knife
food colouring
ruler
beaker

Get ready

⚠️ Safety first

Cutting must be done by an adult, or with a blunt tool on a board.
Food colouring stains — protect the surface and wash hands after.

Step 1 of 4 · Read

Think about what you just learned: say what "surface area to volume ratio" and "diffusion" mean in your own words before you start.

Step 2 of 4 · Do

Cut agar or potato cubes of 1 cm, 2 cm and 3 cm, soak them in food colouring for the same time, then slice each and measure how far the colour reached as a fraction of the cube.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 2

As a cell gets larger, its surface area to volume ratio:

How sure are you?😐 maybe

Question 2 of 2

Organisms grow mainly by:

How sure are you?😐 maybe

Mind = blown

🐙 An octopus has 9 brains and 3 hearts.

Nice work!

You got 0 of 2 right.

Next: Plant Cells Versus Animal Cells All lessons

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✨ More to explore

🌍 Where you see this in real life: Your own body is doing this thousands of times every second.

🏆 Stretch challenge (try it this week): Find one thing about your body that's different from a friend's. Wonder why.

👨‍👩‍👧 For the dinner table: “Tell me something you noticed about your own body today.”

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Why Cells Are Small

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