Lesson 4 ·Class 9 · Cell : The Unit of Life · Flow Lab
Why Cells Stay Small
Grow the cell and watch its surface fall behind.
A cell feeds through its surface but must supply its whole volume. Grow it and see what breaks.
→ → →
🔬 Step 1 of 4 · A small cell
A small cell has a large surface for every unit of volume — food and oxygen reach every corner quickly.
surface ∝ size² · volume ∝ size³ → SA/V ∝ 1/size
5units
Surface area : volume ratio 56%
Free play — step through the process at your own pace, and move the slider to see what changes. Switch to when you're ready to test yourself.
Volume grows faster than surface area, so a big cell has proportionally less surface to feed itself through.
What this lesson covers
What you do
- Set the size to 1, then to 10, and compare the two ratios.
Challenges to clear
- Step through the whole argument.
- Grow the cell to size 8 or more and read what happens to the ratio.
- Shrink it back to 2 or less — the ratio the cell actually lives at.
The task
A cell feeds through its surface but must supply its whole volume. Grow it and see what breaks.
What this lesson covers
- A small cell — A small cell has a large surface for every unit of volume — food and oxygen reach every corner quickly.
- The cell grows — As the cell grows, volume increases as the cube of its size but surface area only as the square.
- Surface falls behind — The surface can no longer take in enough material for the volume inside. Exchange becomes too slow.
- So it divides — Cells stay small and divide instead of growing — that is why an elephant has more cells than a mouse, not bigger ones.
- surface ∝ size² · volume ∝ size³ → SA/V ∝ 1/size
- Small cell — a big surface for its volume. Exchange is easy.
- Large cell — very little surface per unit of volume. The inside starves.
Hint
Volume grows faster than surface area, so a big cell has proportionally less surface to feed itself through.