Cell Size
| English | Chinese | Pinyin |
|---|---|---|
| volume | 体积 | tǐ jī |
| surface area | 表面积 | biǎo miàn jī |
| surface-area-to-volume ratio | 表面积与体积比 | biǎo miàn jī yǔ tǐ jī bǐ |
| diffusion | 扩散 | kuò sàn |
Why cells stay tiny
- Almost every cell is microscopic, whether in a mouse or an elephant.
- A big animal is not made of big cells — it is made of more cells.
- There is a hard physical reason cells cannot grow large.
- It comes down to a race between surface and volume.
Surface versus volume
- A cell takes in food and oxygen across its surface area 表面积.
- It uses those materials throughout its whole volume 体积.
- As a cell grows, its volume grows faster than its surface.
- So a big cell has too little surface to feed its large inside.
A cube of side 2 has surface area 24 and volume 8. What is its SA:V ratio (surface ÷ volume)?
$24 \div 8 = 3$. A smaller cube (side 1) would give $6 \div 1 = 6$ — a higher ratio.
The SA:V ratio falls
- The surface-area-to-volume ratio 表面积与体积比 measures surface per unit of inside.
- Double the size of a cube and this ratio is cut in half.
- A small cell has a high ratio — plenty of surface for its volume.
- A large cell has a low ratio — not enough surface to keep up.
Surface area : volume
Grow the cube and watch the SA:V ratio fall — the reason cells and exchange surfaces stay small.
Why must most cells stay very small?
A small cell has a large surface area relative to its volume, so materials diffuse in and out fast enough.
As a cell gets bigger, its surface-area-to-volume ratio falls.
Volume grows faster than surface area, so the SA:V ratio falls as size increases.
Diffusion sets the limit
- Materials cross the cell by diffusion 扩散, which is only fast over short distances.
- In a large cell, the centre is too far from the surface.
- Supplies would arrive too slowly to keep the cell alive.
- So staying small keeps every part close to the surface.
What sets the size limit for a cell relying on diffusion?
Diffusion is only fast over short distances, so a cell too large cannot supply its centre quickly enough.
Select all true statements about cell size.
Bigger cells exchange materials more slowly per unit volume, not faster. The other three are correct.
Bigger is not better for exchange. A larger cell has less surface for each unit of its volume, so it exchanges materials more slowly, not faster. This is exactly why cells divide instead of growing without limit.
Why exchange surfaces are folded:
- Your lungs must swap huge amounts of oxygen quickly.
- A smooth balloon would have far too little surface.
- Instead the lung is folded into millions of tiny sacs — a giant surface packed into a small volume.
Cells stay small because a small cell has a high surface-area-to-volume ratio. As size grows, volume rises faster than surface area, so the ratio falls and diffusion can no longer supply the centre fast enough. Big organisms use more cells, not bigger ones.