Cells stay small because increasing cell size decreases the surface area-to-volume ratio, making exchange across the plasma membrane too slow to meet the cell’s metabolic needs. Smaller cells also have shorter diffusion distances, allowing substances to move through them more rapidly.
The Reasoning
A cell exchanges materials such as oxygen, carbon dioxide, nutrients and metabolic waste across its plasma membrane. The rate at which these substances can be exchanged depends mainly on the cell’s surface area, whereas demand for nutrients and production of waste depend mainly on its volume.
For a cube-shaped cell with side length :
- Surface area =
- Volume =
- Surface area-to-volume ratio =
| Side length | Surface area and volume | Surface area-to-volume ratio |
|---|---|---|
| and |
Doubling the side length therefore halves the surface area-to-volume ratio. Volume increases faster than surface area because volume is proportional to length cubed, while surface area is proportional to length squared.
As a cell becomes larger, substances must also travel farther between the membrane and the cell’s interior. This increases diffusion distance, reducing the speed at which materials reach their destinations.
A common misconception is that large cells simply have too little surface area. They actually have more total surface area, but too little surface area relative to their volume.
IB Exam Technique
This is SL and HL content in B2.3 Cell specialization. For an explain question, link the complete sequence: increasing size → lower surface area-to-volume ratio and longer diffusion distance → exchange cannot meet metabolic demand. Do not merely state that “small cells diffuse faster” without explaining why.