Cell size matters because, as a cell grows, its surface area-to-volume ratio decreases. The plasma membrane therefore has less surface area available for exchange relative to the cell’s increasing metabolic requirements.
This concept is assessed at both SL and HL in B2.3 Cell specialization.
Why the ratio decreases
A cell exchanges substances such as oxygen, carbon dioxide, nutrients and waste across its plasma membrane. Exchange capacity depends mainly on surface area, while the cell’s demand for resources and production of waste depend mainly on volume.
For a cube with side length :
- Surface area =
- Volume =
