Why are cells small, and how does size limit exchange with the environment?
Explain how the surface area to volume ratio limits cell size and affects the rate of exchange
As a cell grows, volume increases faster than surface area, so the surface area to volume ratio falls, limiting the rate of exchange and therefore cell size.
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What this dot point is asking
You need to explain the relationship between surface area, volume and cell size, and use the SA:V ratio to explain why cells stay small and why exchange surfaces are adapted.
Why surface area to volume matters
A cell takes in oxygen and nutrients and removes wastes across its surface (the cell membrane). But the demand for these substances depends on the volume of living material inside.
For exchange to keep up, the surface must be large enough to service the whole volume. The key measure is the surface area to volume (SA:V) ratio.
How the ratio changes with size
Imagine a cell as a cube of side length :
- Surface area = (grows with the square of length)
- Volume = (grows with the cube of length)
As increases, volume increases faster than surface area, so the SA:V ratio decreases. Small cells therefore have a large SA:V ratio, and large cells have a small one.
Consequences of a falling ratio
A low SA:V ratio means:
- The membrane surface cannot absorb nutrients or release wastes fast enough for the volume inside.
- The centre of a large cell may be starved of oxygen or accumulate waste.
This is why most cells are small - staying small keeps the SA:V ratio high enough for efficient exchange. It also explains why cells divide rather than keep growing.
Adaptations to increase surface area
Cells and tissues that exchange a lot of material are adapted to increase surface area without increasing volume:
- Microvilli on intestinal cells increase the absorptive surface.
- Root hair cells are long and thin for water uptake.
- Alveoli in lungs and folded gill surfaces provide huge gas-exchange areas.
Being thin and flat also helps because it shortens diffusion distance and raises the SA:V ratio.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 SACE Stage 21 marksWhich one of the following graphs shows the change in the surface area-to-volume ratio (SA : V) of a cell as it grows until time X, when it divides in two?Show worked answer →
This is a multiple-choice item worth 1 mark; the correct graph shows SA : V falling as the cell grows, then rising sharply at time X when the cell divides.
As a cell grows, its volume increases with the cube of length while its surface area increases only with the square, so the SA : V ratio steadily decreases (a downward curve) up to time X. When the cell divides into two smaller cells, each daughter cell has a smaller volume and therefore a higher SA : V, so the ratio jumps back up at X. The mark is for selecting the graph that decreases up to X and then increases at the division point.