Exchange surfaces, transport systems and surface area : volume ratio

Exchange and transport in animals · Transport and exchange surfaces · note 2 of 3

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Exchange surfaces, transport systems and surface area : volume ratioSpec 8.2

In short

The surface area : volume ratio compares how much surface an organism has with its size, and it gets smaller as organisms get bigger. A single-celled organism has a large ratio, so diffusion across its surface is fast enough. Multicellular organisms have a small ratio, so they need exchange surfaces and a transport system.

The surface area : volume ratio tells you how much surface an organism has compared with its size. The bigger the organism, the smaller this ratio becomes.

surface area : volume ratio = surface area ÷ volume

Surface area : volume ratio of a cube

A cube-shaped block of tissue has sides of 3 cm. Calculate its surface area : volume ratio.

A cube has 6 faces. Area of one face = 3 × 3 = 9 cm².
Surface area = 6 × 9 = 54 cm².
Volume = 3 × 3 × 3 = 27 cm³.
Ratio = 54 : 27. Divide both sides by 27.

Answer: 2 : 1

As a cube gets bigger, its surface area : volume ratio gets smaller
Side of cube	Surface area	Volume	Surface area : volume
1 cm	6 cm²	1 cm³	6 : 1
2 cm	24 cm²	8 cm³	3 : 1
3 cm	54 cm²	27 cm³	2 : 1

Single-celled and multicellular organisms

A single-celled organism has a large surface area : volume ratio. Substances only have a short distance to diffuse, so diffusion across the surface is fast enough.
A multicellular organism has a small surface area : volume ratio. Many cells are deep inside the body, so the diffusion distance is long and diffusion across the body surface is far too slow.

Multicellular organisms therefore need two things. An exchange surface has a large surface area so that enough oxygen can enter and enough carbon dioxide can leave. A transport system (the circulatory system in humans) then carries substances between the exchange surface and all the cells in the body.

Maths skill:

Write the ratio in its simplest form, in the form x : 1, unless the question asks otherwise. Check that the surface area is in cm² and the volume in cm³.

Three cubes drawn to scale with sides of 1 cm, 2 cm and 3 cm, labelled with surface areas of 6, 24 and 54 cm², volumes of 1, 8 and 27 cm³, and surface area : volume ratios of 6 : 1, 3 : 1 and 2 : 1. — As the cube gets bigger, its surface area : volume ratio gets smaller.

Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026

Frequently asked questions

How are alveoli adapted for gas exchange?

Alveoli are adapted for gas exchange by having a very large surface area, from millions of alveoli, and walls only one cell thick for a short diffusion distance. A rich network of capillaries and ventilation keep a steep concentration gradient, and a moist lining lets gases dissolve, which helps them to diffuse.

Why is the surface area to volume ratio important for cells?

The surface area to volume ratio matters because substances enter and leave across the surface. A single-celled organism has a large ratio and a short diffusion distance, so diffusion is fast enough. A large multicellular organism has a small ratio and many cells deep inside the body, so diffusion across its body surface alone would be far too slow.

How do you calculate surface area to volume ratio?

To calculate the surface area to volume ratio, divide the surface area by the volume and write the answer in the form x : 1. For a cube with 3 cm sides, the surface area is 6 × 9 = 54 cm² and the volume is 27 cm³, so the ratio is 54 ÷ 27 = 2 : 1.

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