What is the SA:V of a cube of side 2 cm?

Surface area 24 cm², volume 8 cm³, so 3 : 1.

What is the SA:V of a cube of side 4 cm?

Surface area 96 cm², volume 64 cm³, so 1.5 : 1.

Why can a single-celled organism exchange gases by diffusion across its surface?

Its SA:V is large and its diffusion distance is short.

Why can large organisms not rely on diffusion across the body surface alone?

Their SA:V is small and the diffusion distance to the centre is long, so diffusion is too slow.

Give two body-shape adaptations that increase SA:V.

A flattened body; thin or branching extensions such as large ears.

Name two systems that large organisms have developed to overcome a small SA:V.

Specialised exchange surfaces (lungs, gills) and a circulatory (mass transport) system.

Why do small mammals have a high metabolic rate per gram?

They have a large SA:V and lose heat quickly, so they must respire faster to keep their body temperature constant.

Why do small mammals need to eat so much for their size?

They must release enough energy by respiration to replace the heat lost across their large relative surface.

How does body shape differ in animals from cold and hot climates?

Cold: compact, rounded, small extremities (small SA:V). Hot: large extremities such as ears (large SA:V).

Why is the elephant's problem often overheating?

It has a small SA:V, so it loses heat slowly.

How do you calculate the SA:V of a cuboid?

Add the areas of all six faces, then divide by the volume.

Surface area to volume ratioAQA A-Level Biology: Flashcards

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What happens to SA:V as an organism gets larger?

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What happens to SA:V as an organism gets larger?: It decreases, because volume increases faster than surface area.
What is the SA:V of a cube of side 2 cm?: Surface area 24 cm², volume 8 cm³, so 3 : 1.
What is the SA:V of a cube of side 4 cm?: Surface area 96 cm², volume 64 cm³, so 1.5 : 1.
Why can a single-celled organism exchange gases by diffusion across its surface?: Its SA:V is large and its diffusion distance is short.
Why can large organisms not rely on diffusion across the body surface alone?: Their SA:V is small and the diffusion distance to the centre is long, so diffusion is too slow.
Give two body-shape adaptations that increase SA:V.: A flattened body; thin or branching extensions such as large ears.
Name two systems that large organisms have developed to overcome a small SA:V.: Specialised exchange surfaces (lungs, gills) and a circulatory (mass transport) system.
Why do small mammals have a high metabolic rate per gram?: They have a large SA:V and lose heat quickly, so they must respire faster to keep their body temperature constant.
Why do small mammals need to eat so much for their size?: They must release enough energy by respiration to replace the heat lost across their large relative surface.
How does body shape differ in animals from cold and hot climates?: Cold: compact, rounded, small extremities (small SA:V). Hot: large extremities such as ears (large SA:V).
Why is the elephant's problem often overheating?: It has a small SA:V, so it loses heat slowly.
How do you calculate the SA:V of a cuboid?: Add the areas of all six faces, then divide by the volume.

Exam questions on Surface area to volume ratio

Biology students make cube-shaped models of organisms to investigate how size affects the exchange of substances. They compare a small cube with sides of 3 cm and a larger cube with sides of 6 cm.
Explain why an organism the size of the 6 cm cube is less able to exchange substances by diffusion across its body surface alone than one the size of the 3 cm cube.2 marks
A shrew has a body mass of about 8 g and must feed almost continuously. An elephant has a body mass of about 4000 kg and eats for much of the day, but eats far less food for each kilogram of its body mass. Both are mammals that maintain a constant body temperature.
Elephants have very large, thin ears with many blood vessels near the surface. Explain how this helps an elephant to lose heat despite its small surface area to volume ratio.2 marks
Two cube-shaped model organisms are made. Organism A has sides of 1 cm and organism B has sides of 4 cm. The cells in both models respire at the same rate for each cm³ of volume.
Calculate the surface area to volume ratio of organism A and of organism B. Show your working.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).