Surface Area to Volume Ratio (AQA A Level Biology): Revision Note
Exam code: 7402
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Surface area to volume ratio & size
Surface area and volume are factors that affect the exchange of materials in organisms
Surface area: the total area of an organism that is exposed to the external environment
Volume: the total internal volume of an organism
As the overall size of an organism increases, the surface area to volume ratio decreases
This is because volume increases more rapidly than surface area as size increases
Calculating SA:V ratio
The surface area : volume ratio can be calculated for objects of different shape
Different shapes may be used to represent the dimensions of different organisms, e.g. a bacterial cell may be represented by a cylinder
Worked Example
A researcher calculated that the diameter of a spherical toad egg was 1.794 mm.
Use the formulae provided to calculate the surface area : volume ratio of the toad egg.
Surface area of a sphere = 4πr2
Volume of a sphere = 
πr
Answer:
Step 1: determine r
The radius is half the diameter
1.794 ÷ 2 = 0.897
Step 2: determine the surface area of the toad eg
surface area = 4 x π x 0.8972
= 10.111 mm2
Step 3: determine the volume of the toad egg
volume = x π x 0.897
= 3.757 mm3
Step 4: determine the SA:V ratio
SA:V ratio is normally notated as x:1, so we need to convert 10.1:3.8 to this notation
First we convert the volume value to 1:
3.8 ÷ 3.8 = 1
Then we divide the SA value by the same factor:
10.1 ÷ 3.8 = 2.7
So our final ratio is:
2.7:1
Examiner Tips and Tricks
Make sure that you know how to calculate surface area and volume for the following shapes:
cube
cuboid
cylinder
sphere
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