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Surface Area to Volume Ratio (AQA AS Biology)
Revision Note
What is Surface Area to Volume Ratio?
- Surface area and volume are both very important factors in the exchange of materials in organisms
- The surface area refers to the total area of the organism that is exposed to the external environment
- The volume refers to the total internal volume of the organism (total amount of space inside the organism)
- As the surface area and volume of an organism increase (and therefore the overall ‘size’ of the organism increases), the surface area : volume ratio decreases
- This is because volume increases much more rapidly than surface area as size increases
As size increases, the surface area : volume ratio decreases
Maths Skill: Calculating SA:V Ratio
How to calculate surface area to volume ratio
- The surface area : volume ratio calculation differs for different shapes (these shapes can reflect different cells or organisms)
Worked example
A researcher calculated the surface area of a large number of toad eggs. She calculated the mean surface area to be 10.12 mm2. Toad eggs are generally spherical. She calculated the surface area using the following equation: 4πr2Use this equation to calculate the mean diameter of a toad egg. Give your answer to 3.s.f.
Step 1: Rearrange the equation to find the radius
r2 = surface area/4π
Step 2: Sub in relevant figuresr2 = 10.12 ÷ 12.56 = 0.805
Step 3: Find the square root of r2Square root of 0.805 = 0.897
Step 4: Find the diameter from the radiusD = 2r
2(0.897) = 1.794
Step 5: Round to three significant figures
1.79 mm
Examiner Tip
You are expected to be able to calculate the SA:V ratio for different shapes and explain how the increasing size of an organism affects the SA:V ratio.
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