AQA AS Biology

Revision Notes

3.1.1 Surface Area to Volume Ratio

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

11. Principles of surface area _ volume, downloadable AS & A Level Biology revision notes

As size increases, the surface area : volume ratio decreases

Maths Skill: Calculating SA:V Ratio

  • The surface area : volume ratio calculation differs for different shapes (these shapes can reflect different cells or organisms)

Calculating SA_V ratios, downloadable AS & A Level Biology revision notes

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: 4πr2

Use 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 figures

r2 = 10.12 ÷ 12.56 = 0.805

       Step 3: Find the square root of r2

Square root of 0.805 = 0.897

       Step 4: Find the diameter from the radius

D = 2r

2(0.897) = 1.794

       Step 5: Round to three significant figures

1.79 mm

Exam 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.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top