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Biology AS AQA

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  1. 1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    9 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    14 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    4 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-1-dna-genes-and-chromosomes
    10 主题
  20. 4-2-dna-and-protein-synthesis
    3 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
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Exam code:7401

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

Three green cubes with dimensions 1 cm, 2 cm, and 3 cm are shown. A table displays their surface areas, volumes, and surface area to volume ratios.
As size increases, the surface area : volume ratio decreases

Calculating SA:V ratio

  • The surface area : volume ratio can be calculated for objects of different shapes

    • Different shapes may be used to represent the dimensions of different organisms, e.g. a bacterial cell may be represented by a cylinder

Comparison of surface area and volume formulas for a cube, cuboid, and cylinder, with example calculations for each shape in a tabular layout.

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 = 4 over 3πr

Answer:

Step 1: Determine r (radius)

  • The radius is half the diameter

1.794 ÷ 2 = 0.897

Step 2: Determine the surface area of the toad, e.g.

surface area = 4 x π x 0.8972

= 10.111 mm2

Step 3: Determine the volume of the toad egg

volume = 4 over 3x 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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