4.2.5

Surface Area to Volume Ratio

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Surface Area to Volume Ratio

The surface area to volume ratio influences how substances and heat energy can be transferred around multicellular organisms.

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Surface area

  • The surface area of an organism is the total surface over which substances and heat can be exchanged.
  • Surface area alone cannot influence the rate of exchange.
  • The surface area of an organism relative to their volume is more important.
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Volume

  • The volume of an organism determines the number of substances that need to be taken in and transported out.
  • As the volume increases, more materials are needed for metabolism because there are more cells.
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Surface area to volume ratio

  • When an organism increases in size, its volume increases because there are more cells.
  • The increase in the number of cells means that there is less surface area for the exchange of materials because many cells are in contact with one another.
    • The surface area to volume ratio (SA : V) decreases with increasing organism size.

Exchange

Single-celled organisms exchange directly with their external environment. Multicellular organisms need adaptions to sufficiently exchange materials with the environment.

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Need for exchange

  • Single-celled organisms need to obtain substances from their environment to be used in processes inside the cell (e.g. oxygen for respiration).
  • Waste substances also need to be removed from the cell (e.g. carbon dioxide) to avoid harming the cell.
  • Substances are exchanged by all organisms for this reason.
    • So, the surface area of the plasma membrane needs to be sufficiently large enough to adequately exchange useful substances and waste.
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Body surface exchange

  • Single-celled organisms can exchange gasses and other substances using their cell membrane.
  • The rate of gas exchange is increased by a larger surface area to volume ratio.
    • Single-celled organisms can be adapted to increase their surface area to volume ratio (e.g. by making themselves wide, flat or with multiple folds).
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Diffusion rate

  • Diffusion rate is rapid for single-celled organisms because the substances only have to move across one cell-surface membrane.
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Larger organisms

  • As cells or organisms increase in size, their surface area to volume ratio decreases.
  • This decreases the rate of exchange with the environment.
  • This means larger cells and multicellular organisms need adaptions to allow the effective exchange of materials with the environment.
    • Examples of adaptions included having highly folded membranes to increase the surface area to volume ratio.

Jump to other topics

1Cell Structure

2Biological Molecules

3Enzymes

4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

7Transport in Plants

8Transport in Mammals

9Gas Exchange

10Infectious Diseases

11Immunity

12Energy & Respiration (A2 Only)

13Photosynthesis (A2 Only)

14Homeostasis (A2 Only)

15Control & Coordination (A2 Only)

16Inherited Change (A2 Only)

17Selection & Evolution (A2 Only)

18Classification & Conservation (A2 Only)

19Genetic Technology (A2 Only)

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