2.2.3

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.

Surface area

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.
Volume

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.
Surface area to volume ratio

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.

Adaptations to Surface Area to Volume Ratio

Some organisms have evolved adaptations that allow them to overcome a decrease in surface area to volume ratio. These adaptations include:

Large surface area

Large surface area

  • A large surface area allows more of a substance to diffuse at the same time.
  • Prokaryotic organisms have enormous SA:V, allowing them to meet the cell’s energy requirements through respiration that occurs across the plasma membrane only.
  • Eukaryotic cells are far larger so have a smaller SA:V, this necessitates the need for specialist organelles for respiration (mitochondria) that have huge membrane surface area in which enough proteins for respiration can be embedded to meet the larger cell’s needs.
Thin membrane

Thin membrane

  • A thin membrane reduces the diffusion distance.
Transport systems

Transport systems

  • Some organisms have transport systems that move substances around the organism or facilitate exchange.
    • E.g. Blood vessels, lungs, gills, xylem.
  • Transport systems allow a higher concentration gradient to be maintained.
    • A steep concentration gradient increases the rate of transport.
  • Transport systems also allow substances to move closer to the cells that need to exchange them.
Heat transfer

Heat transfer

  • Some large animals have adaptations that allow heat to be transferred more efficiently.
    • E.g. Elephants have large, highly vascularised ears that allow heat to be transferred rapidly.
  • Lower latitudes have warmer climates so the animals that live there need to transfer heat to the surroundings in order not to overheat. Animals living in the polar regions must try to conserve as much heat as possible due to cold external temperatures.
  • A general trend in animal biology is for occupants of equivalent niches in low latitudes to be smaller than their high latitude counterparts. This is because smaller animals have larger SA:V so can transfer heat to the surroundings more efficiently.
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Practice questions on Surface Area to Volume Ratio

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    Why does the surface area of cells decrease when the number of cells increases?Multiple choice
  2. 2
    Which of the following statements about surface area to volume ratio are true?True / false
  3. 3
    Which of the following animals has the largest surface area to volume ratio?Multiple choice
  4. 4
    What is the adaptive advantage of a thin membrane?Multiple choice
  5. 5
    Which of these statements about surface area to volume ratio adaptations are true?True / false
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Cell Size & Surface Area

Eukaryotic vs Prokaryotic Cells