Size & Surface Area
Biology: AQA A Level, topic 3.1.1
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.
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
- 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
- A large surface area allows more of a substance to diffuse at the same time.
Thin membrane
- A thin membrane reduces the diffusion distance.
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
- 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.
Test yourself on Size & Surface Area
Why does the surface area of cells decrease when the number of cells increases?
Which of the following statements about surface area to volume ratio are true?
Which of the following animals has the largest surface area to volume ratio?
What is the adaptive advantage of a thin membrane?
Which of these statements about surface area to volume ratio adaptations are true?