Surface Area to Volume Ratio

Biology: Cambridge International A Level, topic 4.2.5

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

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.

Exchange

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

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.

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).

Diffusion rate

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

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.

Test yourself on Surface Area to Volume Ratio

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?
Which of the following statements about substance exchange are true?
Which of these is NOT an adaptation of single-celled organisms to increase their surface area to volume ratio?

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