AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
A single-celled organism has a relatively large surface area to volume…
Full specification wording
Diffusion: A single-celled organism has a relatively large surface area to volume ratio.
Surface area to volume ratio compares the total external surface area of an organism with its volume.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 6 of 11
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Surface area to volume ratio compares the total external surface area of an organism with its volume. As an object becomes smaller, its surface area falls, but its volume falls much faster, so the ratio of surface area to volume rises. A single-celled organism such as Amoeba is very small, so it has a relatively large surface area to volume ratio. This means that for each unit of its volume there is a comparatively large area of cell surface membrane through which substances can diffuse. The ratio is calculated by dividing surface area by volume, and the same principle explains why large multicellular organisms need specialised exchange surfaces and transport systems.
What earns the marks
- Define surface area to volume ratio as surface area divided by volume.
- Explain that as size decreases, volume decreases more rapidly than surface area, so the ratio increases.
- State that a single-celled organism is small and therefore has a relatively large surface area to volume ratio.
- Link the large ratio to a large area of cell surface membrane relative to the volume of cytoplasm that must be supplied.
- Contrast this with a large multicellular organism, which has a smaller surface area to volume ratio and needs specialised exchange surfaces.
- Use a simple numerical example, such as a cube of side 1 unit having a ratio of 6 compared with a cube of side 2 units having a ratio of 3.
Common mistakes and how to put them right
Saying a single-celled organism has a large surface area in absolute terms.
Put it right by saying it has a large surface area relative to its volume.
Inverting the ratio by dividing volume by surface area.
Put it right by dividing surface area by volume.
Believing that all small organisms have a large surface area.
Put it right by specifying that the ratio, not the absolute area, is large.
Exam tips
- Write the ratio as surface area divided by volume and show the substitution if values are given.
- Use the phrase 'relative to its volume' whenever you describe the ratio of a small organism.
- Compare a small and a large organism in the same answer to demonstrate understanding of the trend.
Try the lesson
The start of the Diffusion: A single-celled organism has a relatively large surface area to volume ratio. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the surface area to volume ratio of a typical single-celled organism?
Listen
Surface area to volume ratio compares the total external surface area of an organism with its volume. As an object becomes smaller, its surface area falls, but its volume falls much faster, so the ratio of surface area to volume rises. A single-celled organism such as Amoeba is very small, so it has a relatively large surface area to volume ratio. This means that for each unit of its volume there is a comparatively large area of cell surface membrane through which substances can diffuse. The ratio is calculated by dividing surface area by volume, and the same principle explains why large multicellular organisms need specialised exchange surfaces and transport systems. A strong answer earns marks for these points. Define surface area to volume ratio as surface area divided by volume. Explain that as size decreases, volume decreases more rapidly than surface area, so the ratio increases. State that a single-celled organism is small and therefore has a relatively large surface area to volume ratio. Link the large ratio to a large area of cell surface membrane relative to the volume of cytoplasm that must be supplied. A common slip: Saying a single-celled organism has a large surface area in absolute terms. Instead, saying it has a large surface area relative to its volume. Exam tip: Write the ratio as surface area divided by volume and show the substitution if values are given.
Key words
Say what each word means before you turn the card.
Surface area to volume ratio
Try answering aloud or in your head before turning the card.
Scale and cell size
Try answering aloud or in your head before turning the card.
Exchange surfaces
Try answering aloud or in your head before turning the card.
See every definition
- Surface area to volume ratio
- Compares the total external surface area of an organism with its volume, calculated by dividing surface area by volume.
- Scale and cell size
- As an object becomes smaller, its surface area falls, but its volume falls much faster, so the ratio of surface area to volume rises.
- Exchange surfaces
- Specialised surfaces that large multicellular organisms need because they have a smaller surface area to volume ratio.
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