AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The relationship between the size of an organism or structure and its…
Full specification wording
Surface area to volume ratio: The relationship between the size of an organism or structure and its surface area to volume ratio.
Surface area increases with the square of a linear dimension while volume increases with its cube, so as an organism or structure gets larger its surface area to volume ratio falls.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 3
Study notes
From the checked teaching notes for this specification point.
The idea
Surface area increases with the square of a linear dimension while volume increases with its cube, so as an organism or structure gets larger its surface area to volume ratio falls. A 1 mm cube has 6 mm² of surface and 1 mm³ of volume, a ratio of 6:1; a 2 mm cube has 24 mm² over 8 mm³, a ratio of 3:1. The biological consequence is that exchange happens across surfaces while demand is generated throughout the volume. A large organism therefore has proportionally less surface through which to absorb oxygen and nutrients and to lose heat and waste, and the distance from the surface to the innermost cells becomes too great for diffusion to supply them in time. Small organisms can rely on diffusion across the body surface alone.
What earns the marks
- Correct calculation of surface area and of volume in the same units.
- Dividing surface area by volume and expressing the answer as a ratio to 1.
- Each correct row where the question supplies a table to complete, using the values given in that question.
- Stating that as size increases volume increases faster than surface area, so the ratio falls.
- Linking the falling ratio to exchange: proportionally less surface for the same demand, and longer diffusion distances.
Common mistakes and how to put them right
Giving the ratio the wrong way round, as volume to surface area.
Mixing units, for example µm² with mm³, before dividing.
Quoting an unrounded value when the question asks for a given number of significant figures.
Saying a larger organism has a smaller surface area, when it has a larger area but a smaller ratio.
Writing no working, so the method mark is lost when the arithmetic slips.
Exam tips
- Write surface area divided by volume as a line of working before using the calculator; a method mark is available even if the final value is wrong.
- Read the significant figures instruction twice and round only at the end of the calculation.
- Compare ratios, not raw surface areas, whenever the question involves two organisms of different size.
Try the lesson
The start of the Surface area to volume ratio: The relationship between the size of an organism or structure and its surface area to volume ratio. lesson, made from checked teaching notes for this specification point.
Have a go first
As an organism increases in size, what happens to its surface area to volume ratio?
Listen
Surface area increases with the square of a linear dimension while volume increases with its cube, so as an organism or structure gets larger its surface area to volume ratio falls. A 1 mm cube has 6 mm² of surface and 1 mm³ of volume, a ratio of 6:1; a 2 mm cube has 24 mm² over 8 mm³, a ratio of 3:1. The biological consequence is that exchange happens across surfaces while demand is generated throughout the volume. A large organism therefore has proportionally less surface through which to absorb oxygen and nutrients and to lose heat and waste, and the distance from the surface to the innermost cells becomes too great for diffusion to supply them in time. Small organisms can rely on diffusion across the body surface alone. A strong answer earns marks for these points. correct calculation of surface area and of volume in the same units. dividing surface area by volume and expressing the answer as a ratio to 1. each correct row where the question supplies a table to complete, using the values given in that question. stating that as size increases volume increases faster than surface area, so the ratio falls. Exam tip: Write surface area divided by volume as a line of working before using the calculator; a method mark is available even if the final value is wrong.
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