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3.1.1Exchange and Transport

Surface Area to Volume Ratio — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 4 of 48 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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3.1.1 Surface Area to Volume Ratio — AQA A Level Biology 7402 notes: Calculate and explain surface area to volume ratios and relate them to the need for specialised exchange surfaces in larger organisms.All organisms must exchange substances with their environment: oxygen, carbon…

  • diffusion across the body surface
  • Small organisms
  • mass transport system
  • large surface area
  • steep concentration gradient
  • Exam-style question 1

Section 1 of 9Why exchange matters

All organisms must exchange substances with their environment: oxygen, carbon dioxide, water, nutrients, heat and waste products such as urea.

1Diffusion across the body surface
In a small organism, such as a single-celled amoeba, this can occur by diffusion across the body surface because the distance from the surface to the centre is short, and the surface area is large relative to the volume of the cell that needs supplying.
2
In a large organism, materials cannot diffuse fast enough to reach all the cells, so specialised exchange surfaces and transport systems are needed.

Section 2 of 9Calculating surface area and volume

As an organism increases in size, its volume increases faster than its surface area. This can be shown with cubes. For a cube of side length l: surface area = 6l2, volume = l3, so the ratio is 6 / l.

Side (cm)Surface area (cm2)Volume (cm3)SA : V
1616 : 1
22483 : 1
496641.5 : 1

Doubling the side length multiplies the area by 4 but the volume by 8, so the ratio halves.

Calculating surface area and volume
For a sphere, surface area = 4πr2 and volume = (4/3)πr3, so the ratio is 3 / r.
In an exam, show formulae, working and units, and give the ratio in the form x : 1 so you can compare.

Section 3 of 9Consequences for organisms

1
Small organisms
(unicellular, and flat worms) have a large SA:V ratio. Diffusion across the body surface supplies their needs, so they do not need specialised systems.
2
Larger organisms
Larger organisms have a smaller SA:V ratio.
  • The diffusion distance is greater and the surface area is too small to supply the volume of cells.
  • They need specialised exchange surfaces (lungs, gills, villi),
    • and a mass transport system to move substances to and from the exchange surfaces.
      • (circulatory system)
3
Organisms can also reduce the problem by changing their shape to increase surface area: flat shapes (flatworms), folded surfaces, or branching structures.

Section 4 of 9Features of a good exchange surface

Whatever the organism, effective exchange surfaces have the same features, related to Fick's law:

Features of a good exchange surface
large surface area
(folding, such as villi and microvilli; many small units, such as alveoli);
thin
thin often one cell thick - so there is a short diffusion distance;
steep concentration gradient
steep concentration gradient maintained by a blood supply or by ventilation;
permeable
permeable and often with transport proteins in the membrane.

These are explored in later lessons on the alveoli, fish gills, insect tracheae and leaves.

Section 5 of 9Links to heat and body size

Surface area to volume ratio also affects heat exchange.

Links to heat and body size
Small animals lose heat rapidly relative to their size, so they need a high metabolic rate to stay warm (see the lesson on SA:V and metabolic rate), whereas large animals lose less heat per unit mass.
Animals in cold climates tend to be larger and compact (Bergmann's rule) with small extremities, and those in hot climates tend to have large ears, thin bodies or long limbs to lose heat.
These ideas are applied in questions that ask you to explain adaptations of animals to their environment.

Section 6 of 9Worked problems and organisms with unusual shapes

Most organisms are not cubes, so in exams you may be given a simple shape and asked to treat it as a cuboid, a cylinder or a sphere.

1
For a cuboid of sides a, b and c, surface area = 2(ab + bc + ac) and volume = abc.
2
For a cylinder of radius r and length l, area = 2πrl + 2πr2, volume = πr2l.
3
Compare shapes of equal volume: a flattened or elongated shape always has a larger surface area than a compact one, which is why flatworms, leaves and the folded gut lining are efficient at exchange.
4
Notice also that the ratio can be misleading if the units differ; keep all lengths in the same unit.
5
A good tip is to calculate a ratio for each organism and then comment on how it relates to its need for a transport system, a ventilation mechanism or insulation.

Microscopy skills link herea cell seen in a micrograph can be measured, the actual size calculated as image size divided by magnification, and then used to estimate its SA:V ratio.

Section 7 of 9Exam-style questions

  1. A cube has sides of 3 cm. Calculate its surface area to volume ratio. [2]2 marks

  2. Explain why a large multicellular animal needs a specialised gas exchange system, but an amoeba does not. [3]3 marks

  3. Describe the features of an efficient exchange surface. [3]3 marks

Section 8 of 9Key terms

TermMeaning
Surface area to volume ratioTotal surface area of an organism divided by its volume
Exchange surfaceRegion where substances are exchanged between an organism and its environment
Diffusion distanceDistance a substance has to travel by diffusion
Mass transportMovement of substances in bulk over long distances in a transport system
Concentration gradientDifference in concentration between two regions
Metabolic rateRate at which an organism uses energy

Section 9 of 9Exam technique

1
Saying that large organisms have a smaller surface area - they have a larger area but a smaller area relative to volume.
2
Quoting the ratio without units or in an unusable form.
3
Forgetting that volume increases faster than area.
4
Thinking all organisms need a transport system - small ones do not.
5
Not mentioning diffusion distance when explaining why diffusion is too slow.

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