1.14 Surface area to volume ratio and exchange surfaces
7 exam questions worth 20 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A cube has sides of 3 cm.
What is its surface area to volume ratio?
Choose one answer.
Correct Not quite: the answer is 2 : 1
Mark scheme
A (2 : 1)
Model answer
A: 2 : 1. Surface area = 6 × 32 = 54 cm2 and volume = 33 = 27 cm3, so the ratio is 54 : 27 = 2 : 1.
A cuboid is 4 cm long, 2 cm wide and 1 cm high.
Calculate its surface area to volume ratio.
Mark scheme
- surface area = 28 (cm2) (1)
- volume = 8 (cm3) (1)
- ratio = 28 : 8 = 3.5 : 1 (1)
Model answer
Surface area = 2 × (4 × 2) + 2 × (4 × 1) + 2 × (2 × 1) = 16 + 8 + 4 = 28 cm2. Volume = 4 × 2 × 1 = 8 cm3. The ratio is 28 : 8 = 3.5 : 1.
Explain why a single-celled organism does not need an exchange surface, but a large multicellular organism does.
Mark scheme
- a single-celled organism has a large surface area to volume ratio (1)
- so substances can diffuse across its surface fast enough to meet its needs (1)
- a multicellular organism has a small surface area to volume ratio / a long diffusion distance to the inner cells (1)
- so it needs specialised exchange surfaces and a transport system (1)
Model answer
A single-celled organism has a large surface area compared with its volume, so enough oxygen and food can diffuse across its surface to meet its needs. A large multicellular organism has a small surface area to volume ratio and the distance to its inner cells is long, so diffusion across the outside is too slow. It needs specialised exchange surfaces and a transport system.
Describe three features of the alveoli that make gas exchange efficient.
Mark scheme
Any three from:
- many alveoli / large surface area (1)
- thin walls / one cell thick, so a short diffusion distance (1)
- good blood supply / many capillaries (1)
- ventilation / breathing keeps a steep concentration gradient (1)
Model answer
There are millions of alveoli, which give a large surface area. Their walls are only one cell thick, so the diffusion distance is short. They are surrounded by many capillaries, which give a good blood supply.
Explain how the villi help the small intestine to absorb digested food.
Mark scheme
- villi increase the surface area (1)
- so more digested food / glucose can be absorbed into the blood at the same time (1)
- allow: thin walls give a short diffusion distance / a rich blood supply keeps the concentration gradient steep
Model answer
The villi increase the surface area of the small intestine, so more digested food, such as glucose, can be absorbed into the blood at once.
A mouse loses heat much faster than an elephant, in proportion to its size.
Use ideas about surface area to volume ratio to explain why.
Mark scheme
- the mouse has a larger surface area to volume ratio (1)
- heat is lost from the surface (1)
- so the mouse loses more heat for each unit of volume / body mass (1)
Model answer
The mouse has a much larger surface area to volume ratio than the elephant. Heat is lost through the surface, so the mouse loses more heat for each unit of its volume.
Explain how the structure of a fish gill is adapted for gas exchange.
Mark scheme
Any four from:
- many gill filaments / lamellae (1) give a large surface area (1)
- lamellae are thin / one cell thick (1) so a short diffusion distance (1)
- a good blood supply (1) carries oxygen away and keeps a steep concentration gradient (1)
- water flows over the gills (1) so there is a continuous supply of oxygen (1)
Model answer
A gill has many thin filaments covered in lamellae, which give a very large surface area. The lamellae are very thin, so oxygen has a short distance to diffuse. A rich blood supply carries the oxygen away and keeps the concentration gradient steep, and water flows over the gills, which keeps supplying fresh oxygen.
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