Surface area to volume ratioEdexcel A-Level Biology B: Mind map

Exam questions on Surface area to volume ratio

1. A student made cubes of agar jelly containing a pH indicator, with sides of 2 mm, 4 mm and 8 mm, to model cells of different sizes. She placed them in dilute acid and timed how long the acid took to change the colour of the whole cube. (Surface area of a cube = 6 x side squared; volume = side cubed.)
   The 8 mm cube took much longer to change colour completely than the 2 mm cube. Explain this result.2 marks
2. The planarian is a flatworm about 15 mm long, 3 mm wide and less than 0.5 mm thick. It has no lungs, gills or blood. Its cells use oxygen from the water around it, which diffuses in across its body surface. A human, by contrast, is about 1.7 m tall and has lungs and a circulatory system.
   Explain how the shape of the planarian allows it to obtain enough oxygen without a circulatory system.2 marks
3. A spherical bacterium has a diameter of 2 µm, and a spherical human cell has a diameter of 20 µm. A human adult is a large multicellular organism made of many cells of this size. (Surface area of a sphere = 4πr²; volume of a sphere = 4/3 πr³.)
   Calculate the surface area to volume ratio of each cell and state how many times greater it is for the bacterium than for the human cell.3 marks