1. 1
   A student makes three cubes of agar jelly containing a pH indicator, with sides of 1 cm, 2 cm and 4 cm. She places them in dilute acid and times how long it takes for the colour change to reach the centre of each cube. The times are 4 minutes for the 1 cm cube, 15 minutes for the 2 cm cube and 60 minutes for the 4 cm cube.

   (a)

   What is the surface area-to-volume ratio of the 2 cm cube?

   [1 mark]

   • A0.33 cm⁻¹
   • B1.5 cm⁻¹
   • C3 cm⁻¹
   • D6 cm⁻¹

   (b)

   Which statement best explains why the 4 cm cube took the longest time?

   [1 mark]

   • AIt has the lowest surface area-to-volume ratio and the greatest distance from surface to centre
   • BIt has the largest surface area, so acid entered more slowly
   • CAcid diffuses more slowly through large pieces of agar than through small ones
   • DIts larger volume made the acid more dilute before it entered

   (c)

   Suggest why a large, active animal cannot rely on diffusion across its body surface for gas exchange.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student paints clear nail varnish on the lower surface of a sun leaf and a shade leaf taken from the same beech tree (Fagus sylvatica), peels off the dried casts and counts the stomata visible at high power. The circular field of view has a diameter of 0.40 mm. Her counts in five fields of view for the sun leaf are 18, 22, 25, 19 and 21. The mean count for the shade leaf, also from five fields, is 12.

   (a)

   What is the mean stomatal density of the sun leaf?

   [1 mark]

   • A42 mm⁻²
   • B53 mm⁻²
   • C131 mm⁻²
   • D167 mm⁻²

   (b)

   Why did the student count stomata in five fields of view rather than one?

   [1 mark]

   • ATo make the measurement more accurate by using a higher magnification
   • BBecause stomatal numbers vary across a leaf, so repeated counts give a more reliable mean
   • CTo avoid counting the same stoma twice
   • DBecause the shade leaf has fewer stomata than the sun leaf

   (c)

   Explain why the sun leaf is likely to lose water by transpiration faster than the shade leaf.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A 17-year-old swimmer breathes into a spirometer. At rest, each breath moves 0.5 dm³ of air in and out. After a normal inhalation she can breathe in a further 3.0 dm³ with maximum effort, and after a normal exhalation she can force out a further 1.2 dm³. After 12 weeks of swim training the measurements are repeated once: each resting breath is still 0.5 dm³, the extra volume she can breathe in is 3.3 dm³ and the extra volume she can force out is 1.4 dm³. Her coach claims the training has increased her lung capacity.

   (a)

   Explain how the muscles and ribs cause the forced exhalation used to measure the extra 1.2 dm³.

   [3 marks]

   (b)

   Using the data, evaluate the coach's claim.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Babies born before about 30 weeks of pregnancy often produce too little pulmonary surfactant, and many develop respiratory distress: their alveoli collapse at the end of each exhalation and breathing is laboured. In one hospital study of babies born at 26 to 28 weeks, 60% of those given no artificial surfactant developed severe respiratory distress, compared with 25% of those given artificial surfactant into the trachea shortly after birth.

   (a)

   Explain the adaptations of the mammalian lung, including surfactant, that allow efficient gas exchange.

   [6 marks]

   (b)

   Using the data, discuss the importance of surfactant and the extent to which the study shows that artificial surfactant prevents respiratory distress.

   [6 marks]

   Total for question 4: 12 marks

End of questions