1. 1
   A leafy shoot of cherry laurel (Prunus laurocerasus) is cut under water and fitted into a bubble potometer. The rate at which the air bubble moves along the capillary tube is recorded in three conditions: still air at 20 °C, 12 mm per minute; air moved by a fan at 20 °C, 30 mm per minute; shoot enclosed in a clear plastic bag at 20 °C, 4 mm per minute.

   (a)

   From which tissue of the leaf does most of the water evaporate before diffusing out through the stomata?

   [1 mark]

   • AUpper epidermis
   • BPalisade mesophyll
   • CSpongy mesophyll
   • DWaxy cuticle

   (b)

   Which statement explains the faster rate when the fan is switched on?

   [1 mark]

   • AThe fan raises the temperature of the leaf
   • BMoving air forces extra air into the stomata
   • CThe fan causes the guard cells to close
   • DMoving air removes water vapour from around the stomata, keeping the concentration gradient steep

   (c)

   The potometer measures the rate of water uptake. Explain why this is not exactly the same as the rate of transpiration.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In the placenta, the partial pressure of oxygen is about 4 kPa. At this partial pressure adult haemoglobin is about 55% saturated with oxygen, whereas foetal haemoglobin is about 75% saturated. In the mother's lungs, at about 13 kPa, adult haemoglobin is about 98% saturated. When percentage saturation is plotted against partial pressure of oxygen, both types of haemoglobin give an S-shaped curve, with the foetal curve lying to the left of the adult curve.

   (a)

   Which explains the S-shape of the curves?

   [1 mark]

   • AEach of the four haem groups has a different fixed affinity for oxygen
   • BBinding of one oxygen molecule changes the shape of haemoglobin, increasing the affinity of the remaining haem groups
   • CCarbon dioxide binds to the haem groups at low partial pressures of oxygen
   • DHaemoglobin can only bind oxygen once all four haem groups are empty

   (b)

   What does the position of the foetal curve to the left of the adult curve show?

   [1 mark]

   • AFoetal haemoglobin has a higher affinity for oxygen at any given partial pressure
   • BFoetal haemoglobin has a lower affinity for oxygen
   • CFoetal haemoglobin cannot become fully saturated
   • DFoetal haemoglobin carries carbon dioxide instead of oxygen

   (c)

   Using the data, explain how oxygen passes from the mother's blood to the foetus in the placenta.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Each 100 cm³ of human blood carries 20 cm³ of oxygen when its haemoglobin is fully saturated. Blood leaving the lungs is 98% saturated. In a vigorously exercising leg muscle the partial pressure of oxygen falls to 3 kPa. At 3 kPa, haemoglobin would be 40% saturated if the partial pressure of carbon dioxide were 6 kPa, as in resting tissue, but in the exercising muscle the partial pressure of carbon dioxide is 9 kPa and haemoglobin is only 30% saturated.

   (a)

   Explain how the higher partial pressure of carbon dioxide causes haemoglobin in the exercising muscle to release more oxygen.

   [3 marks]

   (b)

   Calculate the volume of oxygen released from each 100 cm³ of blood in the exercising muscle, with and without the effect of carbon dioxide, and evaluate the benefit of this effect.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   At sea level, the partial pressure of oxygen in a mountaineer's alveoli is about 13 kPa and her arterial haemoglobin is about 98% saturated. She climbs to 5000 m, where atmospheric pressure is about half that at sea level. Her alveolar partial pressure of oxygen falls to about 6.5 kPa, yet her arterial haemoglobin is still about 85% saturated. Within hours of arriving, her breathing becomes deeper and faster.

   (a)

   Explain how the lungs are ventilated, including forced exhalation, and how faster, deeper breathing helps gas exchange at 5000 m.

   [6 marks]

   (b)

   Explain, in terms of cooperative binding, why her haemoglobin saturation fell only from 98% to 85% although alveolar oxygen partial pressure halved, and discuss the consequences for oxygen delivery to her tissues.

   [6 marks]

   Total for question 4: 12 marks

End of questions