Haemoglobin and oxygen transportEdexcel International A Level Biology: Subtopic test

1. 1
   A mountaineer climbs from sea level to a camp at 5,000 m, where the partial pressure of oxygen in the air she breathes is about half of its value at sea level.

   (a)

   Each molecule of adult haemoglobin contains four haem groups. What is the maximum number of oxygen molecules that one haemoglobin molecule can carry?

   [1 mark]

   • A1
   • B2
   • C4
   • D8

   (b)

   Compared with sea level, what happens to the percentage saturation of haemoglobin with oxygen in her blood leaving the lungs at 5,000 m?

   [1 mark]

   • AIt rises, because the diffusion gradient into the blood is steeper
   • BIt falls, because the lower partial pressure of oxygen in the alveoli means less oxygen binds to haemoglobin
   • CIt stays at 100%, because haemoglobin always has a high affinity for oxygen
   • DIt rises, because haemoglobin has a higher affinity for oxygen at low partial pressures

   (c)

   Explain how haemoglobin is able to pick up oxygen in the lungs and release it in respiring tissues.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During a marathon, a runner's leg muscle cells respire rapidly and release large amounts of carbon dioxide into the blood.

   (a)

   Which statement describes the Bohr effect?

   [1 mark]

   • AHaemoglobin has a higher affinity for oxygen when the carbon dioxide concentration is high
   • BHaemoglobin binds carbon dioxide more readily than it binds oxygen
   • CA high carbon dioxide concentration shifts the oxygen dissociation curve to the left
   • DA high carbon dioxide concentration shifts the oxygen dissociation curve to the right, so more oxygen is released at a given partial pressure

   (b)

   In which form is most of the carbon dioxide carried in the blood from the muscles to the lungs?

   [1 mark]

   • AAs hydrogencarbonate ions in the plasma
   • BAs dissolved carbon dioxide in the plasma
   • CAs carbaminohaemoglobin in the red blood cells
   • DAs carbonic acid in the plasma

   (c)

   Explain how the carbon dioxide released by the muscle cells increases the supply of oxygen to them.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Oxygen passes from a mother's blood to her fetus's blood across the placenta. In the placenta the partial pressure of oxygen is about 3 kPa. At this partial pressure the mother's (adult) haemoglobin is about 45% saturated with oxygen, whereas the fetal haemoglobin is about 65% saturated.

   (a)

   Explain how the difference in saturation at 3 kPa allows the fetus to obtain enough oxygen from its mother's blood.

   [3 marks]

   (b)

   Describe how the oxygen dissociation curve of fetal haemoglobin differs from that of adult haemoglobin, and suggest why fetal haemoglobin is gradually replaced by adult haemoglobin after birth.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A physiologist studies haemoglobin in blood samples taken from adult volunteers at rest and during vigorous exercise. She exposes the samples to different partial pressures of oxygen and carbon dioxide and records the percentage saturation of haemoglobin with oxygen.

   (a)

   Explain why the oxygen dissociation curve of adult haemoglobin is S-shaped and how this shape helps haemoglobin to transport oxygen efficiently.

   [6 marks]

   (b)

   During vigorous exercise the muscle cells respire much faster. Explain how the transport of carbon dioxide in the blood and the properties of haemoglobin ensure that the muscle cells receive more oxygen.

   [6 marks]

   Total for question 4: 12 marks

End of questions