Transport of gases in the bloodEdexcel A-Level Biology B: Subtopic test

1. 1
   Adult haemoglobin is a protein made of four polypeptide chains. Each chain contains a haem group with one iron(II) ion at its centre. Experiments show that the oxygen dissociation curve of haemoglobin is S-shaped, because the first oxygen molecule binds slowly but later molecules bind more easily.

   (a)

   Which level of protein structure describes the combination of four polypeptide chains in a haemoglobin molecule?

   [1 mark]

   • AQuaternary structure
   • BPrimary structure
   • CSecondary structure
   • DTertiary structure

   (b)

   What is the maximum number of oxygen molecules that one adult haemoglobin molecule can carry?

   [1 mark]

   • A2
   • B3
   • C4
   • D8

   (c)

   Explain why the oxygen dissociation curve of haemoglobin is S-shaped.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During vigorous exercise the muscle cells of an athlete respire rapidly. The partial pressure of carbon dioxide in the muscle capillaries rises and the pH of the blood in the capillaries falls. The oxygen dissociation curve of the haemoglobin in these capillaries changes position.

   (a)

   In which direction does the oxygen dissociation curve of the haemoglobin shift in the muscle capillaries of the athlete?

   [1 mark]

   • ATo the left
   • BIt does not change
   • CDownwards, so the maximum saturation is lower
   • DTo the right

   (b)

   What is the effect of this change on the haemoglobin in the muscle capillaries?

   [1 mark]

   • AHaemoglobin binds oxygen more tightly
   • BHaemoglobin releases more oxygen to the muscle at a given partial pressure of oxygen
   • CHaemoglobin loses its iron(II) ions
   • DHaemoglobin releases less carbon dioxide

   (c)

   Explain how the rise in carbon dioxide concentration in the muscle capillaries causes oxygen to be released from haemoglobin more readily.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Seals dive for long periods without breathing. Their muscles contain a very high concentration of myoglobin. Myoglobin is a single polypeptide with one haem group. At a partial pressure of oxygen of 2 kPa, myoglobin is more than 80% saturated with oxygen, whereas adult haemoglobin is only about 25% saturated.

   (a)

   Compare the structure and function of myoglobin with those of adult haemoglobin.

   [3 marks]

   (b)

   Explain how the high concentration of myoglobin in the muscles allows a seal to stay underwater for a long time.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the placenta, oxygen passes from the blood of a pregnant woman to the blood of her fetus. At a partial pressure of oxygen of 4 kPa, the mother's haemoglobin is about 60% saturated with oxygen, whereas the fetal haemoglobin is about 80% saturated.

   (a)

   Explain how the mother's haemoglobin picks up oxygen in her lungs and releases it to her respiring tissues, referring to the Bohr effect.

   [6 marks]

   (b)

   Explain the significance of the higher oxygen affinity of fetal haemoglobin, and explain why an adult who had only fetal haemoglobin would be disadvantaged.

   [6 marks]

   Total for question 4: 12 marks

End of questions