Haemoglobin and oxygen transportAQA A-Level Biology: Subtopic test

1. 1
   Haemoglobin is carried inside red blood cells. A student is studying the structure of human haemoglobin and how it takes up oxygen in the lungs. She notes that the oxyhaemoglobin dissociation curve for haemoglobin is S-shaped, with a shallow start and then a steep rise as the partial pressure of oxygen increases.

   (a)

   Which statement describes the quaternary structure of human haemoglobin?

   [1 mark]

   • AA single polypeptide chain containing one haem group
   • BFour polypeptide chains, each containing a haem group with an iron ion (Fe²⁺)
   • CTwo polypeptide chains joined by a single disulfide bond, with no haem group
   • DFour haem groups joined to each other by peptide bonds, with no polypeptide chains

   (b)

   Which term describes the effect whereby the binding of one oxygen molecule to haemoglobin makes it easier for further oxygen molecules to bind?

   [1 mark]

   • AThe Bohr effect
   • BCompetitive inhibition
   • CCooperative binding
   • DDenaturation

   (c)

   Explain why the oxyhaemoglobin dissociation curve is S-shaped, with a shallow start and then a steep rise.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During vigorous exercise, the muscle cells of a runner respire rapidly and release large amounts of carbon dioxide. This lowers the pH of the blood in the muscle capillaries. A sports scientist is investigating how this affects the way haemoglobin loads and unloads oxygen.

   (a)

   Compared with the curve at rest, how is the oxyhaemoglobin dissociation curve for the muscle capillaries affected?

   [1 mark]

   • AIt shifts to the left because the affinity of haemoglobin for oxygen is higher
   • BIt shifts upwards because haemoglobin can become more than 100% saturated
   • CIt does not change because the curve depends only on the partial pressure of oxygen
   • DIt shifts to the right because the affinity of haemoglobin for oxygen is lower

   (b)

   Which statement correctly compares haemoglobin in the capillaries of the lungs with haemoglobin in muscle capillaries?

   [1 mark]

   • AIn the lungs the lower carbon dioxide concentration gives haemoglobin a higher affinity for oxygen
   • BIn the lungs the lower carbon dioxide concentration gives haemoglobin a lower affinity for oxygen
   • CIn muscle capillaries haemoglobin loads oxygen and in the lungs it unloads oxygen
   • DThe affinity is the same in both because the haemoglobin molecules are identical

   (c)

   Explain how the effect of carbon dioxide on haemoglobin benefits the exercising muscle.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the placenta, oxygen passes from the mother's blood to the blood of the fetus. Adult haemoglobin is 50% saturated at a partial pressure of oxygen of 3.5 kPa, whereas fetal haemoglobin is 50% saturated at 2.4 kPa. At a partial pressure of oxygen of 4 kPa, similar to that in the placenta, adult haemoglobin is about 59% saturated and fetal haemoglobin is about 80% saturated. After birth, fetal haemoglobin is gradually replaced by adult haemoglobin over the first few months of life.

   (a)

   Use the information to explain how fetal haemoglobin allows the fetus to obtain oxygen from its mother's blood.

   [3 marks]

   (b)

   Suggest why it is an advantage for a baby to have haemoglobin with a lower affinity for oxygen after birth.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Llamas live at altitudes above 4000 m, where the partial pressure of oxygen in the air is only about 60% of that at sea level. Llama haemoglobin is 50% saturated at about 3.0 kPa, compared with 3.5 kPa for human haemoglobin. A shrew is a very small mammal with a very high metabolic rate. Its haemoglobin is 50% saturated at about 5 kPa.

   (a)

   Explain how the haemoglobin of the llama is adapted to its environment.

   [6 marks]

   (b)

   Explain how the haemoglobin of the shrew is adapted to its way of life, and why llama haemoglobin would be unsuitable for the shrew.

   [6 marks]

   Total for question 4: 12 marks

End of questions