Haemoglobin dissociation curves

Question 1

Explain the quaternary structure of a haemoglobin molecule (3)

Answer 1

1. Comprised of 4 polypeptide chains - 2 alpha - 2 beta
2. Each polypeptide associated with a haem group (Fe2+ ion) - total 4 haem groups
3. Each Fe2+ ion can bind with one O2 molecule - total of 4 O2 molecule per haemoglobin

Question 2

State how CO2 affects haemoglobin’s affinity for O2 (2)

Answer 2

1. Low partial pressure of CO2 = high affinity for O2

2. High partial pressure of CO2 = low affinity for O2

Question 3

Explain the shape of the Oxygen dissociation curve (4)

Answer 3

1. Sigmoid shape
2. Shape of haemoglobin molecule makes it difficult for first O2 molecule to bind so low affinity initially
3. Binding of first O2 changes shape of haemoglobin making it increasingly easier for second and third O2 to bind (Conformational change) - affinity increases
4. After binding of 3rd O2, probability of 4th O2 binding decreases although it is easier to do so, as the no. Of binding site decreases dramatically - affinity decreases

Question 4

Explain the effects of a shift in O2 dissociation curves (2)

Answer 4

1. A shift to the left is an increase in O2 affinity

2. Shift to right is a decrease in O2 affinity

Question 5

Explain the effects of O2 affinity of haemoglobin (2)

Answer 5

1. A high O2 affinity means O2 is loaded more readily but unloads less easily
2. A low O2 affinity means O2 is loaded less readily but unloads more easily

Question 6

Explain how Oxygen is loaded at the lungs and unloaded at tissues by haemoglobin (Bohr effect) (4)

Answer 6

1. At the lungs, concentration of CO2 is low as it diffuses across the gas exchange surface and is excreted from the organism
2. A lower concentration of CO2 means haemoglobin has a higher affinity for O2, and with a larger partial pressure of O2 in the lungs, O2 is loads forming ‘oxyhaemoglobin’
3. In respiring tissues, the concentration of CO2 is high and the affinity of O2 is decreased
4. Low partial pressures of O2 in respiring tissues also means that O2 is unloaded (dissociated) to muscle cells

Question 7

Explain how Carbon dioxide aids in the (un)loading of O2 by haemoglobin (4)

Answer 7

1. CO2 forms an acidic solution so that it lowers pH
2. A lower pH changes the shape of haemoglobin reducing its affinity for O2
3. A higher concentration of CO2 produces a larger decrease in pH so that haemoglobins affinity for O2 is further reduced
4. This way, when a cell is respiring more, it produces more CO2 which causes haemoglobin to release more O2 that it needs

Question 8

Explain how foetal haemoglobin is adapted (3)

Answer 8

1. Foetal haemoglobin has a higher affinity for O2 at low partial pressures of O2 than adult haemoglobin
2. At low partial pressures at placenta, mother’s haemoglobin dissociates releasing O2 and foetal haemoglobin associates with the O2 (as higher affinity)
3. There is a higher partial pressure of O2 in mother’s blood than in foetal blood entering placenta so concentration gradient maintained