IAL Edexcel Biology - Haemoglobin

1.9(i) Understand the role of haemoglobin in the transport of oxygen and carbon dioxide

1.9(ii) Understand the oxygen dissociation curve of haemoglobin, the Bohr effect and the significance of the oxygen affinity of fetal haemoglobin compared with adult haemoglobin

• A large globular protein molecule found in red blood cells
• Made up of 4 subunits (peptide chains), each with an iron-containing heme group
• Plays a role in transportation of oxygen and carbon dioxide in blood
• Each heme group can bind 1 oxygen molecule, so each haemoglobin molecule can bind at most 4 oxygen molecules

• Oxygen binds reversibly to haemoglobin and is carried in the form of oxyhaemoglobin

• The oxygen dissociation curve is sigmoidal
  • In the lungs (top right part of the curve), a small increase in partial pressure causes a large increase in % saturation (i.e. oxygen bound rapidly)
  • In the body tissues (bottom left part of the curve), a small decrease in partial pressure causes a large decrease in % saturation (i.e. oxygen released rapidly)
• The affinity of haemoglobin for oxygen is affected by CO2 level (Bohr Effect), pH and temperature

👇 For IAL Edexcel Biology, you would need to know how the oxygen dissociation curve changes as a result of carbon dioxide level changes 👇

• When carbon dioxide level in the surrounding rises (e.g. in body tissues), curve shifts downwards and to the right
  • Reduced affinity of haemoglobin for oxygen
  • Haemoglobin needs higher levels of oxygen to be saturated and releases oxygen more easily
• When carbon dioxide level in the surrounding drops (e.g. in lung capillaries), curve shifts upwards and to the left
  • Increased affinity of haemoglobin for oxygen 
  • Easier for oxygen to bind to haemoglobin

• Higher affinity for oxygen as compared to adult (or maternal) haemoglobin
• Can remove oxygen from maternal blood even when proportion of oxygen is relatively low

• Majority react with water in plasma (catalyzed by carbonic anhydrase) and carried in the form of carbonic acid
• Some bind to haemoglobin to form carbaminohaemoglobin
• Some dissolve in plasma

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Written by Vivian | GCE Biology Specialist @ TUTTEE

  References:

1. Edexcel AS Biology Revision Guide. Edexcel.

2. https://alevelbiology.co.uk/notes/haemoglobin-structure-and-function/

3. https://media.lanecc.edu/users/driscolln/RT127/Softchalk/Oxygen_transport_softchalk/Oxygen_Transport_Lesson2.html

4. https://ib.bioninja.com.au/options/option-d-human-physiology/d6-transport-of-respiratory/bohr-shift.html

5. https://embryology.med.unsw.edu.au/embryology/index.php?title=File:Haemoglobin_comparison_oxygen_saturation_curve.png

6. http://www.pathwaymedicine.org/carbon-dioxide-transport