Haemoglobin

Haemoglobin definition

Water-soluble globular protein composed of 2 alpha and 2 beta polypeptide chains

Haem group function

Each haem group contains Fe2+ ion which binds one oxygen molecule

Oxygen carrying capacity

One haemoglobin molecule can carry four oxygen molecules

Partial pressure of oxygen (pO2)

Measure of oxygen concentration; higher dissolved O2 → higher pO2

Affinity for oxygen at high pO2

Haemoglobin binds O2 tightly (occurs in lungs, called loading)

Affinity for oxygen at low pO2

Haemoglobin releases O2 (occurs in respiring tissues, called unloading)

Loading definition

Oxygen binds to haemoglobin in lungs where pO2 is high

Unloading definition

Oxygen released from haemoglobin in tissues where pO2 is low

Oxygen dissociation curve definition

Graph showing % saturation of haemoglobin vs pO2

Initial part of curve

Shallow because first O2 molecule binds slowly

Middle part of curve

Steep due to positive cooperativity; first O2 binding increases affinity for next O2 molecules

Final part of curve

Flattens because fourth O2 molecule binds with difficulty

Positive cooperativity

Binding of one O2 changes haemoglobin shape, making it easier for next O2 to bind

Fetal haemoglobin definition

Haemoglobin in foetus with higher O2 affinity than adult haemoglobin

Reason for fetal haemoglobin

Higher affinity needed to absorb O2 from maternal blood at lower pO2 in placenta

Bohr effect definition

High CO2 concentration lowers haemoglobin’s affinity for O2

Mechanism of Bohr effect

CO2 increases acidity, alters haemoglobin shape, facilitates O2 release

Importance of Bohr effect

Ensures oxygen is released in respiring tissues where CO2 is high

Effect of saturation on affinity

After first O2 binds, haemoglobin changes shape, making binding of second and third O2 easier

Saturation at high pO2

In lungs, haemoglobin nearly 100% saturated

Saturation at low pO2

In tissues, haemoglobin saturation decreases, releasing oxygen

Effect of CO2 on dissociation curve

High CO2 shifts curve right → more O2 released; low CO2 shifts curve left → O2 binds more tightly

Relationship of haemoglobin to respiration

Oxygen transported efficiently from lungs to tissues; returns to lungs for reloading