Haemoglobin

Haemoglobin and Oxyhaemoglobin

• Human haemoglobin is found in red blood cells - it carries oxygen around the body. 
• It is a large protein with a quaternary structure - it has four polypeptide chains. Each chain has a haem group which contains iron and gives it it's red colour. 
• Each molecule can carry four oxygen molecules. 
• In the lungs, oxygen joins to the iron to form oxyhaemoglobin. 
• This is a reversible reaction. 
• When an oxygen molecule joins it is known as association or loading. and when it leaves it is dissociation or unloading. 

Affinity for Oxygen and pO2

• Affinity means the tendency a molecule has to bind. 
• The affinity varies depending on the conditions - one being the partial pressure of oxygen (pO2)
• This is a measure of oxygen concentration. 
• The greater the concentration of dissolved oxygen in cells, the higher the partial pressure. 
• As pO2 increases, haemoglobin's affinity for oxygen increases:
• Oxygen loads onto haemoglobin to form oxyhaemoglobin when there's high pO2. 
• Oxygen enters blood capillaries at the alveoli in the lungs. 
• Alveoli have a high pO2 so oxygen loads onto haemoglobin to form oxyhaemoglobin. 
• When cells respire, they use up oxygen - lowering pO2.
• Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen.
• The haemoglobin then returns to the lungs to pick up more oxygen. 

Dissociation Curves

• Shows how saturated the haemoglobin is with oxygen at any given partial pressure. 
• The affinity of haemoglobin affects its saturation. 
• Where partial pressure is high, haemoglobin has a high affinity…