Transport of oxygen

• what does the word affinity mean?


  attraction between the oxygen and haemoglobin
• why can't complex organisms simply rely on dissolving oxygen in plasma?

  • simply dissolving oxygen in plasma is inadequate in complex organisms
  • even if plasma is 100% saturated with oxygen, this will not supply enough oxygen to tissue as a fast enough rate
• which mechanism have complex organisms developed to overcome the problem of simple oxygen dissolving in plasma?

  • respiratory pigment (haemoglobin) evolved to allow adequate supply of oxygen to tissues
  • the oxygen transported by these pigments is in addition to that transported by the plasma
• what are the features of haemoglobin?

  • prosthetic haem group
  • quaternary structure (4 polypeptide chains)
  • each prosthetic prosthetic group contains an iron molecule
  • haem group has a high affinity for oxygen
• what is the word equation to show the reaction between haemoglobin and oxygen?

  haemoglobin + oxygen (reversible) oxyhaemoglobin
• why is it essential that the reaction between haemoglobin and oxygen reversible?

  so that the oxygen can bind to the haemoglobin and can be transported to where it is required, but then the oxyhaemoglobin needs to be disassociated so that the oxygen can be used in respiring tissues, this allows the erythrocyte to be reused
• what are the key features of haemoglobin which enable it to be efficient?

  • readily associate with oxygen at the gaseous exchange surface
  • readily disassociate with oxygen at those tissues requiring it
  • this means the affinity of haemoglobin for oxygen changes depending on the oxygen concentration
• what does partial pressure mean?

  the proportion of total pressure provided by a particular gas as a part of a mixture of gases

  • pO2 - partial pressure of oxygen
  • pCO2 - partial pressure of CO2
• what does oxygen tension mean?

  the amount of oxygen in the air expressed as the pressure created by the presence of oxygen, expressed as kilopascals (kPa)
• describe the affinity of haemoglobin for oxygen at gas exchange surfaces

  • high oxygen tension (high pO2)
  • low CO2 tension (low pCO2)
  • high affinity
  • results in oxygen binding to haemoglobin
• describe the affinity of haemoglobin for oxygen at respiring tissue

  • lower oxygen tension (low pO2)
  • high CO2 tension (high pCO2)
  • low affinity
  • oxyhaemoglobin dissociates
• why is the affinity for oxygen low at low oxygen tension?

  • so that oxyhaemoglobin can dissociate at respiring tissues which will need more oxygen for aerobic respiration
• why does the binding of one oxygen molecules make it easier for subsequent molecules to bind?

  • haemoglobin takes one oxygen molecule out of solution and so maintains a steep diffusion gradient, allowing more oxygen to bind
• what is meant by positive cooperativity?

  • the arrangement of the haemoglobin molecule means that as soon as one oxygen molecule binds to a haem group, the molecule goes through a conformational change, making it easier for the next oxygen molecules to bind
• how does positive cooperativity maintain diffusion gradients of oxygen?


  as the oxygen is bound to the haemoglobin, the free oxygen concentration in the erythrocyte stays low, so a steep diffusion gradient is maintained until all the haemoglobin is saturated with oxygen
• what happens once the third molecule of oxygen has bound to the haemoglobin?


  its more saturated meaning there is a conformational change, making it difficult for the last oxygen molecule to bind
• where partial pressure of oxygen is high = high affinity of haemoglobin
• what is an oxygen dissociation curve?

  • an important tool for understanding how the blood carries and releases oxygen, they show the affinity of haemoglobin for oxygen
  • the percentage saturation haemoglobin in the blood is plotted against the partial pressure of oxygen
• what is an oxygen dissociation curve also know as?

  sigmoid curve
• why does foetal haemoglobin have a higher affinity for oxygen than that of adults?

  • foetus is completely dependent on its mother to supply it with oxygen
  • oxygenated adult blood runs close to foetal deoxygenated blood in placenta
  • if affinity was same, then little or no oxygen would be transferred to the blood of the foetus