Animal Transport 3

• Haemoglobin binds to oxygen in the lungs and releases it in the respiring tissue
• 4O2 + Hb -> (reversible) Hb.4O2

  oxygen + haemoglobin -> oxyhaemoglobin
• Haemoglobin must readily associate with oxygen where gas exchange takes place (e.g. alveoli) and readily dissociate from oxygen at the respiring tissue (e.g. muscle)
• Haemoglobin can perform the requirements by changing its affinity for oxygen as it changes its shape
• Affinity = degree to which two molecules are attracted to each other
• Each haemoglobin molecule has 4 haem groups

  • each haem contains an ion of iron (Fe2+)
• Cooperative binding:

  • one oxygen can bind to each iron -> 4O2 can bind to each haemoglobin
  • first oxygen that attaches changes the shape of the haemoglobin molecule -> making it easier for the second molecule to attach
  • second oxygen that attaches changes the shape of the haemoglobin molecule -> making it easier for the third molecule to attach
  • third oxygen molecule doesn’t change the haemoglobin shape so it takes a large increase in oxygen partial pressure to bind the fourth oxygen molecule
• Cooperative binding = increasing ease with which haemoglobin binds its second and third oxygen molecules
• Cooperative binding allows haemoglobin to pick up oxygen very rapidly in the lungs
• % oxygen saturation = how much haemoglobin is occupied with all 4 oxygens
• • Red blood cells load oxygen in the lungs where the oxygen partial pressure is high -> haemoglobin is saturated with oxygen
  • cells carry the oxygen, as oxyhaemoglobin, to respiring tissue, e.g. muscle
  • At respiring tissue, oxygen partial pressure is low -> oxyhaemoglobin unloads oxygen and it dissociates
• Oxygen affinity of haemoglobin is high at high partial pressure of oxygen -> oxyhaemoglobin doesn’t readily release
• Oxygen affinity reduces as the partial pressure of oxygen decreases and oxygen is readily release, meeting respiratory demands
• The more to the left, the more readily haemoglobin associates with oxygen and less easily it dissociates from it
• High metabolic rate -> lower affinity of oxygen -> curve to the left
• Bigger the animal -> slower metabolic rate (heart rate) -> higher affinity
• If haemoglobin has a higher affinity -> it loads oxygen more readily into oxyhaemoglobin
• As levels of carbon dioxide increases, haemoglobin has a lower affinity for oxygen so oxygen is released more readily
• Haemoglobin unloads oxygen from oxyhaemoglobin in respiring tissue as the partial pressure of carbon dioxide is high so oxygen is needed
• Bohr effect = movement of the oxygen dissociation curve to the right at a higher partial pressure of carbon dioxide, as at a given oxygen partial pressure, haemoglobin has had a lower affinity for oxygen
• Carbon dioxide is transported in 3 ways:

  • in solution, in the plasma (approximately 5%)
  • as the hydrogen carbonate ion, HCO3- (approximately 85%)
  • bound to haemoglobin as carbamino-haemoglobin (approximately 10%)
• Transported as HCO3-:

  • Carbon dioxide in the blood diffuses into the red blood cell
  • carbonic anyhydrase catalysss the combination of carbon dioxide and water, making carbonic acid
  • carbonic acid dissociates into H+ and HCO3- ions
  • HCO3- ions diffuses out of the red blood cell into the plasma, chlorine diffuses into the red blood cell to maintain electrochemical neutrality, known as the chloride shift
  • H+ ions cause oxyhaemoglobin to dissociate into oxygen and haemoglobin
  • H+ ions combine with haemoglobin to form haemoglobinic acid
  • oxygen diffuses out of the red blood cell into the tissue