Haemoglobin

• The role of haemoglobin:

  • Haemoglobin is an important part of the circulatory system, human haemoglobin is found in red blood cells - its role is to carry oxygen around the body
  • There are many chemically similar types of haemoglobin found in many different organisms, all of which carry out the same function
  • As well as being found in all vertebrates, haemoglobin is found in earthworms, starfish, insects, plants and even in some bacteria
• • Haemoglobin is a large protein with a quaternary structure - its made up of four polypeptide chains
  • Each chain has a haem group which contains an iron ion and gives haemoglobin its red colour
  • Each molecule of human haemoglobin can carry four oxygen molecules
• • In the lungs, oxygen joins to haemoglobin in red blood cells to form oxyhaemoglobin
  • This is a reversible reaction - near the body cells, oxygen leaves oxyhemoglobin and it turns back to haemoglobin
  • When an oxygen molecule joins to haemoglobin its referred to as association or loading, and when oxygen leaves oxyhaemoglobin its referred to as dissociation or unloading
  • Hb + 4O2 <> HbO8
• Affinity for oxygen means the tendency a molecule has to bind with oxygen. Haemoglobin affinity for oxygen varies depending on the conditions that affects it is the partial pressure of oxygen.

  pO2 is a measure of oxygen concentration. The greater the concentration of dissolved oxygen in cells, the higher the partial pressure

  As pO2 increases, haemoglobin affinity for oxygen also increases:

  • Oxygen loads onto haemoglobin to form oxyhaemoglobin where there's a high pO2
  • Oxyhaemoglobin unloads its oxygen where there's a lower 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 - this lowers the 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
• An oxygen dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure. The affinity of haemoglobin for oxygen affects how saturated the haemoglobin is:

  • Where pO2 is high, haemoglobin has a high affinity for oxygen, so it has a high saturation of oxygen
  • Where pO2 is low, haemoglobin has a low affinity for oxygen, so it has a low saturation of oxygen
• • Weirdly, the saturation of haemoglobin can also affect the affinity - this is why the graph is S shaped and not a straight line
  • When haemoglobin combines with the first O2 molecule, its shape alters in a way that makes it easier for other O2 molecules to join too
  • But as the haemoglobin starts to become saturated, it gets harder for more oxygen molecules to join
  • As a result, the curve has a steep bit in the middle where its really easy for oxygen molecules to join and shallow bits at each end where its harder
  • When the curve is deep, a small change in pO2 causes a big change in the amount of oxygen carried by the haemoglobin
• • The partial pressure of carbon dioxide (pCO2) is a measure of the concentration of CO2 in a cell.
  • To complicate matters, pCO2 also affects oxygen unloading
  • Haemoglobin gives up its oxygen more readily at a higher pCO2
  • Its a cunning way of getting more O2 to cells during activity
• • When the cells respire they produce carbon dioxide, which raises the pCO2
  • This increases the rate of oxygen unloading (the rate at which oxyhaemoglobin dissociates to form haemoglobin and oxygen) - so the dissociation curve shifts right (but remains the same shape)
  • The saturation of blood with oxygen is lower for a given pO2 meaning that more oxygen is being released. This is called the Bohr effect
• • Different organisms have different types of haemoglobin with different oxygen transporting capacities - it depends on things like where they live, how active they are and their size.
  • Having a particular type of haemoglobin is an adaptation that helps the organism survive in a particular environment
• Low oxygen environments

  • Organisms that live in environments with a low concentration of oxygen have haemoglobin with a higher for affinity of oxygen that human haemoglobin
  • This is because there isn't much oxygen avaliable, so the haemoglobin has to be very good at loading any available oxygen
  • The dissociation curve of their haemoglobin is to the left of ours
• High activity levels

  • Organisms that are very active and have a high oxygen demand have haemoglobin with a lower affinity oxygen than human haemoglobin
  • This is because they need their haemoglobin to easily unload oxygen, so that its available for them to use
  • The dissociation curve of their haemoglobin is to the right of the human one
• Size:

  • Small mammals tend to have a higher surface area to volume ratio than larger mammals.
  • This means they lose heat quickly, so they have a high metabolic rate to help them keep warm - which means they have a high oxygen demand
  • Mammals that are smaller than humans have haemoglobin with a lower affinity for oxygen than human haemoglobin, because they need their haemoglobin to easily unload oxygen to meet their high oxygen demand.
  • The dissociation curve of their haemoglobin is to the right of the human one
• Describe and explain the effect of increasing carbon dioxide concentration on the dissociation of oxyhaemoglobin

  • Increases/more oxygen dissociation/unloading OR Deceases haemoglobin’s affinity for O2
  • (By) decreasing (blood) pH/increasing acidity;