Haemoglobin

• Many organisms have to transport substances over large distances to get them to and from their exchange surfaces. Mass transport systems such as the circulatory system in animals, ensure the efficient movement of substances through the organism
• 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.
• Haemoglobin (Hb) is a large protein with a quaternary structure - it is made up of 4 polypeptide chins. Each chain has a Haem group which contains an iron ion and gives haemoglobin its red colour. Each molecule of human haemoglobin can carry 4 oxygen molecules
• In the lungs, oxygen joins to haemoglobin in red blood cells to form oxyhaemoglobin. This is a reversible reaction - the the body cells, oxygen leaves oxyhaemoglobin and it turns back into haemoglobin. When an oxygen molecule joins to haemoglobin it's referred to as association or loading, and when oxygen leaves oxyhaemoglobin its referred to as dissociaton or unloading
• Hb + 4O2 = HbO8
• Affinity for oxygen means the tendency a molecule has to bind with oxygen. Haemoglobin's affinity for oxygen varies depending on the conditions it's in - one of the conditions that affects it is partial pressure of oxygen (pO2)
• 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's affinity for oxygen also increased

  1. oxygen loads onto haemoglobin to form oxyhaemoglobin where there is a high pO2
  2. Oxyhaemoglobin unloads its oxygen where there is 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 for oxygen affects how saturated the haemoglobin is
• The shape of the oxygen dissociation curve depends on the affinity of haemoglobin for oxygen. If the affinity is high, the curve will be steep. If the affinity is low, the curve will be shallow
• Where pO2 is high (e.g. in the lungs), haemoglobin has a high affinity for oxygen, so it has a high saturation of oxygen. Where pO2 is low (e.g. in respiring tissues), haemoglobin has a low affinity for oxygen, so it has low saturation of oxygen
• Saturation of haemoglobin can also affect affinity - this is why the graph is 'S-shaped'. When haemoglobin combines with the first O2 molecule, its shape alters in a way which makes it easier for other O2 molecules to join too. But as the haemoglobin becomes more saturated, it gets harder for more oxygen molecules to join. As a result - curve is steep in the middle where it is easy for oxygen molecules to join and shallow at each end where it is harder. When the curve is steep, as 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
• pCO2 affects oxygen unloading. Haemoglobin gives up its oxygen more readily at higher pCO2. When cells respire they produce carbon dioxide which raises the pCO2. This increase the rate of oxygen unloading (i.e. the rate at which oxyhaemoglobin dissociates to form haemoglobin and oxygen). So the dissociation curve shifts right (stays shame shape). The saturation of blood with oxygen is lower for a given pO2, meaning that more oxygen is being realeased - the Bohr effect
• Organisms that live in environments with a low concentration of oxygen (e.g. lugworm) have haemoglobin with a higher affinity for oxygen than humans. This is because there isn't much oxygen available, so the haemoglobin has to be very good at loading any available oxygen. The dissociation curve is to the left.
• Organisms that are very active and have a high oxygen demand have haemogobin with a lower affinity for oxygen than humans. This is because they need their haemoglobin to easily unload oxygen, so that is is available for them to use. The dissociation curve is to the right
• Small mammals tend to have higher surface area to volume ratio that large 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 humans because they need their haemoglobin to easily unload oxygen to meet their high oxygen demand. The dissociation curve is to the right.