3.3.4.1 Mass transport in animals

• Haemoglobin is the oxygen-transporting red pigment inside erythrocytes (red blood cells).
• Organisms that are very active (high respiration rate) have a high oxygen demand and haemoglobin with a lower affinity for oxygen than human haemoglobin.
• Haemoglobin is a globular protein with a structure that curls up so that hydrophilic side chains face outwards and hydrophobic side chains face inwards, making it soluble and good for transport in the blood.
• A prosthetic haem group with an Fe2+ ion is associated with each polypeptide chain in mammal haemoglobin. Each haem group can combine with two atoms/one molecule of oxygen.
• In each haem group there is one Fe2+ ion. Each ion in a haemoglobin molecule can combine with a single oxygen molecule, meaning a single haemoglobin molecule can carry 4 oxygen molecules, which is called oxyhaemoglobin.
• At the respiring tissues, haemoglobin has a low affinity for oxygen due to low pO2 so oxygen is dissociated (unloaded) from haemoglobin.
• The Bohr effect is the phenomenon where the presence of carbon dioxide helps the release of oxygen from haemoglobin.
• Conditions at respiring tissues such as temperature, carbon dioxide, lactic acid, and pH can affect the oxygen dissociation curve.
• Myoglobin acts as an oxygen storage compound and has a greater affinity for oxygen than haemoglobin, being rapidly reloaded after exercise.
• The oxygen dissociation curve favours uptake of oxygen in the lungs and release of oxygen in the tissues.
• Haemoglobin has four polypeptide chains and is made up of two alpha polypeptides and two beta polypeptides.
• The structure of haemoglobin is quaternary.
• When haemoglobin has a high affinity, it is more likely to load oxygen and it is less likely to release oxygen (dissociate).
• Blood arriving at the lungs has a lower partial pressure of oxygen (pO2) than that in the lungs, creating a diffusion gradient and causing oxygen to move from the alveoli into the blood.
• When haemoglobin has a low affinity, it is more likely to unload oxygen (dissociate) and less likely to load oxygen.
• The oxygen dissociation curve has a sigmoid shape, indicating that haemoglobin’s affinity for oxygen changes as the partial pressure of oxygen (pO2) changes.
• Haemoglobin's affinity for oxygen depends on the partial pressure of oxygen (pO2).
• Oxygen combines with haemoglobin to form oxyhaemoglobin where there's a high partial pressure of oxygen (pO2), and oxyhaemoglobin breaks down to haemoglobin and oxygen where there's a lower partial pressure of oxygen (pO2).
• The partial pressure of oxygen (pO2) can be thought of as a measure of oxygen concentration and is higher in the lungs and lower in body tissues such as muscle.
• The oxygen dissociation curve shows the relationship between the partial pressure of oxygen in the surroundings and saturation of haemoglobin.
• A prosthetic group is a non-protein group forming part of or combined with a protein.
• Organisms that live in low oxygen environments have haemoglobin with a higher affinity for oxygen than human haemoglobin.
• Myoglobin, a red pigment in mammalian muscles, has a higher affinity for oxygen than haemoglobin, storing oxygen and releasing it only when pO2 is very low.
• Haemoglobin carries oxygen, also known as oxyhaemoglobin, in red blood cells.
• Haemoglobin loads and unloads oxygen in the blood.
• In the lungs, haemoglobin has a high affinity for oxygen due to high pO2 so haemoglobin readily loads oxygen.
• Oxygen loads onto haemoglobin at high partial pressure.
• Tissues have a low partial pressure of oxygen as it has been used in respiration.
• Haemoglobin is called oxyhaemoglobin when it is fully saturated with oxygen.
• The affinity of haemoglobin for oxygen relates to the tendency of haemoglobin to bind to oxygen.
• Explain a property of iron ions that enables these ions to carry out their role in red blood cells (2)

  • iron ions are charged/polar
  • so can bind to oxygen
• partial pressure = total absolute pressure x volume fraction of gas component 
• Partial Pressure of O2: The partial pressure of oxygen (pO2) can be thought of as a measure of oxygen concentration
• The oxygen dissociation curve is sigmoid shape because when the first molecule of O2 combines, it changes the quaternary structure of haemoglobin which creates another binding site for oxygen to bind to.
• Describe and explain effect of carbon dioxide concentration on the dissociation of oxyhemoglobin

  • increases oxygen dissociation
  • by decreasing blood pH

  (known as Bohr effect)
• Describe how haemoglobin loads and unloads oxygen in the blood.  (4)

  1. Oxygen loads onto haemoglobin at high partial pressure.
  2. In the lungs haemoglobin has a high affinity for oxygen.
  3. Tissues have a low partial pressure of oxygen as it has been used in respiration. 
  4. In tissues haemoglobin has a lower affinity for oxygen.
  5. Haemoglobin unloads oxygen at low partial pressure.
• Two types of circulatory systems:

  • Open
  • Closed
• Closed circulatory system:

  • Blood fully enclosed within blood vessels at all times
  • Blood pumped from the heart through progressively smaller vessels (capillaries)
  • Substances diffuse in and out of blood and into cells in capillaries
  • Blood returns to the heart via progressively larger vessels
• Features of a double circulatory system

  • Blood is confined to vessels and passes through heart twice for every circuit of the body. 
  • One circuit to the lungs (pulmonary) where it’s pressure is reduced
  • Blood returned to heart to boost its pressure before blood is circulated to the rest of the body (systemic)
  • This allows substances to be delivered to the rest of the body quickly
• Double closed circulatory systems are found in mammals and they involve two circuits:

  Pulmonary circuit 

  Systemic circuit