mass transport

• Retrieval question 1: How are red blood cells adapted to their function?
• Retrieval question 2: What is the role of a lymphocyte cell?
• Retrieval question 3: What is the role of platelets?
• Retrieval question 4: What does plasma carry in the blood?
• Specification Points: Describe the need to transport substances into and out of a range of organisms, including oxygen, carbon dioxide, water, dissolved food molecules, mineral ions and urea. Explain how the structure of the blood is related to its function: a red blood cells (erythrocytes) b white blood cells (phagocytes and lymphocytes) c plasma d platelets
• Mass transport systems transport organic and inorganic substances around the body. They are needed by larger organisms to supply organic and inorganic substances to exchange membranes.
• Haemoglobin


  • Has a quaternary structure made of four polypeptides (Alpha- x2 and beta-globin x2), held in place by hydrogen bonds and ionic bonds, contains a haem group (prosthetic group) in the middle of each polypeptide which is flat and circular with iron (Fe2+) in the middle, soluble in water
• Haemoglobin - conjugated protein
• How could this structure be related to haemoglobin's function? The iron in the haem group binds loosely to oxygen. One molecule of haemoglobin can carry up to 4 molecules of oxygen
• Why do we need haemoglobin? Oxygen is not very soluble, so Haemoglobin (Hb) inside red blood cells is required to transport O2.
• Oxyhaemoglobin: Oxygen can be loaded and unloaded onto haemoglobin. When loaded, this forms oxyhaemoglobin. As the partial pressure (oxygen concentration) increases, so does the affinity of haemoglobin for oxygen
• Quick equation: Percentage saturation of haemoglobin = X100 Oxygenated hemoglobin / Maximum saturation
• Positive cooperativity: Oxygen binds to haemoglobin in a process of positive cooperativity. As each subunit of haemoglobin binds oxygen, the affinity of the remaining unbound subunits increases as their shape changes
• How haemoglobin transports oxygen around the body
• Oxygenated hemoglobin


  • Maximum saturation
• Oxygen binds to haemoglobin in a process of positive cooperativity


  As each subunit of haemoglobin binds oxygen the affinity of the remaining unbound subunits increases as their shape changes
• How haemoglobin transports oxygen around the body


  The ability of haemoglobin to take up and release oxygen depends on the amount of oxygen in the surrounding tissues
• The amount of oxygen is measured by the relative pressure that it contributes to a mixture of gases. This is called the partial pressure (pO2) or oxygen tension (kPa)
• Factors affecting the ability of haemoglobin to take up and release oxygen

  • Amount of oxygen in the surrounding tissues
• The sigmoid oxygen dissociation curve of haemoglobin shows the percentage saturation of Hb with O2 in relation to how much oxygen is in the environment
• Binding is cooperative, giving an S-shaped (SIGMOIDAL) curve, allowing haemoglobin to pick up oxygen in the lungs (where O2 concentration is high) and drop off oxygen at the tissues (where O2 concentration is low)
• Percentage saturation = how much oxygen Hb picks up
• A small increase in the partial pressure of oxygen (pO2) causes a disproportionately large increase in the percentage saturation of Hb


  Oxygen has a very high affinity for O2 when partial pressure is high
• A small decrease in the pO2 causes a disproportionately large decrease in the percentage saturation of Hb


  Hb has a low affinity for O2 when the partial pressure is low
• Myoglobin is a protein found in muscle cells and ensures that the high oxygen demand of muscles is met
• Myoglobin has just one subunit (compared to the four of Hb) so there is no cooperative binding
• Myoglobin has a much higher oxygen affinity than haemoglobin and its affinity for oxygen is unaffected by partial pressure, acting like an oxygen store
• Fetal Haemoglobin has a different affinity for oxygen compared to adult haemoglobin, allowing the fetus to take O2 from its mother
• If fetal Hb had the same affinity as adult Hb, no O2 could pass from mother to fetus
• Fetal haemoglobin


  Has a different affinity for oxygen compared to adult haemoglobin
• High affinity of fetal Hb


  Allows the fetus to take O2 from its mother
• If fetal Hb had the same affinity as adult Hb

  No O2 could pass from mother to fetus
• Bohr effect


  Most oxygen is required by the most active cells, which produce higher concentrations of carbon dioxide
• Higher levels of carbon dioxide

  Cause oxyhaemoglobin to release more oxygen
• Bohr effect is the effect of CO2 on the O2 dissociation curve of Hb
• When levels of CO2 are high, Hb binds with O2 less well (reduced affinity) and releases O2 rather than takes it up
• In the lung capillaries, where CO2 is low, Hb binds with O2 more easily (increased affinity) and binds to O2 rather than releasing it
• Summary: pH and temperature are expected to affect affinity due to the Bohr effect
• Organism's oxygen dissociation curve differs from other mammals by being shifted to the left, indicating higher affinity for oxygen at lower pO2
• Birds engaging in sustained flight have oxygen dissociation curves shifted to the right, with lower affinity for oxygen at a given pO2