mass transport in animals

• Cells of all living organisms

  • Need a constant supply of reactants for metabolism, e.g. oxygen and glucose
• Single-celled organisms

  Can gain oxygen and glucose directly from their surroundings, and the molecules can diffuse to all parts of the cell quickly due to short diffusion distances
• Larger organisms


  Made up of many layers of cells, meaning that the time taken for substances such as glucose and oxygen to diffuse to every cell in the body would be far too long
• The diffusion distances involved are too great
• To solve the problem of diffusion distances

  1. Their exchange surfaces are connected to a mass transport system

  2. The digestive system is connected to the circulatory system

  3. The lungs are connected to the circulatory system
• Mass transport

  The bulk movement of gases or liquids in one direction, usually via a system of vessels and tubes
• Mass transport systems

  • Help to bring substances quickly from one exchange site to another
  • Help to maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings
  • Ensure effective cell activity by keeping the immediate fluid environment of cells within a suitable metabolic range
• Haemoglobins

  A group of chemically similar molecules found in many different organisms
• Haemoglobin

  • A globular protein which is an oxygen-carrying pigment found in vast quantities in red blood cells
  • Red blood cells are biconcave discs, meaning that they are concave on both sides
  • This creates a high SA:V ratio for the diffusion of gases
  • Red blood cells do not contain a nucleus, providing more space inside the cell for haemoglobin so that they can transport as much oxygen as possible
• Haemoglobin structure

  • A quaternary structure as it is made up of four polypeptide chains
  • These chains or subunits are globin proteins (two α–globins and two β–globins) and each subunit has a prosthetic haem group
  • The four globin subunits are held together by disulphide bonds and arranged so that their hydrophobic R groups are facing inwards, helping to preserve the three-dimensional spherical shape, and the hydrophilic R groups are facing outwards, helping to maintain solubility
• Changes to the sequence of amino acids in the subunits

  Can change the function of the protein, e.g. in sickle cell anaemia a base substitution that results in the amino acid valine (non-polar) replacing glutamic acid (polar) makes haemoglobin less soluble
• Haem group

  • Contains an iron II ion (Fe2+) which is able to reversibly combine with an oxygen molecule, forming oxyhaemoglobin
  • The presence of oxyhaemoglobin causes blood to appear bright red in colour
  • Each haemoglobin with the four haem groups can therefore carry four oxygen molecules, or eight oxygen atoms
• The haem group is the same for all types of haemoglobin but the globin chains can differ substantially between haemoglobins from different species
• Oxygen transport


  The majority of oxygen transported around the body is bound to the protein haemoglobin in red blood cells
• Red blood cells

  Also known as erythrocytes
• Haemoglobin

  • Each molecule contains four haem groups, each able to bond with one molecule of oxygen
  • Each molecule of haemoglobin can carry four oxygen molecules, or eight oxygen atoms in total
• Oxygen binding to haemoglobin

  1. Oxygen + Haemoglobin Oxyhaemoglobin

  2. 4O2 + Hb Hb4O2
• Cooperative binding


  The binding of the first oxygen molecule results in a conformational change in the structure of the haemoglobin molecule, making it easier for each successive oxygen molecule to bind
• Oxygen dissociates in the tissues

  The reverse of the oxygen binding process happens
• Oxygen dissociation curve


  Shows the rate at which oxygen associates, and also dissociates, with haemoglobin at different partial pressures of oxygen (pO2)
• Partial pressure of oxygen (pO2)


  The pressure exerted by oxygen within a mixture of gases; a measure of oxygen concentration
• Haemoglobin saturation

  When all of its oxygen binding sites are taken up with oxygen; when it contains four oxygen molecules
• Haemoglobin affinity for oxygen


  The ease with which haemoglobin binds and dissociates with oxygen
• High haemoglobin affinity for oxygen


  Binds easily and dissociates slowly
• Low haemoglobin affinity for oxygen


  Binds slowly and dissociates easily
• Binding of the first oxygen molecule to haemoglobin

  • It is difficult due to the shape of the haemoglobin molecule
  • It occurs slowly, explaining the relatively shallow curve at the bottom left corner of the graph
• Binding of the remaining oxygen molecules to haemoglobin

  • After the first oxygen molecule binds, the haemoglobin protein changes shape (conformation)
  • This makes it easier for the next haemoglobin molecules to bind
  • This speeds up binding of the remaining oxygen molecules
  • Explains the steeper part of the curve in the middle of the graph
• Cooperative binding


  The shape change of haemoglobin leading to easier oxygen binding
• Binding of the fourth oxygen molecule to haemoglobin


  • As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind
  • This is due to the shortage of remaining binding sites
  • Explains the levelling off of the curve in the top right corner of the graph
• Oxygen-haemoglobin dissociation curve


  Provides information about the rate at which haemoglobin binds to and dissociates from oxygen at different partial pressures of oxygen
• Reading the curve from left to right


  Provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen
• At low pO2


  • Oxygen binds slowly to haemoglobin, haemoglobin cannot pick up oxygen and become saturated as blood passes through oxygen-depleted tissues, haemoglobin has low affinity for oxygen, saturation percentage is low
• At medium pO2

  • Oxygen binds more easily to haemoglobin, saturation increases quickly, a small increase in pO2 causes a large increase in haemoglobin saturation
• At high pO2

  • Oxygen binds easily to haemoglobin, haemoglobin can pick up oxygen and become saturated as blood passes through the lungs, haemoglobin has high affinity for oxygen, saturation percentage is high, increasing pO2 by a large amount only has a small effect on percentage saturation
• Reading the curve from right to left


  Provides information about the rate at which haemoglobin dissociates with oxygen at different partial pressures of oxygen
• In the lungs, where pO2 is high


  • There is very little dissociation of oxygen from haemoglobin
• At medium pO2

  • Oxygen dissociates readily from haemoglobin, this region corresponds with the partial pressures of oxygen present in the respiring tissues of the body, a small decrease in pO2 causes a large decrease in percentage saturation of haemoglobin, leading to easy release of plenty of oxygen to the cells
• At low pO2


  • Dissociation slows again, there are few oxygen molecules left on the binding sites, the release of the final oxygen molecule becomes more difficult, in a similar way to the slow binding of the first oxygen molecule
• Bohr effect

  Changes in the oxygen dissociation curve as a result of carbon dioxide levels
• When the partial pressure of carbon dioxide in the blood is high


  Haemoglobin's affinity for oxygen is reduced