mass transport

• Mass transport


  Bulk movement of gases or liquids in one direction (usually, via a system of vessels and tubes)
• Need for a circulatory system

  • Constant supply of reactants for metabolism = O2 & glucose
  • Molecules can diffuse to all parts of the cell quickly = short diffusion distances (single cell organisms)
  • Larger organisms = many layers of cells = increases time taken for diffusion = far too long = exchange surfaces connected to mass transport system
  • Digestive system connected to circulatory system
  • Lungs connected to circulatory system
• Purpose of mass transport

  • Transports substances quickly from one exchange site to another
  • Maintains diffusion gradients at exchange sites and between cells and fluid surroundings
  • Effective cell activity = keeps immediate fluid environment of cells within a suitable metabolic range
• Circulatory system


  Mass transport system = one-way flow of blood within blood vessels carries essential nutrients and gases to all cells of body
• Haemoglobin


  • Group of chemically similar molecules
  • Globular protein = O2 carrying pigment found in red blood cells
• Red blood cells


  • Biconcave discs = concave on both sides = creates high sa:v ratio for diffusion of gases
  • No nucleus = provides more space inside cell for O2 to transport
• Haemoglobin molecules


  Protein molecules w/ a quaternary structure evolved to make efficient loading of O2 under one set of conditions but unloading it under a different set of conditions
• Haemoglobin Structure

  • Primary: sequence of amino acids in 4 polypeptide chains
  • Secondary: polypeptide chains coiled into helix
  • Tertiary: each polypeptide chain is folded into a precise shape, important factor for ability to carry O2
  • Quaternary: all 4 polypeptides linked together form an almost spherical molecule
• Prosthetic haem group

  Each polypeptide is associated with a prosthetic haem group that contains an fe2+ ion
• Oxygen binding to haemoglobin


  1. Each fe2+ ion combines with a single O2 molecule

  2. Total of 4 O2 molecules carried by a single haemoglobin molecule

  3. Fe2+ reversibly combines with O2 to form oxyhaemoglobin

  4. Oxygen + haemoglobin -> oxyhaemoglobin

  5. 4O2 + Hb -> Hb4O2
• Oxyhaemoglobin


  Causes blood to appear bright red
• Loading and unloading O2


  1. Haemoglobin binds with O2 = loading/associating = in lungs

  2. Haemoglobin releases O2 = unloading/dissociating = in tissues
• Cooperative binding


  Binding of the first O2 molecule results in a conformational change in structure of haemoglobin molecule = easier for successive O2 molecule to bind
• Reverse cooperative binding


  When O2 dissociates in the tissues
• Haemoglobins with high affinity for O2


  Take up O2 more easily but release it less easily
• Haemoglobins with low affinity for O2


  Take up O2 less easily but release it more easily
• Role of haemoglobin


  Bind and transport O2 to tissues to be used in aerobic metabolic pathways
• O2 not soluble in H2O but Hb are = efficient O2 transport if bound to Hb = Hb must readily associate with O2 at surface where gas exchange takes place & readily dissociate from O2 at those tissues requiring it
• Changes its affinity (chemical attraction) for O2 under different conditions


  Happens because the structure changes in the presence of certain substances
• Presence of CO2


  New shape of haemoglobin molecule binds more loosely to O2 = releases O2
• Shape of O2 dissociation curve

  • Shape of Hb molecules makes it difficult for first O2 molecule to bind to one of the sites on one of four polypeptide subunits because they are closely united
  • At low O2 conc = little O2 binds to haemoglobin = gradient of curve = shallow initially
• Binding of first O2 molecule

  1. Binding site of first O2 molecule changes quaternary structure of Hb molecule

  2. Changes shape

  3. Makes it easier for other subunits to bind to an O2 molecule
• Binding of first O2 molecule

  Induces other subunits to bind to an O2 molecule
• Binding of 2nd O2 molecule

  Takes a small increase in partial pressure of O2 compared to binding of 1st one
• Positive cooperativity


  Binding of 1st molecule makes binding of 2nd easier and so on = gradient of curve steepens
• Binding of 3rd O2 molecule

  • In theory = easier for haemoglobin to bind the 4th O2 molecule
  • In practice = harder = due to probability with majority of binding sites occupied = less likely that a single O2 molecule will find an empty active site to bind to = gradient of curve reduces and graph flattens off
• Haemoglobin (Hb)


  Quaternary protein made up of 4 globin polypeptides & 4 haem groups
• Haemoglobin


  • Structure of haem is identical in all types of haemoglobin but globin chains can differ between species
  • Globin polypeptides determine precise properties of Hb
• Hb oxygen-binding properties


  Bind to and release oxygen in different conditions
• Environmental factors


  Major impact on evolution of Hb within species
• Hb of species living at high altitudes

  • Binds much more readily to oxygen
  • Can obtain sufficient level of oxygen saturation in blood when partial pressure of oxygen in air is low
• Foetal haemoglobin


  • Has higher affinity for oxygen than adult Hb
  • Allows foetus to obtain oxygen from mother's blood at placenta
  • Shifts oxygen dissociation curve to the left - at any given partial pressure of oxygen, foetal Hb has higher saturation % than adult Hb
• After birth, baby begins to produce adult Hb which gradually replaces foetal Hb
• Importance of adult Hb

  Important for easy release of oxygen in respiring tissues of a more metabolically active individual
• Diffusion


  • Fast enough for short distance transport
  • Efficient supply of materials over larger distances requires mass transport system
• All organisms exchange materials between themselves and environment
• Increasing size of organisms

  Surface area to volume ratio decreases to point where needs cannot be met by body surface alone
• Specialist exchange surfaces


  Located in specific regions of organism to absorb nutrients, respiratory gases and remove excretory products
• Transport system


  • Required to take materials from cells to exchange surfaces and from exchange surfaces to cells
  • Materials transported between exchange surfaces and environment and between different parts of organism
• • Mammals = closed double circulatory system = blood confined to vessels & passes 2 through heart for each complete circuit of body 
  • blood passed through lungs = pressure reduced
  • so if it passes immediately to rest of body = slow pressure would make circulation very slow 
  • blood = returned to heart to increase pressure before being circulated to rest of tissues 
  • substances = delivered quick to rest of body
  • mammals = high body temp & high rate of metabolism 
  • vessels that make up circulatory system of mammal are divided into 3 types = arteries, veins, capillaries