7 - Mass transport

• Haemoglobin


  Chemically similar molecules found in a wide variety of organisms
• Haemoglobin molecules


  • Protein molecules with a quaternary structure that has evolved to make it efficient at loading oxygen under one set of conditions but unloading it under a different set of conditions
• Structure of haemoglobin


  1. Primary structure (sequence of amino acids in the four polypeptide chains)

  2. Secondary structure (each polypeptide chain is coiled into a helix)

  3. Tertiary structure (each polypeptide chain is folded into a precise shape)

  4. Quaternary structure (all four polypeptides are linked together to form an almost spherical molecule, each associated with a haem group containing a ferrous ion)
• Loading oxygen


  The process by which haemoglobin binds with oxygen, taking place in the lungs
• Unloading oxygen


  The process by which haemoglobin releases its oxygen, taking place in the tissues
• Haemoglobins with high affinity for oxygen


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


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


  • Readily associate with oxygen at the gas exchange surface
  • Readily dissociate with oxygen at tissues requiring it
• Affinity of haemoglobin for oxygen

  Changes due to shape changes in the presence of substances like carbon dioxide
• Affinity of haemoglobin for oxygen under different conditions

  • High at gas exchange surface (high oxygen, high carbon dioxide)
  • Low in respiring tissues (low oxygen, high carbon dioxide)
• Scientists observed that many organisms possessed haemoglobin and proposed it carried oxygen from the gas exchange surface to the tissues requiring it for respiration
• Different haemoglobins have different affinities for oxygen due to slightly different amino acid sequences resulting in different tertiary and quaternary structures
• Oxygen dissociation curve


  Graph of the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen
• Oxygen binding to haemoglobin

  1. Difficult for first oxygen molecule to bind due to closely united polypeptide subunits

  2. Binding of first oxygen molecule changes quaternary structure, making it easier for other subunits to bind oxygen

  3. After binding of third molecule, harder for fourth molecule to bind due to probability
• Oxygen dissociation curves


  • They all have a roughly similar shape but differ in their position on the axes
  • The further to the left the curve, the greater the affinity of haemoglobin for oxygen
  • The further to the right the curve, the lower the affinity of haemoglobin for oxygen
• Presence of carbon dioxide

  Reduces the affinity of haemoglobin for oxygen (Bohr effect)
• Oxygen loading, transport and unloading

  1. At gas exchange surface: low CO2, high pH, high O2 affinity, O2 readily loaded

  2. In tissues: high CO2, low pH, low O2 affinity, O2 readily unloaded
• Haemoglobin normally becomes saturated with oxygen as it passes through the lungs, but not all molecules are fully loaded
• In active tissues, more oxygen is unloaded from haemoglobin
• Diffusion


  Fast enough for transport over short distances
• Mass transport system


  Efficient supply of materials over larger distances
• Large organisms require a transport system
• Haemoglobin adaptations in different species

  • Lungworm: haemoglobin has high affinity for O2 to extract max from low-O2 environment
  • Llama: haemoglobin has high affinity for O2 to load at high altitudes
• Heart


  Muscular organ that lies in the thoracic cavity behind the sternum (breastbone) and operates continuously and tirelessly throughout the life of an organism
• Materials exchange between organisms and environment

  1. Small organisms - exchange over body surface

  2. Large organisms - specialist exchange surfaces required
• As organisms evolve into larger and more complex structures, tissues and organs become more specialised and dependent on one another
• This makes a transport system essential
• Transport system


  • Suitable medium (e.g. blood, air)
  • Mass transport (more rapid than diffusion)
  • Closed system of tubular vessels
  • Mechanism for moving transport medium (muscular contraction, passive processes)
  • Mechanism to maintain one-way flow (e.g. valves)
  • Mechanism to control flow to suit changing needs
  • Mechanism for mass flow of water/gases (e.g. breathing in mammals)
• Closed, double circulatory system in mammals

  Blood is confined to vessels and passes twice through the heart for each complete circuit of the body
• This is because when blood is passed through the lungs, its pressure is reduced, so it needs to be returned to the heart to boost pressure before circulating to the rest of the body
• Human heart


  • Two separate pumps lying side by side
  • The pump on the left deals with oxygenated blood from the lungs
  • The pump on the right deals with deoxygenated blood from the body
  • Each pump has two chambers
• The final exchange from blood vessels into cells is rapid because it takes place over a large surface area, across short distances and there is a steep diffusion gradient
• Atrium


  Thin-walled and elastic chamber that stretches as it collects blood
• Cardiac cycle


  The sequence of events that is repeated in humans around 70 times each minute when at rest
• Cardiac cycle


  1. Contraction (systole)

  2. Relaxation (diastole)

  
• Contraction


  • Occurs separately in the ventricles and the atria
• Relaxation


  • Takes place simultaneously in all chambers of the heart
• Ventricle


  Chamber with a much thicker muscular wall that has to contract strongly to pump blood some distance, either to the lungs or to the rest of the body
• Transport of water in the xylem

  1. Water absorbed by roots through root hairs

  2. Water transported through xylem vessels

  3. Evaporation of water from leaves (transpiration) pulls water through xylem
• Transpiration


  Evaporation of water from leaves